Reshaping Recyclable Thermosets

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NON-TECHNICAL SUMMARYThe long-term persistence of many synthetic materials and the resulting impact on the environment has made clear the importance of developing new routes to sustainable polymers. This project aims to address the challenges of sustainability via the design of new, robust materials with longer life spans that need to be replaced less frequently. Recyclable and self-repairing materials offer one route for extending the useful lifetime of a polymeric material. Unfortunately, many of the most commonly encountered polymers that are readily recyclable and repairable suffer from poor resistance to solvents or exposure to high temperatures. Conversely, polymers with enhanced solvent, thermal, and dimensional stability are generally not recyclable and cannot be rendered self-healing, because their molecular structure does not allow the mobility required to do so. This project aims to elucidate the fundamentals that will allow bridging the divide between these two classically disparate families of materials. By exercising precise control over key molecular-scale parameters, the factors governing processability, recyclability, self-healing, and utility will be determined to allow the design of next-generation materials. A key component of this project involves outreach and educational activities directed toward local K-12 students as well as training and professional development of graduate and undergraduate students in emerging areas of chemistry and polymer science.TECHNICAL SUMMARYOne of the traditional classification mechanisms for bulk polymeric materials relies on whether or not the chains that comprise the material are crosslinked. Crosslinked polymers (i.e., thermosets) may have significantly enhanced dimensional, chemical, mechanical, and solvent stability compared to their non-crosslinked analogs (i.e., thermoplastics), but these attributes are accompanied by an inability to be reshaped or recycled. There is a significant need for materials that combine the properties of thermosets and thermoplastics by relying on reversible crosslinks that can undergo exchange by either an associative or dissociative mechanism. The former method of crosslink exchange is particularly promising because it allows for network rearrangement without loss of connectivity (i.e., no change in crosslink density). Networks that undergo associative exchange have recently become known as "vitrimers." The goal of this project is the investigation and development of vitrimers that are accessible via a straightforward method that relies on the curing of vinyl monomer-derived prepolymers generated by controlled radical polymerization. Critically, this strategy decouples the network curing and backbone polymerization steps, allowing for precise manipulation of structure, topology, and functionality within the chains comprising the network. Three specific aims will be pursued to (1) interrogate the effect of structural elements of both the polymeric and crosslinker components of vitrimers derived from vinyl polymers, (2) determine the role of chain microstructure and topology on vitrimer (re)processability, and (3) prepare and investigate vitrimers with stimuli-activatable intrinsic catalysts. Successful completion of these aims will reveal fundamental structure-property relationships of dynamically crosslinked networks while generating design principles for vitrimers with unprecedented chemical and mechanical properties..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.