Crystallization in Stretched Polymer Networks. I. trans-Polychloroprene

Abstract

Abstract Changes in tensile stress afford a simple means of studying the rates of crystallization and the melting temperatures in crosslinked polymers subjected to simple extension. The form and magnitude of the stress changes in networks of trans-polychloroprene are closely similar to those observed for cis-1:4-polyisoprene and cis-l:4-polybutadiene networks. They are in accord with the formation of oriented crystallites and incompatible with folded chain crystallization at extensions as low as 15 per cent. It seems likely that the present networks do not crystallize by chain folding even in the unstretched state. The large increases in rate of crystallization with extension are approximately accounted for by corresponding increases in the equilibrium melting temperature. Direct measurements of the melting temperature show similar rises with extension. The rise in melting temperature is in good agreement with Flory's theoretical treatment of oriented crystallization at extension ratios of three and higher (when the crystallite orientation is complete) for three networks having different degrees of crosslinking. When referred to a constant segmental mobility, namely, that obtaining at Tg+50° C, the rates of crystallization at various extensions obey a common dependence upon the degree of supercooling. This relation is in fair accord with theories of nucleation kinetics, except at the lowest temperatures where there is some indication of the appearance of a new crystal form.