Degradation and Crystallization Studies of Branched PLA Prepared by Reactive Extrusion
This thesis proposes strategies to improve the crystallinity and mechanical properties of poly(lactic acid) (PLA) and investigates the effects of these modifications on hydrolytic degradation. The effect of long-chain branching (LCB) on the hydrolytic degradation at 60°C was monitored through the mass loss, molar mass distributions, and thermal properties of degraded specimens. A three-week induction period prior to the onset of mass loss coupled with an immediate loss in molar mass of over 80% in the same timeframe pointed to a bulk erosion mechanism. The highest loss in molar mass was observed in the z-average molar mass (Mz) of LCB PLA, exceeding 90% in the first three-weeks, and was attributed to the cleavage of the LCB segments from the polymer chain. Degradation-induced crystallinity resulted from the enhanced chain mobility at the experimental conditions, owing to the combined influence of annealing and the plasticizing effect of water. Although the hydrolysis profile differed between the linear and branched PLAs, branching did not affect negatively the extent of degradation over a 12-week period. Increases in the crystallinity of PLA from 5% to 20% were achieved through reactive extrusion and the addition of biofiller (BF), a novel type of cross-linked PLA-based nucleating agent. The crystallinity of the nucleated formulations was further increased to over 50% by annealing at temperatures between 80 and 120C. The BF additive was particularly effective in improving the crystallinity due to its PLA-based nature, which provided good compatibility with the matrix material. Owing to the improved crystallinities, the annealed materials demonstrated over 30% increases in flexural moduli compared to the neat material, while impact strength was maintained. Annealing of the modified PLA also resulted in 10°C increases in the glass transition temperatures followed by improvements in the heat deflection temperature (HDT) by as much as 7°C. An annealing temperature of 100C was selected as optimum, due to evidence of thermal degradation taking place above this temperature. When subjected to hydrolysis over a 12-week period, the nucleated and annealed PLA degraded to the same extent as the untreated PLA, with over 30% loss in mass and 90% loss in molar mass.