Anisotropy of Shear Relaxation in Confined Thin Films of Unentangled Polymer Melts

The anisotropic shear relaxation functions of confined thin films of unentangled polymer melts are measured via nonequilibrium step-strain simulations of in-plane and out-of-plane shear using the finitely extensible, nonlinear-elastic (FENE) model. We show that the classical Rouse model unsurprisingly fails to predict the thin-film relaxation functions in response to out-of-plane shear, due in part to non-Gaussian conformation statistics in the dimension perpendicular to the sub/superstrate. Using an alternate empirical model for the out-of-plane response, we quantify decreases in the plateau modulus G⊥^P, relaxation time λ_⊥, and viscosity η_⊥ and an increase in the logarithmic relaxation rate r_⊥ as functions of film thickness, and we discuss these anisotropic changes in stress-relaxation properties in terms of structural/conformation changes on the microscopic level, namely the relative contraction and non-Gaussian quality of polymer conformations in the dimension normal to the substrate and the resulting phenomenon of cooperative relaxation. We then incorporate these into a semiempirical extension to the Rouse model which closely predicts our computational results and which will be useful for further study of polymer thin films.