Molecular crystals aromatic, packing

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. 

Determination of the crystal structure of phase II by Lonsdale in 1929 unequivocally settled over 70 years of debate concerning the geometry and bonding of aromatic molecular systems. The measured bond lengths and crystal structure of hexamethylbenzene are shown in Fig. 9.6.1. The hexamethylbenzene molecules lie within planes approximately perpendicular to (111). Phase III is structurally very similar to phase II, but differs from it mainly by a shearing process between molecular layers that results in a pseudo-rhombohedral, more densely packed arrangement. 

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