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Fluid Phase Equilibria 241 (2006) 155174

Review

Hyperbranched polymers: Phase behavior and new applicationsin the eld of chemical engineering

Matthias Seiler

Degussa AG, Process Technology & Engineering, Fluid Processing VT-F, Rodenbacher Chaussee 4, D-63457 Hanau, Germany

Received 14 October 2005; accepted 6 December 2005Available online 9 February 2006

Abstract

During the lastdecade, hyperbranched polymershave become the focusof intense interdisciplinary research. The endeavor to demonstrate the fullpotential of hyperbranched polymers continuously results in more and more complex hyperbranched structures with new synthetic methodologies.A remarkable variety of applications for hyperbranched polymers has been investigated. Some of them have been already commercially realized. Anew promising area of potential applications for hyperbranched polymers is the eld of chemical engineering. The use of hyperbranched polymersin separation processes involving extractive distillation, solvent extraction, absorption, membranes or preparative chromatography might offerconsiderable potentials for cost savings. Therefore, this review intends to introduce these new potential applications of hyperbranched polymers.Based on a brief description of the synthetic methodologies and properties of hyperbranched polymers, a detailed overview of the investigatedphase behavior of branched polymer systems is given, followed by a discussion of new potential applications of hyperbranched polymers in theeld of chemical engineering. 2006 Elsevier B.V. All rights reserved.

Keywords: Hyperbranched; Dendritic; Polymer; Dendrimer; Phase; Solution; Solubility; Equilibrium; Chemical engineering; Application; Surface; Smart materials;Separation; Extraction; Distillation; Absorption; Adsorption; Membrane; Coating; Modelling; Process

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562. Denitions, synthesis and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

2.1. Denitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562.2. Synthesis and structural characterization of hyperbranched polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1582.3. Properties of hyperbranched polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

2.3.1. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1602.3.2. Mechanical and rheological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1602.3.3. Solution properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

3. Phase behavior of hyperbranched polymer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1613.1. Experimental investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

3.2. Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1644. Applications of hyperbranched polymers in the eld of chemical engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

4.1. Membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1674.2. Extractive distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Abbreviations: VLE, vaporliquid equilibrium; LLE, liquidliquid equilibrium; DB, degree of branching; UCST, upper critical solution temperature; LCST,lower critical solution temperature; UST, upper solution temperature; LST, lower solution temperature; NMR, nuclear magnetic resonance; VPO, vapor pressureosmometry; EOS, equation of state; HX, heat exchanger; THF, tetrahydrofuran; PG, hyperbranched polyglycerol; ED, 1,2-ethanediol; wt.%, weight percent, masspercent; D, dendritic unit; L, linear unit; T, terminal unit; a.m.u., atomic mass unit Tel.: +49 6181 59 3049; fax: +49 6181 59 4697.

E-mail address: [email protected].

0378-3812/$ see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.uid.2005.12.042


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156 M. Seiler / Fluid Phase Equilibria 241 (2006) 155174

4.3. Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1694.4. Absorption, adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

5. Conclusions and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1716. Inuence of John Prausnitz on my work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

1. Introduction

About 50 years after the introduction of the macromolecularhypothesis by Staudinger, the entire eld of polymer sciencecould be described as consisting of only two major architecturalclasses: (I) linear topologies as found in thermoplastics and (II)crosslinked architectures as found in thermosets [1]. Now, atthe beginning of the 21st century, four major domains can bedened and distinguished in accordance with their propertiesand architecture (see Fig. 1):

(I) linear, random coil thermoplastics such as plexiglass ornylon;

(II) crosslinked thermosets such as rubbers or epoxies;(III) branched systems based on long chain branching in poly-

olenssuchas low density poly(ethylene)andotherrelatedbranched topologies;

(IV) dendritic polymers consisting of three subsets that arebased on the degree of structural control, namely (a) ran-dom hyperbranched polymers; (b) dendrigraft polymers;and (c) dendrimers.

Dueto theunique repertoire ofnew properties, dendriticpoly-mers are recognized as a fourth major new architectural class[1], a class with a young but well established body of interdis-ciplinary research exploring a remarkable variety of potentialapplications.

Apart from the dendritic polymers, other important branchedtopologies in polymer science such as comb and star polymers,networks, and microgels are shown in Fig. 2.

The tedious and complex multistep-synthesis of dendrimersresults in expensive products with limited use for large-scaleindustrial applications. For many applications, which do notrequire structural perfection, using hyperbranched polymers

can circumvent this major drawback of dendrimers. Unlikedendrimers, randomly branched hyperbranched polymers withsimilar properties can be easily synthesized via one-step reac-tions and therefore represent economically promising productsalso for large-scale industrial applications. Companies suchas the Perstorp Group (Perstorp, Sweden), DSM Fine Chem-icals (Geleen, Netherlands), BASF AG (Ludwigshafen, Ger-many), and Hyperpolymers GmbH(Freiburg, Germany)alreadyproduce commercially available hyperbranched polymers on alarge-scale 1 (Fig. 3).

1 Currently Perstorp Specialty Chemicals AB (Sweden) produces hyper-

branchedpolymers, known as Boltorn

products, on a ton-scalefor 4 EUR/kg.

Most of the applications of hyperbranched polymers arebased on theabsence of chain entanglements, theglobularshape,and the nature and the large number of functional groups withina molecule. Modication of the number and type of functionalgroups on hyperbranched polymers is essential to control theirsolubility, compatibility, reactivity, adhesion to various surfaces,self-assembly, chemical recognition, and electrochemical andluminescence properties. In other words, the large number of functional groups allows for the tailoring of their thermal, rhe-ological, and solution properties and thus provides a powerfultool to design hyperbranched polymers for a wide variety of applications. Fig.4 gives anoverview of the investigatedapplica-tions for hyperbranched polymers (for further details see [241,68,138] ).

An area of application that, until now, has remained almostunconsidered in scientic discussions is the eld of chemicalengineering.

Unlike conventional linear polymers, hyperbranched poly-mers do not only show a remarkable selectivity and capacity[7,111] , but, because of a lack of chain entanglements, also acomparatively low solution and melt viscosity [4,12,4446] aswell as an enormous thermal stability [4,47] . Since the polarity

of hyperbranched macromoleculescan be adjustedby controlledfunctionalization of the end groups, selective compounds (con-sisting of either pure hyperbranched polymers or fractions of hyperbranched additives) can be tailored. The remarkable selec-tivities and capacities of hyperbranched polymers in combina-tion with their low melt viscosity, high solubility and thermalstability can be used for the optimization of a number of sepa-ration processes.

Therefore, this review focuses on new developments of hyperbranched polymers in the eld of chemical engineer-ing. Based on a brief description of the synthesis andproperties of hyperbranched polymers, a detailed overviewof the research regarding the phase behavior of hyper-branched polymers is given, followed by a discussion of newpotential chemical engineering applications of hyperbranchedpolymers.

2. Denitions, synthesis and properties

2.1. Denitions

Dendritic polymers are recognized as a fourth major class of macromolecular architecture as illustrated in Fig. 1 [1]. Theyrepresent highly branched globular macromolecules, which canbe subdivided according to their degree of structural control into

three different categories, namely (a) random hyperbranched


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M. Seiler / Fluid Phase Equilibria 241 (2006) 155174 157

Fig. 1. Representation of the four major classes of macromolecular architectures [1].

polymers, (b) dendrigraft polymers, and (c) dendrimers (seeFig. 5).

Theterm dendrimer is derived from theGreek words dendron(tree) and meros (part). Dendrimers are highly uniform, three-dimensional, monodisperse polymers with a tree-like, globular

structure and a large number of functional groups. Dendrimerswere introduced at the end of the 1970s by V ogtle and co-workers [41], followed by the fundamental pioneering syntheticmethodologies of Tomaliaet al. andFr echet et al. [1,4143,115] .As shown in Fig. 5, a dendrimer is a symmetrical, layeredmacromolecule which consists of three distinct areas: the poly-functional central core (dendrimer) or focal point (dendron),which represents the center of symmetry, various well-dened,radial-symmetrical layers of repeating units (also called gen-

erations), and the end-standing groups, which are also termedperipherical or terminal groups.

