Information about research done by the Brassart Research Group, Department of Engineering Science, University of Oxford

Chemo-mechanics of biodegradable polymers

Our group’s research on biodegradable polymers combines experimental characterisation with physics-based constitutive modelling.

Biodegradable polymers are materials that are designed to degrade, and eventually disappear, after they have fulfilled their intended structural function. They are increasingly being used for a variety of applications, including packaging, coatings, agriculture, and biomedical implants. To accurately predict the behaviour of these materials over long time scales, it is critical to understand the underlying chemo-mechanical couplings. One the one hand, mechanical properties (stiffness, strength, and toughness) evolve during degradation as the molecular weight decreases due to chemical degradation. On the other hand, mechanical forces impact the kinetics of diffusion and degradation. In our group, we conduct experiments to probe the effect of environmental conditions (water, temperature, applied loads) on the degradation and mechanical properties of degradable plastics, like PLA. We also develop continuum constitutive models that couple hydrolytic degradation to the large deformation, elasto-viscoplastic response of the polymers.

• Constitutive modelling of hydrolytic degradation and viscoplasticity in glassy polymers: an effective temperature approach.
• Shear yielding and crazing of dry and wet PLA at body temperature. 
• A reaction-diffusion framework for hydrolytic degradation of amorphous polymers based on a discrete chain scission model. 

Multiscale modelling of rubberlike materials

Many soft materials, such as elastomers and hydrogels, are made of long chain molecules crosslinked to form a three-dimensional network. The mechanical properties of these materials depend on the response of the individual chains, as well as the topology of the network. In our group, we develop modelling strategies at the chain, network and continuum scales. At the continuum scale, we develop thermodynamically consistent theories for coupled deformation and mass transport. At the network scale, we develop computational discrete network models which explicitly resolve the network structure to investigate the role of heterogeneities and network defects on the deformation and failure response. Such discrete networks can be viewed as ultra coarse-grained models, in which chains are replaced by entropic springs interacting via shared crosslink points. At the chain level, we develop statistical mechanics to better describe energetic contributions to the chain response, including the energy due to bond stretching and bond angle opening, and their impact on the rate of chain scission. Our ultimate goal is to upscale these statistical mechanics model to the macroscopic scale, preserving the essential physics but enabling large-scale simulations. 

• On tube models of rubber elasticity: fitting performance in relation to sensitivity to the invariant I2. 
• Constitutive modelling of hydrolytic degradation in hydrogels.
• The role of chain stretch heterogeneity on the uniaxial failure response of rubbery networks.
• Stretching response of a chain with deformable bonds.

Micromechanics of heterogeneous materials

Our group has expertise in the micromechanical modelling of composites, including inclusion- and fibre-reinforced composites, porous materials, and multiphase materials. Modelling techniques include full-field finite element simulations on Representative Volume Elements of the microstructure, as well as advanced mean-field homogenisation theories in elasto-viscoplasticity. The latter are suited for implementation in finite element simulations at the structure scale. Recent contributions have focused on developing homogenisation techniques for transient diffusion problems and for sintering aggregates. With colleagues from Monash University and RMIT, we are also interested in modelling the behaviour of architected materials, such as brick-and-mortar structures, using analytical models and finite element simulations, validated by experiments.

• Modelling the effect of layer strength distribution on the brick-and-mortar failure regimes and properties.
• Effective transient behaviour of heterogeneous media in diffusion problems with a large contrast in the phase diffusivities.
• Bounds for shear viscosity in Nabarro-Herring-Coble creep. 

Mechanics of Li batteries

Mechanical degradation is a key factor limiting the deployment of high-capacity electrode materials for Li-ion batteries. During the charge and discharge of a battery, lithium atoms are inserted and extracted from the electrode, causing large volume changes and mechanical stresses which can eventually lead to fracture. Our group has expertise in the modelling of coupled diffusion and large elasto-viscoplastic deformation in high-capacity anode materials, such as silicon. Our recent work has proposed constitutive model for amorphous silicon, wherein plasticity is mediated by shear transformation zones. Recently, we have developed a new model coupling creep and diffusion for all-solid-state batteries. This project is in collaboration with colleagues in the Department of Materials.  

• The impact of magnesium content on lithium-magnesium alloy electrode performance with argyrodite solid electrolyte.
• A theory for coupled lithium insertion and viscoplastic flow in amorphous anode materials for Li-ion batteries.
• Cyclic plasticity and shakedown in high-capacity electrodes of lithium-ion batteries.