Metallocene-made linear low density polyethylenes and commercial ethylene 1-alkene copolymers display a strong memory effect of their crystallization path even at temperatures above the equilibrium melting point. On cooling from different melt temperatures above equilibrium, the crystallization temperature (Tc) shifts to higher values, while for the homopolymer Tc remains unchanged. We correlate this phenomenon with remains in the melt of self-seeds from the copolymer’s crystallizable sequence partitioning. Albeit molten, long ethylene sequences that form the initial crystallites remain in the copolymer’s melt at a close proximity, lowering the change in free energy barrier for nucleation. The residual sequence segregation in the melt is attributed to a topology of loops, links and ties that generate in the intercrystalline regions after sufficiently large crystallinity content evolves. This topology restricts the copolymer crystalline sequences to diffuse upon melting and to reach the initial random melt, and hence affects a subsequent crystallization. The effect on melt memory of copolymer molar mass, comonomer content, comonomer distribution, and level of crystallinity has been analyzed by DSC, x-ray, TEM and optical microscopy. The strong effect of melt memory on crystallization rate can be used to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPEs, and it can also identify liquid-liquid phase separation in broadly distributed copolymers.

Melt memory of crystallization and melt structure of random ethylene copolymers

251st National Meeting (2016)

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