Rapid, photomediated healing of hexaarylbiimidazole-based covalently cross-linked gels

The intrinsic healing of covalently cross-linked polymer networks is commonly effected via the utilization of backbone-borne functional groups able to reversibly cleave or rearrange, thereby enabling mixing and coreaction of network strands that bridge contacted interfaces; however, such materials often exhibit slow healing rates and are susceptible to creep under load. To address these deficiencies, we incorporated hexaarylbiimidazole (HABI) functionalities, groups that are homolytically cleavable, to yield relatively low reactivity lophyl radicals under UV or visible light irradiation and which, in the absence of light, spontaneously recombine without significantly participating in deleterious side reactions, into the backbone of poly(ethylene glycol)-based polymeric gels. Whereas the network connectivity of these HABI-incorporating gels was stable in the dark, they exhibited significant creep upon irradiation. The influence of swelling solvent on the reaction kinetics of backbone-borne HABI photolysis and lophyl radical recombination was examined and revealed that gels swollen with 1,1,2-trichloroethane (TCE) exhibited higher radical concentrations than those swollen with either acetonitrile or water under equivalent irradiation conditions, attributable to the relative solvent affinity for the hydrophobic HABI functionalities affording more rapid HABI cleavage and slower radical recombination rates in TCE than in water. The fastest healing rates for cleaved samples brought into contact and irradiated with visible light were observed for TCE-swollen gels, although rapid restoration of mechanical integrity was achieved for gels swollen with any of the solvents examined where tensile strengths approached those of the pristine materials after 1 to 3 min of light exposure.