Orientational relaxation together with polydispersity decides precursor formation in polymer melt crystallization

We studied the athermal relaxation of bulk extended chains and the isothermal crystallization in an intermediately relaxed melt by means of dynamic Monte Carlo simulations of lattice-polymer systems. The melt contained a memory of chain orientations but no more crystalline order. We found that the athermal relaxation is continuous and homogeneous among the bulk chains, and its rate depends on the chain length. The orientational memory in the melt significantly enhances the rate of crystallization. Nevertheless, no precursor of crystallization occurs in the melt of uniform chain lengths. But, in a binary blend of different chain lengths, the crystallization of oriented long chains acts as the precursor to induce epitaxial crystallization of the relaxed short chains. This mechanism explains the formation of shish-kebab crystals observed frequently in the processing of semicrystalline polymers. The results suggest that in flow-induced polymer crystallization the orientational relaxation of chains decides a selection of the long-chain component and then the precursor formation.