Shear forces between tethered polymer chains as a function of compression, sliding velocity, and solvent quality

The shear forces between polystyrene chains end-tethered to opposing surfaces have been measured with the surface forces apparatus (SFA) in both good and near-Theta solvents. When the shearing velocity was varied, the complex polymer/solvent system responded in a Newtonian-like fashion with the shear force increasing linearly with the shear velocity. The effective viscosity of the end-tethered systems in this Newtonian-like regime was found to be an order of magnitude greater than the viscosity of semidilute solutions of equivalent molecular weight free chains. At larger shear velocities or higher extents of compression, Brownian dynamics simulations suggest the interfacial width will thin, leading to a sublinear increase in the shear force with sliding velocity. Experimental limitations prevented exploration of the higher shear velocities simulated with the Brownian dynamics approach, but increasing confinement eventually did lead to sublinear behavior, in agreement with the simulation prediction.