poss reinforced pet based composite fibers: “effect of poss type and loading level”

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Page 1: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

Composites: Part B 53 (2013) 395–403

Contents lists available at SciVerse ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

POSS reinforced PET based composite fibers: ‘‘Effect of POSS typeand loading level’’

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.05.033

⇑ Corresponding author. Tel.: +90 262 316 7258; fax: +90 262 316 7052.E-mail address: [email protected] (S. Gurdag).

Humeyra Sirin a, Dilek Turan a, Guralp Ozkoc a, Sezen Gurdag b,⇑a Kocaeli University, Department of Chemical Engineering, Izmit, Kocaeli 41380, Turkeyb Kordsa Global A.S., Research and Development Center, Izmit, Kocaeli 41310, Turkey

a r t i c l e i n f o

Article history:Received 26 June 2012Received in revised form 22 March 2013Accepted 26 May 2013Available online 11 June 2013

Keywords:A. Polymer (textile) fiberA. Particle-reinforcementB. Mechanical propertiesE. Melt-spinning

a b s t r a c t

In this study, the effects of poly(hedral oligomeric silsesquoxane) (POSS) type and loading level on themechanical, morphological and thermal properties of melt-spun PET-based composite fibers were inves-tigated. Three different epoxy, hydroxyl or amine functional POSS types were compared. The thermal,morphological and mechanical properties were investigated. It was found that all the three POSS typesgenerally demonstrated nucleating agent behavior, suggested by lower cold crystallization temperaturesalong with higher melt crystallization temperatures. Among the three types of POSS particles, addition of1 wt% hydroxyl functional POSS resulted in five times increased tensile modulus in comparison to theneat PET fiber. This can be explained by the combined effect of reinforcement with higher level of crys-talline content in addition to the presence of more oriented amorphous domains. On the other hand,epoxy terminated POSS/PET composite fibers exhibited doubled tensile modulus with respect to the neatPET fiber; however, the results obtained on amine terminated APOSS-PET fiber did not have any signifi-cant trend.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In the last couple of decades, a great deal of attention has beenpaid to the ways to improve the properties of polymers withoutincreasing the density. Nanoparticle inclusion into native polymerin this respect is one of the solutions that was brought about firstby Toyota researchers with their study on polyamide-clay systemand later on made to take on a different stage by the otherresearchers [1,2]. Due to the advantage emanating from the high-aspect ratio, clay particles are one of the widely used nanoparticlesworldwide [3–6]. On the other hand, polyhedral oligomeric sils-esquioxanes (POSS) with its nanometer size cage like structure isa relatively new material receiving considerable attention latelyas reinforcing agents in polymeric nanocomposites [7]. POSS arehybrid structures consisting of an inorganic cage represented as(SiO1.5)n to which ‘‘n’’ organic substituents are linked [8]. POSSchemistry is regarded extremely versatile since it is possible to at-tach side groups of different functionality to the corner Si atoms[9,10]. The functionality, solubility and reactivity of these struc-tures largely depend on the R group represented as hydrogen oran organic group such as alkyl, aryl or any of their derivatives[11,12]. Due to a great flexibility that the POSS structures are offer-ing to the researchers, they have been focused by many research

groups handling a wide array of thermoplastics such as PP, HDPE,PMMA, PET, PA6, and PC through copolymerization, grafting, andconventional compounding processes and in some thermosetssuch as PU and polysiloxane [10,13–18].

It is known that by incorporating foreign entities into the poly-mer such as nanoparticles, one can manipulate the inherent prop-erties of the native polymer. Upon the incorporation of POSSmolecules into the host polymer, in general, improvements in var-ious aspects such as service temperature, moduli, and flammabilitycan be achieved [19,20]. The incorporation of POSS cages into poly-meric materials can be carried out by melt mixing with the hostmatrix as an easier alternative to grafting or copolymerization [21].

One of the daily challenges in polymer researchers is to improvethe properties of fiber making polymers such as PET by the inclu-sion of nanoparticles such as POSS. One of the limited number ofthe studies indicated an increase of 5–10 �C in thermal decomposi-tion temperature of their PET based composites by using two dif-ferent kinds of amine functional POSS, aminoisobutyl POSS andaminoisooctyl POSS. Furthermore, retention in storage modulusat 120 �C was found to be 45% for the aminopropyl isooctyl POSSwhen added to the matrix polymer at a loading of 1 wt% [22].

