Strong Memory Effect of Crystallization above the Equilibrium Melting Point of Random Copolymers

We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (T-c). Following expectations, the T-c of unbranched PE samples was constant and independent of the initial melt temperature The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the T-c of copolymers with higher molar mass depends on the temperature of the initial Melt, T-c increases as the temperature of the melt decreases. We attribute the increase in T-c to a strong crystallization memory in the melt above the equilibrium melting and correlate this phenomenon with remains in the melt of the copolymer's crystallizable sequence partitioning. Albeit molten, long crystallizable sequences 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 restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. Erasing memory of the prior sequence selection in copolymer melts requires much higher temperatures than the theoretical equilibrium value The critical melt temperature to reach homogeneous copolymer melts (T-onset), and the comonomer content at which melt memory above the equilibrium melting vanishes are established. The observed correlation between melt memory, copolymer crystallinity and melt topology offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers.