Development of Sustainable and Recyclable Addition-Type Polymer Networks and Addressing Creep Limitations in Covalent Adaptable Networks

Bin Rusayyis, Mohammed Abdulaziz

doi:https://doi.org/10.21985/n2-b290-tt77

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Development of Sustainable and Recyclable Addition-Type Polymer Networks and Addressing Creep Limitations in Covalent Adaptable Networks

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Conventionally cross-linked polymers, which comprise the vast majority of commercial thermosets, cannot be decross-linked after curing or flow upon heating. Therefore, they cannot be effectively recycled into high-value products at end-of-life. Their lack of recyclability is due to the permanent cross-links, which restrict the flow of the chains in the network even at elevated temperature. The recycling challenge of conventionally cross-linked polymers results in both environmental damages and economic losses. To address this challenge, the concept of reprocessable polymer networks emerged, and research in this area has been active for the past two decades. Reprocessable polymer networks, also known as covalent adaptable networks (CANs) or dynamic covalent polymer networks (DCPNs), are polymer networks that contain sufficient levels of dynamic covalent bonds which are capable of dissociating or exchanging in response to external stimuli, such as heat or light, rendering them malleable. Most past studies on CANs required complex multi-step synthesis and/or pre-functionalization, which limits their scalability and commercial viability. Additionally, most of the few studies that have reported CANs with full cross-link density recovery after recycling were focused on step-growth polymer networks and, prior to research done for this dissertation, no study has ever reported reprocessble addition-type polymer networks synthesized exclusively from vinyl monomers and exhibiting full recovery of cross-linked density after multiple recycling steps. The objective of this dissertation is to exploit dynamic covalent chemistries in the synthesis of reprocessable polymer networks. In particular, this dissertation describes simple and scalable synthetic methods to produce catalyst-free, addition-type recyclable polymer networks made from commercially important vinyl monomers via free radical polymerization and exhibiting full recovery of properties associated with cross-link density after recycling. We have identified three simple, one-step approaches that enable us to achieve our objective. The first approach is to utilize piperdine-based dynamic dialkylamino disulfide chemistry (or BiTEMPS chemistry). The second approach is to employ non-piperdine-based dynamic dialkylamino disulfide chemistry. The third approach is based on dynamic hindered urea bonds. Additionally, despite significant advancements in the development of CANs, their susceptibility to creep at elevated temperature (T) has been considered a major technological stumbling block as creep is highly undesirable and prevents these reprocessable networks from being used in many high-value applications. Thus, there remains a need to develop CANs that show little long-term reactivity at elevated T to avoid creep at those conditions but that can be processed at higher, but reasonable T over an acceptable time scale while maintaining full cross-link density recovery after recycling. This dissertation also seeks to address the often-cited flaw associated with elevated-temperature creep susceptibility of CANs. We have identified and developed three different CAN materials that exhibit excellent long-term creep resistance at elevated T. Two of these CANs are based on dissociative dialkylamaino disulfide dynamic chemistry with high bond dissociation energy. The third CAN is based on dynamic hydroxyurethane linkages, which involve both associative and dissociative dynamic chemistries. In summary, this dissertation offers simple and effective methodologies to produce addition-type reprocessable polymer networks from any monomer and/or polymer containing carbon-carbon double bonds that are amenable to radical polymerization. Given the commercial importance of vinyl polymers, this research offers a sustainable solution to the large plastic waste generated from vinyl cross-linked materials. This dissertation also addresses the creep limitations in CANs, which would extend the range of applications for these materials to cover advanced technological applications where high temperature creep resistance is required.

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