Ambient and Switchable plasticity in thiol-X photopolymers
Matt McBride, University of Colorado
Abstract: The thiol-thioester exchange reaction was implemented into photopolymerizable resins enabling chemically controllable bond exchange and stress relaxation even at ambient temperature. Networks containing such exchangeable bonds are able to covalently adapt to stress from mechanical deformation or polymerization-induced shrinkage effectively mitigating the negative side effects of residual stress. Specifically, the base or nucleophile mediated thiol-thioester exchange, which requires the presence of free thiols, a suitable catalyst, and a thioester containing crosslinker, is a near ideal exchange reaction for this purpose as it occurs rapidly, selectively, and controllably while being orthogonal to photopolymerization processes. This work engineered photopolymerizable polymer networks capable of undergoing this exchange reaction and demonstrated how this exchange enables spatially controlled stress reduction, recycling, and remolding of photopolymerized thiol-ene networks. Through the addition of photoactivated base or acid generators, the exchange was turned on or off highlighting the photo switchable nature of this reaction effectively turning a solid polymer network into a fluid or vice versa. In addition, this work demonstrated recycling of a photopolymerizable resin by addition of excess thiol that depolymerizes the network through the exchange reaction, and with the addition of supplemental monomer, the depolymerized resin was recycled resulting in a polymer network with the same mechanical properties. Lastly, various catalyst and monomers were explored to fully understand the relationship between bond exchange and the mechanical properties. In total, this work present a readily accessible, photopolymerizable polymer network with very low stress and recyclability.