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Research at the Brassart Research Group, Department of Engineering Science, University of Oxford

Research

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, and agricultural films. They are also attractive materials for biomedical implants such as cardiovascular stents and orthopaedic fixtures.  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.

  • Huanming Chen, Zhouzhou Pan, Daohe Yuan, Gregory S. Sulley, Reece N. Oosterbeek, Charlotte K. Williams, and Laurence Brassart. Shear yielding and crazing of dry and wet PLA at body temperature. Under review.
  • Zhouzhou Pan and Laurence Brassart. A reaction-diffusion framework for hydrolytic degradation of amorphous polymers based on a discrete chain scission model. Acta Biomaterialia 167 (2023) 361-373.

Constitutive modelling of soft materials

Many soft materials, such as elastomers and hydrogels, are made of long chain molecules crosslinked to form a three-dimensional network. In our group, we develop micromechanics-based constitutive models to predict the large-deformation behaviour of these materials, explicitly accounting for the response of individual chains and the topology of the network. In particular, we develop computational Discrete Network models, in which key topological features (coordination, chain length) and connectivity defects (dangling ends, loops) are explicitly represented. This approach provides insight into structure-property relationships, and further provides a tool to assess the validity of simplifying assumptions used in analytical constitutive models. We also develop novel continuum constitutive theories for various materials. Ongoing projects focus on the constitutive modelling of thermoplastic elastomers, filled rubbers, liquid crystal composites, and biodegradable hydrogels.  

  • Gordon Kumar and Laurence Brassart. On tube models of rubber elasticity: fitting performance in relation to sensitivity to the invariant I2. Mechanics of Soft Material 5 (2023) 6.
  • Zhouzhou Pan and Laurence Brassart. Constitutive modelling of hydrolytic degradation in hydrogels. Journal of the Mechanics and Physics of Solids 167 (2022) 105016.
  • Ghadeer Alamé and Laurence Brassart. Effect of topological defects on the elasticity of near-ideal polymer networks. Journal of Applied Mechanics 87 (2020) 121006.

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.

  • Georgia Hunter, Lee Djumas, Andrey Molotnikov, and Laurence Brassart. Modelling the effect of layer strength distribution on the brick-and-mortar failure regimes and properties. Mechanics of Materials 187 (2023) 104820.
  • Georgia Hunter, Lee Djumas, Laurence Brassart, and Andrey Molotnikov. Controlling failure regimes in Brick-and-Mortar structures. Extreme Mechanics Letters 51 (2022) 101596.
  • Laurence Brassart and Laurent Stainier. Effective transient behaviour of heterogeneous media in diffusion problems with a large contrast in the phase diffusivities. Journal of the Mechanics and Physics of Solids 124 (2019) 366-391.
  • Laurence Brassart and Francis Delannay. Bounds for shear viscosity in Nabarro-Herring-Coble creep. Mechanics of Materials 137 (2019) 103106.

Mechanics of energy storage materials

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.  

  • Jack Aspinall, Krishnakanth Sada, Hua Guo, Souhardh Kotakadi, Sudarshan Narayanan, Yvonne Chart, Ben Jagger, Emily Milan, Laurence Brassart, David Armstrong, and Mauro Pasta. Light lithium magnesium alloy anodes for solid state batteries. Submitted.
  • Afsar Bagheri, Jamal Arghavani, Reza Naghdabadi, and Laurence Brassart. A theory for coupled lithium insertion and viscoplastic flow in amorphous anode materials for Li-ion batteries. Mechanics of Materials 152 (2021) 103663.