Crystallization and polymorphic behaviour of melt miscible blends of crystalline homopolymers with close melting temperatures under confinement

Abstract

The confined crystallization behavior of the crystalline/crystalline blends of poly(ethylene oxide) (PEO) and poly(butylene adipate) (PBA) in the electrospun nanofibers fabricated from the ternary blends of polystyrene (PS), PEO, and PBA, where PS was present as the majority component, was investigated. The electrospun nanofibers had PS as the matrix whereas the PEO/PBA mixed phase, with total weight fraction of 0.2 or less, constituted the dispersed domains having sizes of the order of only few hundred nanometers. Hence, the crystallization induced phase separation of PEO and PBA occurred within the confined domains. Despite extremely close melting temperature of PEO and PBA, faster crystallization kinetics of PEO led to entrapment of PBA in the intraspherultic regions of the former. Hence, further crystallization of PBA occurred under hierarchical confinement where the larger and smaller length-scale confinements were imposed by the glassy PS matrix and PEO crystals, respectively. Such a scenario resulted in crystallization occurring both via homogenous as well as heterogeneous crystallization mechanism. Interestingly, the homogenous nucleation of PEO was suppressed whereas the heterogeneous nucleation was enhanced in the ternary blend nanofibers even at very low weight fraction. Most significantly, it was observed, that during melt-recrystallization, memory of chain orientation is retained which favoured the formation of α-crystals in PBA. The present study highlights the role of confinement as well as chain orientation, on the crystallization behaviour, in crystalline/crystalline blend systems for which limited studies have been done so far.