Rapid fabrication of polymers and composites via frontal polymerization

Autocatalytic reactions are a powerful tool for material transformation owing to their amplification of minor stimuli to effect dramatic change. Frontal polymerization (FP) is an autocatalytic, propagating reaction wave driven by the exothermic polymerization of a monomer. A small thermal stimulus is applied locally to a solution of monomer and initiator, activating polymerization. The polymerization releases heat, accelerating the reaction, and ultimately producing a propagating wave of polymerization that quickly converts all available monomer to polymer. FP is characterized by its high rate of reaction and minimal requisite input energy. As such, there has been significant interest in utilizing this reaction for polymer synthesis. The majority of previous work in this area focused on FP in test tubes or similar reactors using radical polymerization or epoxy polymerization. It was thus desirable to move beyond this paradigm to achieve direct fabrication of functional, high-performance polymer and composite materials. We first envisioned using the transformation of liquid monomer to solid polymer to reinforce a flexible material. A triacrylate monomer, peroxide initiator, and thixotropic fumed silica filler were used to infiltrate microvasculature embedded in a flexible PDMS matrix. Radical FP of this solution produced a polyacrylate endoskeletal structure that stiffened the overall material. However, it soon became polymers with greater strength and stiffness than these acrylic species would be required to create high-performance materials. Frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (DCPD) produced a better polymer product, since PDCPD is used industrially for products like truck body panels and agricultural equipment. FROMP had previously been limited by high catalyst loading requirements and short working time (