Development of Polyimine-Based Dynamic Covalent Network: From Malleable Polymers to High-Performance Composites

Since the advent of synthetic polymers over a century ago, polymer science and technology development has transformed and enhanced our way of life from clothing to food storage to electronics and automobiles. To suit different applications, two broad categories of synthetic polymers have been developed: linear polymers, or thermoplastics, which can be melted, or solubilized, and reprocessed into a new shape or form; and network polymers, or thermosets, which are usually unable to be melted or reshaped once cured. The irreversible and permanent nature of thermosets was initially desirable for enabling high strength and long service lifetimes, but also makes thermosets inherently unrecyclable. Recently, dynamic covalent chemistry has been employed to form malleable thermosets, a new class of network polymers which can be reprocessed, and recycled like thermoplastics. The focus of this dissertation is the development of robust, catalyst-free malleable thermosets using exchangeable imine (a.k.a. Schiff-base) chemical links, and exploration of their great potential in fabricating high-performance composite materials. Our work in this field began with a collaborative effort to explore the recyclability of catalyst-containing epoxy-acid networks, which were one of the first malleable thermoset materials identified. Our work with epoxies resulted in the discovery of a linear relationship between the glass transition temperature of a material, and the temperature at which the material becomes malleable due to bond exchange reactions. Next we turned our attention to development of malleable thermosets using dynamic imine chemistry. Polyimine networks were developed which exhibit excellent mechanical strength and malleability under mild conditions without an added catalyst. The polymers were found to be completely hydrolytically stable, yet water could be used to catalyze the bond exchange reactions, leading to room temperature malleability. Further development led to both hydrophilic and hydrophobic network polyimines that exhibit a broad range of mechanical properties from elastomers with room temperature malleability, to semi-crystalline polymers with high tensile strength and thermal stability. We found that the reversibility of imine-linked malleable thermosets could enable uniquely efficient recycling processes. This method was used to develop carbon fiber reinforced composites (CFRC’s) in which the fiber and resin materials could be recovered and recycled in their original form. These thermoset composites are also moldable, weld-able and repairable due to crosslink exchange of the polymer binder. In another application, powders of such imine-linked malleable thermosets were combined with ion-conductive sulfide powders to form electrolyte membranes for solid-state lithium ion batteries exhibiting record-setting stability for FeS2 solid state batteries, and a four-fold improvement in energy density over previously-reported solid state rechargeable lithium ion batteries. This work has brought to light a method for fabricating environmentally stable, yet highly functional Schiff-base thermosets, which have the potential to enable a large number of transformative applications particularly regarding composite materials.