Dendrigrafts were introduced in 1991 as Comb-burst poly-mers by Tomalia et al. [48] and as arborescent polymers by Gau-thier and M oller [49]. Dendrigraft polymers may be regarded

as semi-controlled branched polymer architectures intermediatein terms of structure control between dendrimers and hyper-branched polymers [50]. As described by Teertstra and Gauthierdendrigraft synthesesfollowa generation-based growthmethod-ologysimilar todendrimers, butuse polymericchainsasbuildingblocks leading to a rapid increase in molecular weight per gen-eration and therefore, in a few steps, to macromolecules with ahigh molecular weight (typically one to two orders of magni-tude larger than for their dendritic counterparts). In comparison

Fig. 2. Branched topologies in polymer science.

The gure has been supplied very kindly by Prof. B. Voit, Leibniz-Institut f ur Polymerforschung Dresden e.V., Germany.


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Fig. 3. Commercially available hyperbranched polymers.

to dendrimers, dendrigraft polymers are less controlled sincegrafting may occur along the entire length of each generationalbranch and the exact branching densities are arbitrary and dif-cult to control [1].

Hyperbranched polymers (see Fig. 5) represent another classof globular, highly branched macromolecules with a large num-ber of functional groups. However, unlike dendrimers, hyper-branched polymers exhibit polydispersity and irregularity interms of branching and structure. These kinds of polymer struc-

turesare known frompolysaccharides suchas glycogen,dextran,and amylopectin since the 1930s [51].

2.2. Synthesis and structural characterization of

hyperbranched polymers

Hyperbranched polymers and dendrimers share a few com-mon features such as their preparation from AB x monomersleading to highly branched macromolecules with a large number

Fig. 4. Applications of hyperbranched polymers discussed in literature. Bold italic: commercial applications of hyperbranched polymers.


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Fig. 5. Dendritic polymers divided into the subclasses hyperbranched polymers, dendrigrafts, and dendrons/dendrimers.

Branch cell structural parameters: (a) branching angles; (b) rotation angles; (l) repeat unit lengths; (Z) terminal groups [1].

of functional end groups. However, the synthetic approachesfor hyperbranched polymers and dendrimers differ substan-tially; hence differences in molecular shape and architecturesand sometimes also in properties are observed.

The tedious step-wise procedure for dendrimers often resultsin expensive products with limited availability and thereforerestricteduse for large-scale industrial applications. Unlike den-drimers, hyperbranched polymers are often easy to synthesizeon a large-scale and therefore are considered to be alternativesfor dendrimers.

Hyperbranched polymers are prepared in one-step proce-dures, most common by polycondensation of AB x monomers,as reported by Stockmayer [52,53] , Flory [54] and Kim andWebster [55]. If x 2 and the functionality A reacts only withfunctionalitiesB of another molecule, thepolymerizationof AB x monomers results in highly branched polymers [54]. Apart from polycondensation , addition polymerization of monomers thatcontain an initiating function and a propagating function in thesamemolecule [44], ring-opening polymerization [56], and self-condensing vinyl polymerization (SCVP) [57] can be appliedfor the synthesis of hyperbranched macromolecules. By now,a number of excellent reviews on the synthetic approaches for

hyperbranched polymers has been published (see for instance

[24,58,59,68,138] ) giving a detailed insight in the underlyingmethodologies and reaction mechanisms.

The one-step procedures used for the preparation of hyper-branched polymers lead to uncontrolled statistical growth. Con-sequently, the resulting structures are imperfect and polydis-perse. Furthermore, unlike dendrimers, the control over layersor generations as well as over the molar mass deteriorates. Dueto the statistical nature of the coupling steps, steric hindrance of growing chains, and reactivity of functional groups, the propa-gation occurs at only two sites among branching units, whichgives different polymer segments [60]. The different segmenttypes within a hyperbranched macromolecule are depicted inFig. 6.

Based on AB 2 monomers, linear segments, known as defects,show one functional B-group unreacted, whereas the termi-nal segments have two unreacted B-functionalities. Similar todendrimers, the dendritic segments in hyperbranched macro-molecules represent fully incorporated monomers which haveno unreacted functionalities. The most prominent feature of hyperbranched polymers is their degree of branching DBor branching factor, which denes the ratio of branched,terminal, and linear units in the macromolecular structure

[2].


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Fig. 6. A Hyperbranched polymer with its different segment types from poly-merization of AB 2 monomers.

The degree of branching of a perfect dendrimer equals 1,while a linear polymer has a DB of 0. Two different equationshavebeen suggestedfor theaverageDB. Therst denition [61],compares the sum of the dendritic and the terminal repeating

units to the sum of all repeating units in the structure (Eq. (1))

DB(%) =D + T

D + T + L 100 (1)

where D , T and L represent the number of dendritic, terminaland linear units per macromolecule. The second denition byFrey and co-workers (see Eq. (2)) does not include the terminalrepeatingunits and is thereforeclaimed to be more accurate thanEq. (1) at low molar mass [62].

DB (%) =2D

2D + L 100 (2)

Thefractionsof D-, L-and T -repeatingunitsare usuallydeter-mined by NMRspectroscopy. Eq. (3) has been suggested for thecalculation of themolar mass of a hyperbranched polymer usingthe relative fraction of the respective repeating unit [62].

DPn =D + L + T

T D(3)

2.3. Properties of hyperbranched polymers

2.3.1. Thermal propertiesDueto their highlybranchedstructure, dendriticpolymersare

almost exclusively amorphous materials. Therefore, the glasstransition temperature T g is one of the most important thermalproperties. T g is an important parameter for a dendritic poly-mer with respect to potential applications in the eld of powdercoatings or rheology modiers. Upon heating, amorphous com-ponents convert at T g from a glassy state to a liquid state, i.e.,into a melt for low molar mass substances or a rubbery statefor high molar mass compounds. In the melt, thermal energyis sufciently high for long segments of each polymer chainto move in random micro-Brownian motions. In the amorphoussolidstate,on theotherhand,polymerchainsassume theirunper-turbeddimensions as they do in solution under theta-conditions.Below T g , all long-range segmental motions cease. Rotationsaround single bonds become very difcult and the only molec-

ular motions that can occur are short-range motions of several

contiguous chain segments and motions of substituent groups[63].

In the case of dendritic polymers the situation is more com-plex, since segmental motions arealso affected by thebranchingpoints and the presence of numerous functional groups. Theglass transition temperature of a hyperbranched polymer is notonly affectedby thechain-end composition,but also bythemolarmass and the macromolecular composition [64]. According toSchmaljohann et al. it can be understood as a combination of inter- and intramolecular effects. Differences in T g of hyper-branched polymers with different repeating units but the sameend groups demonstrate the intramolecular effect of segmentalmotion, whereas the change of T g through variation of the endgroups (their polarity in particular) can be assigned to transla-tional motion and an intermolecular effect [6].

For dendritic polymer systems T g increases with generationnumber to a limit, above which it remains nearly constant [64].This increase in T g with generation number is assumed to reecta decrease in chain mobility due to branching.

A number of research groups demonstrated that the chemicalnatureof thelargenumber of terminal groupsstronglyaffects theglass transition temperature (see for instance [27,45,6467] ).

By means of DSC measurements Sunder et al. demonstratedthat the exibility, i.e., T g, of a modied highly polar hyper-branched polymer with large number of hydroxyl end groupsis controlled mainly by two factors: (i) hydrogen bonding of the end groups, increasing the rigidity of the molecules and (ii)tendencies of the substituents to form higher ordered phases(mesophases, crystallization) [65]. It is an important informa-tion that the degree of alkyl substitution has hardly any effect onT m, however there is a pronounced effect on T g [65].

For further information on the thermal properties of hyper-branched polymers see the recent studies of Voit [3], Sunder[65], Schmaljohann et al. [6], Rogunova et al. [66], Magnussonet al. [67] and Behera et al. [157] .

2.3.2. Mechanical and rheological propertiesInvestigations on new applications of a polymer are often

closely related to its material and processing properties. There-fore, the mechanical and rheological properties of hyper-branched polymers are of great importance.

