Effect of organic montmorillonite on cold crystallization and hydrolytic degradation of poly(l-lactide)

Abstract

The effect of organic montmorillonite (OMMT) on cold crystallization of poly(l-lactide) (PLLA) in different conditions including during the DSC heating process and during the annealing treatment was evaluated. The variations of crystalline structure of PLLA in different samples were comparatively investigated by using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The results show that OMMT exhibits an apparent nucleation effect for the cold crystallization of PLLA matrix. During the annealing treatment, the degree of crystallinity (Xc) increases with the increase of OMMT content. Specifically, the presence of OMMT leads to the formation of α΄-form crystallites. Study on the hydrolytic degradation behaviours of different samples shows that the hydrolytic degradation rate decreases with the increase of Xc, but it increases with the increase of OMMT content. The enhanced hydrophilicity and the formation of α΄-form crystallites are suggested to be the main reasons for the greatly improved hydrolytic degradation ability of samples which contain high content of OMMT. Furthermore, hydrolytic degradation also induces the change of microstructure of the samples. For pure PLLA, hydrolytic degradation promotes the increase of Xc. However, for PLLA/OMMT nanocomposites, hydrolytic degradation leads to the decrease of Xc and the degree of the decrease is dependent on the content of OMMT.