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Research Collection
Doctoral Thesis
Nucleation and Clarification of Polyethylenes
Author(s): Aksel, Seda
Publication Date: 2015
Permanent Link: https://doi.org/10.3929/ethz-a-010580676
Rights / License: In Copyright - Non-Commercial Use Permitted
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ETH Library
DISS. ETH NO. 22972
Nucleation and Clarification
of Polyethylenes
A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
SEDA AKSEL
Master of Science ETH in Materials Science, ETH Zurich
born on 12.04.1987
citizen of Turkey
accepted on the recommendation of
Prof. Dr. Paul Smith, examiner
Prof. Dr. Hans-Werner Schmidt, co-examiner
Prof. Dr. Jan Vermant, co-examiner
2015
Contents
Summary 1
Zusammenfassung 3
Chapter I Introduction 5
Chapter II Influence of Polymer Chain Architecture 21
on Opto-thermo-mechanical Properties of Polyethylenes
Chapter III Efficiency of Commercial Additives for 33
Nucleation and Clarification of Polyethylenes
Chapter IV New “Designer” Nucleating/Clarifying Agents for Polyethylene 55
Chapter V Phase Behavior of Polyethylene and 79
Selected Nucleating/Clarifying Agent Binary Systems
Chapter VI Influence of Polyethylene Macromolecular Structure on 97
Crystallization onto 1,2,3-trideoxy-4,6:5,7-bis-O-
[(4-propylphenyl)methylene]-nonitol
Chapter VII Conclusions and Outlook 113
Appendix 119
Acknowledgements 125
Curriculum Vitae 129
1
Summary
In general, polymers that can crystallize most often do so in the form of spherulitic structures, which are
known to efficiently scatter visible light (wavelength 400-700 nm). Thus, semi-crystalline polymers
generally exhibit high values of so-called “haze”, which correspond to translucent or even opaque
specimens. “Nucleating agents” – known to enhance heterogeneous nucleation of semi-crystalline
polymers – in some cases can yield small crystalline non-spherulitic entities in the final solid-state
structures which are not of the order of visible light range and reduce haze, in which case are termed as
“clarifying agents”.
Among semi-crystalline polymers, toughness under a wide range of environmental conditions due to its
low glass transition temperature (as opposed to, for instance, isotactic polypropylene, i-PP), and
relatively low melting temperature permitting relatively low energy-consuming production of industrial
artifacts make polyethylenes (PEs) attractive notably for the major packaging industry. Therefore, the
present thesis explores the efficient enhancement of the crystallization process by “nucleating agents”,
seeking optical transparency and, superior thermal and mechanical properties of PEs.
In a first approach, optical, thermal and mechanical properties of neat PEs of widely different chain
architectures were investigated. It is shown that while transparency can be readily achieved for the PEs
with a high degree of branching, a penalty incurs with a major reduction in their equally-relevant thermal
(i.e. melting temperature) and mechanical (i.e. stiffness) properties. In order to seek a compromise
between optical and thermo-mechanical properties, a range of nucleating/clarifying agents were
investigated for the PEs that intrinsically possess superior thermal and mechanical properties (i.e. high-
density polyethylene (HDPE) and linear low-density polyethylene (LLDPE)) to obtain advantageous
optical characteristics such as those observed for the low-density resins. In this study, the widely-used
“sorbitol”- and “1,3,5-benzene trisamide”- based commercial additives for i-PP, and newly designed
molecules based on “1,3,5-benzene trisamides” and “1,4-phenylene bisamides” were explored as
potential nucleating/clarifying agents for polyethylene.
In this study it was found that the sorbitol derivatives 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol
(DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol (TBPMN) enhanced
nucleation of LLDPE evidenced by increasing its crystallization peak temperature by up to 9 °C and,
in the case of TBPMN clarification imparted by decreasing the haze value of 1 mm thick injection-
molded plaque to values as low as 15 % as observed in commercial clarified i-PP. Furthermore, the
newly designed molecule, aramid-based N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide
(BCPCA) – which circumvents the disadvantage of the thermal instability of aldehyde-based sorbitols
– yielded almost the same reduction in haze as for DMDBS and at substantially lower concentrations
2
(i.e. a bulk haze of 30 % with only 0.1 % w/w BCPCA compared with 25 % bulk haze with 0.25 % w/w
DMDBS).
Investigations of the temperature/composition phase behavior of binary PE/additive systems revealed
that favorable dispersion of the additive in the polymer melt at a concentration range, i.e. hyper-eutectic
regime but below the onset of liquid-liquid phase separation, yields fine fibrils of the additive for the
polymer chains to subsequently crystalize onto them into a non-spherulitic arrangement as observed for
i-PP. Further studies on the microstructure of the solid-state material at those favorable additive
concentrations revealed that addition of sorbitol-based DMDBS and TBPMN, as well as the newly
designed additive BCPCA, prevents spherulitic growth of PE during crystallization – instead a molecular
arrangement featuring random rod-like shish-kebab-type structures, which reduce light scattering was
found to form. However, further analysis of experiments with different PEs possessing different
macromolecular structures proved that preventing spherulitic growth is not sufficient for obtaining
improved clarification, and that the additive fibril-widths as well as the polymer lamellar-widths play an
important role to reduce haze.
3
Zusammenfassung
Polymere, die kristallisieren, bilden meist Sphärolithe. Solche Strukturen streuen sichtbares Licht (400-
700nm). Teilkristalline Polymere sind deshalb transluzent oder gar opak, d.h. weisen hohe sogenannte
“Haze”-Werte auf. “Nukleierungsmittel” werden verwendet für die Erhöhung der heterogenen
Nukleierung von teilkristallinen Polymeren. In bestimmten Fällen bewirkt deren Verwendung die
Bildung kleiner, nicht-sphärolithischer Einheiten, welche nicht der Grössenordnung der sichtbaren
Wellenlänge entsprechen. Nukleierungsmittel, welche auf diese Weise eine Reduzierung des Haze-
Wertes ermöglichen, werden auch “Klärungsmittel” genannt.
Die Zähigkeit von Polyethylen (PE) unter verschiedensten Umgebungsbedingungen – bedingt durch
eine tiefe Glasübergangstemperatur (z.b. verglichen mit dem weit verbreiteten isotaktischen
Polypropylen, i-PP), sowie dessen ebenfalls tiefe Schmelztemperatur und die damit verbundene
sparsame Produktion, machen es zu einem attraktiven Material für die Verpackungsindustrie. Motiviert
durch diese Vorteile untersucht die vorliegende Dissertation die Optimierung des
Kristallisationsprozesses von Polyethylen durch Nukleierungsmittel mit dem Fokus auf die
Verbesserung der Lichtdurchlässigkeit sowie der thermischen und mechanischen Eigenschaften.
In einem ersten Teil werden die optischen, thermischen und mechanischen Eigenschaften purer PEs mit
verschiedensten Kettenarchitekturen aufgezeigt. PEs mit hoher Verästelung können bereits transparent
hergestellt werden, jedoch mit signifikanten Einbussen bezüglich Schmelztemperatur und Steifigkeit.
Mit dem Ziel, einen vergleichbar attraktiveren Kompromiss zu finden, wird im Anschluss die
Verwendung einer Reihe von Nukleierungs-/Klärungsmittel mit PEs mit intrinsisch besseren
thermischen und mechanischen Eigenschaften (z.b. Hochdichte-Polyethylen (HDPE) und lineares
Polyethylen niederer Dichte (LLDPE)) analysiert, um deren Lichtdurchlässigkeit zu verbessern. Weit
verbreitete “Sorbitol”- und “1,3,5-Benzoltrisamid”-basierte kommerziell erhältliche Zusatzstoffe für i-
PP, sowie neuentwickelte Moleküle basiert auf “1,3,5-Benzoltrisamid” und “1,4-Phenylenbisamid”
werden untersucht im Hinblick auf deren Potential als Nukleierungs-/Klärungsmittel für PEs.
Für LLDPE wird eine Erhöhung der Kristallisationstemperatur um ~9°C erreicht unter Verwendung der
Sorbitol-Derivate 1,3:2,4-Bis(3,4-Dimethylbenzylidene)Sorbitol (DMDBS) und 1,2,3-Trideoxy-
4,6:5,7-Bis-O-[(4-Propylphenyl)Methylen]-Nonitol (TBPMN). Im Falle von TBPMN wurde eine
Minderung des Haze-Werts auf bis zu ~15% beobachtet, ähnlich zu kommerziell geklärtem i-PP.
Darüber hinaus bewirkt das neuentwickelte Aramid-basierte N,N’-Bis(cyclohexylmethyl)-1,4-phenylen
dicarboxamid (BCPCA) – welches den Nachteil der thermischen Instabilität von Aldehyd-basierten
Sorbitolen umgeht – fast dieselbe Haze-Reduktion wie DMDBS, jedoch bereits bei substantiell kleineren
Konzentrationen (z.b. Bulk Haze 30% bei 0.1% w/w BCPCA, Bulk Haze 25% bei 0.25% w/w DMDBS).
4
Aus einer Untersuchung des Phasenverhaltens im Hinblick auf Temperatur und Mischung für binäre
PE/Zusatzsoff-Systeme geht hervor, dass die optimale Dispersion des Zusatzstoffes in der
Polymerschmelze bei Konzentrationen im hyper-eutektischen Bereich, jedoch unter der flüssig-flüssig-
Phasentrennung geschieht. Dabei bildet der Zusatzstoff feine Fäserchen, auf welchen die Polymerketten
kristallisieren und nicht-sphärolithische Strukturen bilden, ähnlich zu Beobachtungen für i-PP.
Weitergehende Studien über die entstehenden Mikrostrukturen zeigten zudem, dass die Verwendung
von Sorbitol-basiertem DMDBS und TBPMN, sowie dem neuentwickelten BCPCA unter den genannten
bevorzugten Konzentrationen die Bildung von stäbchenförmigen Molekülarrangements vom
sogenannten “Shish-Kebab-Typ” hervorruft, welche die Streuung reduzieren. Experimente mit
verschiedenen makromolekularen Strukturen der PEs bringen anschliessend hervor, dass die
Vermeidung von sphärolithischem Kristallwachstum alleine nicht genügt, um die Lichtdurchlässigkeit
zu verbessern, sondern dass darüber hinaus die Breite der Zusatzstoff-Fäserchen und Polymer-Lamellen
eine entscheidende Rolle in der Haze-Reduktion spielen.
5
Chapter 1
Introduction
6
1 Preface
Whilst during the past decades much progress has been made in reducing processing cycles of
thermoplastic polymers, and in special cases – most notably isotactic polypropylene (i-PP) – enhancing
their optical properties, through the use of small-molecular additives (“nucleating” and “clarifying”
agents), applying those advances to the most-common, most-produced and most-used polymer, i.e.
polyethylene (PE), has been largely unsuccessful for reasons ill- or not understood. Due to its favorable
chemical resistance and thermal properties, among other things, a very low glass transition temperature
– which offers toughness under a wide range of environmental conditions (as opposed to i-PP), and a
relatively low melting temperature permitting relatively low energy-consuming production of artifacts,
it would be of high benefit to enhance nucleation and – ideally – also clarification of polyethylene,
especially for applications in the packaging industry.
Hence, the objective of the research described in this thesis is to explore the use of known, as well as
novel additives for enhancement of nucleation of polyethylenes, as well as improving their optical
characteristics – in particular reducing scattering of light, which is of obvious interest for the above-
mentioned use – while maintaining their advantageous mechanical properties; and if unsuccessful,
clarify the underlying cause(s).
2 Background
1) Nucleation
When an equilibrated molten crystallizable, flexible polymer solidifies, for instance by cooling below
its crystalline melting temperature, at a certain supercooling the initially randomly coiled chain
molecules form embryonic ordered entities (“nuclei”) that above a critical size grow into semi-
crystalline structures often in the form of radially oriented lamellae, commonly referred to as
“spherulites” 1-4. Importantly, the size, shape, orientation, molecular connectivity of these crystalline
entities and the overall degree of crystallinity dictate the macroscopic physical properties of the
solidified polymer 5. Therefore, understanding the nucleation and crystallization processes, and
controlling the final microstructure are of critical interest in order to direct these characteristics.
As mentioned above, the formation of (semi-)crystalline structures commences with the formation of
entities in which chain molecules are locally ordered, so called “primary nucleation”, until a critical-
size nucleus is reached. Once supercritical-size nuclei are formed, additional chains deposit onto the
surface of the primary nuclei, – a process referred as “secondary nucleation or crystal growth”. Primary
nucleation requires an energy barrier to be overcome for the generation of stabile nuclei. In the absence
7
of foreign matter such as solid particles, critical-size nuclei are formed by the polymer chains
themselves, in the event known as “homogeneous nucleation”. In this case, the energy barrier is
overcome when the polymer melt is supercooled to a temperature well below the crystalline melting
temperature of the polymer. The presence of foreign bodies in the molten phase may reduce the energy
barrier by acting as pre-existing nucleation sites onto which chains can deposit; in such a case the term
“heterogeneous nucleation” is used. The aforesaid foreign species, called “nucleating agents”, initiate
crystallization at higher temperatures, i.e. at reduced supercoolings, and may increase the number of
nucleation sites which results in increased solidification rates 6-12, and therewith reduce processing cycles
of industrial production of artifacts. For instance, in manufacturing injection-molded products, higher
crystallization temperatures require less cooling of the mold and higher rates of crystallization permits
faster removal of the articles 13. In addition to the above-mentioned processing advantages, nucleating
agents can cause changes in the polymer solid-state structure which affects its macroscopic physical
properties. For instance, increased nucleation and resulting decreased size of spherulitic structures were
reported to cause improvement in mechanical properties such as elastic modulus, tensile strength, yield
strength and impact strength of the material 14-18. N.B. Apart from nucleating agents, predetermined
nucleation can also be achieved by polymer crystallites themselves which are not completely molten and
can act as a foreign surface, in the polymer melt (self–seeding) 5. For nucleating agents to be most
efficient, they should exhibit sufficient thermal stability, be well dispersed in the polymer melt and form
a large solid surface area to provide polymer chains to nucleate on it 19-22.
The overall rate of solidification of crystallizable polymers is controlled by a competition between
thermodynamic and kinetic factors. At high temperatures (i.e. close to the equilibrium melting
temperature), solidification is slow due to the small thermodynamic driving force – which is required to
overcome the energy barrier for the formation of stable nuclei and subsequently to drive secondary
nucleation (crystal growth). On the other hand, at very low temperatures, the mobility of chains to diffuse
into favorable positions is highly reduced, leading to low rates of crystallization. In the case of
polyethylene, nucleation and crystallization occur extremely quickly due to its very simple molecular
structure, especially for unbranched resins such as high-density polyethylene (HDPE) 5. These high
nucleation and crystallization rates render interference with and control of the solidification of this
particular polymer difficult. This is most clearly illustrated by the fact that known nucleating agents
were found to increase the crystallization temperature of polyethylene by 2-5 °C only, while this values
has been recorded as high as ~20 °C for the case of i-PP 23.
8
2) Analysis
There exist a number of analytical methods to examine and quantify the effect of the addition of
nucleating agents on the crystallization process. Besides observation of the altered morphology by,
among others, optical microscopy (e.g. decreased size of spherulites – see Figure 1), crystallization can
conveniently be analyzed with differential scanning calorimetry (DSC), for instance by recording the
increase in peak or onset crystallization temperature (Tc) and a concomitant decrease in the supercooling,
(∆T) of the polymer during cooling at a particular rate (cf. Figure 2).
Figure 1.
Polarized optical micrographs of a solidified compression-molded film of a neat linear low-density polyethylene
(LLDPE) (left) and the same polymer comprising the nucleating agent 1,2,3-trideoxy-4,6:5,7-bis-O-
[(4propylphenyl)methylene]-nonitol (TBPMN) (right). The images illustrate the dramatic influence of the additive
on the size and number of the polymer spherulites; see Chapter 3.
Figure 2.
Differential scanning calorimetry (DSC) thermographs of a neat linear low-density polyethylene (LLDPE)
( ─ ─ ) and that polymer comprising the nucleating agent, 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)
methylene]-nonitol (TBPMN) ( ▬ ) during dynamic crystallization (cooling rate 10 °C/min); see Chapter 3.
9
3) Clarification
Amongst the above-referred nucleating agents, certain additives (unfortunately, only selected few) are
known to not only affect processing cycles and mechanical properties of polymers, but also can
dramatically enhance their optical characteristics as expressed in terms of “haze” and “clarity” – hence
the denotation “clarifying agents” 21, 24-26. In the following, basic issues of these optical characteristics
are addressed in some detail.
Inhomogeneous regions in matter may lead to scattering of incoming light and thus resolution details of
an object seen through the material can be affected. Inhomogeneity can stem from either the differences
in refractive index between adjacent regions, or the roughness on the surface of the material. In order to
characterize optical properties, commonly three terms are employed: i.e. “haze” (related to scattered
light), “clarity” (related to unscattered light transmission) and “gloss” (associated with reflectivity) 5.
In this thesis, the phenomenon of haze is the principle mode of characterization; it quantifies the fraction
of light transmitted through a specimen, which is scattered between the angles of 2.5° and 90° 27.
It is important to note that haze is affected by scattering of light by both internal/bulk and
external/surface inhomogeneities of an object.
Differences in the molecular arrangement in crystalline and disordered phases of typical semi-
crystalline polymers are the main cause of the “internal/bulk haze”. As stated before, most of such
materials, including polyethylene, form lamellar crystallites oriented in a radial manner (i.e. spherulites)
that due to their highly anisotropic structure, efficiently scatter light. Naturally, once diameters of such
spherulites approach the wavelengths of visible light, scattering becomes even more pronounced. Hence,
as was demonstrated by Bernland 28 for the case of i-PP, reduction of the size of spherulitic entities is
not the cause of reduced haze by “clarifying agents”. But, prevention of the formation of that structure
is a path reducing light scattering.
In addition to scattering of light by inhomogeneities in the solid state, surface irregularities often due to
processing issues are another contributor to scattering of light, referred to as “external/surface haze”.
The latter expectedly becomes of increasing importance for objects of decreasing thickness, such as
blown films 29-32, as discussed in the following in some more detail. But before entering into that, it is
important to point out that – most conveniently indeed – the two types of contributions to the overall
haze (bulk and surface) can be readily distinguished and separated by eliminating the haze due to
external/surface issues by, for instance, applying a liquid, which has a similar refractive index as the
material of interests, onto the material surface as shown in Figure 3 5, 33-35.
10
Figure 3.
Illustration of haze measurement with a hazemeter according to ASTM Standard D 1003 27. The hazemeter records
the “overall haze” of the specimen with surface irregularities (top). Immersion oil, which has similar refractive
index as the specimen has, eliminates “external/surface haze” and hazemeter measures “internal/bulk haze”
(bottom).