Due to the highly branched, globular structure, the congu-ration of hyperbranched polymers and dendrimers is coined bya lack of chain entanglements. The non-entangled state imposespoor mechanical properties, resulting in brittle dendritic poly-mers with limited use as thermoplastics [4]. The stressstrainbehavior of hyperbranched polymers can be similar to that of ductile metals as observed by Rogunovaet al. forhyperbranchedpolyesters. Likeductile metals, hyperbranched polyestersdo notstrain harden [66]. This is due to their globular structure, whichdoes not permit the process of chain extension and orientation(the usual mechanisms of strain hardening). However, inter-molecular associations, such as hydrogen bonding and possiblyintermolecular crystallization of a few linear segments, provideconnections between the hyperbranched macromolecules [66].

Apart from the mechanical properties also the viscosity

behavior of linear and branched polymers shows remarkable


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Fig. 7. Melt viscosity vs. molar mass of linear and dendritic polymers [77] :a.m.u., atomic mass unit.

differences. This was already noted by several scientists at theend of the 1960s [6974] . Fig. 7 illustrates the melt viscosityof linear and dendritic polyethers as a function of the molarmass (given in atomic mass units). For linear polymers, abovea critical molar mass, a drastic increase in melt viscosity isobserved. However, the line for dendritic polymers shows acontinuous slope of 1.1 up to 100,000 a.m.u. with no criticalmass limit being observed. This behavior can be explained bythe different macromolecular structure of linear and dendriticmacromolecules. At low molar mass, linear polymers consist of random coil chains which, as the molar mass increases, start toentangle at a critical molecular size, leading to a sharp increasein melt viscosity. Unlike linear polymers, the globular, highlybranched architecture of both dendrimers and hyperbranchedpolymers prevents chain entanglements, resulting in consid-

erably smaller melt viscosities and a continuous slope of the-function (see Fig. 7). There are a number of studies conrm-ing the strikingly low melt and solution viscosities of dendriticpolymers in comparison to linear polymers [4,7,12,4446] .

2.3.3. Solution propertiesA number of excellent research studies have been published

focusing on dilute and semi-dilute properties of branched poly-mer systems [46,7588] .

Hyperbranched polymers have a signicantly lower intrin-sic viscosity, MarkHouwink exponent, hydrodynamic volume,and ratio of radius of gyration to hydrodynamic radius in com-parison to their linear analogues of the same molar mass. Alsothe osmotic second virial coefcient A2 was object of numer-ous research studies to characterize the effect of branching.As observed for many branched polymer solutions branchingdecreases the second virial coefcient in good solvents. There-fore, for a branched polymer, A2 is always lower than that forthe homologous linear polymer [84,85] .

For many potential applications of hyperbranched polymersin the eld of chemical engineering (see Section 4) the phasebehavior of concentrated hyperbranched polymer solutions isof great importance. Therefore, the following section gives anoverview of the measured andmodelled phase equilibria of con-

centrated hyperbranched polymer solutions.

3. Phase behavior of hyperbranched polymer solutions

The comprehension of the phase behavior is an essential pre-requisite for contemporary polymer science and engineering.Phase separation and segregation often occur during the pro-duction andprocessing of polymers, either due to their necessityor owing to undesirable circumstances such as the incompati-bility between polymers or an insufcient solvent power. Theimpact of high-performance polymer blends on modern mate-rial science is signicant and continuously growing. Especiallyimprovements in performance characteristics such as rigidity,toughness, abrasion resistance, chemical and ame resistance,heat resistance, and ease of processing are of great importance[89]. Even though in production and processing the equilibriumstate is usually not reached, it is nevertheless of great impor-tance to know, what the equilibrium condition of the regardedsystem would be like, in order to understand the properties of aplastic, to operate a production process optimally, or to modifypolymeric materials successfully [90].

The large body of interdisciplinary research on dendriticpolymers, i.e., dendrimers, dendrigrafts, and hyperbranchedpolymers, is a guarantee for emerging applications (see Fig. 4).However, the understanding of essential fundamentals such asthe phase behavior of dendritic polymer solutions is still in itsinfancy. The experimental investigation of the phase behaviorof hyperbranched polymer systems is a crucial requirement fora successful introduction of new applications to highly compet-itive markets. In this context, thermodynamic models, whichaccurately account for the impact of polymer branching onthe phase behavior of polymer systems, play a very impor-tant role; they enable the optimization of new applications

of hyperbranched polymers without requiring an unjustiableamount of experimental phase equilibriumdata.However,so far,despite the scienticeffort of thermodymists in this eld, almostno gE-model or equation of state has been developed, whichproved its suitability in considering explicitly the inuence of thedegree of polymer branchingon thephase behaviorof highlybranched polymer systems over a wide temperature andpressurerange.

This section summarizes the most important studies on thephase equilibria of hyperbranched and other branched polymersystems that have been investigated so far. In the rst section,experimental thermodynamic studies on the miscibility behav-ior/phase behavior are described, followed by a second sectionfocussing on the modelling of the phase behavior.

3.1. Experimental investigations

Severalauthors studied the inuenceof polymerbranchingonthe miscibility behavior of polyolensolvent systems [9196] .Kleintjens et al. investigated the inuence of polymer chainbranching on liquidliquid phase equilibria in polymer solu-tions. For the system polyethylenediphenyl ether they foundthat the upper critical solution temperature (UCST)-curve of a branched polyethylene solution may be shifted by more than10 K comparedwith that of a linearpolyethylene sampleof about

equal number and mass average molar mass [91].


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Charlet andDelmasdemonstrated that the lowercritical solu-tion temperature (LCST)-curve shifts towards higher tempera-tures with an increasing degree of polymer branching [92]. Thisincrease in polymer solubility is due to a difference in acces-sible surface area of the polymer, which, in comparison to alinear polymer of the same molar mass, is larger for branchedmacromolecules.

Chen et al. investigated the LCST and UCST phase behav-ior of branchy poly(ethylene-propylene) copolymersin differentsolvents and the inuence of supercritical ethylene on the loca-tion of the phase boundaries [94]. The experimental data couldbe modelled well using the statistical associating uid theory(SAFT). Further studies were carried out by Chen et al. [95],Whaley et al. [96], De Loos et al. [97,99] , and Han et al. [98].

Polyetheramide hyperbranched polymers having alkyl termi-nated end groups blended with linear low density polyethylenes(LLDPE) were investigated by Sendijarevic et al. [156]. Theblends were extruded as thin lms so that surface properties andeffectsof lengthand concentration of thealkyl groupson themis-

cibility of the lms could be studied. Sendijarevic et al. foundthat the miscibility of the hyperbranched polymers in LLDPEcan be controlled by varying the length of the alkyl terminatedend groups [156] .

Gitsov and Fr echet demonstrated that hybrid linear-dendriticpolyether copolymers are able to form mono- and multimolec-ular micelles depending on the dendrimer generation and thecopolymer concentration of watermethanol solutions [100] .Ishizu et al. showed that alternating maleimide/styrene hyper-branched copolymers exhibit the behavior of nanospheres suchas dendrimers with a uniform inner density distribution and anaverage volume density close to their bulk density [82].

Mooreeld and Newkome demonstrated that the solubil-ity of dendrimers can be tailored by introducing appropriatefunctionalities into a dendrimers surface groups. By meansof hydrophilic functional end groups, hydrophobic dendrimerssuch as polyethers and polycarbosilanes can become water sol-uble, and also water-soluble dendrimers can turn into hydropho-bic molecules after functionalization with hydrophobic groups[101]. Some dissolved dendritic polymers exhibited consider-able variable hydrodynamic radii which predominantly dependon solvent properties such as pH [101] .

Regarding dendritic polymers, there are some reports in lit-erature, which discuss the inuence of the macromolecularstructure as well as the nature and number of chain-end func-tionalities on the polymer solubility in selected solvents (see forinstance [77,101108] ). It was found that hyperbranched poly-mers and dendrimers are highly soluble in solvents which arecapable of solvating the numerous chain ends. For these kindsof solvents, Hawker and Fr echet investigated the solubility of isomeric polyesters. In all cases, the dendrimers were more sol-uble than their hyperbranched analogues and both proved to besignicantly more soluble than the linear polyester [77]. More-over, the solubility of a polyether dendrimer in tetrahydrofuran(THF) was approximately 50 times higher in comparison to itslinear analogue [103].

