Effect of Heterogeneous Short Chain Branching Distribution on Acceleration or Retardation of the Rate of Crystallization from Melts of Ethylene Copolymers Synthesized with Ziegler-Natta Catalysts

摘要

Ethylene 1-alkene copolymers with the same molar mass, same molar mass distribution and very similar average comonomer content of 3.8 +/-0.4 mol % but different broad interchain comonomer content distribution (CCD), are found to display different extents of melt memory effect on crystallization rate. With decreasing the initial melt temperature, all broadly distributed copolymers show enhanced crystallization followed by a decrease of crystallization rate. However, copolymers with a higher content of molecules in the medium composition range (6-2 mol%comonomer) and a lower content of lightly branched molecules (<2 mol% comonomer), display the highest effect of memory of a prior crystallization in their high temperature melts. The increase in crystallization peak temperature is up to 4 ℃. The acceleration and further retardation of the crystallization are associated with self-seeds that remain in one-phase or two liquid phases melts respectively. The magnitude of the effect on crystallization is proportional to the number of surviving seeds in the initial melt.