Polymer chain scission, oligomer production and diffusion: A two-scale model for degradation of bioresorbable polyesters

This paper presents a computer model for the biodegradation of polyesters such as poly(lactic acid), poly(glycolic acid) and their copolymers. The model can take polymer details such as molecular weight distribution, different end and random scission rates and copolymer ratio as input data. A multi-scale approach is developed: polymer chain scission and oligomer production which occur at the molecular scale are modelled using a kinetic Monte Carlo scheme, oligomer diffusion which occurs at the device scale is modelled using a diffusion equation, and the two are connected at the finite difference nodes of the diffusion equation. The two-scale model can be used to predict the temporal evolution and spatial distribution of molecular weight distribution in a device as well as the weight loss as a function of time. It is shown that the kinetic Monte Carlo scheme can accurately predict the effect of copolymer ratio on the degradation rate. Grizzi and co-workers observed in their experiments that a PLA film 0.3 mm thick degrades much more slowly than one that is 2 mm thick. The numerical study shows that the conceptional reaction diffusion model suggested by Grizzi et al. needs to be extended in order to explain the size effect fully.