Seiler and Arlt studied the low- and high-pressure phase

behavior of polymer solutions containing commercially avail-

able hyperbranched polymers such as hyperbranched Boltorn

polyesters, hyperbranched Hybrane poly(ester amides) andhyperbranched polyglycerols [7,109116] : At moderate pres-sures, the investigated vaporliquid equilibria (VLE) of differ-entlybranchedpolyglycerols and commercially available hyper-branched polymers such as polyesters and poly(ester amides) inethanol or in water allowed for a discussion on how the degreeof polymer branching, the nature and number of polymer func-tionalities, and the solvent polarity determines the slope of thebubble point curve and the solvent activity. Furthermore, specialattention was devoted to the inuence of commercially avail-able hydroxyl-functional hyperbranched polymers, dendrimersand linear polymers on the vaporliquid equilibria of azeotropicsystems such as ethanolwater, tetrahydrofuranwater, and 2-propanolwater. The extent of inter- and intramolecular hydro-gen bond formation has a dominant impact on the solventactivity and therefore determines partition coefcients and rel-ative volatilities. At large polymer concentrations, hydroxyl-functional hyperbranched polyesters tend to form agglomer-

ates, limiting their polymer solubility and separation efciency.On the other hand, highly soluble hyperbranched polyglyc-erols and the hyperbranched poly(ester amide) Hybrane S1200break the azeotropic system behavior of a variety of aque-ous azeotropic mixtures in a remarkable manner making thempotential entrainer candidates for the extractive distillation (seeSection 4) [7,109116] .

The inuence of hyperbranched polymers exhibiting ahydrophobic shell (acetylated polyglycerol, esteried hydroxyl-functional polyester) on the liquidliquid equilibrium (LLE)of the azeotropic tetrahydrofuranwater system has also beeninvestigated [7]. The system hyperbranched polyester Boltorn

H3200tetrahydrofuranwater shows a remarkably distinctsolutropicphenomenon. Moreover, the hyperbranched polyesterBoltorn H3200 exhibits a great selectivity and capacity allow-ing for the breaking of the tetrahydrofuranwater azeotrope bycombining single stage extraction with distillation [7,109115] .

Seiler et al. demonstrated that thehigh-pressure phase behav-ior of nonaqueous hyperbranched polyester solutions shows thecharacteristics of a type-IV system according to the classica-tion of van Konynenburgand Scott. In thesystemhyperbranchedpolyester Boltorn H20ethanolCO 2 the location of the uppersolution temperature (UST) curve proved to be dependenton thepolymer molar mass and the solvent polarity and independentof the system pressure. However, the lower solution tempera-ture (LST) of the respective system strongly depends on thepressure. An increase in CO 2 concentration leads to a consid-erable shift of the lower solution temperature curve to lowersystem temperatures (see Fig. 8). For the system hyperbranchedpolyesterethanolCO 2 , the merging of the UST and the LSTcurve into the hourglass miscibility gap is observed at the CO 2concentration of 50.5wt.% [7,109] .

Garasmuset al. studied thestructure of hyperbranched polyg-lycerol and amphiphilic poly(glycerol ester)s in aqueous andnonaqueous solution. Similar particles sizes and molar massesof the hyperbranched polyglycerols in D 2O and CD3OD wereobserved. The polymers show a compact structure and well-

dened entities in both solvents. For the nonpolar solvent


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Fig. 8. High-pressure phase behavior of the system hyperbranched polyester (Boltorn H20)ethanolCO 2 [109].

C6D6 amphiphilic derivates of the hyperbranched polyglycerolsbecome more compact with increasing degree of esterication[86].

Polese et al. measured innite-dilution activity coef-cients of polar and nonpolar solvents in comb polymers andpoly(propylene imine) dendrimersof generation25 [105] . Sol-vent activity coefcients at innite dilution change with respectto thedendrimer generation number, reachinga minimum atgen-eration 4 in the temperature range of 333413 K [105] . Due tothe exothermic formation of hydrogen bonding, alcohols formhydrogen bonds with the dendrimer more easily at low tem-peratures and hence, the solubility decreases with increasingtemperature [105] . Since Polese et al. used dendrimers with abasic character, slightly acidic solvents showed better solventqualities than others (THF, toluene, ethyl acetate).

Boogh et al. demonstrated that hyperbranched polymers rep-resent very effective low viscosity liquid tougheners for epoxyresins requiring no solvent during blending and processing.The multifunctional epoxidised hyperbranched polymers used,phase separate during the curing of the thermosetting resin. Thechemical architecture of the shell, i.e., its polarity and reactiv-ity, controls the initial miscibility and the kinetics of the phaseseparation process [117] . UCST and LCST data are reported fora 5 wt.% modied (solvent-free) epoxy resin using a less polar

hyperbranched polymer than the reference modier.

Pruthikul et al. determined the relative accessibility of inter-nal and terminal carbonyl groups of hyperbranched polyestersfrom the degree of hydrocarbon bonding measured by FT-IR spectrometry. The results show that in molecules of low-generation number, the terminal groups are about as accessible(e.g. for solvent molecules) as those in linear polymers. How-ever, these functional groups become less accessible as thegeneration number increases, presumably because the moleculefolds back on itself to some degree resulting in a more compact,globular shape [106].

Tianet al. studied a novel amphiphilic biodegradable cationichyperbranched copolymer with polyethyleneimine (PEI) asbackbone and poly(ethylene glycol) (PEG) and poly( -benzyll -glutamate) (PBLG) as hyperbranched arms. The copolymerswere found to be self-assembled in water with a critical micelleconcentration (CMC) in the range of 0.003680.0125g/l and ahigh hydrophobic micelle core. With increasing content of thehydrophobic PBLG-block, the CMC value of PEGPEIPBLGmicelles decreased. It was also shown that protonated environ-ments play a critical role in determining the size and CMC of PEGPLIPBLG micells [118] .

Hay et al. compared thermal bulk properties and the PvT-behavior of dendrimer melts based on benzyl ether with lit-erature values for monodisperse, linear polystyrenes. Further-

more, property measurements are presented for an exact linear


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analogue to the fth-generation dendrimer [120] . The investiga-tions suggest that, unlike the linear polystyrene, some form of structural transition occurs in thedendrimerbulk at a molar massnear thatof the fourth-generationdendrimer. Dendrimersexhibitan increased packing efciency as evidenced by a decreasedspecic volume (increased density) when compared with anexact linear analogue of the fth-generation dendrimer [120] .A crystalline state can be formed by both the lower-generationdendrimer and the linear analogue. This crystalline state is notobserved in dendrimers above the third generation [120] .

Garamus et al.investigated thesolutionstructure ofmetalpar-ticles prepared in unimolecular reactors of amphiphilic hyper-branched polymers by means of small-angle neutron scattering.They demonstrated that amphiphilic hyperbranched polymerscan serve as molecular conned environments for the synthesisand stabilization of inorganic metal particles [86].

Prausnitz and co-workers measuredVLEof polyamidoamine(PAMAM) dendrimers and benzyl ether polymers (dendriticand linear) in several polar and nonpolar solvents at 308343 K

[104]. ThestronglyhydrophilicPAMAMdendrimersshow com-plete miscibility with water, lower alcohols, glycols, ethylene-diamine and they are insoluble in nonpolar solvents [104]. Theabsorption behavior of the latter dendrimers does not dependon the generation number. The linear analogues of the den-drimers absorb less solvent, since, under experimental condi-tions (303362 K), they were partially in the crystalline state[104].

Lieu et al. presented VLE data of binary mixtures of arbores-centpolystyrene and linear polystyrene dissolvedin chloroform,toluene or cyclohexane at temperatures between 323K and343K [102]. Only for cyclohexane, a dependence of solvent

absorption on polymer generation was found. Unlike chloro-form and toluene, cyclohexane is a poor solvent for arborescentpolystyrene. Therefore, a signicant entropic contribution anda low enthalpic contribution in cyclohexane are believed to beresponsible for the absorption behavior, whereas for chloroformand toluene the opposite applies [102].

