Smart Polymers for Performance and Sustainability

Polymers form the foundation of materials used across many applications. However, despite their versatility and essential functions, polymers are running into some tough challenges. Industries today are demanding materials to perform in use and be more sustainable while remaining cost effective. To meet these current demands, researchers are stepping up to discover new chemistries and smarter ways to create these materials. Thermoplastic polymers provide adequate performance in a range of applications but fail in demanding environments where creep resistance and solvent resistance are necessary. Thermosetting polymers provide the best performance in structural applications providing high mechanical strength, dimensional stability and durability since they are crosslinked by permanent bonds forming a strong network. Unsaturated polyesters (UPEs) are a prevalent class of thermosets which find application in construction, transport, marine and wind energy markets. UPEs are commonly sold as a liquid resin using styrene as a reactive diluent which forms the crosslinks upon initiation. These materials are highly durable when in service, but they cannot be reshaped or recycled at their end of life since crosslinking is permanent and irreversible. Chemical recycling involving pyrolysis or solvolysis is possible, but these methods are currently inefficient and expensive. Therefore, a major challenge is finding the right balance between performance and environmental impact, especially for thermosetting polymers. This new research paper in European Polymer Journal – conducted by Jonathan Gregg, James Wilson, and led by Professor Andrew Slark from the University of Sheffield in collaboration with Steven Brown from Scott Bader Company Ltd – aims to target new crosslinked polymers which perform in use and can be triggered to depolymerize at their end of life to enable recycling. These Covalent Adaptable Networks offer a more sustainable alternative to irreversible networks.

The researchers began by synthesizing UPEs via copolymerization of maleic anhydride using different polymerization methods, where the molecular weight was readily controlled. Independently, multifunctional furan crosslinkers (MFCs) were synthesized from commercially available precursors. All of the UPEs and MFCs were made using commercially available monomers by industrially relevant techniques where reagents were reacted in bulk without solvents. All reactions proceeded to high conversion in one step with high atom efficiency and no side products. NMR, Infra-red Spectroscopy, Mass Spectroscopy and Size Exclusion Chromatography all confirmed the composition of the UPEs and MFCs. These components were then simply added together in bulk to make a homogeneous molten mixture which was then cooled to allow crosslinking reactions between the UPEs and MFCs. Copolymerization and crosslinking could be performed at ambient temperature or accelerated by mild heating at 65 °C. The crosslinked networks were evaluated by various characterization techniques. Soxhlet extraction showed that the optimal networks were robust with high solvent resistance and a gel fraction of 99%. Thermal analysis by Differential Scanning Calorimetry demonstrated that the crosslinked network dissociated with bond breaking occurring at 130 °C. This was corroborated by Dynamic Mechanical Analysis which showed a large reduction in stiffness during dissociation. This was further proven by rheological measurements, illustrating that melt viscosity was low (20 P) after network dissociation, which allows facile reprocessing of the material. Critically by heating the material to 150 °C and cooling to room temperature, the crosslinked network was reformed. Dissociation of the crosslinked network by heating and reformation by cooling was achieved multiple times. By continuously cycling between low temperature and high temperature, the material repeatedly switched between a crosslinked network and a reprocessable low viscosity melt. The materials were also designed with Safety, Health and Environment in mind. The UPEs and MFCs were both made in 1-pot reactions without solvent or free monomer. The crosslinked networks were also made without harmful monomers like styrene used for classic thermosets. The range of potential polymer compositions is broad, meaning that it is possible for material properties to be varied in a facile manner and finely tuned to meet a range of application requirements. In principle, in future the UPEs and MFCs could also be obtained from bio-based monomers, potentially reducing the reliance on fossil fuels and lowering the carbon footprint.

In conclusion, the new study led by Professor Andrew Slark and his team addresses the need to develop materials to have high performance and be more sustainable while remaining cost effective. The smart design of unsaturated polyesters and multifunctional furan crosslinkers enables materials which crosslink at low temperature yet depolymerise at high temperature, enabling reprocessing and recycling. This can be achieved multiple times. The unsaturated polyesters, multifunctional crosslinkers and crosslinked networks are made in bulk via industrially relevant techniques and a range of compositions are potentially feasible. Overall, this makes the approach facile, versatile and cost-effective.