Cleavable epoxy networks using azomethine-bearing amine hardeners

This work is a proof-of-concept of the use of azomethine-bearing diamines as novel hardeners of standard epoxy compounds to yield cleavable and thermoformable covalent adaptable networks (CANs), with functional properties otherwise comparable to conventional epoxy networks. A suitable aromatic diamine (TPA-o-PD) was synthesised at acceptable purity for the intended use and successfully reacted with DGEBA. The resulting azomethine-bearing cured epoxy networks exhibited glass transition temperature values and a thermal stability profile similar to conventional epoxy network counterparts. In contrast to their conventional counterparts however, the azomethine-bearing networks were shown to dissolve in mixtures of chloroform and methanesulfonic acid, due to acid hydrolysis of at least some of the azomethine bonds of the network. The resulting recyclate material after evaporating the solvent was consistent with the profile of a thermoplastic polymer of high molecular weight, suggesting limited depolymerisation/network cleavage during dissolution in the chloroform/methanesulfonic acid mixture. The recyclates were soluble in polar aprotic solvents and showed good thermal stability, high T_g and molecular weight values, consistent with the attributes of engineering thermoplastics. Lastly, the cured networks were shown to be thermoformable at 200 °C, yielding self-standing films with only minor reduction of properties.