Returning to the issue of haze caused by surface roughness, two causes have been proposed in production
technologies of, for instance, melt-blown films – known as free-surface flow processes: 1) “extrusion
roughness” resulting from flow-induced irregularities and 2) “crystallization roughness” due to the
formation of crystalline entities on, or close to the surface. In an in-depth and most revealing study,
Sukhadia et al. investigated the correlation between surface haze of melt-blown films of a variety of
polyethylenes and their diverse melt elasticities 34. These authors reported three different regimes of
haze in a parabolic dependence of recoverable shear strain (Υ∞) of the polyethylenes processed: at very
low values of Υ∞ of the resins, blown films exhibited high haze values due to the development of distinct
spherulitic superstructures, which result in pronounced surface roughness and hence haze
(crystallization haze/regime I); at somewhat higher values of Υ∞, films featured the lowest haze values
due to a change in morphology from the aforementioned spherulitic structure into a fibrillar, row-
nucleated type texture (intermediate haze/regime II); in resins of higher values of Υ∞, i.e. those of high
melt elasticity (for instance due to long-chain branching or a broad molecular weight distribution) a very
fine-scale, elasticity-driven surface roughness was induced, which increased haze (extrusion
haze/regime III). In order to minimize surface roughness due to the latter rheological instabilities
changes in processing conditions have been explored, such as repeatedly extruding the polymer melt
prior to film blowing, lowering the throughput or increasing the extrusion temperature 29, 30, 36-38.
Proposed solutions to reduce growth of spherulitic entities, as well as reduction of surface roughness
resulting from flow instabilities are discussed in the following section with the principal focus on
nucleation and clarification of polyethylenes.
11
3 State-of-the Art of Nucleating and Clarifying Polyethylenes
The very many reported attempts to enhance nucleation and/or clarification of polyethylenes – based on
fundamentally different approaches – can be classified as follows:
1) Modification of the macromolecular structure
2) Polymer additives
3) Inorganic additives
4) Organic additives
5) Processing aids
6) Orientation
7) Lamination
In the following each of these approaches will be briefly reviewed.
1) Modification of the Macromolecular Structure
The term “polyethylene” in the literature may refer a polymer made of 100 % ethylene-monomer repeat
units, the homopolymer, or to copolymers produced with ethylene and minor amounts of other
monomeric moieties such as α-olefins, e.g. propylene, 1-butene, 1-pentene, 1-hexene or 1-octene, etc.
This modification of the regular linear polyethylene chain macromolecules via incorporation of unlike
repeat units inevitably leads to different molecular order in their solid state and changes the degree and
nature of crystallinity and therewith, among other characteristics, optical properties of the material. In
addition, related effects can be obtained by employing synthetic conditions yielding longer chain-
branched polyethylenes 39. However, as will be demonstrated in Chapter 2, the above modifications of
the main-polyethylene chains – whilst at times leading to enhanced optical properties in a most
impressive manner – are often at an unacceptable expense in terms of reduced melting (i.e. use-)
temperature and mechanical characteristics.
2) Polymer Additives
Polymers of a relatively high degree of crystallinity and high crystalline melting temperature relative to
a host polymer have been used as a nucleating agent in different polyethylene matrixes. For instance,
addition of high-density polyethylene (HDPE) to low-density polyethylene (LDPE) has been shown to
provide a high nucleation density, yielding a shish-kebab-type structure comprising HDPE pre-nucleated
fibrils which LDPE lamellae crystallize onto it 40. Similar nucleating fibrillar entities (shish) and
lamellae (kebabs) were observed by cooling of bimodal HDPE under a pulse of shear. Under such
conditions, flow-induced nucleation of the high molecular weight tail of the bimodal grade becomes a
substrate for the nucleation of lower molecular weight material 41. Furthermore, friction fibrillated ultra-
12
high molecular weight poly(tetrafluoroethylene) (PTFE) added to HDPE, generated during controlled
shear in compounding, resulted in HDPE crystallizing onto the PTFE nanofibrils 42. In another polymer-
polymer blend study, linear low-density polyethylene (LLDPE) was mixed with i-PP which itself was
pre-nucleated with nucleating agents 43.
In addition, LLDPE produced with a chromium catalyst (Cr-LLDPE) which features a high molecular
weight tail was introduced into metallocene LLDPE (m-LLDPE) to manufacture blown films 34. Prior
to this study, it was found that spherulitic-like superstructures are the dominant factor that contributes
to surface roughness (hence surface haze) of m-LLDPE in comparison to different LLDPE resins of
similar melt index 33. It was proposed that the relatively narrow molecular weight distribution of m-
LLDPE results in fast relaxation of flow-induced orientation and yielding spherulitic-like
superstructures and decrease of the melt elasticity. In order to broaden the molecular weight distribution
to increase the macromolecular relaxation time, a high molecular weight tail material was added to m-
LLDPE, and indeed it was found that by increasing the melt elasticity of the polymer blend, the surface
haze decreased as described in aforesaid parabolic “recoverable shear strain (Υ∞)” versus “haze” curve’s
regime II in the section on Clarification 34.
In still another approach, it has been tried to decrease the surface haze of m-LLDPE by adding high-
pressure, long-chain branched low-density polyethylene (HP-LDPE) to increase melt elasticity,
therewith reducing the formation of spherulitic-like superstructures 33. However, long-chain branched
structures can promote directional orientation which may lead to imbalances in mechanical properties
such as reduced impact- and tear resistance.
In alternative studies, HDPE was blended with plastomer-type ethylene (PEP) and a copolymer of
ethylene/vinyl acetate (EVA). Whilst EVA alone showed the same effect as LDPE in terms of decreasing
the haze, it deteriorated the dart impact- and tear strength; therefore PEP was added to attempt to
maintain mechanical properties of HDPE 44.
13
3) Inorganic Additives
The nucleation efficiency of a vast collection of small-particle sized minerals such as talc
(3MgO.4SiO2.H2O) 45, calcium carbonate (CaCO3) 46, 47 or whiskers (B2Mg3O6, SiO2-MgO-CaO) 48 have
been investigated as nucleating agents for PEs. Selected, prominent results of these studies are listed in
Table 1.
Table 1.
Inorganic additives as nucleating/clarifying agents for PE: polymer type used; molecular composition of the
additive; maximum peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its
corresponding additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin
samples (Δhaze) and its corresponding additive concentration; sample type according to processing; reference
(ref). n.a. = not available.
14
4) Organic Additives
A variety of small organic molecules (e.g. anthracene, p-terphenyl) 49, organic acid derivatives (as listed
in Table 2) 50-54, sorbitol derivatives (as listed in Table 3) 55, 56 and phosphorus containing species (i.e.
phenyl phosphate compound) 57 have been investigated for improving nucleation and clarification of
polyethylenes.
Table 2.
Organic acid derivatives as nucleating/clarifying agents for PE; molecular composition of the additive; maximum
peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its corresponding
additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin samples (Δhaze)
and its corresponding additive concentration; sample type according to processing; reference (ref). n.a. = not
available.
15
Table 3.
Sorbitol derivatives as nucleating/clarifying agents for PE: polymer type used; molecular composition of the
additive; maximum peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its
corresponding additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin
samples (Δhaze) and its corresponding additive concentration; sample type according to processing; reference
(ref). n.a. = not available.
5) Processing Aids
Processing aids have been widely used in commercial polymer processing operations to eliminate,
among other things, flow instabilities, e.g. melt-fracture, which leads to distortions of the surface of
extruded products 58-62. The commercially most applied example is fluoropolymer-based materials for
eliminating melt-fracture induced sharkskin formation of LLDPE 63-68. In terms of decreasing the
extrusion roughness and hence the surface haze, Pruss et al. examined such boron nitride based
processing aids in film-blowing of m-LLDPE and showed improved clarification 69.
6) Orientation
Prevention of the formation of the efficiently-light-scattering spherulitic structures, or destruction
thereof, can also be achieved by inducing orientation by stretching polyethylene films to enhance
clarification. Solid-state drawing of blown films between two heated rolls at the temperatures below the
crystallization melting point is one method to achieve reduced haze 70-72. A similar method was also
employed with three rolls to provide temperature differences between the rolls to enhance rapid cooling
73. Apart from solid-state stretching by heated rolls, cast films of LLDPE blended with a nucleating agent
16
(i.e. Hyperform HPN-20E from Milliken & Company) were stretched to a 4x4 areal draw ratio resulting
in improved haze relative to that of the unoriented blend films 74. Furthermore, gel sheets, obtained by
dissolving and quenching polyethylene from a solvent, were drawn in another study with the same
objective 75.
7) Lamination
As it was previously reported in the section on “Modification of the Macromolecular Structure”, the
trade-off between the optical and thermo-mechanical properties of polyethylenes is one of the major
issues for further development of the polyethylene market. Therefore, also a multi-layer approach,
comprising extrusion of two or more materials through a single die with two or more orifices to merge
laminar structured blown films with different properties, has been explored in several studies 76-78.
4 Objective and Scope of the Thesis
From the above introductory review it is evident that a vast amount of research has been conducted to
generate polyethylenes with feature both the superior mechanical properties of HDPE and the attractive
optical characteristics of, for instance, ultra-low-density polyethylene (ULDPE). Whilst the mechanisms
to reduce haze, and maintain mechanical characteristics, such as stiffness, are now reasonably well
understood, a solution to achieve the above desired combination of properties for polyethylene has still
not materialized – particularly in blown film production or typical molding operations where the material
cannot be quenched fast enough to restrict crystal growth – which is, hence, the principal objective of
this thesis. In order to elucidate the trade-off issue between haze and stiffness in more detail; optical,
thermal and mechanical properties of PEs with different chain architectures are investigated in Chapter
2. In Chapter 3, widely-used “sorbitol”- and “1,3,5-benzene trisamide”-based commercial additives
for i-PP are explored as a nucleating/clarifying agent for the polyethylenes, possessing superior thermal
and mechanical properties (i.e. LLDPE and HDPE). In addition to the commercial compounds, further
studies in Chapter 4 explore newly designed molecules based on “1,3,5-benzene trisamides” and “1,4-
phenylene bisamides” as a potential nucleating/clarifying agent for PEs.
In Chapter 5, the phase behavior of different PEs with selected additives – which display efficient
clarification in previous chapters – is presented and the solid-state microstructure of these binary systems
is investigated. In more elaborate studies, influence of the macromolecular structure of PEs on the final
solid-state microstructure and, associated optical properties of the most effective additive and the
polymers are investigated in Chapter 6.
Finally, thesis is terminated with general conclusions and an outlook in Chapter 7.
17
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Patent 0,048,179 A1, 2001.
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20
21
Chapter 2
Influence of Polymer Chain Architecture on
Opto-thermo-mechanical Properties of Polyethylenes
22
23
1 Introduction
It is well-known that modification of the degree of crystallinity, as well as the size and shape of
crystalline entities, have a major effect on the macroscopic physical properties of polymers. As
mentioned in the previous chapter, one rather simple manner to manipulate the above-mentioned
structural elements of a given polymer is through modification of the macromolecular structure; that is
introduction of “foreign” repeat units in an otherwise structurally regular chain molecule (i.e.
homopolymer). This approach inevitably leads to altered molecular order in the solid state,
accompanied by changes in the degree and nature of crystallinity and therewith, among other
characteristics, the mechanical and optical properties, and melting temperature of the material 1, 2.
Focusing on polyethylene (PE), comonomers can be added in the polymerization reactor at a judiciously-
chosen ratio with the ethylene monomer. By incorporation of a specified amount of an α-olefin
comonomer, the molecular order of the polymer can be changed and its crystallinity in the solid state
can be controlled from ~ 0 % (i.e. plastomer) up to ~ 90 % in the case of ethylene homopolymer 3. As
summarized in Figure 1, linear, essentially unbranched PE chain molecules are termed “high-density
polyethylene” (HDPE); PE backbones with shortly alkyl groups typically attached at random intervals,
are termed “linear low-density polyethylene” (LLDPE); highly-branched polymers with, on the order of
50 branch points per 1000 carbon atoms, are termed “low-density polyethylene” (LDPE); a particular
form of LLDPE that has a much higher concentration of short or longer chain branches, are termed
“very low-density polyethylene” – also known as “ultralow-density polyethylene” (VLDPE/ULDPE) 3,
4.
Figure 1
Schematic representation of different classes of PE structures (reproduced from Ref. 3, 4)
In this chapter, polyethylenes comprising a range of comonomers and degrees of branching, were
examined to develop an “opto-thermo-mechanical matrix”, with a focus on four “base” polyethylene
resins supplied by The Dow Chemical Company.
24
2 Experimental
1) Materials
The base polyethylene resins used throughout this study were supplied by The Dow Chemical Company
and used as received. Selected properties of these resins are listed in Table 1, Section 3.1. In addition, a
number of other PE resins from different polyethylene suppliers were also included to explore possible
trends in the “opto-thermo-mechanical matrix”: polyolefin plastomer with ethene –1– octene
copolymer, AFFINITY EG 8100 G, melt index (MI) (190 °C/2.16 kg) = 1 g/10 min, density (d) = 0.870
g/cm3 (The Dow Chemical Company); polyolefin plastomer with ethene –1– octene copolymer,
AFFINITY PL 1280 G, MI (190 °C/2.16 kg) = 6 g/10 min, d = 0.900 g/cm3 (The Dow Chemical
Company); LDPE, 42,803-5, MI (190 °C/2.16 kg) = 7 g/10 min, d = 0.918 g/cm3 (Aldrich); HDPE,
Seetec CJ563, MI (190 °C/2.16 kg) = 4.7 g/10 min, d = 0.955 g/cm3 (Lotte Daesen Petrochemical Corp.)
and HDPE, Hostalen GC 7260, MI (190 °C/2.16 kg) = 8.0 g/10 min, d = 0.960 g/cm3 (LyondellBasell).
2) Processing
Samples for mechanical testing were prepared by melt-compression molding at 220 °C, followed by
quenching to room temperature in a cold press, yielding films of ~100 µm thickness. For optical
characterization, plaque samples were prepared by injection molding. As shown in Figure 1, neat
polyethylene resins were processed at 220 °C under a nitrogen blanket in a laboratory co-rotating mini-
twin-screw extruder (Xplore (DSM), 15 ml) at 40 r.p.m. for 5 min. Subsequently the molten polymers
were extruded into a laboratory mini-injector (Xplore (DSM), 12 ml), followed by injection into a mold
kept at room temperature, to yield circular plaque samples (thickness 1 mm, diameter 25 mm).
Figure 1.
Processing scheme for the injection-molded plaques: co-rotating mini-twin-screw extruder (a), mini-injector (b),
laboratory-scale injection-molding machine (c), final circular plaque sample (d).
25
3) Analysis
Optical characteristics
Haze of the injection-molded samples was determined at room temperature with a Haze-Gard Plus®
instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 5. In addition to
“overall haze”, in order to eliminate the effect of surface scattering, “bulk haze” measurements were
also conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner GmbH,
Germany) with non-drying immersion oil (Cargille Series A refractive index oil, n = 1.5150 ± 0.0002)
which has a refractive index similar to the polymer plaques. Haze values reported here correspond to the
average of measured for five samples.
Thermal analysis
Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,
Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at standard
heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure
complete melting of the polymer and to prevent self-nucleation, the samples were kept for 5 min at the
maximum temperature (i.e. 220 °C) prior to cooling. The reported melting temperatures correspond to
the peak temperatures in the DSC thermograms. The melting temperature values of the resins other than
the base PEs were obtained from the technical data sheets of their respective suppliers. The degree of
crystallinity of the polymer was calculated from the enthalpy of fusion, derived from the endothermic
peak, adopting a value of 293 J/g for 100 % crystalline polyethylene 6.
Mechanical properties
Uniaxial tensile testing of the base polyethylene resins was performed on an Instron 5864 tensile testing
machine equipped with pneumatic clamps. The instrument was set up with a ±100 N static load cell and
was used in constant rate of elongation mode (i.e. 12 mm/min). All tests were carried out at room
temperature on dogbone-shaped specimens of ~100 µm thickness, 2 mm width and 12 mm gauge length.
All reported Young’s modulus (E) values of the “base resins” correspond to an average of five
measurements. The Young’s modulus values of the other resins were obtained from the technical data
sheets of their respective suppliers.
Scanning electron microscopy
Samples for scanning electron microscopy (SEM) studies were prepared by casting low concentration
solutions (~1 % w/w) of neat PE in p-xylene, yielding thin films after evaporation of the solvent; these
were subsequently molten at 220 °C and quenched to room temperature. The solidified films were coated
with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning electron
microscope (LEO Elektronenmikroskopie GmbH, Germany).
26
3 Results and Discussion
1) Properties
Characteristics of the various “base” polyethylenes used in this study, including the grades of
copolymers comprising ethylene-butene (C4), hexene (C6) or octene (C8) comonomers, are presented in
Table 1.
Table 1.
Summary of principal characteristics of the “base” PE resins studied: density; melt index; number-average
molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI, Mw/Mn);
comonomer type and comonomer content; peak melting temperatures (Tm); crystallinity; Young’s modulus;
“overall haze” and “bulk haze”.
27
Density is one of the most commonly used measures for characterization of polyethylene resins. Its value
is a function of molecular weight, crystallization kinetics and, most importantly, comonomer and branch
content. In the case that all other factors are constant, generally the density of a specimen decreases as
the branch content, molecular weight or rate of crystallization increase 1, 7. In the following, a broad
range of polyethylenes possessing different branch content and molecular weight will be assessed and
expressed in the form of density on prominent macroscopic physical material properties, i.e. optical,
thermal and mechanical, will be presented.
2) Optical Properties
Figure 2 shows a plot of “overall haze” of 1 mm thick injection-molded plaques versus the density of
various PEs. Clearly, haze is generally found to dramatically increase with their density.
Figure 2.
Plot of “overall haze” of 1 mm thick injection-molded polyethylene plaques versus density of the polymer. Solid
symbols denote experimental data obtained for the “base” PE resins and open symbols refer to data from Ref. 8.
The solid line is a guide to the eye only.
On the other hand, resins possessing approximately the same density can feature a relatively large spread
in the value of haze (cf. Figure 2: ~ 0.92 g/cm3 and ~ 0.95 g/cm3). In this context, it should be noted that
“surface haze” resulting from processing imperfections affects the overall haze values. As can be seen
from Table 1, plaques produced with pellets of LLDPE – 6 and LLDPE - 8, 1MI resins, which have
similar comonomer content and molecular weight characteristics, exhibit similar bulk haze (57% and
62%, respectively). However, their “overall haze” values are different and much higher than their “bulk
haze” (i.e. 81 % and 73%). The influence of “surface haze” on the optical properties of the selected
injection-molded plaque samples is illustrated in Figure 3. For instance, while resolution details of the
numbers seen through the plaques reveal variation for the low melt index resins such as LLDPE – 6,
28
LLDPE – 8, 1MI (cf. Figure 3. a, b, c), LLDPE – 8, 25MI possessing a high melt index has a
homogeneous appearance for the resolution of numbers. This result can be contributed to the varying
extent of surface imperfections in the final product due to issues in processing, which is affected by the
molecular architecture of PE.
Figure 3.
Illustration of the effects of inhomogeneous surface imperfections on “overall haze” (O) and “bulk haze” (B)
values of the samples. The 1 mm thick, injection-molded plaques of neat LLDPE – 6, powder; LLDPE – 6, pellet;
LLDPE – 8, 1MI; LLDPE – 8, 25MI (a, b, c, d, respectively) have respectively: O 80%, B 54; O 81%, B 57; O
73%, B 62; O 90%, B 88. Images on the right with white-written text background are the identical samples shown
on the left.
