Impact of solvent quality on tribological properties of polymer brushes

Polymer brushes, as modifying coatings, significantly improve the tribological properties of various contacting surfaces. The friction that arises when an external load is applied and the polymer brushes laterally shift relative to each other is determined by their interaction energy and the depth of interpenetration. If the brushes are immersed in a lowmolecular-weight solvent, the friction force can be controlled by varying the solvent quality through changes in external conditions, such as temperature, chemical composition of the solution, and so on. It should be noted that theoretical studies on the effect of solvent quality on the tribological properties of brushes are practically absent. To determine the influence of solvent quality on the interaction of flat polymer brushes, two complementary approaches were used: analytical and numerical self-consistent field methods. In both cases, a coarse-grained model of polymer brushes was employed. The solvent quality in the model was defined through the Flory-Huggins parameter for polymer-solvent interaction. A quantitative assessment of the overlap zone width, osmotic pressure, and friction force arising when the brushes approach each other was conducted. A theoretical description of the friction force in the low shear rate regime was proposed based on the Brinkman equation for two compressed brushes sliding against each other. It was shown that, with constant total polymerization degree, grafting density, and lateral sliding speed of the brushes relative to each other, the width of the overlap zone decreases following a power law with increasing inter-plane distance, regardless of solvent quality. Under conditions of strong compression of flat polymer brushes, the friction force approaches a certain limiting value, while the friction coefficient tends to zero, independent of solvent quality. In the moderate pressure region, the friction coefficient significantly increases with a decrease in the solubility of the grafted polymers under the same applied external load and the composition of the flat polymer brushes. The analytical method showed high agreement with the data from the numerical simulations. The obtained results allow for predicting the tribological properties of polymer brushes depending on solvent quality and, consequently, predicting the effect of external conditions on the friction force between modified surfaces.