The experimental results described above represent valu-able information to test or develop thermodynamic models forhyperbranched polymer systems.The most important modellingresults are summarized in the following section.

3.2. Modelling

van Vliet et al. applied the mesoscopic simulation methoddissipative particle dynamics to study the dynamics of polymersolvent liquidliquid phase separation. It was foundthat the degree of polymer branching has a pronounced effecton the radius of gyration and the centre of mass diffusion of thepolymer [119] . The simulation results show that the differencein chemical potential between the mixed and the demixed stateis the main driving force behind the centre of mass diffusionand thus the phase separation, rather than the reduced radius of gyration due to polymer chain collapse [119] .

Lue performed Monte Carlo simulations for athermal solu-tions of dendritic polymers (generation 05) and investigated

the structural and thermodynamic properties of these systems

from the dilute to the concentrated regimes. At low polymerconcentrations, dendritic polymer solutions have a lower sys-tem pressure than solutions containing linear polymers of thesame molar mass, owing to the more compact dendritic archi-tecture [85]. In the concentrated polymer regime, solutions con-taining low-generation dendritic polymers behave similarly tolinear polymers, while those containing high-generation poly-mers have a system pressure that increases more rapidly withconcentration [85]. Monte Carlo simulations were also used byTimoshenko and Kuznetsov to study conformational structuresformed by star and comb hetero-polymers during kinetics of folding from the coil to the globule, as well as the correspondingequilibrium states on going from the good to the poor solution[121].

Apart from Monte Carlo simulations, Steinhauser also usedMolecular Dynamics simulation to investigate the inuence of chain branching of various chain topologies on the static prop-erties of polymers. Several important quantities that are suitablefor the quantitative characterization of branched polymer struc-

turesare discussed.Steinhauser demonstrated that starpolymersin a good solvent are more spherical than in a theta solvent. Thiseffect becomes stronger with increasing arm number. The rheo-logical and thermodynamic properties of dendrimers in solutiondepend on the location of the terminal groups and the densitydistribution of the macromolecule [122] . For dendrimers it wasfound that not all the terminal groups lie near the exterior of the macromolecule. The functional end groups can signicantlypenetrate the interior of the dissolved dendrimer. Particularlyfor higher-generation dendrimers ( G 5) in theta solvents, theterminal groups come very close to the core of the macro-molecule. Furthermore, Steinhauser found that the segments

of monomers pertaining to the inner dendrimer generations aremore expanded than the ones at the periphery of the polymer.This effect increases with solvent quality andwith the total num-ber of dendrimer generations.The stretching of monomer bondscan be understood as a consequence of molecular crowding inthe core region. The structure functions of dendrimers exhibit astructure that approaches the one of a solid sphere with increas-ing generation number [122] .

Jang and Bae introduced several modications of amodel based on the lattice cluster theory (LCT) and theVeytsman hydrogen-bonding model to describe liquidliquidphase behavior of binary hyperbranched polyolwater sys-tems [107,108,123,124] . As the number of polymer end groupsincreases exponentially with the generation number, the effectof hydrogen bonding between the solvent molecules and theend groups is growing. The solventsolvent hydrogen bondingdominates the phase behavior of hyperbranched polymerwatersystems [107,108,123,124] . In their LCT model, the dendriticstructure is characterized by three parameters: (a) the gener-ation number; (b) the separator length, which represents thenumber of bonds between branching points; and (c) the coresegments between zeroth-generation points. The model of Jangand Bae does not account for hydrogen bonding between thesolvent molecules and the inner polymer functionalities. Therecent version of their model [108] qualitatively accounts for

the dependence of pressure, structure and hydrogen bonding


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on liquidliquid equilibria of dendrimersolvent systems. How-ever, until recently, no comparison was reported underlining theapplicability of this model for thedescription of LCST andcoex-istencecurvesofdifferentlybrancheddendriticpolymersolventsystems. For further information concerning the associationbehavior of hyperbranched polymers in aqueous solution see[125,126] .

Mezzenga et al. developed a new thermodynamic modelfor the prediction of phase equilibrium behavior of thermoset-reactive modier polymer blends. The model combines theFloryHuggins lattice theory with a group contribution theoryfor the prediction of the free energy of mixing as a func-tion of blend composition, temperature, and degree of poly-merization. The model was tested using a diglycidyl etherbisphenol-A epoxy cured with isophorone diamine, blendedwith epoxy functionalized hyperbranched polymers. After con-sidering the enthalpy and entropy variations of both the ther-moset and the modier, an excellent experimental agreementbetween the model and the experimental cloud points was

obtained. However, the entropy reduction caused by physicalpolymerpolymer interactions had to be expressed as an addi-

tive term to the enthalpic interaction parameter [127]. Fig. 9shows some modelling results of the free energy of mixing for aresin-hyperbranchedpolymer blendat 80 Casafunctionofcur-ing time, temperature and composition. Initially, the free energyof mixing is negative at all compositions (see Fig. 9a), so thatno two points on the free energy curve have a common tan-gent and both components are fully compatible. However, at theend of the reaction, the free energy of mixing is positive for allcompositions, indicating that the system is in an unstable state[127]. The curves shown in Fig. 9b illustrate the transition froma homogeneous to a heterogeneous system for discrete curingtimes corresponding to the appearance of inection points in thecurve [127] .

The modication of epoxy resins with reactive hyper-branched polymers is of great interest since this class of modi-ers allows for easily processable tough thermoset resins andthermoset based composites. The structural build-up duringreticulation of thermoset systems containing reactive modierscan strongly inuence the nal properties of such blends as

shown by Mezzenga et al. in a study of the rheological behav-ior during cure of an epoxy/amine thermoset system blended

Fig. 9. Resin-hyperbranched polymer blend at 80 C [127]: (a) 3D plot of free energy as a function of curing time and composition; (b) 2D plot of free energy as a

function of composition at discrete times. Composition is expressed in volume fraction of thermoset pseudocomponent.


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with reactive hyperbranched polymers. The phase separationand gelation behavior of these epoxy resin/hyperbranched poly-mer blends was studied as well [127] .

Jang et al. also proposed a LCT-based model to describe den-dritic vaporliquid equilibria of dendrimer solutions [124] . Bymeans of three model parameters, the model accounts for thestructure of dendrimers, solventsolvent specic interactionsandsolvent-endgroup specic interactions. Jang and Bae exam-ined VLE of polyamidoamine dendrimermethanol and benzyldodecyldendrimertoluene systems.They demonstrated that thesolvent-end group specic interaction dominates VLE of den-drimer solutions, i.e., the nature of end groups has a crucialinuence on the VLE of dendrimer solutions [124] .

Ko et al. derived a thermodynamic model based on latticecluster and melting point depression theory to account for thestructural effects on the phase behavior of hyperbranched solidpolymer electrolyte/salt systems. For these kinds of systems theproposed model accounts for the structural dependence of thefree energy of mixing. However, the model does not consider

the end-group effect and the specic interaction between thepolymer molecules and the salt molecules [128] .

Yoon et al. investigated the phase behavior of the nematicliquid crystal 4-ethoxybenzylidene-4 -n-butylaniline in hyper-branched polymers and adopted Freeds lattice cluster theoryto describe the structures of the hyperbranched polymers. Byintroducing an additionalparameter, highly oriented interactionsbetween segments could be described resulting in a very goodagreement between the calculation and the experiment [129] .