It must be also taken into account that, apart from the branch content, molecular weight characteristics,
which is another parameter that affect the density, can also alter the haze. As can be seen in Table 1,
LLDPE – 8, 1MI and LLDPE – 8, 25MI resins, which have identical comonomer content and side chain
length, higher haze is found for the LLDPE of the lower molecular weight.
Furthermore, additives in the resin (i.e. stabilizers, anti-oxidants, etc.) can also affect haze. For instance,
injection-molded plaques of HDPE – 4, powder resin, which is free of additives, show ~15 % lower haze
in comparison with those produced with its pellet version. Therefore, the effect of additives on light
scattering by the molten polymer was investigated by melt-intrinsic haze measurements. For this
29
purpose, compression-molded molten specimen, sandwiched between 3 mm glass plates with 1 mm
spacers were placed onto the haze port of the vertically-oriented hazemeter (cf. Figure 4.a). As shown
in Figure 4.b, the sample of HDPE – 4, pellet containing additives exhibited significantly more
pronounced light scattering by the molten polymer when compared with that of the additive-free
polymer. SEM images of films of HDPE – 4, pellet reveal relatively large spherulitic solid-state
structures (cf. Figure 4.c), consistent with the higher haze value of the respective resin, and the absence
of those features in films of neat HDPE – 4, powder.
Figure 4.
Schematic of the design of the melt-intrinsic haze measurements (a); melt-intrinsic haze values of HDPE – 4, pellet
( ■ ) and neat HDPE – 4, powder ( □ ) at room and above the melting temperature (b); SEM images of films
produced with HDPE – 4, pellet (left) and neat HDPE – 4, powder (right) samples (c).
30
3) Thermo-mechanical Properties
Low haze, which can be readily obtained with very low-density PEs, as shown in Figure 2, is generally
preferable for the packaging industry. However, thermal and mechanical properties of PEs of such low-
density polyethylenes are compromised.
Figure 5.a shows a plot of the melting temperature versus density revealing the major penalty in terms
of melting – and, associated service temperature. A plot of the Young’s modulus as a function of the
density of compression-molded PE films is shown in Figure 5.b. Similar to the penalty in melting
temperature, the stiffness also dramatically diminishes for the lower density, more transparent PEs,
which is consistent with previous studies 3, 9. A compromise must, therefore, be found between optical
and thermo-mechanical properties. For this purpose, in the following chapters a range of
nucleating/clarifying agents will be investigated for the polyethylenes with the objective that they exhibit
superior thermal and mechanical properties, i.e. HDPE or LLDPE, in combination with advantageous
optical characteristics such as those that are typically observed for the low-density resins.
Figure 5.
Plot of melting temperature versus density of different polyethylenes (a) and Young’s modulus versus density of
compression-molded polyethylene films (b). Solid symbols denote experimental data obtained for the “base” PE
resins and open symbols refer to data obtained from the technical data sheets of the respective PE resins described
in the materials section. The solid line in (a) is a guide to the eye only, and extrapolates to equilibrium melting
temperature of PE at ~141.5 °C 10.
31
4 Conclusions
In this chapter, the ways in which optical, thermal and mechanical properties of polyethylenes can be
altered by tailoring the chain architecture of the polymer were investigated. The interdependence
between mechanical, thermal and optical properties is summarized in Figure 6. It can be seen that highly
transparent polyethylenes can readily be produced, but this desirable property is achieved at the expense
of a major reduction of their equally-relevant thermal (i.e. melting temperature) and mechanical (i.e.
stiffness) characteristics, which are of a paramount importance for many applications. Therefore,
developing a polyethylene-based material with optimal optical and thermo-mechanical properties
remains a major challenge.
Figure 6.
Plot of Young’s modulus (blue-square symbols) and melting temperature (red-triangle symbols) versus “overall
haze” for polyethylenes, showing the trade-off between optical and thermo-mechanical properties. Solid symbols
denote experimental data obtained for the “base” PE resins and open symbols refer to data obtained from Ref. 8
and the technical data sheets of the respective PE resins described in the materials section.
32
5 References
1. Popli, R.; Mandelkern, L. J. Polym. Sci. Pol. Phys. 1987, 25, (3), 441-483.
2. Kaplan, W. A. Ed. Modern Plastics Encylopedia '98. McGraw-Hill: New York, 1997; p 52.
3. Peacock, A. J. Handbook of Polyethylene: Structures, Properties, and Applications. Marcel
Dekker: New York, 2000; p 2, 304.
4. Kaiser, W. Kunststoffchemie für Ingenieure: Von der Synthese bis zur Anwendung. Hanser:
Munich, 2011; p 236.
5. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM
Standard D 1003-07el, 2007.
6. Wunderlich, B.; Czornyj, G. Macromolecules 1977, 10, (5) 906.
7. Peacock, A. J.; Mandelkern, L. J. Polym. Sci. Pol. Phys. 1990, 28, (11), 1917-1941.
8. Loiseau, E. Master Thesis ETH Zurich, 2010.
9. Saavedra, J. V.; Patel, R.; Ratta, V. US Patent 8,092,920, 2012.
10. Wunderlich, B. Thermal Analysis. Academic Press: San Diego, 1990; p 418.
33
Chapter 3
Efficiency of Commercial Additives for
Nucleation and Clarification of Polyethylenes
34
35
1 Introduction
Sorbitol-based and 1,3,5-benzene trisamide-based additives currently are the most widely-used
commercial nucleating/clarifying agents for isotactic polypropylene (i-PP). Sorbitol derivatives such as
(1,3:2,4)-dibenzylidenesorbitol (DBS), 1,3:2,4-di-p-methylbenzylidenesorbitol (MDBS), 1,3:2,4-
bis(3,4-dimethylbenzylidene)sorbitol (DMDBS), 1,3:2,4-di-p-ethylbenzylidenesorbitol (EDBS) and
1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol (TBPMN) have been
comprehensively investigated as efficient i-PP nucleators/clarifiers 1-13. The remarkable performance
and the commercial success of sorbitol derivatives result from, among other things, their chemical
structure that allows them to easily dissolve in, and recrystallize from the polymer melt, yielding a three-
dimensional nanofibrillar network of a large surface area for subsequent nucleation of the polymer
chains 14, 15. Recently, a new group of substituted 1,3,5-benzene trisamides, i.e. 1,3,5-tris(2,2-
dimethylpropionylamino)benzene (TDMPAB), has been advanced, providing superior nucleation and
imparting drastically improved optical properties at ultra-low concentrations, accompanied by
outstanding thermal stability and excellent solubility in the polymer melt, thus facilitating and improving
homogeneous dispersion of the additive during processing 16-20.
Even though the aforementioned additives were presented as nucleating and clarifying agents for
polyolefins, there are few documented studies about their use for polyethylenes (PEs) 21-23, which are
capable of providing an increased toughness over a wider temperature range due to their lower glass
transition temperatures and allowing for the production of low-energy-consuming artifacts as a result of
their relatively low melting temperatures when compared with i-PP. In this chapter, in a first attempt,
different commercial additives from the 1,3,5-benzene trisamide families (i.e. TDMPAB) and sorbitol
derivatives (i.e. DMDBS and TBPMN) have been screened for potential use with the four “base” PE
resins employed in this thesis (previously introduced in Chapter 2). The chemical structures of the
additives are shown in Figure 1.
Figure 1.
Chemical structures of the additives used in this study: 1,3,5-tris(2,2-dimethylpropionylamino)benzene
(TDMPAB) (left), 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) (middle) and 1, 2, 3-trideoxy-4,6:5,7-
bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN) (right).
36
2 Experimental
1) Materials
The “base” polyethylene resins listed in Chapter 2 were supplied by The Dow Chemical Company and
used as received. The compounds, 1,3,5-tris(2,2-dimethylpropionylamino)benzene (TDMPAB,
Irgaclear XT386, CAS Registry Number: 745070-61-5) from Ciba Speciality Chemicals, now BASF
SE, Basel; 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS Registry
Number: 135861-56-2) from Milliken Chemicals and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-
propylphenyl)methylene]-nonitol (TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from
Milliken Chemicals, were used as received.
2) Processing
Injection-molding plaques
The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molded
plaques. PE/additive blends were compounded in a laboratory co-rotating mini-twin-screw extruder
(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Series of PE with different additive
concentrations were prepared commencing with a PE masterbatch (12.7 g) comprising 2 or 5 % w/w
additive, which was subsequently diluted to lower concentrations. For the masterbatch preparation,
pellet grades (LLDPE – 8, 1MI and LLDPE – 8, 25MI) were compounded with the mini-twin-screw
extruder and powder grades (HDPE – 4 and LLDPE – 6) were dry blended. For each concentration, 8.1
g of the compounded mixture was extruded into a laboratory mini injector (Xplore (DSM), 12.0 ml).
Subsequently, the desired amount of PE/additive was added to the remaining 4.6 g masterbatch. By
repeating this procedure, blends of PE and the additives were prepared with decreasing additive
concentrations in the range of 2 or 5 % w/w to as low as 0.005 % w/w. Thereafter, the molten
polymer/additive blends were injected into a mold kept at room temperature to produce plaque samples
(thickness 1.0 mm, diameter 25.0 mm). The entire micro-scale polymer processing was conducted at
220 °C under a nitrogen blanket. Reference samples of the neat polymer were produced according to the
identical procedure.
37
3) Analysis
Optical characteristics
Haze of the injection-molded samples was determined at room temperature with a Haze-Gard Plus®
instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 24. A circular area on
the plaque samples, 18.0 mm in diameter, was illuminated by the light beam; the recorded haze values
hereafter are referred to as “overall-area haze”. In addition, haze values of circular area with 8.0 mm
diameter on samples (those free of apparent surface irregularities) were recorded and, hereafter, are
referred to as “small-area haze”. In order to eliminate the effect of surface scattering, “bulk haze”
measurements were conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner
GmbH, Germany) with non-drying immersion oil (Cargille Series A refractive index oil of n = 1.5150
± 0.0002) which has a similar refractive index as the polymer plaques. Haze values reported here
correspond to the average of values measured for five samples.
As a complementary method to haze measurements, photographs of the plaques possessing minimum
“overall-area haze” values of each PE/additive series were taken with a digital camera and
quantitatively analyzed by the method described below. As depicted in Figure 2, plaque samples were
placed onto a 0.1 mm thin patterned stainless steel background mask. The samples were illuminated
from the bottom with a light source (“Micron” Tavola Luminosa, Osram L 15W/10 daylight fluorescent
tube) and subsequently a digital image was taken with a Canon EOS 20D DSLR camera from the top.
The background mask pattern consists of a simple grid of 2 mm stripes separated by 2 mm wide gaps,
resulting in an alternating series of lit (bright) and unlit (dark) sample areas. Analysis of the
corresponding digital images were performed by quantifying the sharpness of the transition from bright
to dark areas along any perpendicular to the grid pattern. But before presenting the results obtained, it is
important to point out the definition of pixel intensity which quantifies the sharpness of these transitions.
Digital images are two-dimensional grids of pixel intensity values with the width and height of the image
being defined by the number of pixels in x (rows) and y (columns) direction. Thus, pixels are the smallest
single components of images, holding numeric values, referred as “pixel intensities” that range between
black and white. The characteristics of this range, i.e. the number of unique intensity (brightness) values
that can exist in the image are defined as the “bit” (depth of the image) and specify the level of precision
in which intensities are coded 25. In this study, 8 bit RGB (a widely used color space) files obtained by
the camera were converted to 8 bit gray-scale files which display 256 (28) gray levels (integers only).
Subsequently, images were quantitatively analyzed with ImageJ software. 5 pixel wide line was drawn
perpendicularly to the mask grids and its “gray value” profile was plotted (cf. Figure 2.c-left). The
sharpness of the transitions from bright (gray value > 100) to dark (gray value < 100) and vice versa was
quantified by plotting the “derivative of the gray value” profile (cf. Figure 2.c-right): inflection points
38
in the crossovers appear as sharp positive/negative peaks and taken as a complementary method to haze,
i.e. with higher absolute peak values indicating less haze/better transparency. In order to quantitatively
analyze the differences between the samples, values of the “gray value derivative” are compared for
different clarifying agents for each resin in Figure 4.b, 6.b, 8.b, 10.b.
Figure 2.
New method to determine qualitative and quantitative optical properties of injection-molded plaque samples to
complement haze measurements. Samples are placed onto a background mask and illuminated from the bottom
(a). Digital image (b) taken with a camera is analyzed by evaluating the pixel intensities along the arrow shown in
the photo graph: “Pixel intensities/gray values” along the “distance” are plotted (left) and the corresponding
“derivative” curve (right) is shown in (c).
39
Thermal analysis
Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,
Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at a standard
heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure
complete melting of the polymer and prevent self-nucleation, samples were kept for 5 min at the
maximum temperature prior to cooling. The reported crystallization temperatures correspond to the peak
temperatures in the DSC thermograms.
Scanning electron microscopy
Samples for scanning electron microscopy (SEM) studies were prepared by casting low concentration
solutions (~1 % w/w) of neat PE and PE containing 2 % w/w of the additives in p-xylene, yielding thin
films after evaporation of the solvent; these were then molten at temperatures above the melting
temperatures of the additive in the blend and subsequently quenched to room temperature. The solidified
films were coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini
scanning electron microscope (LEO Elektronenmikroskopie GmbH, Germany).
40
3 Results and Discussion
Nucleation efficiency and optical properties of the solidified binaries comprising “base” polyethylenes
(i.e. HDPE – 4, LLDPE – 6, LLDPE – 8, 1MI and LLDPE – 8, 25MI) and the aforementioned additives
TDMPAB, DMDBS and TBPMN were investigated at additive concentrations ranging from 0 % w/w
up to 2 or 5 % w/w, and are presented in the following sections. In each section, peak crystallization
temperatures of the polyethylenes (Tc, PE) in PE/additive blends and increase in Tc, PE with respect to that
of the neat resins (ΔTc, PE) at concentrations where maximum crystallization peak temperature (Tc, max) is
obtained are presented (Figure 3, 5, 7 and 9). Additionally, optical properties of the blends are shown
by plotting “overall-area haze”, “small-area haze” and “bulk haze” versus additive content and their
decrease with respect to the neat resins (Δhaze) at concentrations where minimum haze (Hazemin) is
obtained are shown (Figure 4.a, 6.a, 8.a and 10.a).
1) HDPE – 4
In Figure 3, are presented the polymer peak crystallization temperatures (Tc, PE) at low additive content
mixtures in HDPE – 4. The data indicates no significant differences relative to the neat polymer. In other
words, introduction of the additives does not affect the nucleation efficiency, presumably due to the high
intrinsic nucleation- and crystallization growth rate of HDPE – 4.
Figure 3.
Peak crystallization temperatures of HDPE – 4 containing TDMPAB, DMDBS or TBPMN (from left to right).
Red, dashed lines indicate the Tc, PE of the neat resin.
On the other hand, haze measurements of HDPE – 4 plaques containing the sorbitol clarifying agents
(i.e. DMDBS and TBPMN) presented in Figure 4.a show improvement of clarification of the polymer,
whilst 1,3,5-benzene trisamide based TDMPAB induced no reduction in haze.
41
Figure 4.
Haze of HDPE – 4 containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze” (■), “bulk
haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive content. Red,
dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed haze values of
the neat HDPE – 4, and values of the decrease in haze with respect to the neat resin (Δhaze) at concentrations (%
w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-area haze”, *** = “bulk
haze” (a). Quantitative analysis of the optical properties of the HDPE – 4 injection-molded plaques comprising
0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.5 % w/w TBPMN (from left to right): “gray value derivative”
plotted versus the “distance” along the arrow direction as shown in the corresponding plaque photos (b).
42
Most interestingly, reduced haze with TBPMN, for instance ~40 % “small-area haze” at concentrations
of 0.5–3 % w/w, is fairly successful for HDPE resins. Quantitative analysis of the digital images of the
HDPE – 4 plaques according to above described method in Figure 2 are shown in Figure 4.b. Plaques
comprising DMDBS and TBPMN have sharper transitions and a higher level of “gray value derivative”
in comparison with TDMPAB, which is consistent with the reported haze values. Finally, as revealed
by the data presented in Figures 3 and 4, there appears to be no consistent connection between nucleation
efficiency and the ability of compounds to reduce haze, which is in accord with previous reports 16, 19, 26.
2) LLDPE – 6
Corresponding data regarding the nucleation efficiency for LLDPE – 6 compositions are presented in
Figure 5. As can be seen, a minor increase in Tc, PE (i.e. ~2 °C) and an insignificant decrease in haze (i.e.
~5 %) (cf. Figure 6.a) for the binaries with TDMPAB reveal its poor nucleation and clarification ability
in LLDPE – 6. On the other hand, the sorbitol derivatives, DMDBS and TBPMN, induced a modest
increase in Tc, PE at fairly low concentrations (i.e. ~5 °C at 0.5 % w/w and ~6 °C at 0.25 % w/w,
respectively).
Figure 5.
Peak crystallization temperatures of LLDPE – 6 containing TDMPAB, DMDBS or TBPMN (from left to right).
Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat LLDPE – 6,
the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at concentrations (% w/w)
where Tc, max is observed.
43
Regarding the effect of the addition of sorbitols on optical properties to this PE resin, from the data
presented in Figure 6.a, it can be concluded that DMDBS and TBPMN are poor clarifying agents for
the reduction of “overall-area haze” of LLDPE – 6, but are significantly effective in decreasing the
“bulk haze”. This observation points to the processing issues causing the imperfections on the surface
of injection-molded plaques which was previously discussed in Chapter 2-Figure 3. It should be noted
that a high degree of surface imperfections spread over the sample surface did not permit to measure
“small-area haze” for the mixtures with LLDPE – 6. The results observed with “bulk haze”
measurements of LLDPE – 6 with DMDBS and TBPMN are highly encouraging, manifest in significant
reductions, i.e. 32 % and 25 %, at concentrations of 0.25 % w/w and 0.5 % w/w, respectively.
Figure 6.
Haze of LLDPE – 6 containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze” (■) and
“bulk haze” (●) of injection-molded plaques, plotted as function of the additive content. Red, dashed lines indicate
the “overall-area haze” values of the neat resin. In the table below are listed haze values of the neat LLDPE – 6,
and values of the decrease in haze with respect to the neat resin (Δhaze) at concentrations (% w/w) where minimum
haze (Hazemin) is observed: * = “overall-area haze”, *** = “bulk haze” (a).
44
Figure 6.
Continued; quantitative analysis of the optical properties of the LLDPE – 6 injection-molded plaques comprising
0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.5 % w/w TBPMN (from left to right): “gray value derivative”
plotted versus the “distance” along the arrow direction as shown in the corresponding plaque photos (b).
As it was previously found for HDPE – 4, quantitative analysis obtained by the digital images of the
plaques in Figure 6.b reveal that DMDBS and TBPMN have higher “gray value derivative”, in accord
with haze measurements. Changes in the positive/negative peak values are likely due to surface
imperfections of the samples.
Clarification with TBPMN of LLDPE – 6 is observed over a relatively wide concentration range of the
additive (i.e. 0.5–3 % w/w), as it was previously observed with HDPE – 4. This result can possibly be
attributed to the high solubility of TBPMN in PE during processing over a large concentration regime.
