Enhanced Heat Deflection Resistance via Shear Flow-Induced Stereocomplex Crystallization of Polylactide Systems

Abstract

Stereocomplex crystals (SCs) of polylactides (PLAs) with melting points over 220 °C show great potential to improve the heat deflection resistance of PLAs. However, it is still a challenge to fabricate PLA materials with high SC contents due to the requirement for high production efficiency and thus an extremely large cooling rate. In the present work, an upgraded injection molding method, i.e., oscillation shear injection molding (OSIM), was employed to impose intense shear flow on poly( l -lactide) (PLLA)/poly( d -lactide) (PDLA) samples. It is proved that even though a large cooling rate existed, the intense shear flow provided by OSIM induced higher crystallinities of the SCs and well-defined lamellar structure in comparison with conventional injection-molded ones, which subsequently resulted in a high Vicat softening temperature (close to 200 °C) and superb heat deflection resistance in boiling water. To clarify the mechanism of shear-induced SC formation, in situ characterization with precisely controlled parameters was done by performing rheological measurements. A more sensitive response of SC crystallization kinetics to shear flow is observed compared to that of shear-induced homocrystallites, which is attributed to shear-induced stereoselective interaction and the existence of a transiently cross-linking network built through hydrogen bonds in sheared PLLA/PDLA melts. These findings provide an effective method to prepare PLA samples with promising heat deflection resistance without introducing any extra component and reducing its environmental friendliness.