Memory of crystallization in the melt of commercial linear low density polyethylenes processed in an open twin-screw extruder

Abstract

Compared with a constant recrystallization rate of commercial linear low density polyethylenes (LLDPE) as a function of melt temperature, copolymers reprocessed in an open twin-screw extruder, display the expected strong melt-memory features upon melting followed by recrystallization. Precipitation from dilute solution had the same effect on melting-recrystallization of commercial copolymers. In the twin-screw melt re-processing, a small content of long-chain branches and crosslinks are incorporated to the copolymer's structure, but especially a further consumption of additives uncovers self-nucleation features that speed up recrystallization and are related to the major copolymer microstructure. The observed difference in re-crystallization behavior between metallocene type and Ziegler-Natta (ZN) type copolymers, both with equivalent average comonomer content and molecular mass, is due to the inter-chain comonomer distribution. While for metallocene-made and low density polyethylenes, below a critical melt temperature the recrystallization rate increases continuously with decreasing melt temperature, in broadly distributed ZN copolymers there is an inversion of the temperature gradient of the rate at the onset of liquid-liquid phase separation (LLPS). The mass fraction of highly branched molecules required to observe the onset of LLPS via melting followed by recrystallization was evaluated from the TREF profiles, and the effect of self-nucleated melts at different levels on the overall spherulitic morphology was observed via polarized optical microscopy.