The Engineering of Reversible Thermoset Resins Based on the Reversible Photodimerization of Coumarin

Abstract

This dissertation details the development of new UV light triggered reversible thermoset polymers utilizing the reversible photodimerization of coumarin. Thermoset polymers like epoxy resins are widely used for their many favorable properties including high dimensional stability, strength, and the ability to be cured from nonvolatile liquid precursors. However, the insolubility and intractability of these crosslinked polymers limit their use in applications where removability and repairability are necessary features. To address this limitation, our research explores the use of coumarin to create thermosets with photo-reversible crosslinks. The molecule coumarin is capable of forming a dimer when exposed to long wavelengths of UV light, and when exposed to short UV wavelengths this dimer is cleaved back into monomeric form, offering a promising way to make or break polymer crosslinks on-demand. In this research, three complementary approaches are taken to create materials that take advantage of three functions provided by coumarin: photodimerization, dimer photocleavage, and dimer thermal cleavage. Special attention is paid to the design of UV-reversible adhesives.In Chapter 1, a review of coumarin’s fundamental photochemistry is presented, followed by a discussion of how this chemistry can be utilized to engineer light-responsive polymers. Chapter 2 describes the design, synthesis, and evaluation of a new liquid photocurable resin using coumarin photodimerization. An epoxy-functionalized coumarin monomer was reacted with diamines to produce tetrafunctional precursor materials, leading to a liquid precursor resin that could be photocrosslinked and utilized as a photocurable adhesive. Adhesive strength was shown to increase with exposure time as a direct consequence of photodimerization. Inefficiencies in the network forming ability of the material were studied, suggesting that photooxidative chain scission and intramolecular dimerization are partly responsible for limiting efficient photocuring of the material. In Chapter 3, pre-assembled coumarin dimers bearing epoxy groups were used to create resins that could be cured like a conventional epoxy resin. Upon irradiation with 254 nm UV light, the coumarin dimers could be efficiently photocleaved and the thermoset network broken into fragments, yielding a liquid at the surface being irradiated. Irradiation with 254 nm light was demonstrated to release an adhesive bond to a UV transparent substrate within minutes of exposure. The adhesive strengths of these coumarin dimer epoxy resins were comparable to those made with the conventional epoxy resin, diglycidyl ether of bisphenol-a. In Chapter 4, the thermal cleavage of coumarin dimers and its potential use in reversible thermoset polymers was studied. Coumarin dimers are known to undergo cycloreversion back to monomers at high temperatures, but this has never been studied in detail. Kinetic parameters for the thermal cleavage of syn head-to-head and syn head-to-tail 4-methyl-7-glycidyloxycoumarin dimers were measured, allowing the behavior of materials crosslinked by coumarin dimers to be predicted at high temperatures. The syn head-to-tail dimer was found to be significantly more thermally robust than the syn head-to-head dimer. The epoxy resins developed in Chapter 3 were used to demonstrate that thermal dissociation of coumarin dimers is an effective way to break crosslinks in a thermoset polymer; upon heating the polymer was reverted to liquid, releasing adhesive bonds.