It is also essential to evaluate the effect of side functionality ofPOSS structure on the final composite.

To this end, the effect of POSS functionality on the fiber proper-ties has been investigated by some researchers [14,13]. Gonzalezet al. focused on the POSS particles possessing various side groups

Page 2: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

396 H. Sirin et al. / Composites: Part B 53 (2013) 395–403

in order to evaluate their contribution to the fiber mechanical anddynamic mechanical properties. It was demonstrated that non-reactive isooctyl-POSS compounded PET had the highest tensilemodulus and strength; on the other hand, trisilanol isooctyl POSScompounded PET fibers had slightly lower mechanical propertiesin comparison to the former one, but still possessing bettermechanical properties than the neat PET fiber with no obviousphase separation in the compound [14]. In another study, the ef-fects of organically modified mica and trisilanol isobutyl POSS onthe properties of PET fibers obtained by high-speed spinning werecompared. It was found that even though mica particles enhancedthe crystallization of PET more than POSS in the static state duringDSC, its composite fiber resulted in lower modulus in comparisonto both neat PET and POSS-PET nanocomposite. On the other hand,POSS incorporation into the matrix yielded fibers with almostequal mechanical properties to the neat PET fiber [13].

In this study, three types of POSS having different reactive sidegroups, such as epoxy, amine or hydroxyl were used to examinethe type of POSS and effect of loading level on the properties ofPET based composite fibers. The POSS types were suggested to ex-hibit various levels of reactivity with the end groups of PET. To thebest our knowledge, there have not been any systematical spun-fiber work reported in the literature dealt with the selected threedifferent reactive POSS types that aims to improve the mechanicalproperties of the PET fiber. In the current study, composite fibers ofPET/POSS have been prepared via melt-mixing and subsequentfiber spinning method. The resultant fibers were evaluated inrelation to the type of functionality in terms of mechanical, mor-phological and thermal behavior.

2. Materials and methods

2.1. Materials

Three types of POSS derivatives; trisilanol isobutyl POSS(TPOSS), aminopropyl isobutyl POSS (APOSS) and glycidyl isobutylPOSS (GPOSS) were purchased from Hybrid Plastic-USA and usedas received without any further treatment. Fig. 1 shows the chem-ical structures of the POSS particles used. Poly(ethylene tere-phthalate) (PET) was technical commercial grade with anintrinsic viscosity (IV) of 0.9 and obtained from Invista.

2.2. Composite preparation and monofilament spinning

Melt-mixing of each type of POSS particles at concentrations of0.1, 0.5 and 1 wt% was carried out in a 15 ml DSM Xplore Micro-compounder with co-rotating vertical screws. The residence timeof melt mixing was kept constant at 3 min at 280 �C with a screwspeed of 50 rpm. All the compounds were prepared under inert Aratmosphere to prevent the thermo-oxidative degradation. The PETpellets were dried prior to processing. Monofilament spinning unit

Fig. 1. Representation of the chemical

(DSM Xplore Fiber Spinning Unit) was directly coupled with thecompounder. Following the extrusion of the compounds from the1 mm circular die, it was transferred onto the bobbins on thewinder section of the spinning unit operated at 175 m/min. Afterspinning the monofilaments, the bobbin was transferred onto thelet-off roll in the drawing unit (DSM Xplore Fiber Drawing Unit),which was followed by drawing the fiber to 10 times of its initiallength at a speed of 100 m/min at 83 �C by passing the fiberthrough a heated tunnel.

In order to observe the properties of the compounds, just aftercompounding, they were directly injection molded into standardshapes using a DSM Xplore Micro-injection molding machine.

2.3. Characterization

Differential scanning calorimetry (DSC) analyses were carriedout on a Mettler Toledo DSC1 StarSystem instrument under inertN2 atmosphere with a flow rate of 80 ml/min. Approximately10 mg sample was heated from 25 �C to 300 �C with a rate of10 �C/min.%-crystallinity values, xc%, were calculated from the firstheating cycles as follows:

xc% ¼ðDHm � DHcÞ

uDH0f

� 100 ð1Þ

where DH0f is the value of the heat of fusion for the totally crystal-

line PET (specific heat of fusion, which was taken as 140 J/g [23])and u is the weight fraction of the polymer in the composite,DH0 and DH0

f were the heat of crystallization and heat of fusion ob-tained from the DSC analysis, respectively.

