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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, (2) Manav, Anil Kumar, P., Phani, A.S., 2018, Mechanics of polymer brush based soft active materials—theory and experiments, 121, 296—312, (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,

This talk is part of the Engineering - Mechanics and Materials Seminar Series series.

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