The effect of cross-linking on the density and melting of Polythene

Abstract

The density of Polythene has been studied in relation to temperature and the degree of cross-linking produced by pile irradiation. Cross-linking results in a progressive reduction in the percentage of crystalline material at room temperature. The transition temperature corresponding to melting in ordinary Polythene is only very slightly decreased with increasing cross-linking, so that the temperature at which all crystalline structure vanishes is little affected. However, because of the reduced amount of crystalline material present in the cross-linked polymer, the magnitude of the change in density at the transition temperature is considerably lessened. There are certain anomalies between the transition temperature deduced from density measurements and from other methods. The density of cross-linked Polythene in the amorphous state is found to obey a law very similar to the van der Waals law for gases: { P + P o /1- β ( c )}{ V ( c ) - V o } = RT/M , where P o is the internal pressure arising from binding forces and V o is a constant corresponding to a ‘residual’ volume per gram at 0°K; both are independent of the degree of cross-linking c . P represents the external pressure, V(c) the specific volume (or volume per gram) of cross-linked polymer, and M the molar weight 28 (—CH 2 —CH 2 —). β(c) is a function approximately equal to c .