Unexpected crystalline memory effect in poly(l-lactide)-based block copolymers

The crystalline memory effect is frequently observed in polymer crystallization, but the melt structure remains unresolved. In this work, the preserved spherulites in the melt of poly(l-lactide)-based block copolymers are observed by polarized optical microscopy (POM) unexpectedly. These spherulites are not erased instantly on reaching the melting temperatures, but it takes tens of minutes at a designed melting temperature to completely erase the spherulites. These preserved spherulites in the melt, where some crystalline traces are lost but the Maltese cross appearance is still obvious, are possibly attributed to the long range of molecular orientation enhanced by microphase separation between two blocks. Moreover, these preserved spherulites have a significant effect on the subsequent crystallization, i.e. a faster radius growth rate and denser nucleation. This study highlights that a thermodynamically unstable structure can be fully preserved in a supercooled melt for a long time, which has never been reported elsewhere.