Abstract
A strong memory effect of crystallization has been observed in melts of random ethylene copolymers even above the equilibrium melting temperature. Melt memory is correlated with self-seeds that increase the crystallization rate of ethylene copolymers. The seeds are associated with molten ethylene sequences from the initial crystals that remain in close proximity and are unable to diffuse quickly to the randomized melt state. Fast diffusion is restricted by topological chain constraints (loops, knots, and other entanglements) that build in the intercrystalline region during crystallization. The effect of topological constraints on melt memory, or on number of remaining self-seeds in the melt, is analyzed studying the melting and subsequent crystallization of a model ethylene 1-butene copolymer with 2.2 mol% ethyl branches prepared with different levels of crystallinity. There is a critical threshold level of crystallinity of 6-12% to observe the effect of melt memory on the subsequent crystallization rate of this copolymer. A faster development of the initial crystallinity may more efficiently trap knots and loops around the crystallites, leading to a lower crystallinity threshold than for slow or isothermally crystallized copolymers.