Therefore, solubility limits of TBPMN were systematically investigated in a more detailed study of the
phase behavior of the binary system with LLDPE – 6 in Chapter 5.
45
3) LLDPE – 8, 1MI
From the data presented in Figure 7 and Figure 8, it can be concluded that, similar to HDPE – 4 and
LLDPE – 6, TDMPAB displays an insignificant enhancement of nucleation and clarification efficiency
(i.e. 3 °C increase in Tc, PE and 11 % decrease in haze) with LLDPE – 8, 1MI. By contrast, as shown in
Figure 7, DMDBS and TBPMN enhance the nucleation of LLDPE – 8, 1MI by a ~9 °C increase in Tc,
PE with respect to the neat polymer. Even though efficient nucleation occurs at fairly high concentrations
(i.e. respectively above 1.25 % w/w and above 1 % w/w), this value is an encouraging result for reducing
the processing cycles of polyethylenes.
Figure 7.
Peak crystallization temperatures of LLDPE – 8, 1MI containing TDMPAB, DMDBS or TBPMN (from left to
right). Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat
LLDPE – 8, 1MI, the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at
concentrations (% w/w) where Tc, max is observed.
All three types of haze measurements in Figure 8.a: “overall-area haze”, “small-area haze” and “bulk
haze”, reveal a substantial decrease in haze for the compositions of DMDBS and TBPMN with LLDPE
– 8, 1MI. In a most impressive manner, “small-area haze” decreases to the level which is comparable
to the clarified i-PP (i.e. DMDBS: 14 % for LLDPE – 8, 1MI, ~15 % for i-PP 10 and TBPMN: 17 % for
LLDPE – 8, 1MI, 10 % for i-PP 13). In addition to the improved haze of LLDPE – 8, 1MI with DMDBS
and TBPMN, quantitative analysis obtained from the corresponding plaque images in Figure 8.b
consistently show higher “gray value derivative” values.
46
Figure 8.
Haze of LLDPE – 8, 1MI containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze”
(■), “bulk haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive
content. Red, dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed
haze values of the neat LLDPE – 8, 1MI, and values of the decrease in haze with respect to the neat resin (Δhaze)
at concentrations (% w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-
area haze”, *** = “bulk haze” (a). Quantitative analysis of the optical properties of the LLDPE – 8, 1MI injection-
molded plaques comprising 0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.25 % w/w TBPMN (from left to
right): “gray value derivative” plotted as function of the “distance” along the arrow direction as shown in the
corresponding plaque photos (b).
47
For the two sorbitol-based additives, concentrations at which Hazemin values were found closely match
those for the LLDPE – 6 resin (i.e. 0.25 % w/w for DMDBS and 0.25–1.50 % w/w for TBPMN in
LLDPE – 8, 1MI). The change in haze with its strong dependence on additive concentration was also
investigated in a more detailed study of the phase behavior of LLDPE – 8, 1MI/TBPMN binaries in
Chapter 5.
4) LLDPE – 8, 25MI
As it was found for the previous PE resins, TDMPAB also does not cause an increase in Tc, PE of LLDPE
– 8, 25MI (cf. Figure 9) and decreases the haze of neat polymer only to a rather modest degree (i.e. 27
% decrease in haze, to 62 %) (cf. Figure 10). As can be seen in Figure 9, DMDBS and TBPMN also
improve nucleation of LLDPE – 8, 25MI (i.e. ~7 °C and ~8 °C increase in Tc, PE, respectively) – as in
the case of LLDPE – 6 and LLDPE – 8, 1MI – from additive concentrations exceeding ~0.5 % w/w.
Figure 9.
Peak crystallization temperatures of LLDPE – 8, 25MI containing TDMPAB, DMDBS or TBPMN (from left to
right). Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat
LLDPE – 8, 25MI, the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at
concentrations (% w/w) where Tc, max is observed.
As it was previously observed for LLDPE – 8, 1MI, all three types of haze measurements reveal
substantial amounts of decrease for the compositions with sorbitols and LLDPE – 8, 25MI (see Figure
10.a). Outstanding clarification performance of the sorbitols was also found for LLDPE – 8, 25MI (i.e.
48
24 % and 15 % “small-area haze” for respectively DMDBS and TBPMN at concentrations as low as
0.25 % w/w) comparable to clarified i-PP 10, 13. Beneficially, and in contrast to results observed with the
previous resins, the relatively small differences between the “overall-area haze” values and the “small-
area haze” and “bulk haze” values, permits to use this particular resin for injection-molding, high-
thickness applications. This result indicates that rheology of the low molecular weight LLDPE – 8, 25MI
resin in injection-molding is more suitable for producing plaques and other artifacts with homogeneous
surfaces compared with the other resins. Again quantitative analysis of the plaque samples in Figure
10.b reveal higher “gray value derivative” values for DMDBS and TBPMN as in the case of previous
resins, which is also consistent with the above haze results.
As for the previous resins, in Chapter 5, the phase behavior of the LLDPE – 8, 25MI/TBPMN binary
was also studied in order to understand the concentration dependency of the additive on haze of the
samples in more detail.
Figure 10.
Haze of LLDPE – 8, 25MI containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze”
(■), “bulk haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive
content. Red, dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed
haze values of the neat LLDPE – 8, 25MI, and values of the decrease in haze with respect to the neat resin (Δhaze)
at concentrations (% w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-
area haze”, *** = “bulk haze” (a).
49
Figure 10.
Continued; quantitative analysis of the optical properties of the LLDPE – 8, 25MI injection-molded plaques
comprising 0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.25 % w/w TBPMN (from left to right): “gray value
derivative” plotted as function of the “distance” along the arrow direction as shown in the corresponding plaque
photos (b).
5) Structure
In order to gain further understanding of the nucleation and clarification efficiency of the aforementioned
additives with different PEs, scanning electron microscopy (SEM) was conducted with a focus on
LLDPE – 8, 25MI, as optimum performance was observed with this particular grade. For this purpose,
solution-cast samples of neat LLDPE – 8, 25MI and polymer/additive blends were produced, dried,
melted and subsequently quenched. Representative results of this study are presented in Figure 11. The
SEM image of the neat polymer shows clear, classical spherulitic structures (upper-left), while the
polymer containing the additives feature significantly different microstructures. The self-assembly and
crystallization of the additives in PE varies in shape and size for different molecular compounds. The
polymer comprising 2 % w/w 1,3,5-benzene trisamide based TDMPAB reveals large crystal domains
of the additive (upper-right), which significantly scatter visible light. Besides, PE lamellae both grow
onto the additive and at the same time macromolecular organization arises similar to that in the neat
polymer. By contrast, structures obtained with the sorbitol derivatives, DMDBS and TBPMN
(respectively lower-left and lower-right), at the same additive content are seen to feature fine-fibrils of
the additive and, consequently, form a highly beneficial fibrillar, large topological surface-network for
PE to grow predominantly onto them. Finally, it should be noted that the sorbitol derivatives form fibrils
with dimensions of which are not of the order of the wavelength of visible light, thus reducing light
scattering further leading to enhanced clarification performance.
50
Figure 11.
SEM images of the neat LLDPE – 8, 25MI (upper-left) and the same resin containing 2 % w/w of the additives
TDMPAB (upper-right), DMDBS (lower-left) and TBPMN (lower-right).
4 Conclusions
In summary, industrially widely-used additives for i-PP were explored as potential nucleating and
clarifying agents for the “base” polyethylenes used in this thesis. In general, the 1,3,5-benzene trisamide
based TDMPAB exhibits poor nucleation and clarification for all PE resins. On the other hand sorbitol
derivatives, DMDBS and TBPMN, improve nucleation of the PEs, (i.e. an increase in crystallization
peak temperatures up to 9 °C), and induce a drastic enhancement of optical properties (i.e. reduction in
haze to values as low as 14 %), which are similar to that of the clarified i-PP.
Additionally, as shown in Appendix-Section 1, addition of DMDBS to LLDPE – 8, 1MI for thin 50 μm
thin blown-film applications, yields similar properties as observed for the 1 mm thick injection-molded
plaques in this chapter. For instance, “bulk haze” of LLDPE – 8, 1MI blown-films comprising 0.2 %
w/w DMDBS is 3.5 times less than that of the neat polymer (i.e. 1.9 % and 7 %, respectively); this ratio
is 2.5 for the injection-molded plaques comprising 0.25 % w/w DMDBS (i.e. 25 % and 62 %,
respectively).
51
SEM studies demonstrated that the size and shape of the aggregates of the additives have an important
influence on the level of light scattering by the final solid material. While large crystal domains of
TDMPAB do not cause clarification, the fine-fibrillar networks of DMDBS and TBPMN reduce haze
in a most beneficial manner. Additionally, investigating the crystal structure of polyethylene (i.e.
LLDPE – 8, 25MI) in Appendix-Section 3 revealed similar WAXD patterns for the neat polymer and
polymers comprising commercial additives.
However, it should be reemphasized that the macromolecular architecture of the PEs by itself also plays
an important role in nucleation and clarification performance of the additives. For instance, as can be
concluded from the results obtained with HDPE – 4, identical additives are not as effective as in other
resins in terms of nucleation and clarification. This is summarized more quantitatively in Table 1 in
which data are presented regarding minimum and maximum values of haze observed for the different
neat polymers and those to which TBPMN is added, as well as their Δhaze values. Remarkably, when
comparing haze of HDPE – 4 and LLDPE – 8, 1MI – which are of similar magnitude – addition of
TBPMN leads to a reduction of 20-25 % in the former case and 43-19 % in the latter. Equally striking
is a comparison between LLDPE – 8, 1MI and LLDPE – 8, 25MI for which Δhaze is observed to be 43-
19 % and 73-58 %, respectively. Therefore, the influence of the macromolecular structure of PE on the
efficiency of a particular additive will be explored in Chapter 6 in more detail.
Table 1.
Summary of the results presented in this chapter for four “base” PE resins: haze of neat PE, minimum haze value
observed for TBPMN comprising PEs (Hazemin) and corresponding value of the decrease in haze with respect to
the neat resin (Δhaze). The range of the values were recorded according to different haze measurements: * =
“overall-area haze”, ** = “small-area haze”, *** = “bulk haze” (a).
52
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Spontak, R. J. J. Polym. Sci. Pol. Phys. 1997, 35, (16), 2617-2628.
16. Blomenhofer, M.; Ganzleben, S.; Hanft, D.; Schmidt, H.-W.; Kristiansen, M.; Smith, P.; Stoll,
K.; Mader, D.; Hoffmann, K. Macromolecules 2005, 38, (9), 3688-3695.
17. Kristiansen, P. M.; Gress, A.; Smith, P.; Hanft, D.; Schmidt, H.-W. Polymer 2006, 47, (1), 249-
253.
18. Wang, J. D.; Dou, Q. Colloid Polym. Sci. 2008, 286, (6-7), 699-705.
19. Abraham, F.; Ganzleben, S.; Hanft, D.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys.
2010, 211, (2), 171-181.
20. Abraham, F.; Kress, R.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys. 2013, 214, (1), 17-
24.
21. Schmidt, H.-W.; Blomenhofer, M.; Stoll, K.; Meier, H.-R. US Patent 0,149,663 A1, 2007.
22 Cheruvu, S.; Lo, F. Y.-K.; Ong, S. C.; Su, T.-K. World Intellectual Property Organization
95/13317, 1995.
53
23 Miley, J. W.; Carroll, C. C.; Lever, J. G.; Mehl, N. A.; Salley, J. M. US Patent 5,973,043, 1999.
24. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM
Standard D 1003-07el, 2007.
25, Ferreira, T.; Rasband, W. S. ImageJ User Guide — IJ1.46, imagej.nih.gov/ij/docs/guide/,
2010–2012; p 10, 142.
26. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal Institute
of Technology Zurich, Nr 19388, Zurich, 2010.
54
55
Chapter 4
New “Designer” Nucleating/Clarifying Agents for
Polyethylene
56
57
1 Introduction
The use of nucleating/clarifying agents in processing of polyolefins is a most industrially relevant
method to efficiently convert these commodity polymers into consumer products 1. Addition of small
amounts of members of the most successful group of nucleating agents, i.e. certain sorbitol derivatives
such as 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-
propylphenyl)methylene]-nonitol (TBPMN), raises the peak crystallization temperature of isotactic
polypropylene (i-PP) 2, 3 and polyethylene (PE) as previously shown in Chapter 3, which leads to
reduction in the production cycle of melt-processed polymer artifacts. In addition, sorbitol family-
nucleated polymer has lower values of haze (i.e. <20 %) than neat i-PP 2 and neat PE (as shown in
Chapter 3) which makes these blends attractive for packaging applications. However, members of the
sorbitol family with their sugar-based chemical structure suffer from thermal instability during polymer
processing which may lead to discoloured, odorous end-products 4-10.
As pointed out in Chapter 3, a new class of additives based on 1,3,5-benzene trisamides recently was
presented as i-PP nucleating/clarifying agents that circumvents the above disadvantages. This family is
generally capable of raising the crystallization temperatures of i-PP, and some of them providing drastic
improvement to optical properties at ultra-low concentrations, accompanied by outstanding thermal
stability and excellent solubility in the polymer melt, thus facilitating and improving homogeneous
dispersion of the additive during processing 11-15. It has been also found that C3-symmetric
supramolecular entities, which can be synthesized by the reaction of primary amines with 1,3,5-benzene
tricarboxylic acid chloride, preferentially form 1-dimentional, columnar aggregation and the surface
repeating distance between the rod-shaped aggregates of the additive allows for possible epitaxial
interactions with i-PP 16.
In the presented chapter, a wide range of 1,3,5-benzene trisamide and 1,4-phenylene bisamide-based
additives were examined as nucleating/clarifying agent candidates for PE. An extensive library of
compounds was synthesized as described in the experimental section. Four different families of species
were designed to comprise three functional moieties (as illustrated in Figure 1.a, b, c, d) with the generic
structure of A—(X—R) 2, 3:
i. a central core, A – here phenyl;
ii. moieties capable of forming hydrogen bonds, X – here amides, which promote one-dimensional
growth with columnar self-assembly (a) or two-dimensional growth with lamellar self-assembly
(b, c, d); in the latter series the direction of the amide bond was systematically inversed;
iii. a peripheral group, R – here apolar substituents to enable dissolution of the (semi-polar)
compounds in the molten, hydrophobic polymer and possibly provide epitaxial interactions with
it.
58
Figure 1.
Generic structure of novel, potential nucleating/clarifying agents: 1,3,5-benzenetricarboxylic acid derivatives,
compounds 1.1-1.10 in Table 1 below (a); terephthalic acid derivatives, compounds 2.1-2.13 in Table 1 (b);
aminobenzoic acid derivatives, compounds 3.1-3.4 in Table 1 (c); 1,4-diaminobenzene derivatives, compounds
4.1-4.4 in Table 1 (d); and two compounds with a derivative of the central phenyl core, compounds 5.1and 5.2 in
Table 1 (e, f).
59
2 Experimental
1) Materials
Additives
All 1,3,5-benzene trisamide and 1,4-phenylene bisamide based additive materials were synthesized by
and obtained from the group of Prof. Hans-Werner Schmidt, University of Bayreuth, Germany and used
as received.
Synthesis of 1,3,5-benzenetricarboxylic acid derivatives
The compounds 1.1-1.10 shown in Table 1, were prepared from 1,3,5-benzenetricarboxylic acid
trichloride according to a general procedure described in Ref 12. 12.
Synthesis of terephthalic acid derivatives
The compounds 2.1-2.10 shown in Table 1, were prepared according to the general procedure as follows.
Terephthaloyl chloride was added to the solution of the corresponding amine, anhydrous LiCl,
triethyleneamine, which were previously dissolved in dry tetrahydrofuran (THF) under inert atmosphere
and cooled to 0 °C. The reaction mixture was refluxed for 12 h and subsequently was cooled to room
temperature. The resulting precipitate was filtered off, dried under vacuum and recrystallized.
The compounds 2.11-2.13 shown in Table 1 were prepared by mixing the corresponding amine and
dimethyl terephthalate in a reaction vessel. The reaction mixture was subjected to microwave irradiation
at 150 °C for 3 h. Isolation of the products and purification were performed in the same way as above.
Synthesis of aminobenzoic acid derivatives
For the preparation of compounds 3.1-3.4 shown in Table 1, different acid chlorides were reacted under
the above-described conditions (as explained for the compounds 2.1-2.10) with 4-amino-N-(R)-
benzamide derivatives, and purified as above.
Synthesis of 1,4-diaminobenzene derivatives
The procedure followed for the compounds 2.1-2.10 was also applied to synthesize the compounds 4.1-
4.4 shown in Table 1 by reacting 1,4-diaminobenzene with different acid chlorides.
60
In addition, the compounds 5.1-5.2 shown in Table 1 were prepared by mixing
cyclohexyanemethylamine and, 2-propyloxy dimethyl terephtalate and 2-propyl-terephthalic acid
dimethylester, respectively. The reaction mixture was kept at 120 °C for 1 day and 130 °C for 5 days,
respectively. Isolation of the products and purification were performed in the same way as above for the
compounds 2.1-2.10.
The reference compounds, 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS
Registry Number: 135861-56-2) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol
(TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from Milliken Chemicals, were used as
received.
Polyethylene
The selected “base” polyethylene resin was LLDPE – 8, 25MI as listed in Chapter 2 and used
throughout the present chapter. This particular grade was chosen as it featured the most promising
properties to achieve clarification, as demonstrated in Chapter 3. Some additional experiments were
carried out with LLDPE – 6 and LLDPE – 8, 1MI. All grades were supplied by The Dow Chemical
Company and used as received.
2) Processing
Injection molding plaques
The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molded
plaques. PE/additive blends were compounded in a laboratory co-rotating mini-twin-screw extruder
(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Series of PE with different additive
concentrations were prepared commencing with a PE masterbatch (12.7 g) comprising 2 % w/w additive,
which was subsequently diluted to lower concentrations. For the masterbatch preparation, pellet grades
(LLDPE – 8, 1MI and LLDPE – 8, 25MI) were compounded with the mini-twin-screw extruder and
powder grade (LLDPE – 6) was dry blended. For each concentration, 8.1 g of the compounded mixture
was extruded into a laboratory mini injector (Xplore (DSM), 12.0 ml). Then the desired amount of
PE/additive was added to the remaining 4.6 g. By repeating this procedure, blends of PE and the additive
were prepared with decreasing additive concentrations in the range of 2 % w/w to as low as 0.005 %
w/w. Eventually, the molten polymer/additive blends were injected into a mold at room temperature to
produce plaque samples (thickness 1.0 mm, diameter 25.0 mm). The entire micro-scale polymer
processing was conducted at 220 °C under a nitrogen blanket. Reference samples of the neat polymers
were produced according to the corresponding procedure.
61
3) Analysis
Optical characteristics
Haze of the injection-molded samples was determined at room temperature with a Haze-Gard Plus®
instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 17. A circular area on
the plaque samples, 18.0 mm in diameter, was illuminated by light beam; the recorded haze values
hereafter are referred to as “overall-area haze” as before. In addition, haze values of circular area with
8.0 mm diameter on samples (those free of surface irregularities) were recorded and hereafter are
referred to as “small-area haze”. In order to eliminate the effect of surface scattering, “bulk haze”
measurements were conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner
GmbH, Germany) with non-drying immersion oil (Cargille Series A refractive index oil of n = 1.5150
± 0.0002) which has a similar refractive index as the polymer plaques. Haze values reported here
correspond to the average of values measured for five samples.
