Hydrolytic degradation of amorphous PLA: Effect of mechanical loads

Hydrolysis is the primary degradation mechanism of poly(lactic acid) (PLA) in aqueous media. It involves the scission of polymer chains, leading to a gradual reduction in mechanical properties and mass loss. Although the hydrolysis of PLA under various conditions of temperature and pH has been widely studied, the impact of mechanical loads on the degradation rate has comparatively received limited attention. In this study, we investigated the degradation behaviour of initially-amorphous PLA subjected to mechanical loads. We first examined the hydrolysis degradation behaviour at three different temperatures (45 °C, 50 °C and 60 °C) without applied loads to establish its baseline hydrolytic degradation behaviour. The reduction in molecular weight, water uptake and mass loss were measured as a function of degradation time, along with changes in the glass transition temperature and degree of crystallinity. We next characterised the effect of static tensile and compressive loads on the degradation rate at 45 °C. We found that the hydrolysis rate increases with the magnitude of the compressive load when the molecular weight falls below a threshold of approximately 10 kg mol−1. Furthermore, both hydrolytic degradation and mechanical loads significantly contribute to the reduction in yield stress. This PLA also experienced very significant creep during degradation under small loads, suggesting that this polymer is susceptible to creep failure before significant degradation can take place. These results have implications for the prediction of long-term mechanical performance of degradable polymers in load-bearing applications.