Kouskoumvekaki et al. tested the UNIFAC-FV andEntropic-FV models to predict vaporliquid equilibria of dendrimer solu-tions [130] . It is shown that both models can predict the solvent

activity in dendrimer solutions with acceptable accuracy.Calculation results by Seiler et al. demonstrated the suitabil-ityof theUNIFAC-FVmodel to describe vaporliquid equilibriaof hyperbranched polymer and dendrimer solutions [113] . Amodied approach for a determination of the residual solventactivity was suggested allowing for an indirect considerationof dendritic topologies. This approach is based on the obser-vation that, depending on the solvent polarity and the natureand number of functional polymer groups, only certain kindsof the structural polymer units (linear, dendritic, terminal) dom-inate the polymersolvent interactions and thus determine thesolvent activity. Hydroxyl-functional hyperbranched polyethersand polyesters dissolved in good, polar solvents such as waterand ethanol represent soft globular structures with a compara-tively large hydrodynamic volume allowing for penetration of the solvent molecules into the interior of the hyperbranchedpolymers. Hence, for the latter systems, it seemedappropriate toaccount for the contribution of the linear, dendritic, and terminalunits to the residual solvent activity. This led to a remark-able agreement between experimental and UNIFAC-FV results.However, for other polymer solutions where the hyperbranchedpolymer, for instance, is dissolved in a bad solvent, the consid-eration of the terminal group contribution to the residual sol-vent activity is sufcient [113] . Binary and ternary vaporliquidequilibria of polymer solutions consisting of an OH-terminated

polyamidoamine dendrimer, water and/or ethanol can best be

Fig. 10. PvT-data of hyperbranched polymer melts. Filled symbols, hyper-branched polyglycerol PG 2000: M n = 2000g/mol, M w / M n = 1.5. Unlled sym-bols, hyperbranched polyester Boltorn H 2O: M w = 2100g/mol, M w / M n = 1.3.

predicted by UNIFAC-FV if one only accounts for the domi-nant contribution of the terminal groups to the residual solventactivity [7,113] .

Further simulation studies on dendritic polymer systems canbe found elsewhere [88,131137] .

As already pointed out, thermodynamic models, which accu-rately account for the inuence of polymer branching onthe phase behavior of polymer systems, play a very impor-tant role; they enable the optimization of new applications of hyperbranched polymers without requiring a large amount of experimental phase equilibrium data. However, many modelsdescribed above (equations of state, g E-models) do need theinformation of experimental PvT-data for the adjustment of parameters. Since these kinds of PvT-data are usually not avail-able in the literature, the author wants to provide new experi-mental PvT-results [measured at the chair of Prof. G. Sadowski(University of Dortmund) within the scope of M. Seilers PhD-work which was supervised by Prof. W. Arlt (University of

Table 1PvT-data of hyperbranched polymer melts

Pressure(Pa 105)

Density at343.15 K (kg/m 3)

Density at383.15K (kg/m 3)

Density at423.15K (kg/m 3)

Boltorn H2050 1241.6 1214.7 1188.3

100 1245.1 1217.6 1191.5200 1249.2 1222.3 1197.0350 1255.6 1229.4 1205.4600 1265.2 1240.2 1216.2

PG50 1263.5 1234.9 1211.8

100 1266.7 1238.8 1216.8200 1270.5 1244.7 1222.4350 1274.7 1250.4 1226.2600 1282.4 1258.8 1235.9

Hyperbranched polyglycerol PG: M n = 2000g/mol, M w / M n = 1.5; Hyper-

branched polyester Boltorn H20: M w = 2100g/mol, M w / M n = 1.3.


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Erlangen-N urnberg)] fora hyperbranched polyesteranda hyper-branched polyglycerol (see Fig. 10 and Table 1 ).

4. Applications of hyperbranched polymers in the eldof chemical engineering

4.1. Membranes

A wide variety of polymeric materials have been studied toevaluate their potential use as membrane materials for separa-tion processes [139] . However, all the commercially availablemembrane materials developed so far are limited to linear poly-mers. During the last years, a number of research groups startedfocusing on hyperbranched polymer membranes [140147] .Among these studies, the use of hyperbranched polymers ashigh-performance gas separation membranes seems particu-larlypromising.Membrane-based gas separations have attractedmuch attention in the past decade since they offer many advan-tages over traditional separation processes such as low capital

investment cost, low energy consumption, and simple opera-tion [146]. In this context, Fang et al. and Suzuki et al. stud-ied the performance of hyperbranched polyimide membranes[141,143] . In general, polyimide membranes are of consider-able interest in gas separation applications due to their highgas selectivity and excellent mechanical and thermal stability.Fang et al. prepared hyperbranched aromatic polyimide mem-branes by condensation polymerization of a triamine monomer,tris(4-aminophenyl)amine, and a series of commercially avail-able dianhydride monomers [141]. Two types of hyperbranchedpolyimides, an amine-terminated and an anhydride-terminatedsample, were obtained. According to Fig. 11 , lms of these

hyperbranchedpolyimideswere fabricatedby crosslinking treat-

ment during lm cast. Such a lm formation technique is basedon the chemical reaction between the terminal functional groupsof the hyperbranched polyimide and a difunctional crosslinkingagent, which leads to the connection of globular hyperbranchedmacromolecules via chemical bonds [141] . The resulting lmswere transparent, tough and suitable for gas permeation mea-surements.

Crosslinking has a great impact on the gas permeationproperties of the hyperbranched polyimide membranes. Lowercrosslinking density (i.e., lower concentration of crosslinkingagent) results in a higher gaspermeability coefcient but similarideal selectivities [141] . For the same hyperbranched polyimide,the membrane with rigid crosslinking linkages displayed a simi-lar selectivity of CO 2 over N 2 but a much larger CO 2 permeabil-ity coefcient than the one with exible crosslinking linkages.The amine-terminated hyperbranched polyimide membranesshowed larger gas permeability coefcients than the anhydride-terminated membranes. Furthermore, most importantly, Fanget al. pointed out that the O 2 /N2 separation performance of

terephthaldehyde-crosslinked hyperbranched polyimide mem-branes is better than that of the linear analogues and manyother commercially available linear polymeric membranes andcomparable to that of the linear polymeric membranes whichhave been reported to have the highest separation performance[141].

Suzuki et al. investigated the physical andgas transport prop-erties of hyperbranched polyimide membranes prepared from atriamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and adianhydride, 4,4 -(hexauoroisopropylidene) diphthalic anhy-dride (6FDA). These 6FDATAPOB hyperbranched polyimidemembranes exhibited a remarkable thermal stability with a 5%

weight-loss temperature of 510

C. The gas permeability and/or

Fig. 11. Crosslinking scheme for amine-terminated hyperbranched polyimides [141].

Crosslinking agents: ethylene glycol diglycidyl ether (EGDE) and terephthaldehyde (TPA).


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Fig. 12. Sketch map of the structure and proton conductivity mechanism of thehybrid membranes [144].

diffusivity of a polymer depend on the free volume. The frac-tional free volume value of 6FDATAPOB was higher thanthose of the linear analogues, indicating looser packing of themolecular chains. The hyperbranched membrane exhibits a con-siderably high gas solubility, and as a result, shows high gaspermeability [143]. Therefore, Suzuki et al. suggest that thelow segmental mobility and the unique size and distribution of free volume holes arising from the characteristic hyperbranchedstructure of 6FDATAPOB provide an effective O 2 /N2 selectiv-ity. Dueto thehigh permeabilityandO 2 /N2 selectivity, Suzuki etal. conclude that the 6FDATAPOB hyperbranched polyimidemeets the requirements of a high-performance gas separationmembrane [143] .

Shi and co-workers studied the preparation and properties of organicinorganic hybrid membranes based on phosphoric acidand hyperbranched aliphatic polyesters used as electrolytes forpolymer electrolyte membrane fuel cells (PEMFC) operated ata temperature above 100 C [144] . Due to their higher energytransfer efciency, lower maintenance requirements and loweremission of environmental pollutants such as CO,NO x ,andSO x compared with that of internal combustion engines, PEMFCare extremely attractive energy conversion systems to be usedin many industrial applications including electric vehicles andon-site power plants [144] . Shi andco-workers doped thehyper-branched polyester Boltorn H20 into the hybrid membranes atdifferent levels to provide high hydrophilic and water conserva-tion property. According to Fig. 12, a high proton conductivitywas obtained with suitable doping levels of the hyperbranchedpolyester, and the membrane with the highest proton conductiv-ity remained a conductivity of 5 10 4 S/cm even after holdingfor 7h at 130 C under 30% relative humidity. The hybrid mem-branes prepared via a solgel process were stable up to 200 C

[144].

