Molecular Weight Dependence of Primary Nucleation Rate of Polyethylene I. An Extended Chain Single Crystal

Molecular weight (M) dependence of the primary nucleation rate (I) of an extended chain single crystal of polyethylene crystallized at high pressure (P=0.4 GPa) was studied. We obtained for the first time an experimental formula that I∝M−1 which we named “power law of primary nucleation”. We showed a well-known experimental formula that I=I0exp(−κ/ΔT2), where I0 is a constant proportional to the diffusion constant of molecules (D), κ is related to the activation free energy for forming a critical nucleus (ΔG*) and ΔT is the degree of supercooling. We showed that only I0∝D decreases with increase in M and κ does not depend on M. From this results we concluded that M dependence of I is mainly controlled by the “chain sliding diffusion” process not by the formation process of a critical nucleus, that is, I(M)∝D(M)∝Ps(M) and ΔG*=const. Here D can be regarded as an “expanded diffusion constant” and Ps is a survival probability defined in “chain sliding diffusion” theory presented before by one of authors (MH). Based on these results we proposed a “chain sliding diffusion theory of primary nucleation” that the primary nucleation is a process of “chain sliding diffusion” within the nucleus which requires disentanglement of molecular chains within the interface between the nucleus and the melt. The theory explained well the observed power law.