Stress in a Stimuli-Responsive Polymer Brush

Abstract

The application of a polymer brush in sensing, actuation, self-folding, among others acutely depends on the tuneable bending of a brush-grafted substrate caused by the stress in the brush. However, the stress in a stimuli-responsive brush has not been investigated. In this work, we study the stress in the stimuli-responsive planar polymer brushes of neutral water-soluble polymers with low to very high graft densities using strong stretching theory (SST). First, SST with the Langevin force-extension relation for a polymer chain is extended to the study of stimuli-responsive brushes. Stress profile and other properties of a Poly(N-isopropylacrylamide) (PNIPAm) brush are then obtained using the extended SST and an empirical Flory-Huggins parameter. The model predicts that the stress in a PNIPAm brush is inhomogeneous and compressive at all temperatures and graft densities. The resultant stress is predicted to increase in magnitude with increasing graft density. Moreover, it decreases in magnitude with an increase in temperature before plateauing in low graft density brushes. In contrast, its magnitude increases weakly with increasing temperature in high density brushes. This contrasting behavior is traced to the minimum in interaction free energy density vs polymer volume fraction curve for PNIPAm solution at a large volume fraction, and stiffening of chains due to finite extensibility. Furthermore, our results indicate that the ability to tune the resultant stress by changing temperature diminishes with increasing graft density.

Stimuli-responsive plasmonic core–satellite hybrid nanostructures with tunable nanogaps

“…In addition, the thermal response of the p(NIPAM) block would be affected by the graft density of the copolymers on the MNPs, such that the collapse process above the LCST was hindered due to the increase of interchain interactions at high graft density. 39,40 Likewise, SERS intensities would be potentially affected by both the molar ratios of GNSs to GNRs and the graft density of the copolymers. Future works need to experimentally derive graft density from thermo-gravimetric analysis and optimize it to have high SERS signal enhancement attributed to a sharp transition of the copolymers embedded at the spatial gaps in response to temperature change.…”

Demonstrating the Interfacial Polymer Thermal Transition from Coil-to-Globule to Coil-to-Stretch under Shear Flow Using SFG and MD Simulation

“…The body of the polymer density profile became closer to the substrate surface via a parabolic to a steplike manner upon increasing the temperature. It even apparently looked similar to the brush thickness decrease in a poor solvent, − where the decline in the density profile was actually induced by the shear flow. In addition, the distribution of the order parameter also approached the substrate surface with increasing temperature; meanwhile, as shown in Figure G, at 67 °C (the maximum in the simulation), the order parameter profile of the adjacent water molecules exhibited a remarkable increase, in contrast to those at 12 and 37 °C.…”

Pressure Anisotropy in Polymer Brushes and Its Effects on Wetting

“…18,19 This grafting-induced lateral pressure can even bend a flexible substrate. 17,20 No such lateral pressure exists in nongrafted polymer films.…”