Heterogeneous distribution of entanglements in the polymer melt and its influence on crystallization

Recently we demonstrated that it is possible to obtain single chains forming single crystals, where chains are adjacently re-entrant. It is feasible to melt these crystals, either by simple consecutive detachment of chain stems from the crystalline substrate or by cluster melting, where several chain stems are involved. The consecutive detachment of chain stems occurs at the melting point predicted from the Gibbs-Thomson equation, whereas the cluster melting takes place at much higher temperatures. Melting by consecutive detachment of chain stems from the crystal substrate and their diffusion in the melt ultimately results in a new melt state having a heterogeneous distribution of physical entanglements. Because of differences in the transverse relaxation times of the chains in the entangled and disentangled domains, solid-state NMR is the technique used to follow differences in the molecular mobility of the two domains. In this paper, with the help of solid-state NMR, we follow the mechanism involved in the development of the heterogeneous melt state. Observations are that the entangled and disentangled domains are maintained at higher temperatures resulting into a thermodynamically nonequilibrium melt state. On cooling, the heterogeneous melt influence of entanglements on the initial stages of crystallization is followed. It is found that the disentangled chains segments crystallize faster than the entangled chains, which is suggestive for the homogeneous nucleation to occur faster than the heterogeneous nucleation. Rheological studies are performed to follow the influence of disentangled domains on crystallization. With the increasing number of entanglements per unit chain, the time required for the onset of crystallization increases. In the sample from the same batch having lesser number of entanglements per unit chain, the crystallization time can be reduced by a decade.