Dissociative covalent adaptable networks from unsaturated polyesters

Unsaturated polyesters (UPEs) are a class of thermosetting resins which are prevalent in the polymer industry, offering excellent durability in many applications. However, their permanent crosslinks prevent re-use or recycling at their end-of-life. This problem is addressed using UPEs to generate dissociative covalent adaptable networks (CANs). Poly(propylene maleate) and poly(propylene fumarate) were copolymerised with furan-functionalised crosslinkers to form networks comprising reversible Diels-Alder cycloadducts. The thermal properties of each material were analysed by differential scanning calorimetry (DSC), showing that the furan-maleate networks dissociated at 130 °C compared to the furan-fumarate networks which dissociated at a significantly lower temperature (100 °C). The network formed from poly(propylene maleate) using a flexible trifunctional furan crosslinker provided the largest increase in Tg and TrDA, producing the highest gel fraction (99 %) and a stable plateau modulus of 10 MPa by DMA. The networks reform at ambient temperature, although the rate of formation can be increased significantly by mild heating at 65 °C. Overall, the results indicate that furan-maleate CANs behave like reference furan-maleimide networks. Thermal analysis indicated that the furan-maleate cycloadduct was retained by heating to 150 °C, whereas isomerisation of maleate to fumarate was observed after repeated heating cycles to 180 °C. Mechanical testing showed that a bond strength of almost 10 MPa was achieved using these CANs as adhesives, which could be readily modified through changes in copolymer composition.