PHBA, homopolymer

Fig. 6a-c. Electron diffraction pattern of the PHBA homopolymer (DP > 100), c-axis diffraction a before heating, RT b at 375 °C, furnace temperature c after cooling to RT... 

From a DSC scan of the homopolymer of PHBA one can observe a major endotherm at 350 °C and a much smaller one at 445 °C (see Fig. 3) [29], The first transition has been examined by electron diffraction [27-30], X-ray diffraction [28] and proton and 13CNMR [30]. Additional insights have been provided by synthesizing much lower molecular weight samples which permit study of these transitions at appreciably lower temperatures [29]. These low molar mass homopolymers can also go into a nematic phase under a modest shear. 

Turning to the low temperature transition of the homopolymer of PHBA at 350 °C, it is generally accepted that the phase below this temperature is orthorhombic and converts to an approximate pseudohexagonal phase with a packing closely related to the orthorhombic phase (see Fig. 6) [27-29]. The fact that a number of the diffraction maxima retain the sharp definition at room temperature pattern combined with the streaking of the 006 line suggests both vertical and horizontal displacements of the chains [29]. As mentioned earlier, Yoon et al. has opted to describe the new phase as a smectic E whereas we prefer to interpret this new phase as a one dimensional plastic crystal where rotational freedom is permitted around the chain axis. This particular question is really a matter of semantics since both interpretations are correct. Perhaps the more important issue is which of these terminologies provides a more descriptive picture as to the nature of the molecular motions of the polymer above the 350 °C transition. As will be seen shortly in the case of the aromatic copolyesters, similar motions can be identified well below the crystal-nematic transition. 

Fig. 5a-b. Nematic texture of the homopolymer PHBA a DP 15, specimen quenched from 440 °C, no external shear applied b DP > 100, specimen quenched from above 480 °C, external shear was applied to increase the rate at which the fluid specimen sheared between the glass surface above... 

The potential for rapid randomizing processes in the copolyesters at elevated temperatures has been demonstrated conclusively by heating a mixture of the two homopolymers of PHBA and PHNA at 450 °C at a pressure of around several hundred psi [40]. Within a few seconds a viscous melt was observed to extrude from the cracks in the mold. Analysis of this material showed a structure consistent with the random 50/50 copolymer of HBA/HNA (see Figs. 18 and 19). We estimate that at this very high temperature the rate of interchain transesterification reactions corresponds to 1000 ester interchanges/chain/10 s. 

Figure 7. Comparison of DSC Spectra a) HBA/HNA (50/50) Copolymer Annealed for 16 hrs at 250°C, b) Single Crystals of the Alternating (HBA-HNA) (50/50) Copolymer, c) Homopolymer of PHBA or PHNA.

By regression analysis of the isothermal TGA data. Crossland et al. [31] got the overall Ea at different weight loss stages for homopolymer of pHBA and two copolyesters from pHBA, 4,4 -biphenol with TA and mPA, respectively. The E values of all three polymers showed an increase at the beginning stage of weight loss in N2, and this increase was ascribed to the cross-linking and stabilization... 

In Figure 10 is shown the carbon-13 spectrum obtained from a polycrystalline sample of the homopolymer of p-hydroxybenzoic acid (PHBA). The spectrum is resolved into three distinct resonances of 1 1 5 relative intensities with the resonance of five-fold intensity having partially resolved components. The various carbons of the repeat unit are readily assigned to specific resonances on the basis of model compound studies and cross-relaxation studies (see below). Clearly, the... 

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