Strain-induced Stereocomplex Formation from the Initially Amorphous Poly(L-lactic acid)/Poly(D-lactic acid) Samples

Abstract

The strain-induced crystallization behaviors of ultrasonic micro-injection molded poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) samples from an amorphous state were investigated by stress-strain relations and in situ wide angle X-ray diffraction (WAXD) measurements. The formation of direct strain-induced stereocomplex (SC) was evident. In samples molded at 50 and 80 °C, this phenomenon can be attributed to the acceleration of the ordered structures due to the existence of a large number of SC nuclei. The SC nuclei are assumed to serve as the transient physical cross-links to initiate the strain-induced crystallization. The onset of strain-induced crystallization is analogous to the heating induced structural reorganization. Consequently, the observed strain-induced SC process can be considered a pseudo process, which is actually thermally induced. Upon further stretching, the actual strain-induced crystallization occurs with the exclusive formation of the homocrystallite (HC), while the preceding formed SC crystals undergo slight fragmentation during subsequent tensile deformation. At 120 °C, due to the reduced number of SC nuclei within the sample, the occurrence of cold crystallization during stretching plays a more significant role than SC nuclei with respect to the strain-induced SC process, as demonstrated by in situ WAXD measurements upon annealing in both the static and stretched states.