Thermal Recycling of a Diels-Alder Functional Epoxy: Evolution of Thermomechanical Properties

An epoxy network bearing Diels–Alder functional groups (e.g., furan and maleimide) was synthesized and degradation of its thermomechanical properties over repeating thermal recycling was observed. The reverse reaction allows the cross-links to break apart at elevated temperatures and reform when cooled. However, a thermally initiated side reaction occurs, consuming free maleimide via homopolymerization. This reaction results in permanent crosslink sites and a lowered number of reversible crosslinks, thus reducing recyclability. Addition of a radical scavenger is partially helpful enabling the network to be recycled once more. Remarkably, when using a new bismaleimide with ethyl and methyl substituents (EM2M), the recyclability is improved and it enables repeated recycling up to a dozen cycles. The EM2M samples exhibit significantly delayed onset of the maleimide homopolymerization both in temperature-sweeps and isothermal runs in rheometry. FTIR analysis found that the rate of maleimide homopolymerization was slower in EM2M. Therefore, the network using EM2M can be held at elevated temperatures for longer times without losing recyclability. Mechanical properties such as viscoelastic modulus, tensile strength, and elongation at break were stable over the twelve cycles. However, a thirteenth cycle was not possible. This sudden change is attributed to the accumulation of non-reversible bonds formed during previous recycling processes.