Pseudohexagonal packing, chain conformation

One modification of polyethylene (PE), which appears to be stable at high temperatures and pressures (see Sect. 3.2), also presents a pseudohexagonal packing and a disorderd chain conformation [59],... 

Polymorphism is observed relatively frequently in long-chain macromolecules, for which approximately isoenergetic structures exist. The stable crystal form of poly(ethylene), for example, possesses an orthorhombic lattice, but on elongation, triclinic and monoclinic modifications are observed. Three modifications are known in it-poly(propylene) a (monoclinic), P (pseudohexagonal), and y (triclinic). Since the molecules are in a 31 helix conformation in all the modifications, differences in the packing of the chain must be responsible for this polymorphism. The three modifications appear at varying crystallization temperatures. In it-poly(butene-l), however, the various modifications correspond to different kinds of helix, so that variations in conformation must be important (see also Table 5-5). 

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