Crystal hardness and average distance between stable entanglements in melt crystallized polyethylene

Microhardness of a series of linear PE samples with a wide range of molecular weights has been determined. The influence of the mode of crystallization from the melt on the mechanical properties has been examined. It is shown that crystal hardness is correlated to the thickness of the amorphous layer for each series of samples. A new expression which describes the microhardness (H) of polyethylene as a function of the average distance between stable entanglements (knots) s is proposed. A value of s = 360 Angstrom, for the mean length between knots in the network is obtained. Analysis of data reveals that the H value can be expressed in terms of the following morphological parameters: 1) average number of chain folds in a lamellar crystal, 2) the nature of the interface between the crystalline and the amorphous layer and 3) the linear crystallinity.