Shear-stress relaxation in free-standing polymer films

Using molecular dynamics simulation of a polymer glass model we investigate free-standing polymer filmsfocusing on the in-plane shear modulus μ and the shear-stress relaxation modulus G(t) as a functionof temperature T, film thickness H and sampling time t. Various observables, such as the glass transitiontemperature Tg, are seen to vary inversely with H demonstrating thus the (to leading order) linearsuperposition of bulk and surface properties. Confirming the time-translational invariance of our systems,μ is shown to be numerically equivalent to a second integral over G(t). As shown from the respectivestandard deviations, this is especially important for large times and for temperatures around the glasstransition. Both μ and G are found to decrease continuously with T and a jump-singularity is not observed.Using the successful time-temperature superposition scaling of μ and G the shear viscosity can be estimatedfor a broad range of temperatures.