Influence of graphene nanosheets on stereocomplex crystallization behaviors of star-shaped poly (D(L)-lactide) stereoblock copolymer

The nanocompsites of star-shaped poly(D-lactide)-co-poly(L-lactide) stereoblock copolymers (s-PDLA-PLLA) with two-dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X-ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s-PDLA-PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0 degrees C formed in isothermal crystallization for all samples. The crystallization temperatures of s-PDLA-PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3wt% GNS was about 128.2 degrees C, ie, 15 degrees C higher than pure s-PDLA-PLLA. At isothermal crystallization processes, the halftime of crystallization (t(0.5)) of the sample with 3wt% GNS decreased to 6.4minutes from 12.9minutes of pure s-PDLA-PLLA at 160 degrees C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three-dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s-PDLA-PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s-PDLA-PLLA with good nucleation ability in the s-PDLA-PLLA material.