Thermal analysis
Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,
Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at a standard
heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure
complete melting of the polymer and prevent self-nucleation, samples were kept for 5 min at the
maximum temperature prior to cooling. The reported crystallization/melting temperatures of the
additives and polyethylenes correspond to the peak temperatures in the DSC thermograms.
Thermo-gravimetric analysis (TGA)
Thermal stability of the additives was analyzed using a Mettler Toledo TGA/SDTA851 instrument.
Samples were heated from 50 °C to 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere.
Reported temperatures correspond to the temperature at which 5 % sample weight loss occurred.
Scanning electron microscopy
Samples for scanning electron microscopy (SEM) studies were prepared by casting low concentration
solutions (~1 % w/w) of neat PE and PE containing 0.1 or 2 % w/w additive in p-xylene, yielding thin
films after evaporation of the solvent; these were subsequently molten at temperatures above the melting
temperatures of the additive in the blend and quenched to room temperature. The solidified films were
coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning electron
microscope (LEO Elektronenmikroskopie GmbH, Germany).
62
3 Results and Discussion
1) Properties
In order to explore potential nucleating- and clarifying capabilities of the spectrum of compounds
synthesized, blends of LLDPE – 8, 25MI with additives over a wide concentration regime (i.e. 0.005-2
% w/w) were prepared and their nucleation/clarification efficiency was examined. Most prominent
results of this study are presented in Table 1. For convenient reference, results obtained with the
commercial additives, DMDBS and TBPMN, are reproduced here from Chapter 3.
Table 1.
Different properties of neat PE (I) and PE containing DMDBS (II) or TBPMN (III) (for this page): chemical
structures of compounds synthesized; their melting temperatures (Tm) and their crystallization temperature (Tc, a);
approximate temperatures of the onset of weight loss at 5 % w/w (Twl, 5 %); maximum obtained peak crystallization
temperature of LLDPE – 8, 25MI (Tc, PE), increase in the crystallization temperature relative to neat LLDPE – 8,
25MI (ΔTc, PE) and its corresponding additive concentration; values for “overall-area haze” versus “additive
concentration” (red, dashed line indicates the haze values of neat LLDPE – 8, 25MI); minimum obtained haze
values (Hazemin), decrease in haze relative to neat LLDPE – 8, 25MI (Δhaze) and its corresponding additive
concentration. n.a. = not applicable; dec. = decomposition; e. = evaporation.
63
Table 1.
continued; 1,3,5-benzene trisamides (1.1-1.5).
64
Table 1.
continued; 1,3,5-benzene trisamides (1.6-1.10).
65
Table 1.
continued; terephthalic acid based 1,4-phenylene bisamides (2.1-2.5).
66
Table 1.
continued; terephthalic acid based 1,4-phenylene bisamides (2.6-2.10).
67
Table 1.
continued; terephthalic acid based 1,4-phenylene bisamides (2.11-2.13)
68
Table 1.
continued; aminobenzoic acid based 1,4-phenylene bisamides (3.1-3.4).
69
Table 1.
continued; 1,4-diaminobenzene based 1,4-phenylene bisamides (4.1-4.4).
70
Table 1.
continued; N,N’-Bis-cyclohexylmethyl-2-propyloxy-terephthalamide (5.1) and N,N’-Bis-cyclohexylmethyl-2-
propyl-terephthalamide (5.2).
As can be seen from the data summarized in Table 1, the present family of 1,3,5-benzene trisamides
(1.1-1.10) generally featured limited nucleation efficiency, more specifically a 1-4 °C increase in
crystallization temperature only and unsatisfying clarification with minimum haze values above 75 %.
However, certain of compounds of the 1,4-phenylene bisamide species showed an increase in the
crystallization temperature of 6 °C (e.g. 2.2), which is fairly successful for PE and a moderate decrease
in the haze up to ∆haze = 40 % (i.e. 2.2, 2.3) already at beneficially low additive content. Encouragingly,
these results are comparable to those obtained with the widely commercially-used sorbitol product
DMDBS (II).
2) Structure
In order to further investigate our results regarding the nucleation and clarification efficiency of 1,3,5-
benzene trisamides and 1,4-phenylene bisamides in LLDPE – 8, 25MI, scanning electron microscopy
(SEM) was conducted. More specifically, the structure of the “aggregates” of the additives in LLDPE –
8, 25MI matrix was analyzed. For this purpose, solution-cast samples of LLDPE – 8, 25MI containing
2 % w/w of 1.7, 2.2 and, for comparison, sorbitol derivatives II, III were produced, dried, molten and
subsequently quenched. Representative results of this study are presented in Figure 2. The SEM images
reveal large crystal domains of the 1,3,5-benzene trisamide compound 1.7 (upper-left), which
71
significantly scatter visible light. In contrast, 1,4-phenylene bisamide compound 2.2 (upper-right) at the
same content featured a molecular organization as rod-like, shish-kebab-type structures of a very fine
appearance, which reduce scattering of light. LLDPE – 8, 25MI comprising the sorbitol derivative
compounds II (lower-left) or III (lower-right) is seen to crystallize in the form of shish-kebab-type
fibrils of the additive (also previously shown in Chapter 3-Figure 11) in the same manner as observed
with 2.2 consisting of a fibrillar network although with smaller widths of fibrils. The latter are not of the
order the wavelength of visible light, thus further reducing light scattering, as evident from the haze
measurements presented above in Table 1.
Figure 2.
SEM images of LLDPE – 8, 25MI containing 2 % w/w of 1.7 (upper-left), 2.2 (upper-right), DMDBS (II) (lower-
left) and TBPMN (III) (lower-right).
Most interestingly, compound 2.2 exhibits optimal nucleation efficiency at concentrations as low as 0.05
% w/w, which is in contrasts to DMDBS (II) for which concentrations around 0.5 % w/w are required.
Furthermore, the same compound 2.2 is an efficient clarifier already at concentrations of ~0.1 % w/w,
which is less than half the amount needed for DMDBS (II). The solubility limit of 2.2 was systematically
investigated in a study of the phase behavior of the binary system comprising LLDPE – 8, 25MI in
Chapter 5.
72
3) Side Groups
As is evident from the data collected in Table 1, the role of the side group (R) is of paramount importance
for controlling the optical properties by providing surfaces that promote (possible epitaxial) growth of
the polymer. For instance, unlike i-PP comprising trisamides with tert.-butyl side group 11, 1,4-
phenylene bisamide with the same side group (i.e. 2.7) did not show any clarification effect in LLDPE
– 8, 25MI, possibly due to a non-epitaxial match between polyethylene and this side group. Furthermore,
whilst compounds comprising phenyl-based side groups generally exhibit inefficient clarification (i.e.
2.11, 2.12), cyclohexyl-based moieties mostly feature a more promising decrease in haze values of the
PE (i.e. 2.2, 2.3, 4.2 and 4.3). This effect possibly can be attributed to a change in repeating distance
between and/or along the additive aggregates which is reduced when compared with the phenyl-based
side group species.
4) Core Structure
Introducing asymmetry by inversion of one of the amide bonds (X) for the derivatives of amino benzoic
acid (3.1-3.4) generally did not show a clarification effect in contrast to terephthalic acid derivatives
(2.1-2.4) and 1,4-diaminobenzene derivatives (4.1-4.4). As can be seen from Table 2, contrary to as
clarifying additive for i-PP as previously shown for 1,3,5-benzene trisamides 11, 1,4-phenylene
bisamides based on terephthalic acid, which is bonded to the core with carbonyl group (-C=O) (2.1-2.4),
featured improved clarification performance in comparison to the 1,4-phenylene bisamides based on
trans-1,4-diaminocyclohexane attached to the core with an amine group (-NH).
In more elaborate studies, the influence of inversion of amide bonds in cyclohexylmethyl-substituted
1,4-phenylene bisamides on the phase behavior, clarification performance and morphological structures
as additive in LLDPE – 8, 25MI is presented in Figure 3. As is evident from the data presented in Table
2 and Figure 3 (a), inversion of amide moieties in the core structure leads to pronounced differences in
clarification of PE. When comparing the haze values of the PE/additive series comprising compounds
2.2, 3.2, 4.2 (respectively 0, 1, 2 amide bonds reversed), additives 2.2 and 4.2 show improvement in
clarifying ability to concentrations as low as 0.1 % w/w. Figure 3 (c) reveals that fibrillar structures of
the additives 2.2 and 4.2 reduced scattering of light unlike compound 3.2 which did not prevent
formation of common spherulitic structures of the polymer, indeed consistent with the haze data results.
However, the phase behavior of the PE/additive systems shown in Figure 3 (b) was found to be little
affected by the change of direction of amide bonds, this in contrast to their clarifying performance. In
view of the above noted strong similarities in the phase behavior of the PE/additive mixtures, the change
in optical properties can be attributed to changes in the parameters of the crystal unit cell of the additive,
and, therewith, possibly its epitaxial matching ability with the crystallizing polymer.
73
Table 2.
Chemical structures of 1,3,5-benzene trisamides (left) and 1,4-phenylene bisamides (right) featuring inversion of
one or more of the amide bonds, and minimum “overall-area haze” values of i-PP (left) 11 and PE (right) at 0.15
% w/w additive content.
74
Figure 3.
Values of “overall-area haze” versus “additive concentration” of the compounds 2.2, 3.2, 4.2 (from left to right)
in LLDPE – 8, 25MI and red, dashed lines indicate the haze of neat LLDPE – 8, 25MI (a); respective
melting/dissolution (Tm,a, ▲) and crystallization temperatures (Tc,a, ▼) of the additive and melting/dissolution
(T m, PE, ●) and crystallization temperatures (T c, PE, ○) of LLDPE – 8, 25MI (b); SEM images of LLDPE – 8, 25MI
containing 0.1 % w/w of the respective additive compounds (c).
75
Finally, optical properties of one of these species, i.e. 2.2, N,N’-bis(cyclohexylmethyl)-1,4-phenylene
dicarboxamide (BCPCA), with “base” polyethylene resins LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8,
25 MI are presented in addition to some previously shown for LLDPE – 8, 25MI (cf. p. 65) in Figure 4.
For comparison, the results obtained for the binary systems of the corresponding resins with DMDBS
are also shown in Table 3. For the aforementioned resins comprising BCPCA and DMDBS, the decrease
in “overall-area haze”, “small-area haze” and “bulk haze” with respect to that of the neat resins
(Δhaze), at concentrations where minimum haze (Hazemin) is obtained are listed. Hazemin values of the
BCPCA binaries, especially for the “bulk haze” measurements (shown in bold, cf. Table 3), approach
those of the corresponding values obtained with DMDBS. Most interestingly, already at very low
BCPCA concentrations (e.g. 0.1 % w/w) the “overall-area haze”, “small-area haze” and “bulk haze”
values were found to sharply decrease for LLDPE – 8, 1MI and LLDPE – 8, 25MI, whereas the
corresponding concentration is higher (e.g. 0.25 % w/w) in the case of DMDBS (shown in bold, cf.
Table 3). These results also correspond with the blown-film applications of the same samples, which are
shown in Appendix-Section 1.
From the data presented in Table 3 it can be also concluded that BCPCA as well as DMDBS are equally
poor clarifying agents for reduction of the “overall-area haze” of LLDPE – 6, but equally effective in
enhancing the optical characteristic of “bulk haze” values. This observation distinctly points to
processing issues with the polymer itself. Highly encouraging are the results collected for experiments
with LLDPE – 8, 25MI where major reductions in all three values of haze were found. Here of particular
interest is the strong reduction of the additive concentration at which Hazemin is observed (i.e. by a factor
2.5 for BCPCA in comparison with the classical DMDBS).
Figure 4.
Haze of LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8, 25MI (from left to right) containing BCPCA: “overall-area
haze” ( ■ ), “small-area haze” ( ⋆ ) and “bulk haze” ( ● ) of injection-molded plaques, plotted as function of the
additive content. In the graphs, red, dashed lines indicate the “overall-area haze” of the corresponding neat resins.
76
Table 3.
In this table are listed the haze of the neat polyethylenes (shown in red) and, the values of the decrease in haze with
respect to the neat resins (Δhaze) for BCPCA and DMDBS at concentrations (% w/w) where minimum haze
(Hazemin) is observed: * = “overall-area haze”, ** = “small-area haze”, *** = “bulk haze”.
4 Conclusions
In summary, a family of novel additives were explored as potential nucleating and clarifying agents for
polyethylene in this chapter. Certain features of the presented additives can be tailored through a
judicious selection of the substituents for the side groups and by the inversion of amide bonds in the
core structure.
It is anticipated that certain compounds of the 1,4-phenylene bisamide family can be promising
candidates owing to their improved nucleation efficiency and clarification performance which are
comparable to industrially widely used DMDBS, but -in contrast to the latter- accompanied by thermal
stability at elevated processing temperatures.
77
5 References
1. Zweifel, H.; Amos, S. E. Plastics Additives Handbook. 5th ed.; Hanser Gardner Publications:
Cincinnati, OH, 2001; p 949.
2. Kristiansen, M.; Werner, M.; Tervoort, T.; Smith, P.; Blomenhofer, M.; Schmidt, H.-W.
Macromolecules 2003, 36, (14), 5150-5156.
3. Bernland, K.; Tervoort, T.; Smith, P. Polymer 2009, 50, (11), 2460-2464.
4. Libster, D.; Aserin, A.; Garti, N. Polym. Advan. Technol. 2007, 18, (9), 685-695.
5. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interface Sci. 2006, 302, (1), 322-329.
6. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interface Sci. 2006, 299, (1), 172-179.
7. McDonald, J. G.; Cummins, C. L.; Barkley, R. M.; Thompson, B. M.; Lincoln, H. A. Anal.
Chem. 2008, 80, (14), 5532-5541.
8. Fujiyama, M.; Wakino, T. J. Appl. Polym. Sci. 1991, 42, (10), 2749-2760.
9. Rekers, J. W. US Patent 5,049,605, 1991.
10. Mannion, M. J. US Patent 5,198,484, 1993.
11. Blomenhofer, M.; Ganzleben, S.; Hanft, D.; Schmidt, H.-W.; Kristiansen, M.; Smith, P.; Stoll,
K.; Mader, D.; Hoffmann, K. Macromolecules 2005, 38, (9), 3688-3695.
12. Abraham, F.; Ganzleben, S.; Hanft, D.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys.
2010, 211, (2), 171-181.
13. Abraham, F.; Kress, R.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys. 2013, 214, (1), 17-
24.
14. Kristiansen, P. M.; Gress, A.; Smith, P.; Hanft, D.; Schmidt, H.-W. Polymer 2006, 47, (1), 249-
253.
15. Wang, J. D.; Dou, Q. Colloid Polym. Sci. 2008, 286, (6-7), 699-705.
16. Kristiansen, M.; Smith, P.; Chanzy, H.; Baerlocher, C.; Gramlich, V.; McCusker, L.; Weber, T.;
Pattison, P.; Blomenhofer, M.; Schmidt, H.-W. Cryst. Growth Des. 2009, 9, (6), 2556-2558.
17. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM
Standard D 1003-07el, 2007.
78
79
Chapter 5
Phase Behavior of Polyethylene and Selected
Nucleating/Clarifying Agent Binary Systems
80
81
1 Introduction
The nucleation and clarification performance of sorbitol derivatives such as 1,3:2,4-bis(3,4-
dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)
methylene]-nonitol (TBPMN) for certain polyolefins, most notably isotactic polypropylene (i-PP) 1, 2,
and polyethylenes (PEs) (cf. Chapter 3) are governed by the phase behavior of the polymer/additive
binary systems, and, therefore, strongly dependent on the additive concentration.
It has been shown earlier 3 that fine-fibrillar structures of the additives are generated in the hyper-eutectic
composition region of the monotectic phase diagrams of i-PP/sorbitol binaries. These fine-fibrillar
structures of the additives act as efficient nucleation sites and, therewith, prevent common spherulitic
crystal growth of the polymer. Instead, the resulting microstructure features a more random organization
of long-range structures – so-called rod-like “shish-kebab” type structures – that result in reduced
scattering of light by the material.
Aiming at a somewhat similar combination of features as the above-mentioned sorbitol additives which,
crucially, exhibit relatively poor thermal stability during processing, a library of tricarboxamides and
bisamides have been synthesized that do not feature this drawback and their potential use as
nucleating/clarifying agents has been investigated in Chapter 4. One of these species, i.e. N,N’-
bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA), was found to be an efficient nucleator
for PE and was able to reduce the haze of PE, as a matter of fact, at substantially lower additive
concentrations than in the case of sorbitol-based clarifiers.
In the present chapter, the phase behavior of binary systems of “base” PEs with TBPMN is reported,
accompanied by a comparative study of the binary system of LLDPE – 8, 25MI/BCPCA, by analogy
with previous studies on i-PP/sorbitols and i-PP/tricarboxamides 1-4. Furthermore, the solid-state
structure of the neat polyethylenes and those comprising the nucleating/clarifying agents was
investigated.
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2 Experimental
1) Materials
The “base” polyethylene resins listed in Chapter 2 were supplied by The Dow Chemical Company and
used as received.
The compounds, 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS Registry
Number: 135861-56-2) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol
(TBPMN, NX 8000, CAS Registry Number: 882073-43-0) were used as received from Milliken
Chemicals. N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA) was obtained from
the group of Prof. Hans-Werner Schmidt, University of Bayreuth, Germany as described in Chapter 4.
2) Processing
For thermal analysis, PE/additive blends were melt-compounded in a laboratory co-rotating mini-twin-
screw extruder (Eindhoven University of Technology, the Netherlands) at 100 r.p.m. for 5 minutes at
240 °C (compositions with TBPMN) or 260 °C (compositions with BCPCA) with under a nitrogen
blanket. Series of PE with different additive concentrations were prepared commencing with a neat PE
(5.0 g), which was subsequently blended with PE/additive masterbatch comprising up to 5 or 10 % w/w
of the additive, hereafter are referred to as the “concentration series”. For each concentration, 2.5 g of
the compounded mixture was extruded. Subsequently the desired amount of PE/additive mixture was
added to the remaining 2.5 g. By repeating this procedure, blends of PE and additive were prepared with
increasing additive concentrations in the range of 0.1 to 5 or 10 % w/w. Additional mixtures for thermal
analysis comprising higher contents of the additive were prepared by dry blending and melting in a
differential scanning calorimeter (DSC) crucible, hereafter are referred to as the “crucible blend series”.
3) Analysis
Optical characteristics
High-temperature optical characteristics were determined by first preparing compression-molded
polymer films between glass slides using 1 mm spacers, and subsequently heating them above their
melting temperatures, i.e. between 170 °C and 260 °C. Since it is difficult to place molten samples onto
the detection port with horizontal light beam orientation, the Haze-Gard Plus® instrument was turned
vertically as depicted in Chapter 2-Figure 4. Haze values according to ASTM standard D1003 5 thus
recorded are hereafter referred to as “melt-intrinsic haze”.