Thermogravimetric analyses (TGA) were performed on a TAQ5000 under inert N2 atmosphere on ca. 10 mg samples in plati-num pans. The samples were heated from 25 �C up to 800 �C at aheating rate of 20 �C/min.

For the estimation of crystalline orientation in the fibers, polar-ized FT-IR was employed. Infrared spectra of the fibers in midIR re-gion (4000–400 cm�1) were collected on a Thermo Nicolet 6700FT-IR spectrometer with a resolution of 2 cm�1. Each spectrumwas given as the average of 32 scans collected with an ATR attach-ment under reflectance mode using a diamond crystal. In order toensure the linearity of the filaments with respect to each other, thefilaments were wrapped around a custom-made metal apparatus.Differences in the spectra obtained at 0� and 90� were consideredfor the Dichroic ratio calculations. Crystalline orientation in thespun fibers is given as:

P200i ¼ hP2ðcos hÞi ¼ D� 1Dþ 2

2ð3 cos2 h� 1Þ ð2Þ

where h is the angle between local chain axis and the fiber axiswhereas a is the transition moment angle between the associatedvibrational mode and the chain axis. Aıı and A\ are the measuredabsorbance values for radiation polarized parallel and perpendicu-

structures of POSS particles used.

Page 3: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

H. Sirin et al. / Composites: Part B 53 (2013) 395–403 397

lar to the fiber axis, respectively, while D is the Dichroic ratio offiber and can be calculated from Eq. (3) [24,25].

D ¼ Aıı

A?ð3Þ

Birefringence (Dn) was measured in a polarized light micro-scope, OLYMPUS BX 60 equipped with a U-CTB 7M17865 Bereckcompensator and an OLYMPUS UPOC-condenser filter used to pro-duce monochromatic light. The birefringences of the samples wereobtained by measuring parallel and perpendicular refractive indi-ces. It is known that birefringence is a measure of the global orien-tation of the samples; the orientation of the crystalline and theamorphous regions factoring into the equation proportional totheir fractions. As a result, amorphous orientation can be estimatedfrom the total orientation with the help of% crystallinity andcrystalline orientation function obtained from IR measurement.The total birefringence can be calculated as follows:

Dn ¼ xcfcDn0c þ ð1� xcÞfaDn0

a ð4Þ

where fc and fa are the crystalline and amorphous orientation func-tions, respectively. Dn is total birefringence of the fiber. Dn0

c and Dn0a

are the intrinsic birefringence values for 100% crystalline and 100%amorphous PET, which were taken as 0.22 and 0.275 for PET,respectively, proposed by Dumbleton [26].

Mechanical properties of the compounds were determined on aSchimadzu EZ Servey equipment according to ISO 572-5 standardprocedure. Dog bones were tested using a 10 kN load cell with a1 mm/min strain rate. For each composition, five samples weretested and the results were reported as the average.

Fibers spun and then drawn from these compounds were sub-jected to an Instron tensile tester equipped with 1 kg load cell ata 300 mm/min strain rate with a gauge length of 2.54 cm according

(a) (b)

(d)

(f)

Fig. 2. Fracture surface morphologies of composites containing 1 wt% POSS particles: (a) A(100�), (e) GPOSS-PET (4000�), (f) TPOSS-PET (100�), and (g) TPOSS-PET (4000�).

to ASTM D5591 standard. Ten samples were tested for each fiberand the average was reported. For each fiber, breaking strength(BS) and elongation at break (EB) were determined directly fromthe software whereas the tensile moduli of the fibers were ob-tained from the stress–strain curve below 1% strain.

The extent of thermally induced shrinkage of the drawn fiberswas investigated by Testrite MKV Shrinkage Force tester accordingto ASTM D885.

Morphology of the cryo-fractured surfaces of the compoundswas investigated on a JEOL JSM 6335F scanning electron micro-scope (SEM). Prior to imaging, surfaces were coated with gold toa thickness of about 15 nm to ensure conductivity.

Transmission electron microscopy (TEM) was conveyed on aJEOL JEM-2100 high-resolution electron microscope. Ultrathin sec-tions of about 100 nm thickness was prepared using Leica UC6ultramicrotome equipped with a 35� diamond knife operatedunder cryogenic conditions.

