Nucleation Site Memory in the Spherulite Films of Polydisperse Poly(l-lactic acid)

Abstract

Molecular weight distribution (MWD) is a unique feature of polymers, which endows various properties of polymer materials with broadly regulatable scopes. The various chain dynamics and crystallization behaviors of different molecular-weight polymers coexisting within the same polydisperse system would lead to unique crystallization phenomena and aggregating structures. Recently, an MWD-induced polymorphism texture of poly(l-lactic acid) (PLLA) is reported, called nested PLLA spherulites, where one crystal form is embedded inside the other. Here, an interesting nucleation site memory phenomenon is reported for the PLLA thin films, where the nucleation sites of spherulites recover around the initial positions, even after multiple melting–crystallizing cycles. The nucleation site memory strongly associates with solution processing and the nested spherulites. After the impact of heterogeneous nucleation is excluded, it is demonstrated that the nucleation site memory originates from MWD. The insights provided in this work deepen the fundamental understanding of the influence of MWD on the structure evolution during polymer processing.