4.2. Extractive distillation

In comparison to conventional (low-) volatile entrainers, theuse of highly selective nonvolatile hyperbranched polymersshowing large separation efciencies allows for a reduction inthe required hot and cold process utilities in extractive distilla-tion [7]. However, apart from the thermodynamic suitability interms of selectivity and separation efciency (for VLE and sep-aration factors see [7,111,114] ), hyperbranched polymers alsomeet a number of other important entrainer criteria such as

a remarkable solubility (becauseof their largenumberof func-tional groups);

a comparatively low solution viscosity (due to the highlybranched topology);

a remarkable thermal stability (up to 823K as for hyper-branched polyphenylenes);

an increasing variety and large-scale availability at low cost(currently 4 EUR/kg);

noncorrosive behavior; no or tunable reactivity and toxicity; adjustable physical and chemical properties.

Furthermore, there is a three-fold advantage of using a non-volatile entrainer:

the entrainer cannot pollute the distillate; no main column separation section and internals are required

for the separation of the entrainer from the overhead product; a variety of entrainer regeneration options is feasible.

The entrainer recovery in a conventional extractive distilla-tion process is mostly carried out using another countercurrentdistillation column. Unlike this conventional process, the regen-eration of nonvolatile entrainers such as hyperbranched poly-mers or the recently suggested ionic liquids [114,116] allowsfor the use of other unit operations. Conventional distillationfor the separation of a binary mixture consisting of a nonvolatileentrainer and a volatile component is not feasible, since the non-volatility of a component would lead to a breakdown of thecolumns counterow. A possibility to circumvent this problemis the operation of a stripping column (charge reux fractiona-tor) without rectifying section and reux. A (heated) inert gascan be fed into the bottoms of the stripper and guided throughthe column in countercurrent to the entrainer rich feed, resultingin a concentrated entrainer-bottom product and a solvent richoverhead product (see Fig. 13).

Thin-lm evaporators represent another alternative for aneffective, thermally gentle and continuous recycling of a non-volatile entrainer. Depending on the viscosity of the solution,falling-lm evaporators or, for higher viscosities, rotary thin-lm evaporators appear to be suitable. Although the hyper-branchedpolymer meltsshowcomparatively(tootherpolymers)low viscosities,it might beadvantageous to install a refusewormat the rotory end of the thin-lm evaporator such as those usedfor the stripping of epoxy resins or the degassing of polyolens.

Hyperbranched polymers can also be recycled by convection


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Fig. 13. Separation scheme for an extractive distillation processes using non-volatile entrainers: (1) ethanol; (2) water; (E) hyperbranched polymer of lowviscosity; [A] main column; [B] ash drum; [C] stripping column.

drying, using a spray dryer, a thin-lm evaporation dryer or abelt dryer.

Therecoveryoptions describedfora nonvolatileentrainer arecompetingunitoperations, whichhave to be thoroughlyassessedwith regard to their investment and operation costs. A com-parison with conventional separation processes for azeotropicmixtures shows, that the remarkable separation efciencies of hyperbranched polymers [111,114] as well as the variety of energetically promising recycling options for the entrainer offerconsiderable potential for process and economic optimizations.

As an example, Seiler et al. demonstrated the potential useof hyperbranched entrainers by investigating the ethanolwaterseparation [7,114] . The entrainer used was a hyperbranchedpolyglycerol ( M n = 1400 g/mol, M w / M n = 1.5, 20 OH-groups

per macromolecule). In case of the ethanolwater separation,the entrainer suitability of hyperbranched polyglycerols can bedemonstrated by evaluation of the process scheme shown inFig. 13.

An ethanolwater feed is separated by an extractive distilla-tion main column [A] and an entrainer regeneration, combininga ash drum [B] and an atmospheric stripping column [C]. Theash, operated under vacuum conditions (for process condi-tions see [114] ), allows for a preconcentration of the nonvolatileentrainer. The remaining water fractions are separated from theentrainer using an adiabatic atmospheric stripping column with-out reboiler and condenser. Dry air is used as the strippingmedium. Subsequently, the regenerated entrainer is recycledback to the main column (see Fig. 13).

Energetic process evaluation of the latter process using thehyperbranched polyglycerol and conventional extractive distil-lationusing1,2-ethanediol asentrainerwas carried outby meansof Aspen Plus . For further details see [114] . In comparison tothe conventional ethanediol-process, the use of hyperbranchedpolyglycerol allows a maximum saving in overall heat duty of about 20% [114] . Thisunderlinesthe potentialof hyperbranchedpolymers for the separation of azeotropic mixtures by extractivedistillation. Moreover, in this context, it is worth mentioning thatthe investigated entrainer (a hyperbranched polyglycerol) repre-sents a chemically non-optimized compound whose properties

can be further improved, for instance, by increasing the ratio

number of molecular surface functionalities to molar massof thehyperbranched polymer . Due to the biocompatibility of manyhyperbranched polymers, their useas entraineror extraction sol-vents for the separation of avors, essences, natural substancesand food ingredients appears promising as well.

4.3. Extraction

Liquidliquid extraction is a separation process which is incompetition with distillation. Especially with the requirementof low energy consumption, liquidliquid extraction is gainingimportance. It is often applied if themixture components exhibitlow or high boiling points (thus distillation has to be carriedout under costly vacuum or low temperature operation), if themixture is a close boiling or an azeotropic system, if the com-ponents to be separated are thermally sensitive or if the mixturecomprises a key component of low concentration.

Hyperbranched polymers show many properties which indi-cate their potential use as extraction solvents. As discussed

elsewhere [7,111] , thereare somehyperbranchedpolymerscom-mercially available (even with FDA approval) which show highselectivities, remarkable loading capacities for certain key com-ponents, no vaporpressure, comparatively low melt andsolutionviscosities, non-toxic behavior, as well as remarkable thermaland chemical stabilities. Therefore, only recently, a number of studies have been published discussing the separation of com-ponents by means of hyperbranched polymers as extractant[7,110,111,114,116,148,149] .

Goswami and Singh focused on hyperbranched polyestersand their use as metal ion extractant. They observed pH-dependent extraction efciencies. The optimum pH range for

the maximum extraction of metal ions was found to be 5.07.0for Cu(II) and Pb(II), 4.57.0 for Fe(III), 6.08.0 for Co(II) andNi(II), 6.07.5 for Cd(II) and 6.58.0 for Zn(II) [148] . How-ever, aspects concerning the design and operation of a suitableextraction process have not been discussed.

Arkas et al. investigated the use of alkylated hyperbranchedpolymers as molecular nanosponges for the purication of water from polycyclic aromatic hydrocarbons. These polymersallow for the extraction of toxic polycyclic aromatic compoundsdissolved in water. Due to highly selective hyperbranchednanosponges, the concentration of polycyclic aromatic hydro-carbons in water could be reduced to a few ppb. Structuralfeatures such as symmetry of the polymers, exibility of theirbranches, intermolecular interactions, and chemical moietiesof the nanocavities are the parameters determining the extrac-tion/encapsulationcapability [149]. Theextractedpollutantscanbe removed from the hyperbranched nanosponges by treatingthe saturated hyperbranched extraction medium with organicsolvents (regeneration of the hyperbranched polymer).

Seiler and Arlt investigated the separation of the azeotropictetrahydrofuranwater system. THF is produced by dehydrocy-clization of 1,4-butanediol in the presence of an acid catalyst.The key step in purication is the breaking of the THFwaterazeotrope [150] . Generally, this is achieved by pressure-swingdistillation (low/high pressure) distillation, extractive distilla-

tion or azeotropic distillation.


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Fig. 14. Separation scheme for the THFwater separation using the hyperbranched polyester Boltorn H3200 as extraction solvent [114] ; concentrations in weightpercent, THF/polymer/water.