3. Results and discussion

3.1. Results

3.1.1. MorphologyIn order to evaluate the dispersion quality of the POSS in the

composites, cryogenically fractured samples were subjected toSEM analysis. Fig. 2a–f shows the morphologies of PET/POSS com-posites containing POSSs at 1 wt% loading. The SEM pictures indi-cated that POSS nanoparticles dispersed mostly at submicronscale (i.e. app 100 nm level) throughout the PET matrix; howeverthere still existed some bigger particles rarely visible (see forexample, SEM of APOSS/PET composites). Unlike the conventionalnanoparticles used in composite technologies, POSS particles used

(c)

(e)

(g)

POSS-PET (100�), (b) APOSS-PET (4000�), (c) APOSS-PET (15,000�), (d) GPOSS-PET

Page 4: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

Fig. 3. TEM images of composites: (a) APOSS-PET (0.2 lm), (b) APOSS-PET (100 nm), (c) GPOSS-PET (0.2 lm), (d) GPOSS-PET (100 nm), (e) TPOSS-PET (0.2 lm), and (f) TPOSS-PET (100 nm).

398 H. Sirin et al. / Composites: Part B 53 (2013) 395–403

here were in the liquid phase during compounding with PET at280 �C as suggested by DSC analysis. According to DSC analysis,the melting points of POSSs are 265 �C, 205 �C as APOSS and TPOSS,respectively. GPOSS has three melting peaks appearing at 52 �C,120 �C and 140 �C. This means that PET was mixed with POSSs inviscous melt phase. It is thought that this liquid–liquid mixing ofPET with POSSs might have helped to better dispersion at very finescales. Therefore, it was concluded that POSS particles are well dis-persed in the matrix polymer.

Empty holes observed on fracture surfaces shown in Fig. 2eand g can be assigned to debonding during cooling due to the dif-ference between the thermal expansion coefficient of the PET andPOSS. Indeed, this debonding can be attributed to the inadequatewetting of the PET due to the possible high surface tension result-ing from the difference in polarity. As a result, the inorganic parti-cle expands more than the organic matrix, so when they are cooleddown, the inorganic particle contracts more than the organic ma-trix leaving a hole behind. In the exit of the compounder, the POSSdroplets crystallize and solidify as the fine particles as observed inSEM, which is in line with the earlier observations [18,27].

Furthermore, ultramicrotomed slices were examined underTEM (Fig. 3a–f). Due to the high electron density of the POSS par-ticles, they can be seen as dark round images [28]. Even thoughsubmicron sized particles were present similar to the observationsin SEM micrographs, most of them were dispersed at nanometerscale (about 100 nm) in PET independent of POSS type. Due tothe chemical affinity between reactive side groups POSSs andend groups of PET, it is possible to have a covalent bonding witheither the hydroxyl or carboxylic acid end groups of PET dependingon the POSS functionality [12,18]. On the other hand, nature of theside groups of POSS did not impart any significant effect on the dis-persion quality, in contrary to the earlier observations in POSS-polycarbonate systems [29].

3.1.2. Thermal properties of PET/POSS compositesAll the three POSS particles are white crystalline powders at

room temperature. Thermal properties of the nanoparticles wereinvestigated with calorimetric technique. Melting, glass transition,and crystallization temperatures were determined from the heat-ing and cooling scans of respective materials.

Page 5: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

Fig. 4. DCS thermograms of POSS particles.

Table 1Thermal transitions and % crystallinitiy of the composites and the matrix.

Tm (�C) Tcc (�C) Xc (%)

PET 254.6 209.5 27.11% APOSS-PET 255.4 204.8 27.51% GPOSS-PET 256.3 198.2 24.01% TPOSS-PET 258.7 197.3 25.2

Table 2Thermal decomposition of the composites and the matrix at process temperature.

Nanocomposite composition Weight loss at 280 �C (%)

PET <0.51% APOSS-PET 0.51% GPOSS-PET <0.51% TPOSS-PET 0.5

Table 3Thermal properties of PET and PET/POSS composite fibers.

Material As spun As drawn

First heating Cooling First heating Cooling

Tcc Tm Xc Tmc Tm Xc Tmc

PET 136.5 255.9 30.5 201.8 256 37.6 201.50.1% APOSS-PET 134.0 260.5 26.6 199.5 260.9 33.3 205.20.5% APOSS-PET 131.5 258.0 31.5 202.7 257.8 33.7 202.11% APOSS-PET 134.0 258.7 32.0 201.1 258.2 37.9 207.60.1% GPOSS-PET 135.3 256.7 30.9 199.8 254.4 31.1 211.70.5% GPOSS-PET 130.2 257.0 29.5 199.7 252.4 27.8 199.91% GPOSS-PET 129.2 255.7 27.4 202.5 257.2 40.5 206.20.1% TPOSS-PET 125.2 258.4 27.4 204.7 257.6 34.8 201.40.5% TPOSS-PET 112.0 257.8 33.7 213.6 251.5 34.5 201.71% TPOSS-PET 111.5 258.0 43.3 210.3 256.6 50.3 209.7

