Effects of crosslink density on hydrolytic degradation of poly(l-lactide)-based networks

We have studied the accelerated degradation of three crosslinked poly(l-lactide) (PLLA) networks in 0.1 M NaOH at 37 °C. It was found that the degradation rate depends strongly on the molecular weight between crosslinks (Mc). Compared to a linear PLLA reference sample, all networks displayed much slower mass loss. A clear relationship was found to exist between the rate of mass loss, swelling and Mc. The network with the greatest Mc (3500 g/mol), underwent the fastest mass loss throughout the 4 week study, while the networks with the smallest Mc (1400 g/mol) showed very slow mass loss, with only 28% loss after 4 weeks. SEM images of degraded samples clearly showed that degradation only occurred close to the surface in the sample where the Mc was 1400 g/mol. However, results also showed that with increasing Mc the degradation occurred further into the bulk of the network. The linear PLLA sample appeared to hydrolysis homogeneously. 1H NMR analysis of the soluble degradation products revealed that when the Mc was 1400 g/mol, the succinate groups were released from the networks preferentially to the pentaerythritol groups. The delayed release of pentaerythritol suggests that the degradation products of the star prepolymer cores are trapped within the matrix for longer than the degradation products produced from other regions of the network.