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Polymer architecture effects on mechanochemical reactions

By Preston May


Long chain polymers have a unique ability to become highly extended in elongational flow fields. The forces developed along the backbone give rise to scission of the chains near their center. Recently, this unique property of polymers has been adopted to explore new chemical transformations by embedding structural elements into the backbone designed to undergo site-specific bond cleavage, termed mechanophores. An overarching theme in polymer mechanochemistry is that the polymer chains are the link between macroscopic energy and the mechanophore. Therefore, polymer architecture is thought to play a significant role in influencing these types of mechanochemical reactions in polymers. This research aims to identity the governing rules of force transduction in polymer chains by studying various polymer architectures. To achieve this goal, an efficient characterization technique that allows for in-situ measurement of solution-based mechanochemical reactivity through coupling of ultrasound experiments and UV-Vis spectroscopy in a flow cell was created. Using this technique we were able to perform rigorous kinetic analyses to screen the effects of multiple parameters on mechanophore activation using spiropyran mechanophores as a model. The effects of flow rate and sonication intensity are provided. Furthermore, we isolated the effects of molecular mass and chain length pertaining to polymer architecture by synthesizing a series of polymers containing chain-centered spiropyran mechanophores; poly(methyl acrylate), poly(ethyl acrylate), poly(n-butyl acrylate), poly(iso-butyl acrylate) and poly(tert-butyl acrylate). Results show that chain length contributes more to activation than molecular mass of the individual chains. In addition, it was hypothesized that alternative architectures with polymers containing multiple branches and arms could transmit force to mechanophores more efficiently than their linear counterparts. Spiropyran-linked star PMA with four and eight chains were synthesized via single electron transfer living radical polymerization and were screened in the ultrasonication flow cell and in solid-state tensile tests. Experimental data show that polymers with branched architectures activate slower than linear architectures in solution, yet faster in solid-state tensile experiments. These results provide a greater understanding of mechanotransduction processes in polymers and allow us to design increasingly sensitive mechanoresponsive polymers. Furthermore, heterogenous polymer architecuture was examined by introducing a spiropyran mechanophore at the interface of a glass fiber and PMMA matrix. Interfacial shear forces are applied to the mechanophore by a single fiber microbond testing protocol. Results suggest that covalent attachment as well as frictional force might be able to activate mechanophores at interfaces

Topics: Mechanophore, ultrasound, spiropyran, sonication, polymer mechanochemistry, damage sensing, branched polymers, star polymers
Year: 2013
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