H. Sirin et al. / Composites: Part B 53 (2013) 395–403 399

Fig. 4 shows that all the POSS particles present different meltingcharacteristics with increasing temperature. APOSS demonstratedthe highest melting peak, which starts to appear at about 235 �Cwith a peak temperature of 265 �C. TPOSS demonstrated a meltingpeak lower than that of APOSS appearing at 205 �C. On the otherhand, unlike the other POSS nanoparticles, GPOSS representedmultiple-melting behavior as depicted in Fig. 4. It is seen thatGPOSS has three distinct melting peaks appearing at 52 �C,120 �C and 140 �C. This can be attributed to the different crystallineorganization in crystal core or boundaries [30,31]. By consideringthe relatively high processing temperature of 280 �C used for thecompounding and fiber spinning, it can be said that all three POSSparticles are in a complete molten state as being mixed with thepolymer matrix.

%-Crystallinity values of the composites were calculated fromEq. (1) and given in Table 1. It was seen that, generally the levelof%-crystallinity attained in the composites was slightly lower thanthe reference PET. It was established earlier that even though crys-tallization will start from a higher density of crystallization site,the outcome is not always improved% crystallinity with higherdensity of nucleation sites, but also due to this increased densityleaving less volume for the crystallites to grow. Therefore, theyare smaller in size. As a result, the level of final crystallinity inthe composite may not be affected. This can be ascribed to the hin-dered formation of ordered domains of the chains in the presenceof POSS molecules [10,14,32]. Zeng et al., in contrary, suggestedthat in the dynamic state during monofilament spinning, the POSSspherical particles attached to the polymeric backbone as pendantgroups may have a deteriorating effect on the growth of crystallinelamellae just like clay-like plate shaped particles during the crystalgrowth where the polymer chains try to fold around themselves[14].

The highest melting peak was obtained on in TPOSS-PET com-posites. The difference between the melting peaks of neat PETand that of composite was found to be about 4 �C. Since TPOSS pos-sess three functional groups it can form a networked structurewhere three individual PET chains are attached to the same mole-cules. With the hypothesis that a networked structure is valid forthe composite, melting point in the composite is expected to dem-onstrate a slight increase in comparison to the neat PET matrix[18]. Another argument on such a phenomena brought by variousresearchers suggests that in case of TPOSS, the lamellar thicknesswas increased leading to the increase in the melting peak

[6,12,29,33]. Crystallization temperature of the composites dem-onstrated a lower Tcc value in comparison to the neat polymerproving the nucleating activity of the POSS particles [6,23]. Dueto lower Tcc value obtained especially for GPOSS and TPOSS parti-cles, polymer chains have longer time to crystallize, which willfacilitate the crystal growth. On the other hand, crystalline ratiowas not affected by the altered kinetics suggested by the compos-ites and the neat polymer almost having the same level of crystal-line content.

Page 6: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

Fig. 5. The variation of elastic modulus values with respect to POSS content inPOSS-PET nanocomposite compounds.

Fig. 6. The variation of tensile strength with respect to POSS content in POSS-PETnanocomposite compounds.

Fig. 7. The variation of tensile strength values with respect to POSS content indrawn PET fibers.

400 H. Sirin et al. / Composites: Part B 53 (2013) 395–403

Thermal stability of the composites was tested under both inertN2 and oxidative (air) conditions utilizing thermogravimetric anal-ysis technique. The data shown in Table 2 indicate that there is noobvious change in the thermogravimetric behavior for compositesin comparison to neat PET. It was seen that the thermal degrada-tion at the processing temperature of 280 �C, all the compositionsdemonstrated highly stable features with equal to or less than0.5 wt% reduction in mass, which is in line with the literature sug-gesting that POSS particles have the potential to shift the thermaldegradation temperature of the matrix polymer to higher temper-atures [14,18,34,35]. Furthermore, similar to earlier report byZheng et al., it was seen especially for trisilanol isooctyl POSS thatthe nanoparticle is very stable when processed at the melting tem-perature of PET for short amount of time such as 3 min [34].

