‘‘Equilibrium’’ and dynamic behavior of thin films of a perfluorinated polyether

Abstract

We have measured the equilibrium forces between two mica surfaces separated by a thin liquid film of a perfluorinated polyether. The observed force law was monotonically repulsive and had a range which was on the order of ten times the radius of gyration of the polymer molecules. The results can be explained by hypothesizing that some of the polymer chains are effectively ‘‘pinned’’ to the surfaces and thus are not free to exchange with chains in the bulk reservoir on the time scale of the experiments. We have also investigated the response of the same polymer melt to both sinusoidal and step perturbations in the distance between the two surfaces as a function of the film thickness. We have developed a simple rheological model for a linear, viscoelastic fluid which relates the experimental data to the effective viscosity of the confined fluid. For surface separations greater than approximately 1500 Å, the effective viscosity of the fluid film was found to be the same as the macroscopically measured value reported for the bulk polymer. For a surface separation of 840 Å, the effective viscosity was found to be greater than the bulk value. The results obtained from the dynamic experiments agreed with those obtained from the transient experiments.