Nanoclay and crystallinity effects on the hydrolytic degradation of polylactides

Polylactide (PLA)-montmorillonite micro- and nanocomposites based on semicrystalline and amorphous polymers and unmodified and organomodified clays at 5 wt% content were produced by melt mixing and subjected to accelerated hydrolytic degradation over a temperature range of 50-70 °C. Degradation rate constants were higher for amorphous PLA and its composites than semicrystalline PLA and its composites as a result of increased permeation through the amorphous domains. Since the effective pH of the nanofillers and their hydrophilicity change through treatment with organomodifiers the degradation rate constants of the nanocomposites were significantly higher than those of the unfilled polymers; by contrast, those of the microcomposites were lower or slightly lower than those of the unfilled polymers possibly due to the reduction of the carboxyl group catalytic effect through neutralization with the hydrophilic alkaline filler. Although the degradation rate constants increased with increasing temperature from 50 to 70 °C, based on calculated activation energies the degradation kinetics did not differ significantly above and below the assumed T_g of 58-60 °C. Higher activation energies were observed for the semicrystalline polymer and its composites. It appears that bulk hydrolytic degradation starts from the interface between polymer and fillers for all samples resulting in significant morphological differences between nanocomposites, microcomposites and unfilled polymer.