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Title:Rapid fabrication of polymers and composites via frontal polymerization
Author(s):Robertson, Ian Donald
Director of Research:Moore, Jeffrey S.
Doctoral Committee Chair(s):Moore, Jeffrey S.
Doctoral Committee Member(s):White, Scott R.; Sottos, Nancy R.; Murphy, Catherine J.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Frontal polymerization
rapid fabrication
energy-efficient manufacturing
Abstract: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 (<1 h). We were able to reduce the catalyst loading by 3-5 fold by performing FROMP with the more reactive exo-isomer of DCPD, ultimately performing a successful FROMP with catalyst loadings as low as 10 ppm. We then discovered that the addition of alkyl phosphite inhibitors improved the pot life of the FROMP reaction by up to 140 fold. In addition to enabling an extended processing window for the liquid monomer solution, the alkyl phosphite inhibition facilitated FROMP of the gelled solution. Gels were shown to facilitate applications such as shape-fixing, morphing structures, surface patterning, 3D-printing, and rapid vascularization. Before applying the FROMP chemistry to a composite, we developed a model system to understand the effects of the reinforcing fibers on the reaction. Herein, we showed that conductive elements like carbon fibers (CF) accelerate FP through enhanced thermal transport, while insulating fibers produce no effect. A computational model validated our hypothesis. We used the phosphite inhibition strategy to perform vacuum-assisted resin transfer molding and subsequent FROMP of a PDCPD-CF laminate. The resulting composite was high quality and exhibited promising mechanical properties. We anticipate the versatility and ease of use of this chemistry will greatly facilitate the development of new materials and applications using FROMP. Additionally, we explore the possibility of performing synthetic morphogenesis of functional materials using FP. Many opportunities lie ahead for application of reaction-diffusion systems to generate emergent order during processing.  
Issue Date:2017-12-07
Rights Information:© 2017 Ian Donald Robertson. All rights reserved.
Date Available in IDEALS:2019-08-23
Date Deposited:2017-12

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