3.1.3. Thermal properties of PET/POSS composite fibersThermal analysis of both spun and drawn fibers was performed

using DSC technique. The results of both spun and drawn fibers aregiven in Table 3. Similar to the earlier studies, it was found thatPOSS containing spun fibers demonstrated lower Tcc values, whichwas significant especially for TPOSS-PET composite fiber. Tcc de-crease in this fiber reached up to 22 �C, whereas that for the othertwo POSS types was limited to 6 �C. The decrease in Tcc in spun fi-bers correlated satisfactorily with the concentration of the nano-particle such that with the increase from 0.1 to 1 wt%, thereduction occurred at a greater extent. This phenomenon is attrib-uted to the nucleating activity of the particles [6,23]. On the otherhand, except for TPOSS particle, neither APOSS nor GPOSS particledid have any obvious effect on the level of crystallinity, whereasthat obtained on TPOSS-PET system differed appreciably from theneat polymer up to 13%. From the cooling thermogram of the com-posite fibers, it was seen that both Tc,onset and Tc,peak values forAPOSS and GPOSS-PET composite fibers were found to be compara-ble with those of neat PET. However, those for TPOSS compositewere relatively higher in comparison to the neat matrix, whichpoints out to the fact that particularly TPOSS particles behave asnucleating sites giving rise to the crystalline content. Furthermore,earlier crystallization in cooling cycle observed in TPOSS compositestrengthens the argument on the nucleating activity of theparticles.

With respect to cold crystallization exotherm in the first heat-ing cycle on the drawn fibers, as established earlier, crystallizationpeak is missing due to the stabilization of the molecular chainsupon drawing [36]. As expected, increased stress on the fibers dur-ing stretching led to increased crystallization ratio independent ofthe POSS type [37]. Similar to what was observed with the spun fi-bers, level of% crystallinity is the highest in 1 wt% TPOSS containingdrawn fiber, which was approximately 13% more than neat PETrecording a noteworthy difference with respect to the referencePET fiber.

3.1.4. Tensile properties of compounds, spun and drawn fibersMechanical properties of the compounds are determined by the

orientation levels in addition to the crystallization behavior alongwith the presence of defects caused mainly by the foreign entitiespresent in the polymer matrix in this case the POSS particles [23].This particularly prevails for the fibers where the material possessa highly oriented structure thanks to the drawing process. On theother hand, with the injection molded compounds where the ori-entation is relatively low compared to the fiber spinning, morethan the orientation, the reinforcing effect of the nanoparticle inthe bulk plays a significant role.

Elastic moduli values along with the tensile strength of thecompounds obtained from tensile testing of a dog-bone shapedtensile bar geometry are shown in Figs. 5 and 6. From the stress–strain behavior of the samples, it can be stated that no obvious

effect of either the POSS type or the POSS content has been ob-served in the tensile strength profiles. Such changes in the propertytowards both the increasing and decreasing directions were con-sidered within the experimental errors. Therefore, it was con-cluded that POSS particles did not contribute to mechanicalproperties of the material to any appreciable extent due to rela-tively low loading level. Similarly, tensile modulus calculated with-in the elastic region of the stress–strain profile showed that, it didnot have any significant trend with respect to particle type or con-centration [15].

Page 7: POSS reinforced PET based composite fibers: “Effect of POSS type and loading level”

Table 4Crystalline and amorphous orientation of POSS-PET composite fibers.

Material wt% POSS fa (%) AOFb

PET 0 33.9 0.6APOSS-PET 0.1 29.4 0.5

0.5 38.1 1.11 43.8 1.3

GPOSS-PET 0.1 45.3 0.60.5 32.6 0.51 39.3 0.6

TPOSS-PET 0.1 41.3 0.40.5 38.4 0.61 40.5 0.7

a f: Orientation, %.b AOF: Amorphous Orientation Factor.

Fig. 10. The variation of thermal shrinkage values with respect to POSS content indrawn PET fibers.

Fig. 8. The variation of elastic modulus values with respect to POSS content indrawn PET fibers.

Fig. 9. The variation of elongation at break values with respect to POSS content indrawn PET fibers.

