Conformational polymorphism energetics

Essentially following the procedural stages outlined in Section 5.8, the authors then computationally investigated the energetics of the conformations observed in the six polymorphic forms. Ab initio calculations at the RHF/6-3IG level were used to explore the conformational energy profile about the N-C (thiophene) bond. The... 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

The calculations for poly-a-butene are reported, as an example, in fig. 1. The E(0, 02) map represented is that one relevant to the possible conformations of left-handed helices. Di ferently from the case of PP (above cited lecture) the energetic minimum is split into two. Correspondigly, chain polymorphism is experimentally observed for PB(2). The different crystalline modifications have s(3/l)l, s(ll/3)l, s(4/l)l chain conformations the loci of points corresponding to such symmetries are also represented in the figure. 

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