Exploring the crystallization-induced mesophase evolution in an olefin block copolymer through a rationally designed two-step isothermal crystallization strategy

Abstract

The competition between crystallization and phase separation has always been a major concern for semi-crystalline block copolymers. In this work, in order to examine the mesophase evolution during crystallization in an olefin block copolymer (OBC), a “two-step isothermal crystallization” strategy was designed elaborately: the OBC samples were first isothermally crystallized at different temperatures (T_c) to create diverse crystallization histories; after melting, the second crystallization process was carried out to detect the degree of mesophase separation change during the first crystallization. It was quite interesting that the first crystallization at a lower T_c displayed little influence on the second crystallization process, while the first crystallization at a higher T_c could obviously accelerate the subsequent crystallization. In combination with the observation of the crystalline morphologies and phase morphologies in the melt, we speculated that hard blocks mainly crystallized separately during relatively rapid crystallization with a large quantity of nuclei; while crystallization-induced aggregation of hard blocks occurred during slow crystallization with a limited number of nuclei, changing the distribution of the hard blocks and accelerating the subsequent crystallization as a consequence. This work contributes to the understanding of the phase evolution during crystallization of OBC and such multi-block copolymers under different circumstances.