Molecular weight dependence of the lateral growth rate of polyethylene 2. Folded-chain crystals

Abstract

Molecular weight ( M) dependence of the lateral growth rate ( V) of folded chain crystals (FCCs) of polyethylenes (PE) was investigated. This study was carried out on single (or single crystal-like) crystals using the equilibrium melting temperature ( T m 0) determined by applying Gibbs-Thomson's equation. The well-known relation V= V 0 exp(− B/Δ T) was obtained where V 0 and B are constants and Δ T is a degree of supercooling. V 0 strongly decreased with increase of M, whereas B did not, which indicates that the self-diffusion process of polymer chains mainly controls the M dependence of V, whereas the nucleation one does not. Experimental formula that V∝ V 0∝D∝ M − H where D is self-diffusion constant and H is a constant; H=1.7 was obtained. These results are similar to Hoffman et al.'s results but their H was rather smaller, H=1–1.5. A similar study on extended-chain single crystals (ECSCs) reported in our previous paper gave the same experimental formula but H was much smaller, H=0.7. From the difference in H between FCCs and ECSCs, a new proposal that M dependence of V may be mainly controlled by the surface diffusion process of chain polymers on the growing crystal surface, is discussed briefly.