The origin of memory effect in stereocomplex poly (lactic acid) crystallization from melt state

Three arm poly(propylene oxide) block poly(D-lactide) block poly(L-lactide) (PPO-PDLA-PLLA) stereoblock copolymers melted at various melting temperature and recrystallized. The role of crystallization memory effect of stereocomplex structures was demonstrated utilizing in situ STIR and WAXD. There is a critical temperature (T-cr) for detachment of stereocomplex structures of melt samples. When the melting temperature (T-melt) is lower than T-cr, crystallization temperature (T-c) of samples decreases with T-melt increasing. If the T-melt is higher than T-cr, the samples' T-c is as a constant temperature. We attribute the decrease in T-c to crystallization memory effect and correlate it with the detachment of stereocomplex structures in melt. When T-m < T-melt < T-cr, the stereocomplex crystallites melt, but stereocomplex segments could survive. The remaining stereocomplex segments play as the self-seeds and reduce the crystallization barrier energy. The recrystallization is the same process as a general self-seeds crystallization. Influence of melt time and molecular weight on the stereocomplex behavior in melt were also investigated.