H. Sirin et al. / Composites: Part B 53 (2013) 395–403 401

Since the crystallization kinetics and behavior is strongly gov-erned by the nanoparticle inclusion, it gains much emphasis in caseof nanocomposite fibers as stated earlier. Even though compoundsin the bulk state did not demonstrate any particular preferencewith respect to the particle type, both spun and drawn fibers weresubjected to the mechanical test. Figs. 7–9 show tensile strength,elastic modulus and elongation at break profiles of the respectivefibers. It was seen that elastic moduli of the drawn fibers indepen-dent of the POSS type were significantly higher than those of thespun fibers. This is due to the combined effect of both increasedcrystalline level and orientation of not only the crystals but alsothe amorphous parts as a whole. Another noteworthy point wasthat each POSS type made the improvement in composite fiberproperties at different loading levels. Yet, the most remarkablechange with respect to elastic modulus was observed with TPOSSat 1 wt% concentration. At this loading, the respective drawn fiberhad modulus about five times higher than the neat fiber, whichwas supported by the high crystalline ratio obtained with thiscomposition. Such an observation was also made by otherresearchers, who explain this reinforcing effect by the formationof phase-separated crystals with very small diameter in compari-son to the fiber’s at a particular particle concentration [12].

Apart from elastic modulus, breaking strength (BS) and elonga-tion at break (EB) were determined from the stress–strain profiles.In general, both in the spun and drawn fibers, the maximum loadcarrying capacity increased with increasing particle concentration[12,14]. At the same time, as expected, drawn fibers had a higherthreshold to break than those of the spun fibers emanating from

both the increased level of% crystallinity and highly ordered do-mains. Similar to what was observed with elastic modulus, TPOSShad the highest breaking strength among all with a relatively smallstandard deviation. Elongation behavior of the composite fibersprovided further insights into the orientation of the fiber. Withthe least EB at 1 wt% loading, TPOSS-PET fiber suggested that ithad more oriented domains particularly in the amorphous regions.On the other hand, elongation at break values of the compositedrawn fibers did not significantly vary from the neat fiber. Thiscould be speculated to be a result of a higher level of interactionbetween the end groups of POSS and PET provided by the threefunctional groups of TPOSS hindering the mobility of the amor-phous chains more than the other two POSS types. Since EB valuesrecorded on APOSS and GPOSS composite fibers are either higherthan or equal to the that of neat PET fiber, it can be said that thestiffness of the compound was less in comparison to the neatpolymer as a result of relatively less crystallinity levels summa-rized in Table 4 in line with the observation by Baldi et al. [32].Such behavior correlated well with the thermal shrinkage of thedrawn fibers that was discussed as the following.

As observed from Fig. 10, fibers exposed to 177 �C for 2 minunder constant load demonstrated thermal shrinkage profilesdifferent from one another such that GPOSS did not show anydependency on particle concentration whereas APOSS demon-strated a rather negative effect such that at high concentrationsof the particle high shrinkage values were recorded [12]. On theother hand, TPOSS-PET fiber showed a certain dependency onthe particle concentration with the least shrinkage obtained atthe highest concentration as expected. This was due to the factthat, this fiber did not only have the highest% crystallinity at thisconcentration but also the highest total orientation suggested bythe birefringence measurements.

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402 H. Sirin et al. / Composites: Part B 53 (2013) 395–403

3.2. Discussion

POSS incorporation into PET matrix through melt blending wasstudied in this work. As expected, it was seen that all the POSStypes demonstrated nucleating activity enabling the polymericchains to crystallize at lower temperatures seen in the first heatingcycle of DSC test. Additionally, both higher Tonset and Tpeak valuesfor the crystallization temperatures (not given) from the meltstrengthen the validity of such behavior demonstrated by POSSparticles on the crystallization of neat polymer. The most notableeffect of the nanoparticle was obtained with TPOSS both in the bulkproperties and fibers. The superiority of TPOSS-PET nanocompositefiber relative to the others was also supported by the birefringencemeasurements, which is a measure of the total orientation of thecrystalline and amorphous domains together.

Birefringence values of the drawn fibers have been obtained ona light microscope equipped with a Bereck compensator in order tohave an idea on the relative orientation of the molecular chainsafter stretching [38]. The fibers reinforced with TPOSS particlesdemonstrated a birefringence profile dependent strongly on theparticle concentration such that even though at the lowest concen-tration, it displayed a lower value with very low standard error, i.e.0.11%, than that of neat PET fiber, i.e. 0.14%, increased amount ofTPOSS led to more orientation in the fiber accompanied with high-er birefringence. This was also evident in the total elongation ofthese fibers such that TPOSS-PET composite fiber demonstratedca. 4% lower EB than that of neat PET. On the other hand, the othertwo types of POSS particles produced fibers with similar or higherEB to that of reference PET fiber. As suggested earlier, higher elon-gation in the composite fiber led to lower stiffness in the com-pound in comparison to the neat fiber due to relatively lowercrystallinity levels as discussed before [32]. Therefore, it has beenconcluded that the interaction at the interphase between the poly-mer matrix and the particle is more functional in TPOSS-PET com-posite fiber during elongational deformation. Addition of GPOSSinto PET matrix resulted in fibers with birefringence values compa-rable to the neat PET fiber. On the other hand, APOSS-incorporatedPET fibers did not present a meaningful trend with respect to theparticle concentration.

