Thermal and Environmentally Induced Degradation Behaviors of Amorphous and Semicrystalline PLAs Through Rheological Analysis

Abstract

The degradation behaviors of an amorphous and a semicrystalline PLA (i.e., aPLA and cPLA) with similar molecular weights are compared at elevated temperatures and after being treated under various environmental conditions through using small amplitude oscillatory shear rheological experiments. The degradation behaviors are also studied by differential scanning calorimetry and Fourier-transform infrared spectroscopy analysis. Melt thermal degradation analysis shows that the d-lactic acid content does not affect the degradation behavior of PLAs. This is while the increase in temperature beyond 190 °C dramatically, but still similarly, increases the degradation rate of aPLA and cPLA samples. It is also shown that processing of aPLA through a twin-screw extruder at temperatures below 190 °C and at different screw speeds has a negligible effect on its thermal degradation. The degradation of the PLA samples treated under various humidity levels as well as when exposed to different environments such as tap and sea water, regular garden soil and commercial grade microbial fertilizer, is expedited more dramatically around and beyond the glass transition temperature. This is while the cPLA, with around 50% crystallinity, reveals a much less degradation under the aforementioned conditions. This is because the bulk diffusion of water molecules or micro-organisms is harder in cPLA with a compacted crystalline structure. It should, however, be emphasized that the fertilizer degrades the PLA samples much faster and more pronounced than other environments. The tap and sea water exhibits a similar effect on the degradation of PLA samples. Notably, the temperature is the most effective parameter in expediting the degradation rate of PLA, suggesting that the PLA degradation could gradually occur in the environments with extreme warm weather. The increase in humidity also accelerates such degradation.