Polyethylene motional phase disorder

Comparison between the various condis crystals shows that large variations in the amount of conformational disorder and motion is possible even in similar molecules. The tritriacontane in the condis state possesses about 3 gauche conformations per 100 carbon atoms. For cyclodocosane which is in its transition behavior similar to the tetracosane of Fig. 23, one estimates about 16 gauche conformations per 100 carbon atoms, and for the high pressure phase of polyethylene (see Sect. 5.3.2), one expects 37 gauche conformations per 100 carbon atoms 171). The concentration of gauche conformations in cyclodocosane and polyethylene condis crystals are close to the equilibrium concentration in the melt, while the linear short chain paraffin condis crystals are still far from the conformational equilibrium of the melt. 

The discussion of the behavior of eondis crystals of flexibte linear macromolecules and some of the homolc ous oUgcaners covers a witte range of cdisorder-effects and accompanying motion. For many of the macromolecules in the eondis phase chain-extension afto- erystallization with chain folding is possible [polyethylene, polytetrafluOToethylene, poly(vinylidene fluoride), polydilorotrifluoro-ethylene, some aliphatic nylons and polyphosphazenes]. Stx h extended ehain crystals are exceptionally stable and elose to equlibrium. 

For polyethylene and polytetrafluOToethylene the eondis phase could be traced to the oligomeric homolc ues with s %cial effects due to chain ends (paraffins) and whole molecule rotations (pOTfluoroallcanes). While the plastic crystal phase in cyclo-alkenes (Sect. 3.2) and substituted benzenes (Sect. 5.1.1) is restricted to dynamic disorder of a single conformer about rotation axes normal to the piaMS of the molecules, perfluorobutaM and perfluorohexane have dynamic disordOT restricted to motion about axes parallel to the molecular axis. 

Conformationally disordered crystals (condis crystals) were discovered in the 1980 s. They show positional and orientational order, but are partially or fully conformationally mobile. The condis crystals complete the comparison of mesophases in Figs. 2.103 and 2.107. Linear, flexible molecules can show chain mobility that leaves the position and orientation of the molecule unchanged, but introduces large-amplitude conformational motion about the chain axis. Again, the symmetry of the molecule is in this case increased. Condis crystals have often a hexagonal, columnar crystal structure. Typical examples of condis crystals are the high-temperature phase of polyethylene, polytetrafluoroethylene, frawj-1,4-polybutadiene, and the low-temperature phases of soaps, lipids and other liquid-crystal forming, flexible molecules. 

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