Stereocomplex Crystallite-Assisted Shear-Induced Crystallization Kinetics at a High Temperature for Asymmetric Biodegradable PLLA/PDLA Blends

A series of asymmetric biodegradable poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blends with low PDLA compositions was prepared using a solution blending method. The formation of stereocomplex (SC) crystallites in PLLA/PDLA blends was evidenced by differential scanning calorimetry (DSC), as indicated by the melting point of SC crystallites being about 50 degrees C higher than that of PLLA homocrystallites. Isothermal crystallization kinetics under shear conditions at the specific high temperature of 160 degrees C for the PLLA/PDLA blends was investigated using polarized optical microscopy (POM) and rheometry. It was found that the crystallization process of PLLA in the blends was greatly accelerated under shear conditions due to the existence of SC crystallites and the crystallization kinetics of PLLA was promoted with increasing shear rate or shear time. The crystalline morphology remained spherulitic with the spherulitic growth rates unaltered at the applied shear conditions, and the accelerated crystallization kinetics could be attributed to the significantly enhanced nucleation density, for which the extra number of activated nuclei was correlated to shear as a kinetic model to assess the effects of shear on isothermal crystallization kinetics of PLLA/PDLA blends containing SC crystallites. The discrete Maxwell relaxation time spectra at the applied isothermal crystallization temperature of 160 degrees C were used to obtain the reptation and Rouse times of PLLA chains with high molecular masses. Even though the PLLA chains might be orientated under the applied shear, the relaxation time of the blends was still too short to induce any orientated crystal nuclei.