Integration of B-N Coordinated Boronic Ester Bonds into Epoxy Networks Through One-step Crosslinking

Abstract

Epoxy vitrimers containing boron-nitrogen (B-N) coordinated boronic ester and/or boroxine bonds were obtained with a one-step curing reaction. Toward this end, diepoxide (i.e., diglycidyl ether of 1,4-butanediol) was crosslinked with an aromatic amine bearing boronic acid group (e.g., 3-aminobenzeneboronic acid) as the crosslinker. The crosslinking was accomplished through the following in situ reactions. First, the ring-opening polyaddition of epoxide with primary amino groups afforded a linear poly(hydroxyl ether amine). Second, the reaction of di(2-hydroxyethyl)amino structural units of the linear poly(hydroxyl ether amine) with boronic acid groups led to the crosslinking. To reinforce the epoxy networks, a POSS diepoxide [viz. 3,13-diglycidyl double decker silsesquioxane, DDSQ] was further introduced to prompt the co-crosslinking. It was demonstrated that in the epoxy vitrimers the crosslinking was accounted for: i) the inter-chain dynamic covalent bonds and ii) the inter-chains POSS-POSS aggregation. Benefiting from the exchange of dynamic covalent bonds, the epoxy networks displayed excellent reprocessing (or recycling) properties. Thanks to the generation of POSS nanodomains, the epoxy networks were significantly reinforced. It was found that the thermomechanical properties of epoxy networks can be modulated by taking control of the crosslinking densities of the epoxy networks. In the meantime, the epoxy networks showcased the excellent shape memory properties featuring the reconfigurability. Thanks to the introduction of the organosilicon component (i.e., POSS), the improved surface hydrophobicity was displayed, which is pivotal to the epoxy networks against hydrolysis of boronic ester bonds.