83
Thermal analysis
Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,
Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at standard
heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure
complete melting of the polymer and to prevent self-nucleation, samples of the “concentration series”
were kept for 5 min at the maximum temperature prior to cooling. On the other hand, samples of the
“crucible blend series” were heated and cooled three times with 30 min isothermal steps at the
maximum temperature to obtain satisfactory dispersion of the additive in the polymer melt. The reported
crystallization and melting temperatures correspond to the peak temperatures in the DSC thermograms.
Scanning electron microscopy
Samples for scanning electron microscopy (SEM) studies were prepared by casting low concentration
solutions (~1 % w/w) of neat PE and PE containing 0.1 or 2 % w/w additive in p-xylene, yielding thin
films following evaporation of the solvent; these were subsequently molten at temperatures above the
melting temperatures of the additive in the blend and quenched to room temperature. The solidified films
were coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning
electron microscope (LEO Elektronenmikroskopie GmbH, Germany).
3 Results and Discussion
The melting and crystallization behavior of blends of “base” PEs with nucleating/clarifying agents
covering the entire composition range was investigated. Temperature/composition diagrams of the
binary systems of “base” PEs/TBPMN (cf. Figure 1, 3, 5, 7), accompanied by a comparative study of
LLDPE – 8, 25MI/BCPCA (Figure 9), were constructed from the data obtained by differential scanning
calorimetry (DSC) for cooling and heating (left and right graphs in the corresponding figures,
respectively).
1) HDPE – 4/TBPMN Binary System
The temperature/composition phase diagrams of HDPE – 4/TBPMN mixtures are presented in Figure
1. It can be seen that the system shows a similar, simple monotectic phase behavior as the binaries of i-
PP with DMDBS or TBPMN 1, 2. As in the case of i-PP/TBPMN binaries, TBPMN displays high
compatibility with HDPE – 4, which is evidenced by the onset of liquid-liquid phase separation regime
at a relatively high additive concentration of 5 % w/w for the present system.
84
Figure 1.
Crystallization (left) and melting (right) temperature/composition diagrams for the binary system HDPE – 4/
TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive
crystallization or dissolution/melting ( ■, □ ) and crystallization or melting of the polymer ( ●, ○ ). Solid symbols
denote data obtained for the samples of the “concentration series” and open symbols refer to the “crucible blend
series”. The symbol N refers to TBPMN, PE to the respective resin, L to liquid, S to solid. The drawn lines are
guides for the eye only.
Liquid-liquid phase separation was also investigated by examining light scattering from the
polymer/additive molten blend between glass slides with 1 mm spacers as described in the experimental
section. Subsequently, haze values were recorded at different concentrations of the additive at elevated
temperatures (i.e. below and above the Tm of the additive), and finally these values are presented in the
melting/dissolution binary phase diagram of HDPE – 4/TBPMN in Figure 2. From this “melt-intrinsic
haze” data, it can be seen that there is a significant amount of light scattering by the molten binaries at
a concentration of 5 % w/w (i.e. 96 % haze at 230 °C), in contrast to those of lower concentrations (6,
8, 10 % haze at 230 °C for additive concentrations of 2, 3, 4 % w/w, respectively). This result can be
attributed to the onset of the liquid-liquid phase separation regime, which plausibly generates additive-
rich domains, that strongly scatters light, leading to even more pronounced light scattering just below
the melting/dissolution temperature of the additive (i.e. 100 % haze at 215 °C). Unlike the observations
at low concentrations, enhanced haze is observed at elevated temperatures relative to the room
temperature for the liquid-liquid phase separation regime (i.e. 90 %, 100 %, 96 % haze at 25 °C, 215 °C,
230 °C, respectively for the concentration of 5 % w/w). This result can be understood by a refractive
index difference between molten polymer and additive-rich domains that can induce enhanced light
scattering relative to the refractive index difference between solid polymer and additive in the
microstructure of the material.
85
Figure 2.
Expanded view of the melting temperature/composition diagram of the binary system HDPE – 4/TBPMN; the
numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature. Values
at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.
2) LLDPE – 6/TBPMN Binary System
Corresponding data for the phase behavior of LLDPE – 6/TBPMN compositions are presented in Figure
3. Similar to the HDPE – 4, simple monotectic phase behavior which comprises a broad liquid-liquid
phase separation concentration regime (i.e. starting from 5% w/w additive) was observed for the
present binary system. Below this concentration regime, TBPMN exhibits outstanding solubility in the
molten polymer and upon cooling this homogeneous liquid, the additive forms fine-structures in the
polymer-rich liquid for which light scattering is negligible. As is evident from Figure 4, “melt-intrinsic
haze” data reveals essentially no light scattering just below and above the melting/dissolution
temperature of the additive (i.e. 1 % haze) up to the concentration of 4 % w/w. However, at the
concentration of 5 % w/w, already in the fluid phase, a significant amount of light scattering is observed
(i.e. 85 % haze at 230 °C). Upon cooling, the additive assembles into domains below the additive
melting/dissoultion temperature which significantly scatter light (i.e. 99 % haze at 215 °C) and
ultimately leads to a final solid-state structure with high haze (i.e. 82 % at room temperature). Higher
haze in the molten state relative to the one at room temperature for the present concentration can be
attributed to the change in refractive index difference between polymer and additive for different
temperatures as it was previously found for HDPE – 4.
86
Figure 3.
Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 6/
TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive
crystallization or dissolution/melting ( ■, □ ) and crystallization or melting of the polymer ( ●, ○ ). Solid symbols
denote data obtained for the samples of the “concentration series” and open symbols refer to the “crucible blend
series”. The symbol N refers to TBPMN, PE to the respective resin, L to liquid, S to solid. The drawn lines are
guides for the eye only.
Figure 4.
Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 6/TBPMN; the
numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature. Values
at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.
87
3) LLDPE – 8, 1MI/TBPMN Binary System
In Figure 5, temperature/composition diagrams of TBPMN in LLDPE – 8, 1MI resin are shown. As in
the case of HDPE – 4 and LLDPE – 6, simple monotectic phase behavior exhibiting liquid-liquid phase
separation starting from 5 % w/w additive was also observed for this binary system.
Figure 5.
Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8, 1MI/
TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive
crystallization or dissolution/melting ( ■, □ ) and crystallization or melting of the polymer ( ●, ○ ). Solid symbols
denote data obtained for the samples of the “concentration series” and open symbols refer to the “crucible blend
series”. The symbol N refers to TBPMN, PE to the respective resin, L to liquid, S to solid. The drawn lines are
guides for the eye only.
Again, TBPMN exhibits high solubility within the molten polymer below the onset concentration of the
liquid-liquid phase separation regime. As it can be seen from Figure 5, almost no light scattering is
observed up to 4 % w/w in the molten state (i.e. 1-5 %“melt-intrinsic haze” at 230 °C). However, in the
case of 4 and 5 % w/w, “melt-intrinsic haze” values increase to 67 % and 92 % at the corresponding
temperature, which stems from liquid-liquid phase separation. Further cooling the fluid phase of these
concentrations below the additive dissolution/melting temperature leads to large domains of the additive
in the polymer melt which significantly scatter light (i.e. 98 % and 100 % haze below the additive
dissolution/melting temperature for the concentrations of 4 % w/w and 5 % w/w, respectively). Similar
to results recorded for the previous resins, enhanced haze is observed at elevated temperatures in
comparison to that at room temperature at concentrations in the liquid-liquid phase separation regime
(i.e. 3-5 % w/w).
88
Figure 6.
Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 1MI/TBPMN;
the numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature.
Values at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.
4) LLDPE – 8, 25MI/TBPMN Binary System
As it was found for previous PE resins, simple monotectic phase behavior with the liquid-liquid phase
separation starting around 5 % w/w additive content was also observed for the LLDPE – 8,
25MI/TBPMN binary system, as demonstrated in Figure 7.
Again the solubility of TBPMN in the molten polymer was also examined with “melt-intrinsic haze”;
measurements are shown in Figure 8. Similar to the previous resins, starting at the 4 % w/w
concentration, a high degree of light scattering is observed above and below the additive
melting/dissolution temperatures (i.e. 99 % haze at 220 °C, 100 % haze at 210 °C), which leads to solid-
state microstructure that feature a high haze value (i.e. 88 % haze at room temperature). Again, at these
concentrations in the liquid-liquid phase separation regime, molten blends featured increased haze in
comparison with that at room temperature, as in the case of previous binary systems. However, below
the concentration of 3 % w/w, the homogeneous liquid of the additive and polymer barely scatters light
(i.e. 2-7 %“melt-intrinsic haze” at 230 °C). When these compositions are cooled down below the
additive/melting dissolution temperature, the structures formed by the additive in the polymer melt do
not significantly scatter light (i.e. 3 %, 7 %, 29 % haze below the additive dissolution/melting
temperature for the concentrations of 0.5 % w/w, 1 % w/w, 2 % w/w, respectively) and solid-state
material with low haze is obtained (i.e. 30 %, 35 %, 50 % haze at room temperature for additive
concentrations of 0.5 % w/w, 1 % w/w, 2 % w/w, respectively).
89
Figure 7.
Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8,
25MI/TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive
crystallization or dissolution/melting ( ■, □ ) and crystallization or melting of the polymer ( ●, ○ ). Solid symbols
denote data obtained for the samples of the “concentration series” and open symbols refer to the “crucible blend
series”. The symbol N refers to TBPMN, PE to the respective resin, L to liquid, S to solid. The drawn lines are
guides for the eye only.
Figure 8.
Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 25MI/TBPMN;
the numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature.
Values at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.
90
5) LLDPE – 8, 25MI / BCPCA Binary System
As can be seen in Figure 9, the study of phase behavior of the binary system LLDPE – 8, 25MI/BCPCA
also clearly indicates a similar, simple monotectic phase behavior as previously observed for the binaries
with TBPMN. However, unlike for the sorbitol binary systems, for additive concentrations exceeding 1
% w/w, unexpectedly three different thermal transitions of the additive were observed in both cooling
and heating temperature/composition diagrams. Therefore, these thermal transitions of neat BCPCA
were investigated by differential scanning calorimetry (DSC) and temperature-dependent wide-angle X-
ray diffraction (WAXD) as shown in Appendix-Section 2.
Figure 9.
Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8,
25MI/BCPCA. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive
crystallization or dissolution/melting ( ■, □ ), solid-state transitions of the additive ( ▲, ▼, , ) and crystallization
or melting of the polymer ( ●, ○ ). Solid symbols denote data obtained for the samples of the “concentration
series” and open symbols refer to the “crucible blend series”. The symbol B refers to BCPCA, PE to the respective
resin, L to liquid, S to solid. The drawn lines are guides for the eye only.
Strikingly, and similar to the binary systems of polyethylenes with TBPMN studied in the previous
sections, a broad liquid-liquid phase separation regime was observed for the present blends, starting
from a concentration of around 2 % w/w up to virtually the BCPCA axis. In other words, BCPCA
binaries exhibit the monotectic composition close to the polymer axis, causing already at low
concentrations the additive to crystallize first, and therewith provide the active additive solid surface
suitable for polymer nucleation and clarification. Liquid-liquid phase separation was also investigated
by examining light scattering from the molten polymer/additive blends. From the “melt-intrinsic haze”
91
data, it can be seen that there is a significant amount of light scattering by the molten binary at additive
concentrations approaching 2 % w/w (i.e. 61 % haze at 250 °C) which can be attributed to the onset of
the liquid-liquid phase separation regime. Liquid-liquid phase separation leads to domains of high
additive content which significantly scatter light, leading to even more pronounced light scattering just
below the melting/dissolution temperature of the additive (i.e. 89 % haze at 240 °C) and high turbidity
materials at room temperature (i.e. 93 % “bulk haze” at 25 °C).
Figure 10.
Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 25MI/BCPCA;
the numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature.
Values at 25 °C correspond to “bulk haze” measurements as described in Chapter 4.
6) Structure
In order to gain insight into the structure of the “aggregates” of the clarifying agents and its implications
for clarification of PE, scanning electron microscopy (SEM) was conducted. The SEM images in Figure
11 show clear, classical spherulitic structures for all “base” polyethylenes (left columns), whereas the
polymers containing 2 % w/w TBPMN feature a rod-like, shish-kebab-type morphology (right column)
in the hyper-eutectic composition region of the monotectic phase diagram, in accordance with earlier
observations for i-PP 3. At increased magnification, the radially-ordered lamellae constituting the
spherulites in the neat polyethylenes can be discerned. However, four different polyethylenes of different
macromolecular structures, nucleated with TBPMN, revealed dramatically different size of polymer and
fibrillar-additive crystalline entities. Therefore, in Chapter 6, the influence of the macromolecular
structure of PE on the solidification behavior of both the additive and the polymer, their resulting
structures and the haze of the nucleated blends will be investigated in more detail.
92
Figure 11.
SEM images of neat HDPE – 4; LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8, 25MI (a, b, c, d – left and middle
column) and the same polymers containing 2 % w/w TBPMN (right column). Scale bars 1 μm.
93
The structure of BCPCA crystallized in LLDPE – 8, 25MI was also analyzed by scanning electron
microscopy (SEM) and is shown in Figure 12. The samples of LLDPE – 8, 25MI containing 0.1 and 2
% w/w BCPCA reveal a very broad fibril-width distribution of the additive aggregates. This is different
when compared to the observations for TBPMN, and which is manifested in the modest clarification
performance of BCPCA. However, there are regions, which consist of fine-fibrillar molecular
organization of BCPCA (Figure 12 – left images), that show lower widths of fibrils in PE comprising
0.1 % w/w additive in comparison to that containing 2 % w/w additive, therewith reducing scattering of
light, indeed consistent with the aforementioned phase and haze behavior of the LLDPE – 8,
25MI/BCPCA binary system.
Figure 12.
SEM images of LLDPE – 8, 25MI containing 0.1 % w/w (top) and 2.0 % w/w (bottom) of BCPCA revealing the
variations in additive aggregation within the same sample.
94
4 Conclusions
Monotectic phase behavior was demonstrated for the binary systems of polyethylenes with the sorbitol
derivative, TBPMN and a new nucleating/clarifying agent, BCPCA – similar to that previously reported
for i-PP. Obviously, the existence of liquid-liquid phase separation that typifies both systems is
extremely significant, since it efficiently directs the position of the monotectic composition. For
instance, as shown in Figure 13, the BCPCA binary exhibits the monotectic composition close to the
polymer axis causing, already at low concentrations, the additive to first crystallize, and therewith
provide the active additive solid surface suitable for polymer nucleation and clarification.
Figure 13.
Crystallization (left) and melting (right) temperature/composition diagrams of the binary system LLDPE – 8, 25MI
with BCPCA (red symbols), DMDBS (black symbols) and TBPMN (blue symbols). Different symbols refer to
data for different transitions: additive crystallization or dissolution/melting ( ▲, ▲, ▲ ) and crystallization or
melting of the polymer ( ●, ●, ● ).
Finally, it should be noted that the above presented data clearly indicates that the prevention of the
formation of spherulitic polymer structures by nucleating/clarifying agent is not sufficient for achieving
clarification. Even though the 3D fine fibrillar-network of TBPMN exists in polyethylenes, the size of
the fibril-width and polymer lamellae should not be of the order of the wavelength of visible light.
Therefore, the macromolecular structure of PE and its interaction with additive will be examined further
in Chapter 6. Furthermore, the broad fibril-width distribution of BCPCA is still an issue that causes a
relatively high degree of light scattering and wide variations in values of haze in different parts of the
various samples.
95
5 References
1. Kristiansen, M.; Werner, M.; Tervoort, T.; Smith, P.; Blomenhofer, M.; Schmidt, H.-W.
Macromolecules 2003, 36, (14), 5150-5156.
2. Bernland, K.; Tervoort, T.; Smith, P. Polymer 2009, 50, (11), 2460-2464.
3. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal Institute
of Technology Zurich, Nr 19388, Zurich, 2010.
4. Kristiansen, P. M.; Gress, A.; Smith, P.; Hanft, D.; Schmidt, H.-W. Polymer 2006, 47, (1), 249-
253.
5. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM
Standard D 1003-07el, 2007.
96
97
Chapter 6
Influence of Polyethylene Macromolecular Structure
on Crystallization onto 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-
propylphenyl)methylene]-nonitol
98
99
1 Introduction
The nucleating and clarifying abilities of sorbitol derivatives such as 1,3:2,4-bis(3,4-
dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)
methylene]-nonitol (TBPMN) are governed by the phase behavior of the polymer/additive binary
systems, and therefore, are strongly dependent on the additive concentration 1, 2. It has been shown in
Chapter 5 that fine-fibrillar structures of the additives are generated in the hyper-eutectic regime of the
monotectic temperature/composition diagrams of “base” polyethylenes (PEs)/TBPMN binaries (cf.
Figure 1 – Ns + L1 regime) as in the case of isotactic polypropylene (i-PP) 3. In that preliminary study, a
change in the hyper-eutectic composition and, alteration in the crystal size of the additive and the
polymer were observed for different macromolecular structures.
Independent increase of certain PE characteristics – such as comonomer content, side-chain length and
molecular weight – results in a decrease in the melting/crystallization temperatures of the neat polymer
which leads to change in the melting/crystallization temperatures of the additive in the blend, which in
turn shifts the liquidus line of hyper-eutectic composition 4-6. In Figure 1, the changes in the position of
the liquidus line due to the reduced melting/crystallization temperatures of the polymer are depicted.
Figure 1.
Schematic of the partial, low additive content section of the monotectic temperature/composition diagram of the
PE/TBPMN binary system with a focus of the regime of hyper-eutectic composition and its positioning with a
change in the melting temperature of neat PE. The symbol N refers to TBPMN; PE to the respective resin; L to
liquid; S to solid; Tc, a, 1 and Tc, a, 2 to the crystallization temperature of the additive for the corresponding liquidus
lines.
100
In the most simple case, when there is no change in enthalpy and entropy of mixing, the eutectic
composition shifts downward to the temperature-composition axes (cf. Figure 1-black, dashed curve) 6.
However, in reality, changes in enthalpy and entropy shift the liquidus lines – for example towards the
additive axis (cf. Figure 1-green curve) when the solvating power of the polymer for the additive
increases or towards the polymer axis (cf. Figure 1-blue curve) when the solvating power of the polymer
for the additive decreases 6. Therefore, the crystallization temperature of the additive at a specific
concentration in the hyper-eutectic composition can either increase (Tc, a, 1) or decrease (Tc, a, 2) dependent
on the change of the mixing enthalpy and entropy of the system.
Following classical crystallization arguments, an increase of Tc, a reduces the supercooling, which
generally leads to the formation of larger crystalline entities 7 and the opposite is commonly observed at
decreased values of Tc, a. These differences are expected to influence not only just the size of the additive
crystals, but therewith the area available for subsequent nucleation and growth of the polymer onto them,
with anticipated significant influence on haze of the solidified system.
In the present chapter, a wider range of PEs covering a broad spectrum of macromolecular structures
blended with TBPMN was examined in order to explore the influence on the nucleation/clarification
efficiency of the same additive on them. By changing the particular macromolecular intrinsic parameters
of the PEs (i.e. molecular weight, molecular weight distribution and comonomer content), alterations in
the additive fibril-width and other interactions of the polymers with TBPMN were investigated. That
specific additive was chosen as it was found to be the compound to optimally enhance the optical
properties of PE (cf. Chapter 3).
