2 research outputs found
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 <i>G</i><sub>⊥</sub><sup><i>P</i></sup>, relaxation time λ<sub>⊥</sub>, and viscosity
η<sub>⊥</sub> and an increase in the logarithmic relaxation
rate <i>r</i><sub>⊥</sub> 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