Chain Folding and Free Energy Density in Polymer Crystals

Abstract

Macromolecular chains in freely growing crystals very likely fold back at regular intervals in order to minimize the free energy density of the crystal. The surface energy contribution favors infinite thickness. The smearing out effect of interchain potential field, however, arising from the incoherent part of fluctuation of neighboring chains and increasing with the fold period, favors thin crystals. For sake of simplicity the longitudinal chain vibrations were considered in spite of the fact that in actual crystals the main contribution most likely comes from rotational oscillations. Since the general character of smearing out effect is nearly the same in both cases the present results may be applied at least qualitatively. They fairly well represent the general trend of temperature dependence of polymer crystal thickness and its change with intermolecular forces.