Thermal-oxidation degradation of polylactic acid/cellulose nanocrystal composites: Effects of surface chemistry

Cellulose nanocrystal (CNC) is one of the most attractive renewable nano-fillers for polylactic acid (PLA). Depending on the production routes and starting cellulose sources, CNC possesses different surface chemistry, surface charge density, and morphology. However, the effect of the CNC surface chemistry on the thermal-oxidation degradation behaviors of PLA/CNC composites is still elusive, although it is a crucial factor in determining the hot processing and service of composites. In this work, sulfated (CNC-S), carboxylated (CNC-O), and uncharged CNC (CNC-Cl) were produced and compounded with PLA via a two-step masterbatch method, respectively. The thermal-oxidation degradation behaviors of the obtained composites were systematically investigated to elucidate their degradation mechanisms. Results show that the surface uncharged CNC produced by hydrochloric acid hydrolysis contributes to the thermal-oxidation stability of composites, but is more prone to agglomerate due to the lack of electrostatic repulsion. Compared with uncharged CNC, negatively charged CNCs (CNC-S and CNC-O) appear better dispersion state in the PLA matrix. However, PLA/CNC-S composites present relatively lower weight loss temperature and degradation activation energy due to the acceleration of desulfation process on thermal-oxidation degradation. Contrary to expectations, the addition of carboxylated CNC has almost no negative effect on the thermal oxygen stability of PLA because the decarboxylation reaction during melt-processing converts carboxyl groups to volatile by-products. This study sheds light on the thermal-oxidation degradation mechanism of PLA/CNC composites and will be a valuable reference for further research into the degradation of other bio-based polymeric materials.