Mechanics of polymer brush based soft active materials– theory and experiments

Abstract

Brush-like structures emerge from stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. They arise due to the competition between conformational entropic elasticity of polymer chains and excluded volume interactions from the intra and interchain monomer repulsions. Recently, stimuli responsive polymer brush based soft materials have been developed to produce controllable and reversible large deformations of the host substrate. To understand these systems, and improve their functional properties, we study elastic stress distribution and surface stress-curvature relations of a neutral polymer brush grafted on to an elastic beam, made of a soft material. In the strongly stretched brush regime, we combine mean field theory from polymer physics with a continuum mechanics model and show that the residual stress variation is a quartic function of distance from the grafting surface, with maximum stress occurring at the grafted surface. Idealizing the brush as a continuum elastic surface layer with residual stress, we derive a closed form expression for surface stress and the surface elasticity of the layer as a function of brush parameters, such as graft density and molecular weight. The generalized continuum beam model accounts for the Young-Laplace and Ogden-Steigman curvature elasticity correction terms, and yields a surface stress-curvature relation, which contains existing relations in the literature as special cases. Further, we report experiments on a thermoresponsive random copolymer brush, Poly(N-isopropylacrylamide)-co-Poly(N,N-Dimethylacrylamide) (PNIPAm-co-PDMA) brush, grafted on one side of a plasticized poly(vinyl chloride) (pPVC) thin film. Estimated surface stress from measured curvature is on the order of −10 N/m, and it decreases gradually, and reversibly, with increasing ambient temperature from 15 • C to 55 • C.

Shape Shifting of Cup Shaped Particles on Growing poly (2‐hydroxy ethyl methacrylate) Brushes by “Grafting From” Approach and Dissipative Particle Dynamics Simulation

“…Interestingly, shape of particles was completely changed from cup to spherical ball with a small opening at the centre after modifying the particles with poly(HEMA) brush. Similar bending behaviour due to polymer brush growth on one side of soft polymeric substrate was also reported by other researchers . However, in this study, bending was probably not observed due to hydrophilic polymer brush growth on soft cup surface since other hydrophilic brush did not yield shape shifted cup (discussed later).…”

Spherical nano-inhomogeneity with the Steigmann–Ogden interface model under general uniform far-field stress loading

“…In contrast to the large number of available studies for the Gurtin-Murdoch model, most of the literature on the Steigmann-Ogden model is focused on simple geometries, such as nanobeams (Chhapadia et al, 2011;Manav et al, 2018), nanowires (Zhao et al, 2015), rigid stamps (Zemlyanova, 2018a;Zemlyanova, 2018b), thin films (Ogden et al, 1997;Dryburgh and Ogden, 1999) and half-space materials (Li and Mi, 2018;Mi, 2018).…”

Stress in a polymer brush

“…31 Begley et al 32 studied the deformation in a beam due to a grafted brush layer by simplifying the brush layer as interacting beads on the surface of the beam. A model for the large deformation of a brush grafted beam was developed by Manav et al 24 incorporating curvature elasticity and the Young-Laplace corrections to the classical Stoney's equation. 33 The effect of stimuli on a moderately dense brush of a classical polymer 34,35 has been studied extensively by Zhulina et al 36 using both scaling theory 2,3,5 and SST.…”