Combining Reversible and Permanent Crosslinks in Thermosets for High Technology Applications

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TECHNICAL SUMMARY:Polymer networks composed of thermally reversible crosslinks have become popular in mendable, recyclable, and smart materials. These materials have the ability to respond to stimuli such as light exposure or temperature change with a change in material properties, impacting fields from metamaterials and biomaterials to microdevices, additive manufacturing and photolithography. Here, a class of thermoreversible polymer networks will be developed based on Diels-Alder networks; however, by using those networks in combination with one of two other thiol-vinyl click reactions, a new class of materials offering fundamental and practical advantages will be explored. Understanding of thermoreversible networks will be translated into several approaches for achieving the potential of these polymers to be used as smart, self-healing materials. In particular, controlled elimination of thermoreversiblity (i.e., through a Michael addition reaction) either of a uniform fraction of the crosslinks or with spatiotemporal control of the reversible crosslinks should create novel materials, approaches and understanding that enables the complex fabrication and physicochemical patterning of polymer networks and subsequent devices in 3D. Specifically, (i) new polymeric materials will be created that combine thermoreversible networks with conventional irreversible networks, including the ability to eliminate the thermoreversibility in a spatiotemporally controlled manner, (ii) the network structure will be systematically varied and the relationship between reversible bond structures and rheological, mechanical, and healing behavior will be assessed, (iii) direct-write and layer-by-layer approaches for additive manufacturing approaches to 3D devices will be developed, and (iv) dual-stage shape-memory polymers will be created with two distinct thermal transitions associated with the glass transition and the crosslink reversibility.NON-TECHNICAL SUMMARY:Thermosetting plastic materials, as a multibillion dollar industry, are ubiquitous in high technology applications that range broadly from dental materials to additive manufacturing to photovoltaic coatings to advanced optical materials and many others. These polymers are highly functional, specialty materials whose performance is largely dependent on the underlying molecular structure, which is comprised traditionally of irreversible molecular bonds. While this structure gives rise to many of the desired attributes of these materials, that same structure limits their behavior and prevents them from being self-healing, recyclable, or able to change their permanent shapes. Here, through the development of special molecular approaches to these materials that renders the structure reversible at appropriate times and conditions, thermosetting polymers will be formed that combine the optimal features of traditional thermosets with those of smart, responsive materials. This approach will be used to develop improved materials and approaches for use in the fabrication of complex 3D parts by additive manufacturing, novel optical applications, and use as reformable, recyclable, and healable thermosetting polymers. Beyond the technological advances, this approach simultaneously has significant broader impacts associated with the training of a future workforce in an interdisciplinary combination of polymer chemistry, materials science, and optics. The graduate and undergraduate students involved in this project will also participate in the launching of a new Materials Science and Engineering PhD degree at the University of Colorado that will lead to the training of highly qualified researchers for the materials science and engineering workforce.