SANS Evidence of Liquid-Liquid Phase Separation Leading to Inversion of Crystallization Rate of Broadly Distributed Random Ethylene Copolymers

Aiming to understand the inversion of crystallization kinetics observed by DSC, detailed SANS investigations of the melt structure of a broadly distributed ethylene-l-hexene copolymer have been undertaken in a wide range of temperatures that were reached either by heating the solid or cooling from the homogeneous melt state. In both cases, the observed SANS signal transitions from a scattering cross section consistent with a homogeneous melt state (high temperature range) to an intensity that in the low Q region displays the characteristics of the Porod region for particles dispersed in a homogeneous matrix (low melt temperature range). The latter structure is consistent with demixing of the highly branched molecules and corroborates the postulated liquid-liquid phase separation (LLPS) as an explanation for the peculiar crystallization kinetics observed by DSC. The solution temperature is found at 160 degrees C by heating the solid and at 150 degrees C when cooling from the one-phase melt, thus denoting the effect of copolymer crystallization assisting LLPS kinetics. Irrespective of the path taken to approach the melt, SANS gives evidence of the liquid-liquid phase transition while crystallization by DSC is only sensitive to LLPS when cooling from self-nucleated melts.