Fractionated crystallization and fractionated melting behaviors of poly(ethylene glycol) induced by poly(lactide) stereocomplex in their block copolymers and blends

Abstract

Stereocomplexation has been an effective way to improve the physical properties of block copolymers or blends. However, the effect of stereocomplexation on the confined and fractionated crystallization of other crystallizable block has not been comprehensively investigated so far. In this work, we used poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) as stereocomplexable higher melting component and studied the confined crystallization and melting behaviors of poly(ethylene glycol) (PEG) in their block copolymers and blends. Unexpectedly, marked fractionated crystallization and fractionated melting behaviors of PEG component occurred during the two-step crystallization process, because of the constrained effect of the early formed stereocomplex (SC) crystals. The fractionated crystallization and melting behaviors were further facilitated with decreasing the isothermal crystallization temperatures of PLLA and PDLA (Tic,PLA). As revealed by polarized optical microscopy (POM), the increase of Tic,PLA favors the transformation of PEG component from the interlamellar to interspherulitic regions due to the reduced nucleation density of PLA. We proposed that fractionated crystallization and fractionated melting behaviors of PEG impacted by SC crystals were attributed to PEG component dispersed in and out interlamellar regions of PLA matrix. Moreover, small-angle X-ray scattering (SAXS) results indicated that two scattering peaks were identified for copolymers after two-step crystallization process, one consisted of mixed PEG and PLA lamellae and the other contained almost pure PEG lamellae originated from the crystallization of PEG dispersed in interfibriller/interspherulitic regions. This study has shed light on the unique fractionated crystallization and fractionated melting behaviors of the lower melting component in polymer systems with the higher melting component preferentially co-crystallized into SC crystals.