Effect of natural weather on the structure and properties of polylactide/Cloisite 30B nanocomposites

Abstract

The degradation of polylactide (PLA)/Cloisite 30B nanocomposites under natural weathering was investigated as a function of clay loadings (1, 3 and 5 wt.%) for up to 130 days using Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography (SEC), nanoindentation measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). For comparative purposes, the neat PLA was also considered. The FT-IR results showed that the photo-oxidation mechanism of PLA was not modified in the presence of Cloisite 30B, but only the degradation rates were accelerated. Moreover, the photo-oxidative degradation of PLA nanocomposite samples led to the formation of vinyl unsaturation, carbonyls, anhydrides and hydroperoxides groups as a result of the occurrence of several chemical mechanisms simultaneously. The decrease of the weight-average molecular weight, and the number-average molecular weight associated with an enhanced polydispersity of the nanocomposite samples indicated that chain scission was the most prominent phenomenon in natural weathering. The thermal degradation of the PLA was faster in the presence of clay. Modulus and hardness measured by nanoindentation increased slightly with exposure time for both neat PLA and PLA nanocomposite samples; the increase is also a function of the clay content. Finally, the weathering effect on the morphology of exposed samples observed by SEM revealed that the fractured surfaces exhibited many voids and cracks. These defects were much more pronounced for the PLA nanocomposites.