101
2 Experimental
1) Materials
The polyethylene resins used throughout this study are homopolymers and copolymers of ethylene-
octene; selected characteristic properties are listed in Table 1. The polymers were supplied by The Dow
Chemical Company and used as received. The nucleating/clarifying agent 1,2,3-trideoxy-4,6:5,7-bis-O-
[(4-propylphenyl)methylene]-nonitol (TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from
Milliken Chemicals, also was used as received.
2) Processing
Blend preparation
Mixtures of the polyethylenes comprising 0.5 % w/w TBPMN were compounded in a laboratory co-
rotating mini-twin-screw extruder (Eindhoven University of Technology, the Netherlands) at 100 r.p.m.
for 5 minutes at 240 °C under a nitrogen blanket. Reference samples of the neat polymers were produced
according to the same procedure.
Injection molding plaques
The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molding
plaques. PE/additive blends were compounded in a laboratory co-rotating mini-twin-screw extruder
(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Eventually, the molten polymer/additive
blends were injected into a mold at room temperature to produce plaque samples (thickness 1.0 mm,
diameter 25.0 mm). The entire micro-scale polymer processing was conducted at 220 °C under a
nitrogen blanket. Reference samples of the neat polymers were produced according to the corresponding
procedure.
3) Analysis
Optical characteristics
Haze of the injection-molded samples was determined at room temperature with a Haze-Gard Plus®
instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 8. In addition to
“overall haze”, in order to eliminate the effect of surface scattering, “bulk haze” measurements were
also conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner GmbH,
Germany) with non-drying immersion oil (Cargille Series A refractive index oil, n = 1.5150 ± 0.0002)
which has a refractive index similar to the polymer plaques. Haze values reported here correspond to the
average of measured for five samples.
102
Thermal analysis
Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,
Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at standard
heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure
complete melting of the polymer and to prevent self-nucleation, the samples were kept for 5 min at the
maximum temperature prior to cooling. The reported melting and crystallization temperatures
correspond to the peak temperatures in the DSC thermograms. The degree of crystallinity of the
polymers was calculated from the enthalpy of fusion, derived from the endothermic peak, adopting a
value of 293 J/g for 100 % crystalline polyethylene 9.
Mechanical properties
Uniaxial tensile testing of the samples was performed on an Instron 5864 tensile testing machine
equipped with pneumatic clamps. The instrument was set up with a ±100 N static load cell and was used
in constant rate of elongation mode (12 mm/min). All tests were carried out at room temperature on
dogbone-shaped specimens of ~100 µm thickness, 2 mm width and 12 mm gauge length. All reported
Young’s modulus (E) values of the samples correspond to an average of five measurements.
Scanning electron microscopy
Samples for scanning electron microscopy (SEM) studies were prepared by hot-stage melting of
previously blended material on glass slide at 240 °C and; these were subsequently quenched to room
temperature. The solidified films were coated with a thin conductive layer of platinum and imaged using
a LEO 1530 Gemini scanning electron microscope (LEO Elektronenmikroskopie GmbH, Germany).
103
3 Results and Discussion
1) Properties
In order to investigate the effect of macromolecular structure of the various polyethylenes on the
aggregation behavior of the additive and subsequent crystallization of the PEs onto them, blends of the
different pilot plant resins comprising 0.5 % w/w TBPMN were prepared and their
nucleation/clarification efficiency were examined, as well as solid-state structure of the additive and the
polymers. This particular concentration of TBPMN was chosen as it yielded the most promising
clarification performance, as demonstrated in Chapter 3. For reference purposes, data obtained with one
of the “base” PEs, i.e. LLDPE – 8, 1MI, are also included.
Characteristics of the various polyethylenes used in this study, including the grades of homopolymers
and copolymers comprising ethylene-octene comonomers and their blends comprising 0.5 % w/w
TBPMN, are presented in Table 1: densities; melt index; number-average molecular weight (Mn),
weight-average molecular weight (Mw) and polydispersity index (PDI, Mw/Mn); octene (C8) comonomer
content; peak melting temperatures of the neat resins (Tm, p) and blends (Tm, b); Young’s modulus of the
neat resins (Ep) and blends (Eb); crystallinity of the neat resins (wp) and blends (wb); peak and onset
crystallization temperatures of the polymer (Tc, p, Tc, o) and increase in peak and onset crystallization
temperatures relative to the neat PE resins (ΔTc, p, ΔTc, o); and peak and onset crystallization
temperatures of the additive (Tc, a (peak) and Tc, a (onset)); “overall haze” and “bulk haze” of the blends
and decrease in “overall haze” (∆hazeO) and “bulk haze” (∆hazeB) relative to the neat PE resins.
2) Nucleation
The nucleation efficiency of TBPMN in different PE blends was examined by peak and onset
crystallization temperatures of the additive (Tc, a (peak) and Tc, a (onset)). From the data shown in Table
1, it can be concluded that there is only a few degrees of difference between Tc, a (peak) and Tc, a (onset).
In Figure 2, additive crystallization temperatures are shown by plotting DSC cooling thermograms of
PE/TBPMN blends and Tc, a (onset) values are plotted against the comonomer content of the resins. It
appears that additive crystallization temperature reveals a random trend with comonomer content of the
resins. This result can be possibly attributed to the other variables in the PE macromolecular structures
such as Mw, PDI, and distribution of the comonomer along the backbone chain which alter the system’s
mixing enthalpy and entropy to shift the liquidus either to the polymer axis or the additive axis as
demonstrated in Figure 1.
104
Ta
ble
1.
Su
mm
ary
of
pri
nci
pal
ch
arac
teri
stic
s o
f ex
per
imen
tal
PE
res
ins
and
th
eir
ble
nd
s co
mp
risi
ng
0.5
% w
/w T
BP
MN
in
co
mpar
iso
n w
ith
th
e “
ba
se”
res
in L
LD
PE
– 8
, 1
MI.
105
Figure 2.
DSC cooling thermograms showing exothermic peaks of the additive crystallization (left) and Tc, a (onset) of 0.5
% w/w PE/TBPMN blends versus comonomer content of the PE resins (right). The numbers and corresponding
symbols refer to the resins, which are listed in Table 1.
Subsequently, the nucleation efficiency of PE chains onto the solidified TBPMN was also examined by
crystallization peak and onset temperatures of the polymers in the blends (Tc, p and Tc, o), and their
increase relative to the neat PE resins (ΔTc, p and ΔTc, o). Systematic reduction in Tc, o values with increasing
comonomer content of PE resins can be seen in Figure 3 (left) as expected.
Figure 3.
Tc, o (left) and ∆Tc, o (right) of 0.5 % w/w PE/TBPMN blends versus comonomer content of the PE resins. Symbols
refer to the resins, which are listed in Table 1 and the dotted line is guide to the eye only.
106
As can be seen from Figure 3 (right), the effect of TBPMN on the nucleation efficiency of the polymers
is generally comparable for all the resins (i.e. ΔTc, o = ~4 °C for the samples 1-9). However, a much
higher ΔTc, o = ~8 °C is observed for the sample 10. This result can possibly be attributed to the
crystallization of low molecular weight chains onto the high molecular weight chains of the polymer by
itself due to the broad molecular weight distribution (i.e. PDI = 4.1) of the neat polymer, when compared
with the otherwise similar resin 9.
3) Optical Properties
In Figure 4, “bulk haze” of the blends and their decrease relative to the neat resins (∆hazeB) are plotted
against the comonomer content of the resins. Copolymers, which have similar molecular weight as their
homopolymers (samples 1-3), and comprising 0.3 mol % octene-comonomer (samples 4-6) exhibit
reduced haze and increased ∆haze relative to the homopolymers. Samples 7 and 8 possessing low
molecular weight at higher comonomer contents show higher haze with minor amount of ∆haze. Among
the experimental resins, not surprisingly lowest haze values were found for the blends comprising higher
comonomer contents (samples 9 and 10), but no obvious trend in ∆haze was detected.
Figure 4.
“Bulk haze” of 0.5 % w/w PE/TBPMN blends (left) and the decrease in “bulk haze” relative to that of the neat
resins (right) versus comonomer content of the PE resins. Symbols refer to the resins which are listed in Table 1.
4) Fibril-width and Fibril-width-distribution
In order to further investigate the effect of PE macromolecular structure to the final shish-kebab-type
structures, scanning electron microscopy (SEM) studies were conducted for the PE/TBPMN blends by
examining the additive fibril-widths and PE lamellar-widths. In Figure 5, SEM images of the PEs
107
comprising 0.5 % w/w TBPMN are presented according to Mw and comonomer content of the resins. In
the following, the most prominent results of this study are presented.
Change of comonomer content at the same Mw
The homopolymers (samples 1-3) and the copolymers comprising 0.3 mol % octene-comonomer
(samples 4-6) possessing the same Mw were compared. Samples were categorized as Group 1 (samples
1 and 4); Group 2 (samples 2 and 5); Group 3 (samples 3 and 6).
Figure 5 indicates that copolymers generally feature thinner fibril-widths of the additive accompanied
by reduced haze. Group 1 and Group 2 reveal reduced additive fibril-width for the copolymers indeed
associated with a reduced Tc, a (onsets), i.e. fibril width size: 1 > 4, respectively Tc, a (onsets): 149 °C,
144 °C and 2 > 5, respectively Tc, a (onsets): 144 °C, 142 °C. For the Group 3, the fibril-width of the
additive in copolymer is found to be larger than in the homopolymer, unlike in the former groups.
However, the size of the fibril-width in the present group was found to be also related to the Tc, a (onsets),
as in the case of former groups: i.e. fibril-width size: 6 > 3, respectively Tc, a (onsets): 149 °C, 148 °C.
In other words, additive crystallization occurs at lower temperatures, which means less time for the
additive to grow laterally resulting in a smaller fibril-width in the final solid-state structure.
Change of Mw at the same comonomer content
As can be seen in Figure 5, increasing molecular weight of the copolymers comprising 0.3 mol % octene-
comonomer (samples 4-6), appears to result in an uncorrelated change of the fibril-width. However, like
in the previous cases, the fibril-width of the additive reveals an identical dependence on Tc, a (onsets):
fibril-width size: 6 > 4 > 5, respectively Tc, a (onsets): 149 °C, 144 °C, 142 °C.
Effect of low molecular-weight fraction
Samples 7 and 8, which comprise low molecular weight PE fractions feature a large fibril-width-
distribution of the additive (cf. Figure 5). It can be concluded that low molecular-weights generally cause
an increase of the fibril-width-distribution, which significantly scatters light leading to an increase in
haze.
Effect of high comonomer content
Samples 9 and 10, which have higher comonomer content than all other resins exhibited a more
homogeneous distribution of the width of the additive network and a decrease in the size of fibril-width,
together with a substantial reduction of the polymer lamellar-width (cf. Figure 5). In accordance with
the haze results, a decrease in the widths of the structural features of both components in the final solid-
state reduces scattering of visible light and hence leads to lower haze as shown in the previous section.
108
Fig
ure
5.
SE
M i
mag
es o
f th
e P
Es
com
pri
sin
g 0
.5 %
w/w
TB
PM
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on
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efer
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ed i
n T
able
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Th
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aze
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e b
lend
s.
109
The haze results prove that preventing only the lateral-growth of the fibrils of the additive is not
sufficient to improve optical characteristics. Polymer lamellar-width is another important factor that
must be taken into account. For instance, even though the SEM image of sample 4 in Figure 5 reveals a
fine size of the additive fibrils, the size of the polymer lamellar-width is of the order of the wavelength
of visible light leading to high haze.
5) Mechanical and Thermal Properties
In Figures 6 and 7, the Young’s modulus, the melting temperature and crystallinity of the neat resins
together with the blends are plotted against comonomer content. Similar to the findings in Chapter 2-
Figure 5.b, the corresponding properties decrease with increasing comonomer content which is opposite
to the trend found for optical properties.
Beneficially, and in accord with previous studies 10-12, the Young’s modulus of blends comprising the
additive are generally ~1.5 times higher than that of the neat resins. This value can reach up to 2 for the
ones comprising higher comonomer contents (i.e. samples 9 and 10).
Figure 6.
Young’s modulus of neat resins and 0.5 % w/w PE/TBPMN blends versus comonomer content of the PE resins.
Symbols refer to the resins, which are listed in Table 1 and, the symbols including a cross represent blended resins.
The dotted lines are guides to the eye only.
Finally, the data presented in Figure 7 indicates that addition of the nucleating/clarifying agent TBPMN
results in essential insignificant change in the values of the melting temperature and degree of
crystallinity of the different PE resins.
110
Figure 7.
Melting temperature (left) and degree of crystallinity (right) of neat resins and of 0.5 % w/w PE/TBPMN blends
versus comonomer content of the PE resins. Symbols refer to the resins, which are listed in Table 1 and, the
symbols including a cross represent blended resins. The dotted lines are guides to the eye only.
4 Conclusions
In summary, a family of experimental PE resins with different macromolecular structures was explored
to examine the crystallization behavior of the nucleating/clarifying agent TBPMN and subsequent PE
lamellar overgrowth onto the additive fibrils. It was shown that the nucleation and clarification
performance of TBPMN in the blends can be tailored with different intrinsic characteristics of the PE
grades such as comonomer content, molecular weight or molecular weight distribution. It appears that
there is a noticeable dependency of the lateral-growth of TBPMN fibrils and Tc, a of the additive – for
example, early additive crystallization at high Tc, a leaves more time for the lateral-growth of fibrils to
form long-range structures. However, Tc, a and fibril-width did not exhibit a systematic trend dependent
on the comonomer content. Finally, it should be noted that not only the size of the additive fibrils
determines haze. When either the width of the polymer lamella or additive fibril are of the order of the
wavelength of visible light, samples significantly scatter visible light and hence exhibit a high level of
haze.
111
5 References
1. Kristiansen, M.; Werner, M.; Tervoort, T.; Smith, P.; Blomenhofer, M.; Schmidt, H.-W.
Macromolecules 2003, 36, (14), 5150-5156.
2. Bernland, K.; Tervoort, T.; Smith, P. Polymer 2009, 50, (11), 2460-2464.
3. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal Institute
of Technology Zurich, Nr 19388, Zurich, 2010.
4. Peacock, A. J. Handbook of Polyethylene: Structures, Properties, and Applications. Marcel
Dekker: New York, 2000; p 173.
5. Simpson, D. M.; Vaughan, G. A. Ethylene Polymers, LLDPE. Encyclopedia of Polymer Science
and Technology. 2001, (2), 441-482.
6. Koningsveld, R.; Stockmayer, W. H.; Nies, E. Polymer Phase Diagrams a Textbook. Oxford
University Press: Oxford, 2001; p 109, 176.
7. Young, R. J. Introduction to Polymers. University Press: Cambridge, 1986; p.175, 188.
8. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM
Standard D 1003-07el, 2007.
9. Wunderlich, B. Thermal Analysis. Academic Press: San Diego, 1990; p 418.
10. Pukanszky, B.; Mudra, I.; Staniek, P. J. Vinyl Addit. Techn. 1997, 3, (1), 53-57.
11. Zhang, Y. F.; Xin, Z. J. Appl. Polym. Sci. 2006, 100, (6), 4868-4874.
12. Zhang, Y. F. J. Macromol. Sci. B 2008, 47, (6), 1188-1196.
112
113
Chapter 7
Conclusions and Outlook
114
115
1. Conclusions
This thesis describes a comprehensive study of modifying the crystallization of polyethylenes (PEs) with
the aim of drastically improving their optical transparency.
Investigations of optical, thermal and mechanical properties of PEs, possessing modified chain
architectures, demonstrate that transparency can be readily obtained for the PEs comprising a high
degree of branching, but with the penalty of a major reduction in their equally-relevant thermal (i.e.
melting temperature) and mechanical (i.e. stiffness) properties. As the most prominent conclusion of this
thesis, addition of a particular nucleating/clarifying agent to PE – intrinsically possessing superior
thermal and mechanical properties (i.e. linear low-density polyethylene, LLDPE) – exhibits
advantageous optical characteristics such as those observed for the low-density resins (cf. Figure 1). The
results of this work summarized in Figure 1 demonstrate that the Young’s modulus of the clarified PE
can be ~50 times higher than that for the low-density resin of comparable optical performance, and
features a melting temperature ~65 °C higher than that of the latter polyethylene.
Figure 1.
Plot of Young’s modulus (blue-square symbols) and melting temperature (red-triangle symbols) versus "overall
haze” for polyethylenes, showing the trade-off between optical and thermo-mechanical properties. Solid symbols
on the right correspond to the neat resin and solid symbols on the left which follow the dashed-arrows correspond
to the “bulk haze” of clarified resin (sample 9 from Chapter 6). Solid-arrows correspond to the increase in Young’s
modulus and melting temperature for the clarified resin with respect to the low-density resin possessing comparable
haze.
116
In an attempt to gain deeper understanding of what characteristics a clarifying agent should possess in
order to obtain improved optical performance of polyethylene, scanning electron microscopy (SEM)
was used as a tool to investigate the influence of such additives on the microstructure of the solid-state
material. The additives 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol, DMDBS; 1,2,3-trideoxy-4,6:5,7-
bis-O-[(4-propylphenyl)methylene]-nonitol, TBPMN; N,N’-bis(cyclohexylmethyl)-1,4-phenylene
dicarboxamide, BCPCA that imparted improved clarification of PE, were indeed found to prevent
spherulitic growth of the polymer during crystallization – instead, polymer crystals grew onto the
fibrillar surface of the additives and hence featured random rod-like shish-kebab-type structures as
observed in the case of clarified i-PP 1. However, it was found – and this is another main conclusion of
this thesis – that preventing spherulitic growth is not sufficient for obtaining improved clarification for
polyethylenes. The additive fibril and polymer lamellar-widths – which are of the order of the
wavelength of visible light (400-700 nm) – for different macromolecular structures of PE can already
induce to high degree of light scattering. Therefore, homogeneous microstructures of a fine-fibrillar
network of the additive and polymer lamellar-widths that do not exceed the wavelength of visible light
are pre-requisites to obtain clarified PE as found for the samples 9 and 10 in Chapter 6.
2. Outlook
Epitaxial interaction of additive with polyethylene
Even though it is demonstrated that polyethylenes can be clarified, the exact mechanism leading to the
advantageous microstructure is still not well-understood. It is clear that the surface provided by an
additive should strongly promote nucleation of the polymer and, of course, this can be provided by the
chemical structure of the additive fibrils which can feature matching crystallographic distances for
enabling epitaxial growth 2-6 or minimizing the energy barrier for crystal growth by featuring grooves on
the surface of those fibrils 7-9. Therefore, establishing a more detailed understanding of (meso-)epitaxial
interactions with PEs, particularly concerning the correlation between the chemical structure and surface
topology of additives, could represent an important objective for further studies. Moreover, additional
efforts on the design and development of the aramid-based compounds could be a salvation for thermal
instability of the currently employed sorbitols during polymer processing and their migration issues 10-
16.
