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Chapter 14

Advanced Solution Characterization of Polycarbonate Materials across the Molecular Weight Distribution 1,3

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Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: March 8, 2005 | doi: 10.1021/bk-2005-0898.ch014

Arno Hagenaars , Bernard Wolf , and Christian Bailly 1

Laboratoire des Hauts Polymères, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium Institut für Physikalische Chemie der Universität Mainz, Jakob-Welder 2

Weg 13, 55099 Mainz, Germany Current address: GE Plastics, Plasticslaan 1, 4612PX Bergen op Zoom, The Netherlands

3

The microstructure of polycarbonate homo- and copolymers is investigated across the molecular weight distribution with the help of narrowly dispersed fractions obtained by Continuous Polymer Fractionation (CPF). The experimental distribution of branching units in melt-polymerized polycarbonate is compared with predictions obtained from a simulation model. Experiments and simulations with mixed fractions are also used to study redistribution processes affecting melt-polymerizedpolycarbonate. The applicability limits of C P F for the fractionation of copolymers is evaluated for the first time by a study on PC-siloxane copolymers. Copolymers with statistically distributed segments can be fractionated without compositional bias only to the extent that all chains have the same average composition, which is not the case for chains carrying a small number of long comonomer segments.

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© 2005 American Chemical Society In Advances in Polycarbonates; Brunelle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Many important chemical and physical characteristics of polymers containing structural heterogeneities (e.g. branching units, co-monomers or functional groups) are strongly dependent on details of the molecular microstructure. Unfortunately, this effect is often compounded with the influence of molecular weight distribution (MWD). It is therefore highly desirable to gain access to the distribution of microstructural features across the M W D instead of relying on one "global" structure index. It is even more interesting to obtain sizable amounts of narrow molecular weight fractions since it then becomes possible to test the fractions separately or in combination and hence clarify the interplay between microstructure, molecular weight and properties. While structure-property relationships have been thoroughly investigated for industrially important classes of polymers such as polyolefins and polyesters based on the characterization of narrowly dispersed fractions obtained from preparative fractionation techniques like Temperature Rising Elution Fractionation (TREF) and Successive Solution Fractionation (SSF), no such systematic studies have been carried out for polycarbonate materials. BisphenolA based polycarbonate (BPA-PC, Figure 1) is an especially interesting polymer to study for a number of reasons. Firstly, it is an important thermoplastic engineering material which has uses in a wide range of applications including building and construction, automotive, electronic and consumer products. Secondly, B P A - P C can be made via different polymerization routes and its chemical microstructure can be modified in various ways through incorporation of branching units, co-monomers and functional groups. Hence, a large variety of microstructurally different P C materials is readily available for research. In addition, the amorphous structure of B P A - P C simplifies its chemical characterization.

Figure!:

BPA-PC structure.

In Advances in Polycarbonates; Brunelle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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BPA-PC synthesis Currently, B P A - P C is produced on an industrial scale either by interfacial phosgenation or melt-transesterification processes [1]. In the interfacial process, an aqueous solution containing B P A and an organic solvent containing phosgene are mixed, whereupon polycarbonate is formed at the interface between the two immiscible phases. In the melt-transesterification process, B P A and diphenyl carbonate (DPC) are reacted as pure components under reduced pressure and at high temperatures. The melt-transesterification technology has several advantages over the interfacial process since the use of large volumes of organic solvents and highly toxic phosgene is avoided. Moreover, the final resin contains a much lower level of residual contaminants and has a more consistent colour. In contrast, the elevated temperatures used during the melt-polymerization process may lead to side reactions resulting in the formation of branching products. The most widely accepted mechanism for thermal rearrangements of P C in the melt is a base-catalyzed Kolbe-Schmitt reaction [2]. The following structures have been shown to result from this mechanism: phenyl salicylate (PhSAL, structure I), phenyl salicylate phenyl carbonate (PhSALPhC, structure II) and phenyl-0phenoxy benzoate (Ph-o-PhxBz, structure III) [2-4]. The corresponding P C structures are shown in Figure 2. PhSALPhC structures are being formed upon reaction between the hydroxyl group of a PhSAL unit and a carbonate group of an arbitrary polycarbonate chain. Ph-0-PhxBz structures are being formed in a concerted rearrangement reaction of polymer chains with the concomitant release ofC0 . In addition, interfacially and melt-polymerized PC show two significant structural differences: 2

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Interfacial P C is normally fully end-capped (usually with phenyl carbonate end-groups). B y contrast, melt-polymerized PC contains a significant residual concentration of uncapped end-groups (bisphenol monocarbonate) [2].

2.

Due to fast redistribution reactions occurring during polymerization meltpolymerized P C is characterized by a "thermodynamic" most probable Flory molecular weight distribution (MWD) whereas interfacial polymerization is kinetically controlled, leading to a different (although similar) distribution [1].