Based on the liquidliquid equilibrium results discussedrecently, different process options for the THFwater separationusing hyperbranched polymers as extraction solvents are con-ceivable [111,114] . Energetic optimizations of the latter processoptions by means of a pinch analysis (see [114] ) and compari-sonwithconventionalTHFwater separation processes, indicatea noteworthy potential for the separation process illustrated inFig. 14: A 50 wt.% THF/50 wt.% H 2O mixture of 1000 kg/h isfed into a mixer-settler. A specic amount of the extraction sol-vent, the hyperbranched polyester Boltorn H3200, is added totheTHFwatermixture. 2 At 321.15K, thesolutionsplits into thetwo liquid phases L1 and L2. Due to the large THF-solubilityin the polymer-rich phase L1 and its remarkable low density,the polymer-rich phase represents the upper and the water-rich(polymer-free) phase L2 the lower phase (see also Fig. 14).As described elsewhere [7,114] , the THFwater ratio of thepolymer-rich phase L1 is larger than the azeotropic THFwaterconcentration ( w THF , azeotrope = 0.942 at P = 105Pa). Therefore,when feeding L1 into an atmospheric distillation column (seeFig. 14, THF column), the columns feed composition is locatedat the right-hand side of the THFwater minimum boiling

azeotrope of the corresponding y-, x -diagram. Due to the pres-ence of Boltorn H3200, the azeotropic point shifts to smallerTHF concentrations. Thus, the composition of the overheadproduct of the THF column corresponds to the new locationof the shifted minimum boiling azeotrope [7,114] . The bottomproduct of the THF column contains the remaining THF andthe entire polymer. The latter THF-polymer mixture is sep-arated in a ash drum or thin-lm evaporator into the THF

2 It is worth mentioning, that the choice of the overall mixing point and of thecontinuousphase isof considerable importance forthe viscosityof thecoexistingphases and their settling time. The larger the THF fraction of the polymer-rich

phase, the smaller its solution viscosity [7].

product ( x THF = 0.999) and a concentrated polymer ow. Due toremaining THF fractions and a temperature far above the melt-ing temperature of Boltorn H3200 ( T melt,BoltornH3200 333K),the concentrated polymer ow represents still a pumpable mix-ture of acceptable viscosity, which is recycled to the extractionunit. At 3 104 kPa, the water column (see Fig. 14) separatesthe polymer-free phase L2 within three theoretical stages intowater as bottom product and the THFwater minimum boilingazeotrope as overhead product. Both distillation columns do notrequire a rectifying section.

The process optimization by pinch analysis resulted in arequired heat demand of 1705 kJ/kg THF product. The com-parison of this result with simulation results for the conven-tional THFwater separation processes (azeotropic distillation,pressure-swing distillation) indicates a considerable potentialfor using hyperbranched polymers as extraction solvents [114] .

4.4. Absorption, adsorption

In general, the separation of gases via an absorption processusing hyperbranched polymers as scrubbing agents appears fea-sible. However, this eld is still in its infancy. The challenge isto nd a competitive low-cost hyperbranched polymer having ahigh selectivity and capacity, a comparatively low molar mass,anda melt/solution viscositywhichallowsfor theoperation ofanabsorber even at ambient temperature or below. Arlt and Rolker(University of Erlangen,Germany) currently evaluate the poten-tial of hyperbranched scrubbing agents for absorption processes[151].

Another interesting idea is the use of hyperbranched poly-mers as stationary phases for (preparative) chromatography.A number of research groups started focussing on the sepa-ration of mixtures by adsorption methods involving dendritic

polymers [7,111,116,152155] . Shou et al. studied non-bonded


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hyperbranched polymer-coated columns for capillary elec-trophoresis [152] . Capillary electrophoresis represents a pow-erful separation tool which is often applied for the analysis of biopolymers such as peptides, proteins,DNA fragmentsandcar-bohydrates. However, many biopolymers tend to adsorb to thesurface of the capillary column due to hydrophobic or coulombinteractions [152] . Shou et al. synthesized a series of hyper-branched poly(amine esters) and coated them on the inner sur-face of fused-silica capillaries by physical adsorption. It wasfound that a hyperbranched poly(amine ester) coating of sev-enth generation reduced the electroosmotic ow greatly andsuppressed protein adsorption effectively. For basic proteins,high separation efciencies wereobtained [152]. Chao andHan-son investigated dendritic poly(aryl ethers) as bonded stationaryphases in open tubular capillary electrochromatography. Thebonded dendritic polymers tended to reduce the electroosmoticow and showed promising results in a number of separationexperiments including neutral aromatic hydrocarbons and basicproteins [153]. Based on their results, Chao and Hanson con-

clude that this approach shows promise for the developmentof new capillary electrochromatography methods for separationscience.

5. Conclusions and future work

During the last decadedendriticpolymers haveattractedcon-siderableattention. To date, dendriticpolymers represent a well-established eld in polymer science. A remarkable variety of applications for hyperbranched polymers has been investigatedand the interdisciplinary research activities are still extremelyhigh. Due to the costly and complex multistep-synthesis of

dendrimers and new synthetic developments for hyperbranchedpolymers, a remarkably increasing driving force towards thereplacement of dendrimersby hyperbranched polymerscouldbeobserved during the last 3 years. In this context, developmentsare striving for hyperbranched polymers with higher degrees of branchingandfor syntheticmethodologies that allowfor a bettercontrol over branching,molarmassdistribution and architecture(see for instance [2,3,68,138] ).

However, despite the remarkable pace of interdisciplinaryresearch on hyperbranched polymers and the development of new and more complex hyperbranched structures, it is of impor-tance to note that there are still fundamental research areasthat have not yet been examined properly. As discussed in thiswork, one of these areas represents the phase behavior of hyper-branched polymer systems and the development of thermody-namic models, capable of describing the enthalpic and entropicphenomena of highly branched polymer solutions. The chal-lenge is the explicit and adequate consideration of the degreeof polymer branching. For this purpose, further experimentalinvestigations focusing on the inuence of the degree of poly-mer branching on the high-pressure phase behavior of polymersystems have to be conducted.

In terms of applications, it has been recognized that themodication of the number and type of functional groups onhyperbranched polymers is essential for the control of their sol-

ubility, compatibility, reactivity, thermal stability, adhesion to

various surfaces, self-assembly, chemical recognition, as wellas electrochemical and luminescence properties. Thus, thesestructural modications provide a powerful tool for design-ing hyperbranched polymers for specic applications. Low-viscosity hyperbranched polymers represent promising com-pounds also for the optimization of a variety of separationprocesses. It has been demonstrated that hyperbranched poly-mers can show remarkable separation efciencies, selectivities,and capacities allowing for the elimination of azeotropic systembehavior [7,109,111,114] . This enables applications in the eldof chemical engineering, for instance, as entrainers in extrac-tive distillation processes, as extraction solvents in liquidliquidextraction processes or as scrubbing agents in absorption pro-cesses. However, the author wants to stress that there are only afew hyperbranched polymers commercially available at presentwhich meet the requirements of an entrainer, an extraction sol-vent or a scrubbing agent. Especially the combination of (i) acompetitive large-scale availability of hyperbranched polymers(


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Thanks to Johns special relationship to Prof. Knapp andProf. Arlt, he often visited Berlin. During these visits we alwaysdiscussed new thermodynamic developments, results and chal-lenges in the eld of dendritic polymers intensively. His guid-ance and interest in my work were always an enormous moti-vation for me, and many solutions to the industrial challengesI currently face would not have been possible without his ther-modynamic achievements.

I would like to dedicate this review on hyperbranched poly-mers to John Prausnitz in gratitude and friendship and inacknowledgement of the tremendous achievements which rep-resent his lifes work.

List of symbols M or MWt molecular weight (kg/kmol)P pressure (Pa) R universal gas constant (J/(molK))T temperature (K)w weight fraction x liquid phase mole fraction y vapor phase mole fraction

Acknowledgements

The author gratefully acknowledges the support and guid-ance of Prof. W. Arlt (UniversityofErlangen,Germany). Specialthanks to Prof. B. Voit (Leibniz-Institut f ur PolymerforschungDresden, Germany), Prof. W. Shi (University of Science andTechnology of China, Hefei), Bo H aggman (Perstorp SpecialtyChemicals AB, Sweden), and to Prof. R. Mezzenga (Universityof Fribourg, and Nestl e Research Center, Lausanne, Switzer-land) for valuable discussions and for providing selected guresand research results. The author is indebted to Prof. U. Pl ockerand Dr. A. Kobus (both Degussa AG) for continuous support.

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