Chapter 10 Synthesis and Microstructure of Aromatic Copolyesters 1
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J. Economy , R. D. Johnson , J. R. Lyerla , and A. Mühlebach
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Downloaded by CORNELL UNIV on September 21, 2016 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch010
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University of Illinois, Urbana, IL 61801 IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 CIBA-Geigy AG, Forchungszentrum, 180.053, CH-1701, Fribourg, Switzerland 3
A comprehensive interpretation of the microstructure of the liquid crystalline aromatic copolyesters is presented. The role of the synthetic route and of high temperature processing on the microstructure are clearly defined. As a result of this study a predictive model now exists which permits interpretation of the very subtle chemical processes which can occur at elevated temperatures leading to either randomization or ordering of the microstructure. The preparation of the homopolymers of p-hydroxybenzoic acid (HBA) and of 2hydroxy-6-naphthoic acid (HNA) have been described in the literature (1.2) and both have been shown to polymerize as single crystals. However, reports on the synthesis of copolyesters of HBA with either HNA, polyethylene terephthalate (PET), or biphenol terephthalate (BPT) are not well documented in the published literature. Considering the commercial importance of these melt-processible copolyesters of PHB A it is surprising how little progress has been reported on relating the synthesis to the microstructure of these copolymers. Some of the problems associated with the study of these systems arisefromtheir low solubility in most solvents, which greatly limits use of techniques aimed at relating microstructure to the synthetic pathway. An additional complicating factor is the potential for transestenfication reactions during polymerization and subsequent processing which may also influence the microstructure. In fact, there appears to be some confusion in the case of liquid crystalline polyesters as to whether they are stable in the nematic melt randomize(4). or undergo crystallization induced ordering £5}. In this paper, some of our recent studies in this area are combined to provide a reasonably comprehensive picture of the reactions which occur during polymerization, and subsequent processing(2.6.7.8). The HBA/HNA 50/50 copolymer system was selected for this particular study because of its solubility and relative ease of synthesis. Transesterification Processes during Polymerization The HBA/HNA system is perhaps the simplest system to study among the various commercial systems, since the monomers can either be polymerized in solution or 0097-6156/90/0435-0129$06.00A) © 1990 American Chemical Society Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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LIQUID-CRYSTALLINE POLYMERS
in the melt to low molecular weight oligomers which can be further characterized by NMR techniques. In contrast, preparation of the HBA-PET system proceeds via the transesterification of a melt of PET and the acetoxy benzoic acid monomer(9V The complexity of this reaction has been described recently in a study(lO) showing significant variations in composition and microstructure of the 60/40 PHBA/PET copolyester. In the case of the HB A-BPT polymerization, the initial reaction is heterogeneous suggesting that the oligomers would tend to be blockylll) In our study of the HBA-HNA copolymer we chose to carry out the reaction in an inert high boiling hydrocarbon solvent (Therminol 66). The degree of reaction could be followed directly by measuring the evolution of acetic acid. As a starting point, we examined the kinetics of homopolymerization of the two monomers i.e. p-acetoxy benzoic acid and 6-acetoxy-2- naphthoic acid. There already were published data(12) on the kinetics of polymerization of HBA showing a zero order rate process, as one might predict for a polymerization which is carried out almost completely in the solid state. We were surprised to find that the HNA behaved almost identically, precipitating early in the polymerization at a DP - 4-5 in the form of single crystals, so that most of the reaction was carried out in the solid state(2). As shown in Figure 1, the HNA polymerizes in the solid state two times faster than the HBA suggesting that the acetate end group in the growing HNA polymer is better positioned to react and or to diffuse out through the openings in the structure. On the other hand, the rate of conversion of the 1/1 HBA-HNA mixture proceeds in solution at a rate slightly lower than that for the HBA. A plot of 1/monomer concentration (shown in Figure 2) indicates that the kinetics for copolymerization are best described as 2nd order kinetics. A study of the reactivity ratios of the two comonomers was also undertaken; however, analysis of the oligomer with DP < 6 was complicated by solubility problems which made it difficult to isolate samples representative of the actual composition at these low DPs. To get around this problem the polymerization was run to 90% completion (based on acetic acid). C ^ NMR analysis of the copolymer showed the same ratio of monomers as in the starting mixture indicating in the absence of transesterification that the reactivity ratios were the same. End group analysis showed an M value of 2000. In addition, analysis of the diad sequences in this polymer showed a distribution of the four possible diads identical to what one might predict for a random copolymer. The above conclusion on the role of reactivity ratios on microstructure assumes the absence of rapid transesterification reactions between chains. Since such processes might also tend to randomize the microstructure, it seemed important to isolate the role of interchain transesterification. A unique experiment was designed in which a 13c labeled carbonyl in acetoxy benzoic acid monomer (B*) was reacted with the dimer of HBA-HNA. At 99% C enrichment, the only resonances in the carbonyl region of the spectrum will arisefromthe enriched benzoic acid carbonyl. In the absence of any interchain transesterification the microstructure of the polymer would consist only of B*-B dyads (see scheme 1). On the other hand presence of B*-N dyad in the polymer could only arisefroma transesterification reaction where the B* was inserted between the -BN-. As shown in Figure 3, ^ C NMR displays a small but distinct peak (ca.14%) at the resonance position corresponding to B*-N diad. Since the polymerization was run at the same temperature of 245°C for 170 min and to the same degree of polymerization (M ~2252) as the earlier experiment on the reactivity ratios, one can conclude that the role of interchain transesterification is relatively small and that the monomers have approximately equivalent reactivity ratios. The reaction of B* with the HBAHNA dimer was also examined at 225° and 285°C to determine the possibility of a temperature effect. Thetimesof reaction were 20 hrs and one hour, respectively, n
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Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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