Control of thermal degradation of polylactide/clay nanocomposites during melt processing by chain extension reaction

Polylactide (PLA) and PLA/organo-modified clay nanocomposites were found to experience severe thermal degradation during processing via internal mixer and twin-screw extruder. Rheological dynamic time sweep measurements were used to monitor the thermal degradation. The organo-modified clay was believed to accelerate the PLA matrix thermal degradation due to the presence of the organo-modifier. The acceleration effect of the organo-modified clay was highly dependent on the exfoliation extent, with a higher exfoliation extent resulting in a more intense thermal degradation. Three reactive chain extenders, hexamethylene diisocyanate (HDI), pyromellitic dianhydride (PMDA) and Joncryl® ADR-4368, were mixed to the PLA matrix during processing. It was found that Joncryl and HDI had a much higher chain extension reactivity than PMDA, and that they substantially increased the molecular weight of the PLA matrix. The addition of 0.5 wt% Joncryl during twin-screw extrusion increased the complex viscosity from 4700 Pa s for the neat PLA to about 23,000 Pa s. The chain extender HDI has bi-reactive end groups per molecule; therefore, extended PLA molecules with a mostly linear structure were obtained. Since Joncryl is a multi-functional reactive polymer, the molecular structure of the PLA matrix changed from linear to a branched or cross-linked structure depending on Joncryl loading and processing temperature, as confirmed by H1-NMR analysis. Joncryl also showed a remarkable thermal stabilization effect on PLA/organo-modified clay nanocomposites containing up to 6 wt% of Cloisite® 30B without negatively affecting the clay dispersion.