In order to strengthen the hypothesis that overall orientation inthe fiber containing TPOSS particles reached up to higher levelsboth in the amorphous and the crystalline domains, dichroic ratioswere calculated [38]. With birefringence as a measure of total ori-entation in the fiber and crystalline orientation calculated usingthe FTIR data, amorphous orientation values were calculated andgiven below in Table 4.

In general, all the particles helped to increase the crystalline ori-entation in the fiber as a result of decreasing the Tcc value therebygiving more time to the fibers for them to orient of under stress.Notably, TPOSS particles with the least Tcc value demonstrated adefinite concentration dependent amorphous orientation factor(AOF) increase with the higher orientation levels as concentrationincreases accompanied with an enhancement in% crystallinity incomparison to neat PET fiber. Even though this composite compo-sition had a comparable AOF and crystalline orientation factor(COF) values to those of the other compositions, the high crystal-line content helped it to demonstrate the best mechanical proper-ties among all.

The nanoparticles used in this study were selected so as to en-sure a chemical interaction between the polymer chain and theparticle through the functional groups that both present. Carboxylend group analysis on the compounds prepared with three differ-ent POSS particles have revealed that epoxy and amine terminated,GPOSS and APOSS, respectively, POSS containing compounds, hadcomparable carboxyl end groups with that of unprocessed PET[38]. On the other hand, the compound with TPOSS demonstrated

higher carboxyl end groups. This can be explained by the differ-ences in the reactivity of POSS types; such that with the help of along alkyl chain connecting the functional end group to the cagedcore in epoxy and amine terminated POSS, they can get in contactand result in a bond between the polymer, easily. Furthermore, theflexibility of the alkyl tails of these two POSS types allows the poly-mer chain to conform and relax under stress much more easily.Therefore, Young’s moduli of GPOSS and APOSS composite fiberswere lower in comparison to TPOSS composite fiber. However, incase of TPOSS, with three hydroxyl groups in close proximity tothe core, it will present a steric hindrance against a reaction [27].As a result, while the other two particles had comparable carboxylend groups to that of unprocessed PET matrix, TPOSS addition inmelt processing produced compounds with similar carboxyl endgroups to processed PET, which had these groups at slightly higherdensity. Additionally, since epoxy and amine terminated POSS par-ticles had higher chance to get into any interaction with the endgroups of PET, they possibly prevent the trans-esterifications dur-ing extrusion. Hence, both compound and fiber had lower molecu-lar weight than that of TPOSS containing compound. As a result,these fibers had lower mechanical properties than TPOSS-PET fiber.On the other hand, since the chance of reaction of TPOSS with PETis less, it will be mostly present in the matrix by itself still givingchance to PET chains come together to form longer chains evenduring extrusion. Therefore, this particle produced fibers with bet-ter mechanical properties even in comparison to neat PET fiber.

4. Conclusions

The effect of functionality of POSS particle on the mechanicalperformance of PET-based composite fibers were investigated. Itwas seen that hydroxyl terminated POSS TPOSS demonstrated bet-ter mechanical performance especially seen in the tensile modulus,which was found to be fivefold of that of neat PET fiber. It wassuperior to the other two POSS types as well in this regard. Withrespect to the dispersion quality of the particles, there is no supe-riority between the three POSS types where all the particles werepresent at submicron level as seen in both SEM and TEM micro-graphs. The highest% crystallinity was obtained on the TPOSS-PETcomposite fiber with ca. 10% increase with respect to neat PET fi-ber. Furthermore, this fiber also demonstrated higher birefringenceas a representation of the total orientation in the fiber, which led tolower thermal shrinkage.

Acknowledgement

This study was granted by Republic of Turkey, Ministry of Sci-ence, Industry and Technology (Grant number: SANTEZ 00420.STZ.2009-2).

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