Figure 2: Polycarbonate melt rearrangement structures.

Polycarbonate-Siloxane Copolymers Polycarbonate-siloxane (PC-Si) copolymers were first introduced in the mid 1960's and are characterized by an outstanding thermal stability, good weathering properties, excellent flame retardancy and high impact resistance at low temperature [5-9]. PC-Si materials are now used in numerous applications including windows, roofing, contact lenses and gas-permeable membranes. A large group of bisphenol end-capped oligo-siloxanes are known to form useful copolymers with polycarbonate. A particularly interesting polysiloxane block is obtained by reacting hydrosilane-capped oligo-dimethylsiloxane with eugenol (4-allyl-2-methoxyphenol) using the well known hydrosilation reaction. Eugenol is readily available as a synthetic or as a natural product. The resulting α,α> capped polysiloxane can be copolymerized with bisphenol-A to form a PC-Si copolymer [10,11]. The general structure of a PC/Eugenol-siloxane (PC-EuSi) copolymer is shown in Figure 3. PC-Si copolymers can be made via different synthetic routes including interfacial phosgenation and melt polymerization processes [5].


184 HC 3

3

ο X

CH

3

Figure 3: Generic structure of a BPA-Polycarbonate/Eugenol-siloxane copolymer.


Objectives In this paper, we review our recent studies on the fractionation and structural characterization across the M W D of P C homopolymers and siloxane copolymers [12-14]. The assessment of the branching structure distribution across the M W D is discussed for B P A - P C materials made via a melt transesterification process. Experimental results are compared with predictions obtained from a simulation model. Thermally induced processes affecting the M W D of B P A - P C materials made via interfacial phosgenation and melt transesterification processes are also addressed. The general applicability of C P F for the fractionation of copolymers is discussed, based on the results obtained of the PC-siloxane copolymers.

Preparative Fractionation of Polycarbonate In the early years of polymer research, fractionation was mainly applied for determination of the molecular weight distribution of polymers, a property that nowadays can be satisfactorily obtained by using chromatographic techniques such as Size Exclusion Chromatography (SEC). At present, the interest in polymer fractionation mainly lies in obtaining preparative quantities of narrowly dispersed fractions that can be used for further analysis. The large scale fractionation of polymers presents typical problems such as the necessity to use large volumes of solvents, very long fractionation times and the need for specially designed equipment [15-17]. In many cases, a particular polymer can be fractionated by more than one method and its choice depends on the desired objectives of the fractionation, i.e. the number of fractions, fraction distribution and fraction size. Polycarbonate can be fractionated by most common fractionation methods (Table I).


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Table I: Fractionation methods for polycarbonate materials.

Fractionation method

Reference

Precipitation fractionation Film extraction fractionation Stepwise elution column fractionation Baker-Williams fractionation Coacervate fractionation Preparative Size Exclusion Chromatography High Osmotic Pressure Chromatography Continuous Polymer Fractionation

18-31 2,32 33-36 37 33 38 39 40

For the work presented in this paper, narrowly dispersed, multi-gram size fractions of controlled molecular weight were desirable. Continuous Polymer Fractionation fulfills these requirements the best and was selected as method of choice.

Continuous Polymer Fractionation Continuous polymer fractionation (CPF) has been developed as a technique capable of producing large quantities of narrowly distributed molecular weight fractions. Theoretical principles for C P F were first exposed by Englert and Tompa [41] while Wolf and co-workers developed the actual technique in practical form and applied it to different classes of polymers [42,43]. In C P F , fractions of increasing molecular weights are removed from a concentrated polymer solution by a countercurrent liquid-liquid extraction with a single solvent or solvent/non-solvent mixture.

Experimental Samples P C I and PC2 are lab-synthesized polycarbonate samples made by a melttransesterification (MT) process from B P A and diphenyl carbonate, using literature-described procedures [44,45]. PC3 is an industrial sample obtained


186 from an interfacial phosgenation (IP) process. Characterization data for the three samples are given in Table II. PC-SÏL1 and PC-SIL2 are lab synthesized PC/Eugenol-siloxane copolymers obtained from an interfacial phosgenation process. PC-SIL1 contains 10 weight % Eugenol capped siloxane oligomer with a degree of polymerization of 2 (n=0 on average in Figure 3) and PC-SIL2 contains 5 weight % Eugenol capped siloxane oligomer with a degree of polymerization of 23 (n=21 on average in Figure 3). Characterization data for both samples are given in Table III.


Table II: Characterization data for PC homopolymers.

Sample Polym. name process

Mw (g/mol)

Mn (g/mol)

D

Modified units

Hydroxyl Endgroups (ppm)

PhSAL (ppm)

PhSALPhC (ppm)

PCI

MT

21,700

9,520

2.28

n.d.

1250

570

PC2

MT

23,700

9,210

2.57

1795

3465

1185

PC3

IP

23,200

9,270

2.50

n.d.

n.d.

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