Orientation crystallization of polymers

Abstract

The important point made in the last section was the discovery in the systems of structures of the usual folded kind and of morphological elements which consist of orientation-crystallized and probably straightened macromolecules the presence of which can be predicted from the results of a thermodynamic analysis. Structural analyses showed, however, that only a small part of the total crystallizing material can be crystallized in this manner. Even this small amount of the new type of formation contributes significantly to an improvement in the mechanical properties, and for certain polymeric products this improvement may suffice. On the other hand, for products like fibre and fibrillating film the tenacity and elastic modulus should be at maximum so that the problem arises of the causes of the limits on the formation of fibrillar crystals during the process of orientational crystallization. The explanation may lie in the decrease of the molecular orientation in the bulk of the melt after the onset of crystallization when it is reinforced by a network of nascent crystals with straightened chains or a topomorphism in the amorphous state. A way out of this position can probably be found by an analysis of the interrelation between the rates of molecular orientation and crystallization. In the general case, however, the problem of orientational crystallization is still far from being resolved on the physical and, more especially, on the technological level so that a significant effort in this direction is called for.