Computational modelling framework for predicting the evolution of biodegradable polymers during degradation

Hill, Aoife
Biodegradable polymers are attractive alternatives for many permanent implants, potentially reducing long term risks. Designing optimal devices for their intended purpose, however, is costly and time consuming, with small changes to the sample significantly altering the behaviour. Computational models speed up the design process; however, these models require accurate descriptions of material behaviour and predicting changes in properties during degradation has proved challenging. This thesis aims to develop a computational modelling framework for biodegradable polymers by considering how changes in the material at the microscale affect mechanical properties. First, a kinetic chain-scission model is introduced, predicting the molecular weight distribution evolution as a function of degradation time. A refined kinetic model is also developed which allows for faster solutions with acceptable accuracy. The autocatalytic effect of carboxylic acid ends created via chain scissions and also present in the manufactured polymer are considered. Our framework accounts for molecular weight reduction via the cleavage of monomers from chain ends and from scissions in the middle of the polymer chain. These developments allow for a more complete representation of the molecular weight distribution during degradation. In the second phase of the thesis, the relationship between the molecular weight distribution and ductility of the material is explored. During degradation, biocompatible polymers exhibit a transition from ductile to brittle behaviour. Following a review of the literature, trends in this phenomenon are identified and failure criteria are considered. These predictions can offer insight into material failure, particularly at advanced stages of degradation. Finally, the effect of finite chain extensibility on the mechanical behaviour is investigated. As degradation proceeds, short chains build up in the system and may fully extend at moderate strains. We investigate how this contributes to elasticity changes during degradation and, thus, predict the evolving stress-strain behaviour. Results indicated that molecular weight distributions and proper treatment of fully extended chains should be considered for more accurate predictions of the mechanical properties in degrading samples. The models introduced in this thesis are investigated alongside existing experimental data, with predictions strongly supported by experimental observations of degrading biocompatible polymers. Ultimately, the framework presented offers predictions for evolving molecular weight distributions and evolving stress-strain behaviour, including the point of failure, throughout degradation.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland