Interplay between a Strong Memory Effect of Crystallization and Liquid–Liquid Phase Separation in Melts of Broadly Distributed Ethylene–1-Alkene Copolymers

Abstract

Narrow metallocene-made random ethylene copolymers display a strong memory effect of crystallization above their equilibrium melting point akin to the melt memory effect observed in model ethylene–1-butene copolymers. The onset temperature for self-nucleation or surviving self-seeds displays a bell shape with increasing comonomer content with a maximum at ∼2 mol % branches. Self-seeds do not survive at temperatures above the equilibrium melting point for homopolymers and copolymers either with very low branching or with a branching content >4.5 mol %. The self-seeds are associated with clusters of ethylene sequences that remain in the melt in close proximity and accelerate a subsequent crystallization, as observed by higher crystallization peak temperatures and higher nucleation density. Contrasting this behavior, commercial ethylene–1-alkene copolymers with a broad, bimodal comonomer distribution display an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show a continuous acceleration of the rate. The inversion demarcates the onset of a self-seed assisted liquid–liquid phase separation (LLPS) between comonomer-rich and comonomer-poor molecules. The interplay between number of self-seeds and chain diffusion during LLPS causes a decrease in the crystallization rate with decreasing melt temperature. When crystallites remain in the melt at temperatures <123 °C, the crystallization rate again accelerates quickly. The effect in nucleation density and in overall crystalline morphology of crystallization from one-phase homogeneous melts (region A), one-phase heterogeneous melts (region B), and two-liquid-phase melts (region C) was followed by polarized optical microscopy, transmission electron microscopy, and atomic force microscopy.