Study On Crystallization Property Of PLA/MWNTs Composite

Both poly(lactic acid)(PLA) and multi-walled carbon nanotubes (MWNTs) coated nano-SiO2and grafted silance couple agent KH-550were selected as matrix and modifier respectively to prepare PLA/MWNTs composites by solution blending. Fourier Transform Infrared Spectroscopy (FTIR) and scanning electron microscope (SEM) were used to research the effect of modification for MWNTs and dispersion of MWNTs, respectively. Differential scanning calorimeter (DSC) and polarizing microscope (POM) were used to study crystallization and spherulitic morphology of PLA and PLA/MWNTs composites. Avrami equation, Laurizten-Hoffrnan theory, Jeziorny model, Fireman model and JMA equation were used to analyze the isothermal crystallization kinetic, non-isothermal crystallization kinetic and cold crystallization kinetic for PLA and its composites. The results show that:(1) The agglomeration of MWNTs was prevented and smaller apparent size was maintained by surface coating modification. The modified MWNTs dispersed rather well in PLA matrix and formed stronger adhesion with the matrix.(2) For the non-isothermal crystallization, MWNTs can act as a nucleating agent to significately increase the crystallization rate and crystallinity of PLA/MWNTs composites via enhancing the nucleation activity and reducing activity energy of crystallization of PLA.(3) For the isothermal crystallization, the Avrami exponent n and the half time of crystallization t1/2of PLA/MWNTs composites decrease distinctly and the overall crystallization rate increase, which suggested that the growth mode of PLA spherulites changed gradually to two-dimensional. The increase of chain folding work indicates that MWNTs are helpful to form more stable nucleation and beneficial to the crystallization of PLA.(4) For the cold crystallization, MWNTs can decrease the crystallization temperature obviously, increase the crystallization rate and crystallinity simultaneously. The higher activity energy of spherulites growth for PLA/MWNTs composites from JMA model means that MWNTs have both nucleation and hindering effects for PLA crystallization. The nucleation of MWNTs is much more than that of hindering effect, which increases the overall crystallization rate of the composites.(5) The spherulitic size of PLA from melt cooling crystallization decrease with the increase of MWNTs concentration. The spherulitic size of PLA from the cold crystallization is dependent on the dispersion state of MWNTs in the matrix, which is the minimum at0.3wt%of MWNTs.