Multiscale analysis on multiextruded poly(lactic acid)/organoclay nanocomposites: Insights into the underlying mechanisms of thermomechanical degradation

Due to the growing market and the relatively high price of poly(lactic acid) (PLA), the possibility to recycle PLA has attracted a tremendous attention, aiming to reduce the environmental impact related to the life cycle of PLA-based products. However, the comprehensive structural and properties changes of PLA and PLA/clay nanocomposites induced by reprocessing are still not clear. In this work, PLA and its organoclay nanocomposites (PCN5) were subjected to repeated extrusion for 1 and 4 cycles to reveal their recycling characteristics by the observation of the changes in the microstructure, thermal and mechanical properties. Gel permeation chromatography, rheology and morphology analyses were utilized to deeply explore effects of the cycle number on the evolution of chemical structures and clay dispersion. It is found that reprocessing leads to a significant reduction in the weight average molecular mass of PLA by 24.0% and of PCN5 by 37.3%, whereas a more uniform dispersion of clay and a stronger clay-polymer interaction are also found after reprocessing. A direct correlation between the molecular chain scission and deterioration of thermal/mechanical properties is observed both for PLA and PCN5 subjected to reprocessing cycles. The crystallization rate of PLA is suppressed by the addition of clay, while is accelerated by clay after their processing. Additionally, the presence of clay is found to reduce the differences in the thermal stability between the samples reprocessed after 1 and 4 cycles, which is ascribed to the competition between intensive chain scission and better dispersion of clay in the PLA matrix.