This PhD opportunity will be based at Heriot-Watt University, Edinburgh.
Please contact Prof David Bucknall ([Email Address Removed]) for further details.
Summary: Fibre-reinforced thermoplastics are high-performance composites that are finding increasing applications in automobile, sport and construction industries. Increased uptake of these important materials would be enhanced by improvements to production approaches and importantly in the drive to circularity and achieving net zero targets, the ability to recycling materials. To meet these production and recycling challenges, we will explore the inclusion of thermally activated dynamic covalent bonds into the thermoplastic monomers. These dynamic links will provide the ability to repeatedly polymerize and depolymerize the thermoplastic, thereby allowing both easier production and easier separation of the resin from the fibre reinforcements. This PhD will develop novel thermoplastic fibre-reinforced composites that exploit dynamic linkages and is a collaborative project between the materials chemistry group at Heriot-Watt University and the composite engineering group at Edinburgh University.
Background and Context: The rapid growth and exploitation in the use of composites is coupled with a realisation that there is a significant environmental impact associated with the accumulated waste they generate. The nature of the crosslinked thermoset matrices make them unable to be reshaped or reformed once cured and difficult to treat due to their chemical resistivity. By comparison, thermoplastic composites (TPCs) have a number of benefits over thermoset polymer composites not only because of their superior toughness and fatigue resistance, but also because significantly, they offer the possibility of circularity, since they can be readily repaired and remoulded, offering multiple options for reuse and repurposing, as well as offering viable methods for sustainable recycling routes. Thermoplastic composites can in principle be produced using any thermoplastic, potentially offering composites with a massive palette of properties which would be impossible to achieve with conventional thermoset chemistries. Despite these obvious advantages the take-up for TPCs in manufacturing has been slow. In part this is because of challenges to easily produce fibre reinforcement composites. To avoid these issues and increase compatibility with common processing methodologies exploited to make thermosetting composites, low molecular weight oligomers or monomers can be infused into the dry fibres and then polymerized in-situ, as demonstrated with nylon-6 thermoplastics composites.
Whilst TPCs can be readily reshaped after formation, because of the high melt viscosity of the polymer, full recycling of the composite by complete separation of the polymer from the reinforcement is still challenging. A promising route to developing a more environmentally friendly, easy-to-recycle composite is to utilise dynamic interactions (e.g. H-bonding, dynamic covalent bonds or electrostatic interactions) which can trigger the polymerisation/depolymerisation via an external stimulus. An important consequence of exploiting dynamic interactions within the polymer backbone is that the molecular weight and viscosity of the material can be increased or lowered on demand. This allows for simpler separation of the polymer and the reinforcement and allows both the reinforcement and the polymer to be fully recycled without use of large volume of solvents, loss of either component or their properties. In this PhD project we will focus on temperature dependent dynamic covalent bonding.
Research Concept and Objectives: Dynamic covalent bonds (DCBs) have the potential to overcome both technical and commercial challenges to producing fully recyclable TPCs. They can overcome two of the most significant issues to address: separation of the reinforcement from the polymer by cleaving the polymer into flowable low molecular weight chains and improving the adhesion of the polymer to the reinforcement by allowing us to reversibly, covalently bond the polymer to the reinforcement. Many functional groups could potentially be used for DCBs, but we will focus Diels-Alder (DA) reactions, because the reaction forms stable C-C bonds, is 100% atom efficient and it occurs at readily accessible temperatures. It is important that any DCB polymer we develop must retain the characteristic properties of the parent materials and thus be applicable to use in TPCs.