Introduction

A characteristic property of amorphous polymers is the ability to sustain large strains. For cross-linked three-dimensional networks the strain is usually recoverable and the deformation process reversible. The tendency toward crystallization is greatly enhanced by deformation since chains between points of cross-linkages are distorted from their most probable conformations. A decrease in conformational entropy consequently ensues. Hence, if the deformation is maintained, less entropy is sacrificed in the transformation to the crystalline state. The decrease in the total entropy of fusion allows crystallization, and melting, to occur at a higher temperature than would normally be observed for the same polymer in the absence of any deformation. This enhanced tendency toward crystallization is exemplified by natural rubber and polyisobutylene. These two polymers crystallize very slowly in the absence of an external stress. However, they crystallize extremely rapidly upon stretching.

It is a widely observed experimental fact that crystallites produced by stretching usually occur with their chain direction preferentially oriented parallel to the axis of elongation. The extent of the orientation will depend on the type and amount of the deformation. This is particularly true for crystallization at large deformations. These observations contrast with the crystalline texture that results when the transformation is induced in the absence of an external stress merely by cooling. In the latter case the crystallites are, on the average, randomly arranged relative to one another. When a portion of a deformed chain is incorporated into a crystallite, the average stress that it exerts at its end points is reduced.