Confinement crystallization of poly(L-lactide) induced by multiwalled carbon nanotubes and graphene nanosheets

Two sets of multiwalled carbon nanotubes (MWCNTs)/PLLA nanocomposites and graphene nanosheets (GNSs)/PLLA nanocomposites with various MWCNTs and GNSs loadings prepared by a melt blending approach were investigated in terms of both nonisothermal and isothermal crystallization behaviors. The results indicated that MWCNTs and GNSs not only acted as heterogeneous nucleating agents for PLLA crystallization but also restricted the mobility and diffusion of PLLA chains. At low MWCNTs and GNSs concentrations, the nucleation effect of MWCNTs and GNSs was dominant to achieve accelerated overall crystallization kinetics. As the MWCNTs and GNSs concentration rose up to 2.0 and 2.5 mass%, respectively, the MWCNTs and GNSs network structures were formed in the PLLA matrix, which were proved by solid-like rheological behavior at low frequencies in rheological measurement. With further increasing concentration of MWCNTs above the critical concentration, an enhanced nucleation density but an almost unchanged overall crystallization rate for the MWCNTs/PLLA nanocomposites indicated that the expected increase in the crystallization promoting effect from more MWCNTs was offset by some confining effect. However, for GNSs, the formed network structure provided a more severely confined space for PLLA crystal nucleation and growth in contrast to MWCNTs, resulting in the decreased nucleation density and retarded crystallization rate.