Molecular weight dependence of lateral growth rate of polyethylene (I) — an extended chain single crystal

Abstract

Molecular weight ( M) dependence of lateral growth rate ( V) of an extended chain single crystal (ECSC) of polyethylene (PE) crystallized at high pressure ( P = 0.4 GPa) was studied. We obtained a well-known relation that V=V 0 exp(− B ΔT ), where V 0 and B are constants related to a self-diffusion constant of molecules and free energy of forming a critical nucleus and ΔT is degree of supercooling. We showed that V 0 decreases with increasing M and B does not depend on M, which are similar to results reported by Hoffman et al. for a folded chain crystal (FCC) of PE. This indicates that M dependence of V is controlled by the self-diffusion process of molecules, while that is not done by the nucleation process. We obtained an experimental formula, V( M) ∝ D( M) ∝ M − H , where D is a self-diffusion constant and H is a constant, H=0.7. A similar relation has been shown, reported by Hoffman et al. and by us in a separate paper. But the H given by us was larger. H=1.8. It should be noted that the H of a FCC is much larger than that of an ECSC. We will propose a new mechanism from this significant difference on H in a separate paper, that M dependence of V is mainly controlled by the surface diffusion process of chain molecules on a surface of a crystal not by the self-diffusion process within the melt.