UBC Theses and Dissertations
Mechanics of polymer brush based soft active materials Manav
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. Recently, engineered soft active materials are developed to produce large controllable and reversible bending and stretching deformations. These materials are the focus of this work. New theoretical models, molecular simulations to assess them, and experimental studies are presented in this work. Mechanical stress within a brush and its dependence on the molecular parameters of the brush and external stimulus (temperature) is studied for the first time. A continuum beam model accounting for the Young-Laplace and the Steigman-Ogden curvature elasticity corrections is developed first to understand the large deformation of a flexible substrate due to a brush grafted on it. This model yields a generalized surface stress-curvature relation that enables one to determine stress from curvature measurements. Strong stretching theory (SST) from polymer physics is 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 with 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. Experiments on a thermoresponsive brush grafted onto a soft beam showed the surface stress is ∼ −10 N/m and its magnitude decreases gradually, and reversibly, on increasing solvent temperature. Molecular scale parameters of the brush are estimated experimentally to enable qualitative comparison with SST theories.
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