117
Designing optimized polyethylene macromolecular structure
Whilst injection-molded plaques of the clarified PE – possessing haze comparable to clarified i-PP –
presented in this thesis, haze measurement represents only a relatively small area of the sample; i.e.
those that are free of surface imperfections due to polymer processing issues. Contribution of “surface
haze” to the “overall haze” reveals a major effect of the surface imperfections on the optical properties
of the specimen (i.e. 0.25 % w/w TBPMN in LLDPE – 8, 1MI: “overall haze” 54 %, “small area haze”
17 %, “bulk haze” 20 %). Therefore, designing an optimized PE molecular architecture which reduces
“surface haze” should be a study of interest for industrial scale-up and implementation.
Nucleation studies of TBPMN and subsequent polymer crystallization onto it described in Chapter 6 –
by investigating a wide range of PEs of different comonomer content, molecular weight and molecular
weight distribution – show that there is a noticeable correlation between the lateral growth of the additive
fibrils and the crystallization temperature of the additive (Tc, a). For instance, relatively early
crystallization of the additive at high Tc, a plausibly leads to lateral-growth of the fibrils for a longer
period of time until PE crystals start to grow, and hence the additive-fibrils feature courser structures in
the final solid-state material. However, no systematic trends were found between comonomer content,
molecular weight of the polymer and Tc, a in the polymer melt. In order to establish a more profound
understanding for the dependency of the lateral growth of additive fibrils in the polymer melt and Tc, a,
more detailed systematic studies should be performed by more precisely controlling the macromolecular
structure parameters (i.e. molecular weight, molecular weight distribution, sequence of the comonomers
in the backbone).
Last but not least, having demonstrated significant progress in understanding the microstructure of the
clarified PE, further studies of epitaxial interactions with additives accompanied by designing the
optimal macromolecular structure could open possibilities for obtaining highly-transparent high-density
polyethylene, which would feature the benefits of further improved thermal and mechanical properties.
118
References
1. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal
Institute of Technology Zurich, Nr 19388, Zurich, 2010.
2. Lotz, B.; Wittmann, J.-C. Macromol. Chem. Phys. 1984, 185, (9), 2043-2052.
3. Mathieu, C.; Thierry, A.; Wittmann, J.-C.; Lotz, B. Polymer 2000, 41, (19), 7241-7253.
4. Alcazar, D.; Ruan, J.; Thierry, A.; Lotz, B. Macromolecules 2006, 39, (8), 2832-2840.
5. Thierry, A.; Straupe, C.; Wittmann, J.-C.; Lotz, B. Macromol. Symp. 2006, 241, (1),103-110.
6. Kristiansen, M.; Smith, P.; Chanzy, H.; Baerlocher, C.; Gramlich, V.; McCusker, L.; Weber,
T.; Pattison, P.; Blomenhofer, M.; Schmidt, H.-W. Cryst. Growth Des. 2009, 9, (6), 2556-
2558.
7. Shepard, T. A.; Delsorbo, C. R.; Louth, R. M.; Walborn, J. L.; Norman, D. A.; Harvey, N. G.;
Spontak, R. J. J. Polym. Sci. Pol. Phys. 1997, 35, (16), 2617-2628.
8. Siripitayananon, J.; Wangsoub, S.; Olley, R. H.; Mitchell, G. R. Macromol. Rapid Comm.
2004, 25, (15), 1365-1370.
9. Vaughan, A. S.; Hosier, I. L. J. Mater. Sci. 2008, 43, (8), 2922-2928.
10. Libster, D.; Aserin, A.; Garti, N. Polym. Advan. Technol. 2007, 18, (9), 685-695.
11. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interace Sci. 2006, 302, (1), 322-329.
12. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interface Sci. 2006, 299, (1), 172-179.
13. McDonald, J. G.; Cummins, C. L.; Barkley, R. M.; Thompson, B. M.; Lincoln, H. A. Anal.
Chem. 2008, 80, (14), 5532-5541.
14. Fujiyama, M.; Wakino, T. J. Appl. Polym. Sci. 1991, 42, (10), 2749-2760.
15. Rekers, J. W. US Patent 5,049,605, 1991.
16. Mannion, M. J. US Patent 5,198,484, 1993.
119
Appendix
120
1. Blown-film Applications
Blown-films of neat LLDPE – 8, 1MI and its blends with 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol
(DMDBS) were obtained from Martin Hill, Dow Chemical Company, Tarragona, Spain and used as
received. Monolayer polyethylene films were produced on a single-screw extruder (Collin, Germany)
and fitted with a film-blowing die. Processing parameters are provided in Table 1, and haze results of
the films are listed in Table 2.
Processing parameters
Screw speed (rpm) 60
Melt temperature (°C) 240
Take-up speed (m/min) 4.3
Average thickness (μm) 50
Table 1.
Processing parameters for the production of blown-films comprising LLDPE – 8, 1MI and its mixtures with
DMDBS.
Table 2.
“Overall haze”, “bulk haze” and “contribution of the surface haze” to the overall haze for the blown-films of neat
LLDPE – 8, 1MI and its blends with DMDBS. In the bars, grid-line area represents “bulk haze” and the blank area
represents “surface haze” for the corresponding samples.
Blown-films of neat LLDPE – 8, 1MI and mixtures with N,N’-bis(cyclohexylmethyl)-1,4-phenylene
dicarboxamide (BCPCA) were prepared as follows. Respective amounts of the additive and the polymer
were inserted into a plastic bag and thoroughly mixed. The powder mixtures were compounded and
extruded with a twin screw extruder (Leistritz ZSE MAXX, screw diameter = 27 mm; screw l/d ratio =
121
48). Processing parameters: Cylinder temperature: 170 °C (all heating zones), screw speed: 300 – 400
U/min, output: 25-30 kg/h. The extruded strands were quenched in a water bath and cut with a pelletizer.
Monolayer polyethylene films were produced as above. Additional process parameters are provided in
Table 3, and haze results of the films are listed in Table 4. As a comparative study, blown films supplied
by the DOW Chemical Company was also shown in Table 4. The higher haze values for the films of the
same concentration can be possibly attributed to poor mixing and surface melt fracture.
Processing parameters
Screw speed (rpm) 60
Melt temperature (°C) 220
Take-up speed (m/min) 1.95
Average thickness (μm) 50
Table 3.
Processing parameters for the production of blown-films comprising LLDPE – 8, 1MI and its mixtures with
BCPCA.
Table 4.
“Overall haze”, “bulk haze” and “contribution of the surface haze” to the overall haze for blown-films of neat
“LLDPE – 8, 1MI” and its mixtures with BCPCA. In the bars, grid-line area represents “bulk haze” and the blank
area represents “surface haze” for the corresponding samples. ( * ) represents the results obtained by the blown-
films produced in the DOW Chemical Company.
122
Both additives reveal significant amount of “surface contribution to the overall haze” due to the surface
irregularities that come from the issues of free-surface flow processes as it was previously explained in
Chapter 1-Clarification Section. However, the decrease in “bulk haze” of these films, i.e. 1.3 % for
BCPCA and 1.9 % for DMDBS, points to the clarification efficiency of these additives for different
processing applications.
Young’s modulus of selected blown-film samples are listed in Table 5 according to the experimental
procedure to determine mechanical properties in Chapter 2 and Chapter 5. Films comprising additives
reveal improved mechanical performance relative to the neat polyethylene samples. Naturally,
orientation induced by the film-blowing process imparts improved stiffness in comparison to
compression-molded samples. The modulus in transverse direction (TD) is slightly higher than that
found in the machine direction (MD) for all samples, which is consistent with the previous records 1-3.
Krishnaswamy et al. explained this result by relative contributions of the crystalline and non-crystalline
parts to the deformation along the two directions; i.e., tensile deformation along TD involves a favorable
contribution from the polymer lamellae along their long axis, while the deformation in MD generally
involves the interlamellar non-crystalline phase.
Table 5.
Young’s modulus of selected blown-films in machine direction (MD), transverse direction (TD), and compression-
molded (CM) films of the same blends.
1. Krishnaswamy, R. K.; Lamborn, M. J. Polym. Eng. Sci. 2000, 40, (11), 2385-2396.
2. Simpson, D. M.; Harrison, I. R. J. Plast. Film. Sheet. 1994, 10, (4), 302-325.
3. Zhou, H.; Wilkes, G. L. J. Mater. Sci. 1998, 33, (2), 287-303.
123
2. Solid-state Transitions of N,N’-bis(cyclohexylmethyl)-1,4-phenylene
dicarboxamide (BCPCA)
Thermal transitions of neat N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA) were
briefly investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction
(WAXD). DSC analysis revealed three different endothermic peaks (i.e. ~132 °C, ~162 °C, ~273 °C) in
the heating experiments and four exothermic peaks (i.e. ~122 °C, ~152 °C, ~239 °C ~259 °C) in the
cooling cycle (cf. Figure 1).
Figure 1.
Differential scanning calorimetry (DSC) thermograms of neat BCPCA; cooling from the melt at 300 °C (blue
curve), subsequent heating (red curve) recorded at a cooling and heating rate of 10 °C/min.
In order to further investigate a possible temperature-dependent crystal structure of BCPCA, WAXD
measurements were performed at three different temperatures, which were specifically chosen to be
below the temperatures of DSC exothermic peaks (i.e. 25 °C, 150 °C, 190 °C). Wide-angle X-ray
diffraction (WAXD) was performed on an Oxford Instruments XCalibur PX diffractometer using MoKα
radiation (0.71 Å wavelength). BCPCA powder was sealed inside glass capillary tubes (Hilgenberg; 1.5
mm outer diameter). Sample temperature was controlled using the Cryojet accessory by streaming
temperature-stabilised (± 0.1 °C accuracy) nitrogen gas over the capillary tube. The samples were
equilibrated at each temperature for 15 min prior to each measurement, which were carried out with 10
min integration time. The two-dimensional diffraction patterns were radially integrated following
correction for background signal. Three different WAXD patterns were found at these corresponding
temperatures (cf. in Figure 2), indeed consistent with additive solid-state transitions observed in the
binary phase diagrams presented in Chapter 5-Figure 9 and in the above Figure 1. As these transitions
occurred above the crystallization temperature of the polymers employed in this work, this finding was
deemed to be of academic interest for the pure additive, and hence was not further explored.
124
Figure 2.
Wide-angle X-ray diffractograms (WAXDs) of neat BCPCA at 25 °C (black curve), 150 °C (blue curve), 190 °C
(red curve). a.u. = arbitrary unit.
3. Wide Angle X-Ray Diffraction (WAXD) of LLDPE – 8, 25MI and
Blends of LLDPE – 8, 25MI/additives
WAXD patterns of injection-molded plaques are obtained using a Panalytical Empyrean diffractometer with Cu
Kα source between 5-55° using 48 seconds per step acquisition time and a step size of 0.0066°.
Figure 3.
Wide-angle X-Ray diffractograms (WAXDs) of the blends of LLDPE – 8, 25 MI comprising 0.25 % w/w DMDBS
(red), 0.25 % w/w TBPMN (blue), 0.05 % w/w TDMPAB (green), 0.1 % w/w BCPCA (purple) showing a
similarity with the WAXD pattern of neat LLDPE – 8, 25 MI (black) .
125
Acknowledgements
During the time of my PhD, I realized that the answer is so simple to the question of “where is my
home?” which I was asking to myself in recent years. My working environment became my “home”
for the last three years. I spent much more time there than in my apartment. I saw the people, who
contributed to this work, more than my family and my friends. Huge “THANKS” to everyone, who
contributed to the mess on my desk, which is shown in Figure 1.a.
First and foremost, I would like to thank truly to Paul Smith for giving me an opportunity to breathe the
air of Polymer Technology and to “exploit” his broad scientific and life visions. When I first came to
this lab in 2011 as a master student, I was even not at the level of zero – I was standing in minus. At that
time, I never thought that this person will completely change my life and will become my “doctor father,
intellectual father, senior co-worker and, last but not least, second father”. Since I knew him, he never
let me to say “thank you” to him. He always said “keep your thanks to the end”. And now, it is time to
say “thank you Paul” for growing me up scientifically and personally to be ready for the real world. I
will remain forever grateful for his support, patience and trust in me even in times when I didn’t have it
myself. Furthermore, it was a great experience to participate in his research vision, and I feel immense
gratitude for teaching me the importance of self-criticism and intuition to succeed rather than practicing
techniques. Apart from all those, I would like to say special thanks for his creative presents from his
spectacular sense of humor, which we usually receive together with his “suggestions” for our scientific
drafts. For instance, as it can be seen in Figure 1.b, the cover page of Chapter 3 from this thesis was
redesigned by Paul Smith, after a glass of raki fell down on the papers of the chapter.
Furthermore, I’m especially indebted to “my second doctor father”, Hans-Werner Schmidt, for his
invaluable contributions to this work, teaching me the wonders of “trisamides & bisamides”, being a co-
examiner of this thesis, and, last but not least, always offering me a warm-welcome in his group in
Bayreuth.
I would like say special thanks to Jan Vermant for accepting to be a co-examiner of this thesis and his
interest in this work.
DOW Europe GmbH is gratefully acknowledged as an industrial partner of this work for their financial
and material support (cf. Figure 1.c). A special thanks goes to Martin Hill, Selim Bensason, Rudolf
Koopmans for their contributions to this work in our interesting meetings.
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I would like to say my sincere thanks to the “Bayreuth crowd” for their significant amount of
contributions to this work and their spectacular hospitality. Petra Weiss, for organizing my stay in
Bayreuth by thinking about and arranging every small details. Klaus Kreger, for organizing the synthesis
and characterization of the compounds in Chapter 4 (cf. Figure 1.d). Sandra Ganzleben, Jutta Failner,
Doris Hanft and Rika Schneider, for their experimental support for the synthesis of the compounds and
their invaluable experimental assistance by waiting in front of the DSM machine with me to process
injection-molded plaque samples (cf. Figure 1.e) and at the same time speaking basic German with me
to improve my language skills. A special thanks goes to my lovely friend Julia Singer, for her bicycle
supply and making my time enjoyable outside of the lab in Bayreuth.
Furthermore, I am deeply indebted to the “mothers” of this work, Karin Bernland and Eve Loiseau, for
sharing their previous experiences with me. Especially, their theses were among the fixtures at my desk
during my PhD (cf. Figure 1.f).
There are many people who contributed experimentally to make this thesis possible. First, Kirill Feldman
is thankfully acknowledged for his help and advice for many experiments that I made since my master
studies. I would like to express my sincere gratitude to Stephan Busato for sharing his expertise in
photography for this work, thinking out of the box and, introducing a new photography-based
measurement which is alternative to haze measurements in Chapter 3, and, last but not least, for his
invaluable contributions to the cover of this thesis (cf. Figure 1. g). I am deeply indebted to Derya Erdem
for her assistance to improve my practical skills in SEM; without her it wouldn’t be possible to have
those beautiful pictures in this thesis (cf. Figure 1. h). I would like to thank Werner Schmidheiny for his
equipment support, especially for melt-intrinsic haze measurements in Chapter 5. Martin Hill (from
DOW, Tarragona), Markus Blomenhofer (from Lifocolor Farben GmbH), Thomas Schweizer, Werner
Schmidheiny, Raphael Schaller (from ETH Zurich) are gratefully acknowledged for the film-blowing
samples (cf. Figure 1.i). I would like to thank to Julia Dshemuchadse, Aleksandr Perevedentsev and
Derya Erdem for the WAXD measurements.
I had an immense luck to be a part of the “Chuchihästli and Geordies’ crowd” at the time before and
during my PhD. Aleksandr Perevedentsev (Alexito), thanks for his tireless scientific curiosity, being a
truly intellectual friend, his proof reading of this thesis and his arrogant jokes that come from his warmth,
which colored of our days (cf. Figure 1.j). Raphael Schaller (Raphi), thanks for his endless help, kindness
and generosity both at the work and outside of the work (cf. Figure 1.k). Vappu Hämmerli, thanks for
her endless support for the administrative work, her invaluable friendship, nice accompany on ski slopes,
lunches in Kerala and so many other things… (cf. Figure 1.l). Sebastian Radermacher (Noodle boy),
thanks for his experimental support and dog familiarization therapy with Cody to overcome my phobia
to the dogs (cf. Figure 1.m). Furthermore, I would in particular to express my thanks to Irene Bräunlich,
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Andreas Brändle, Paola Orsolini, Gagik Ghazaryan, Felix Koch, Jan Giesbrecht, Harald Lehmann, Theo
Tervoort, Walter Remo Caseri, Ueli Suter, Han Meijer, Wolfgang Kaiser, Sylvie Smith, Tom Schenkels,
Coen Clarjis, Maike Quandt, Louis Schär, Beniamino Paú-Lessi, Alessandro Ofnär, Fabio Bargardi,
Victoria Blair, Ljiljana Palangetic, Martina Pepicelli and all other members of the “Soft Materials”.
And of course, a special thanks goes to my dear friends, “Altin Kizlar”: Huriye Erdogan, Pinar Senay
Özbay, Ece Öztürk, Gökce Yazgan. I feel blessed by having friends like you. Thanks for always being
ready to laugh with me and having a free shoulder for my tears. Güne baslamadan önce sizin
hediyelerinizi bilgisayar ekraninin önünde görmek, en büyük motivasyon kaynagimdi (cf. Figure 1.n).
Siz olmadan bir Zürih ve doktora nasil olurdu bilemiyorum…
This thesis is the culmination of my scholar and family education, which started 28 years ago on the
lands of Anatolia. I will always feel immense gratitude to my parents, Selma & Mehmet Aksel, and my
brother, Tansel Aksel, for doing their best to shape my personal growth since I was born. Canim ailem,
bugünlere gelmemde verdiginiz tüm maddi ve manevi destekleriniz icin cok tesekkürler…
Finally, I thank Sergio for the weekends he spent his time with me in my office and convincing me to
go climbing wall at the breaks (cf. Figure 1.o); reminding my capabilities when I forget them; calming
me down when I am stressed; stimulating me to think, question, criticize and discuss more about
everything and; last but not least, making my life meaningful over the last year.
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Figure 1.
Photo-collage of working-desk of Seda Aksel in ETH Hönggerberg, HCI H 506 (a) and zoom-in photos of selected
items which evoke memories of the people who made this thesis possible.
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Curriculum Vitae
Seda Aksel was born in Izmir, Turkey, on the 12th of April 1987. She attended high school in
Istanbul and graduated in 2005. Subsequently, she started her studies in Materials Science with a
minor degree in Chemistry at Sabanci University, Istanbul, Turkey. Following a three months
research stay at the University of Cambridge, in the Department of Materials in 2009, she
successfully completed her Bachelor degree. In 2010, she started Master of Science (M. Sc.) studies
in Materials Science at the Swiss Federal Institute of Technology (ETH) Zürich, which she
successfully completed with a master thesis in 2012 on the topic of “Decreasing the crystallinity of
polyethylene oxides for the applications of solid-state Li-battery electrolytes” in the Polymer
Technology Group, headed by Paul Smith. She then again joined the Polymer Technology group at
ETH Zürich where she conducted the doctoral studies on the topic of “Nucleation and Clarification
of Polyethylenes” under the supervision of Prof. Paul Smith in collaboration with the Dow Chemical
Company and Prof. Hans-Werner Schmidt, Bayreuth University.
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“Every clarification breeds new questions.”
Arthur Bloch