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

Abstract

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 FTIR and WAXD. There is a critical temperature (Tcr) for detachment of stereocomplex structures of melt samples. When the melting temperature (Tmelt) is lower than Tcr, crystallization temperature (Tc) of samples decreases with Tmelt increasing. If the Tmelt is higher than Tcr, the samples’ Tc is as a constant temperature. We attribute the decrease in Tc to crystallization memory effect and correlate it with the detachment of stereocomplex structures in melt. When Tm<Tmelt<Tcr, 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.