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Mechanics of Polymer Brushes

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: A brush-like structure emerges from the stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. This structure is due to a competition between the conformational entropic elasticity of grafted polymer chains, and the intra and interchain excluded volume repulsions. Polymer brushes occur in biology: neurofilaments, articulate cartilage, extra cellular biopolymers etc. This seminar will examine the reversible mechanical stresses in such soft active matter. We begin with experiments on a thermoresponsive brush grafted onto a soft beam, which showed the surface stress is on the order of −10 N/m and its magnitude decreases gradually, and reversibly, upon increasing solvent temperature. Strong stretching theory (SST) from polymer physics is then combined with continuum mechanics to obtain stress variation in a neutral brush with Gaussian chains. This theory predicts that the normal stress, parallel to the substrate, is a quartic function of the distance from the grafting surface with a maximum at the grafting surface. Idealizing the brush as a continuum elastic surface layer with a residual stress, closed-form expressions for surface stress and surface elasticity as a function of molecular weight and graft density are derived. At a higher graft density, a more refined (semi) analytical SST with Langevin chain elasticity is advanced. Theoretical predictions are assessed by molecular dynamics simulation of a brush using bead-spring model. The seminar will conclude with the theoretical advances made and opportunities for further progress in theory, experiments, and technological applications of polymer brushes. Key References: (1) Manav, M., Ponga, M.P., Phani, A.S., 2019, Stress in a polymer brush, Journal of the Mechanics and Physics of Solids, 127, pp 125—150, https://doi.org/10.1016/j.jmps.2019.03.009 (2) Manav, Anil Kumar, P., Phani, A.S., 2018, Mechanics of polymer brush based soft active materials—theory and experiments, 121, 296—312, https://doi.org/10.1016/j.jmps.2018.06.021 (3) Zou, Y., Lam, A., Brooks, D.E., Phani, A.S., Kizhakkedathu, J.N., 2011, Bending and stretching actuation of soft materials through surface-initiated polymerization, 50(22), 5116-5119, https://doi.org/10.1002/anie.201008252

This talk is part of the Engineering Department Bio- and Micromechanics Seminars series.

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