Stereocomplex-affected crystallization behaviour of PDLA in PDLA/PLDLA blends

Abstract

Stereocomplex (SC) crystals usually act as nucleating agents in polylactide (PLA). However, we previously found that PLA homo-crystallization can occur prior to SC crystal formation. In this work, to introduce a small amount of SC crystals into poly(D-lactide) (PDLA) and investigate further their effect on PDLA crystallization behaviours, random poly(L,D-lactide) (PLDLA) copolymers with L-lactyl contents of 89%, 83% and 50% as well as poly(L-lactide) (PLLA) were synthesized and then blended into PDLA. The crystallization behaviours and crystal structures of the blends were characterized by polarized optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction, and in situ wide angle X-ray scattering. Interestingly, the spherulite growth rate G of PDLA is much smaller in the blends in the presence of previously formed SC crystals than in neat PDLA, whereas the nucleation density is higher and the crystallization half-time determined by DSC is shorter, revealing a higher overall crystallization rate due to the nucleating effect of the introduced SC crystals. The zero growth temperatures T_zg of PDLA in the blends were determined as 165 °C, about 20 °C below the equilibrium melting point of PDLA and not affected by SC crystals, revealing the same supercooling degrees. It is proposed that prior formation of SC crystals in the blends enhanced the constraints of the added high molecular weight PLDLA copolymers and PLLA on PDLA chain diffusion, resulting in slow chain conformation adjustments for the following observed slow spherulite growth of PDLA.