Microfabrication by X-ray-induced polymerization above the lower critical solution temperature

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Department of Chemical Engineering


A polymer synthesis method is presented in which chain growth driven by exothermic reaction stimulates a gradual chain collapse. The globular precipitates in such systems can be restrained from coalescing by polymerizing in a quiescent environment. Time-resolved small-angle scattering study of the methacrylic acid polymerization kinetics in a quiescent system above its lower critical solution temperature (LCST) in water reveals the following features of this method: (a) growing oligomers remain as rigid chains until a critical chain length is reached, at which they undergo chain collapse, (b) radius of gyration increases linearly with time until a critical conversion is reached, and (c) radius of gyration remains constant after the critical conversion, even while conversion is gradually increasing. Following this self-stabilizing growth mechanism, we show that nanoparticles can be directly synthesized by polymerizing N-isopropylacrylamide above its LCST in water. The average size of nanoparticles obtained from a polymer-solvent system is expected to be the maximum extent of reaction spread at that monomer concentration. This hypothesis was then verified by polymerizing N-isopropylacrylamide above their LCST in water, but by initiating the reaction with X-rays shielded by a mask. The microfabricated patterns conform well to the size and shape of the mask used confirming that the growing chains do not propagate beyond the exposed regions as long as the reaction temperature is maintained above the LCST.

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Journal of Applied Polymer Science