Dynamic Chemistries in Non-linear Polymer Systems: From Applications for Sustainability to Fundamental Theories

Li, Lingqiao

doi:https://doi.org/10.21985/n2-g140-x518

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Dynamic Chemistries in Non-linear Polymer Systems: From Applications for Sustainability to Fundamental Theories

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Conventional polymer network materials, e.g., rubber tires, cannot be efficiently recycled for high-value applications because of their permanent network structures. Therefore, at the end of use, none or only a small fraction of the economic value can be recovered from these materials. Scrapped tires demonstrate well this issue along with other severe environmental losses. This dissertation addresses this issue by developing reprocessable polymer network materials based on dynamic chemistries, which allow the network materials to be efficiently remolded and recycled. Specifically, we firstly describe a simple one-step synthetic strategy for reprocessable polymer network materials based on alkoxyamine dynamic chemistry. The materials obtained through this strategy can be reprocessed and recycled multiple times with full property recovery associated with cross-link density. We also demonstrate that this versatile strategy not only works well with lab-grade, relatively pure monomers/polymers, but also with industrial-grade raw materials with a high loading of fillers. Importantly, we also show that reprocessable network materials based on alkoxyamine dynamic chemistry exhibit essentially no creep at an elevated temperature (80 °C), indicating excellent reliability even under harsh use conditions. This capability of “turning on” and essentially completely “turning off” the dynamic chemistry within a relatively narrow temperature window arises from the high activation energy associated with the alkoxyamine dissociation. These results demonstrate that excellent reprocessability and creep resistance can be achieved simultaneously with a judicious choice of the dynamic chemistry. Secondly, we develop hydroxyurethane and thiourethane dynamic chemistries aiming at reprocessable network substitutes for traditional polyurethane network materials. In particular, both hydroxyurethane and thiourethane dynamic chemistries are found to possess two concurrent mechanisms: they can change the network structure through both associative exchange and dissociative reversion mechanisms. With judicious design, the resulting network materials based on either hydroxyurethane or thiourethane dynamic chemistry can exhibit full property recovery after multiple molding cycles. Moreover, we also demonstrate that the potential of recovering monomeric thiol from spent thiourethane network materials, which provides an alternative route to recycle these materials for high-value applications. Overall, besides providing a potential pathway to recyclable substitutes of traditional PU network materials, our study here is also the first report of dynamic chemistry in reprocessable network materials possessing both associative exchange and dissociative reversion mechanisms, which should spur the consideration of dynamic chemistries possessing multiple concurrent mechanisms. Undesired creep is a critical issue which prevents reprocessable network materials from being used in real-life applications. To address this issue, we developed a strategy to suppress undesired creep via the incorporation of a substantial yet sub-critical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. We also develop a theory based on the classic Flory-Stockmayer analysis, allowing to provide an approximate and quantitative prediction for this critical fraction of permanent cross-links. With a specifically designed experimental system, we demonstrate that an appropriate level of permanent cross-links can significantly suppress creep in reprocessable network materials and maintain excellent reprocessability. Lastly, we employ dynamic chemistries in other applications including adaptive thermoset adhesives and thermo-cleavable bottlebrush polymers. Specifically, we have developed adaptive thermoset adhesives based on disulfide dynamic chemistry, which have the capability to fill the voids at interfaces as well as relax the internal stress generated during the initial curing process, leading to a significant improvement of the adhesion performance. We also develop a facile and versatile strategy to synthesize bottlebrush polymer containing alkoxyamine dynamic bonds through radical coupling. The resulting bottlebrush polymers are also used to investigate the effect of polymer chain architecture on the confinement behavior of ultrathin polymer films. We find that the presence of the bottlebrush structure leads to suppressed effects of confinement on glass transition temperature and fragility, which can be ascribed to the highly aligned molecular configuration associated with bottlebrush.

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