Imine-Functionalized Biomass-Derived Dynamic Covalent Thermosets Enabled By Heat-Induced Self-Crosslinking And Reversible Structures

Recently, the development of bio-derived high-performance thermosets has been widely studied owing to the shortage of raw materials from fossil resources and the ecological and end-life disposal issues of traditional petroleum-derived thermosets. However, bio-derived thermosets combining excellent mechanical, fire-resistance, and degradation performance remain inaccessible to synthesize, because the introduction of functional additives or chemical structures into the polymer matrix generally weakens other properties of thermosets. The versatility (self-crosslinkable, rigid, and reversible properties) of Schiff base (imine) chemistry offers a new possibility for achieving this goal. In this study, for the first time, a fully bio-based monomer (the diglycidyl ether of 1,4-butanediylbis ((4-hydroxy-3,5-dimethoxybenzylidene)imine)) (SH-BDA-EP) was synthesized from naturally occurring syringaldehyde and 1,4-butanediamine via a facile and highly selective route. With 4,4'-diaminodiphenylmethane (DDM) as a curing agent, the obtained bio-epoxy thermoset (SH-BDA-EP/DDM) possesses excellent mechanical and fire-resistance performance, exhibiting a flexural strength of 121.8 MPa and a limited oxygen index (LOI) of 39.6%, which are far better than those of common bisphenol A-type epoxy thermoset (DGEBA/DDM). The Schiff base structure in SH-BDA-EP/DDM possesses the heat-induced self-crosslinking effect during combustion, which catalyzes polymer chains to form stable protective char and thus endows SH-BDA-EP/DDM with excellent fire resistance. In addition, other unique functions (rigidity and reversibility) of Schiff base structure also endows SH-BDA-EP/DDM with highly integrated (excellent mechanical properties and acid-catalyzed degradation) performance. Therefore, with Schiff base chemistry, the functionalization of high-performance thermoset can be easily realized, which is possible for any traditional thermosets and is therefore a promising design principle in the field of functional thermosets.