The strong memory of crystallization observed in melts of random ethylene copolymers has been attributed to self-seeds that remain in the melt as memory of the copolymer’s ethylene sequence partitioning during crystallization. When cooled from seeded melts, the free energy barrier for nucleation of copolymer crystals decreases with a profound increase in crystallization rate. Cooling ethylene copolymers from their melts at a constant rate, the increase in crystallization temperature is observed even at temperatures 65 degrees above the observed melting point and at 30 degrees above the equilibrium melting temperature. This unusual strong melt memory is attributed to the topological constraints that build in the intercrystalline regions during crystallization. The effect of molar mass, long chain branching, and degree of crystallinity on increasing strength of melt memory support the correlation between melt memory and entanglements. As the entanglement molecular weight of isotactic polypropylene is about four times higher than for polyethylene, the number of entanglements per chain is much reduced; consequently, self-nucleation of random propylene copolymers is expected in a melt temperature range comparatively closer to the observed melting point. Self-nucleation data for metallocene-based isotactic polypropylene and a series of random propylene-ethylene copolymers give further evidence for the role of melt viscosity on melt-memory.

Effect of melt viscosity on self-nucleation of ethylene- and propylene-1-alkene random copolymers

253rd National Meeting (2017)

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