Shear-stress relaxation in free-standing polymer films

G George and I Kriuchevskyi and H Meyer and J Baschnagel and JP Wittmer, PHYSICAL REVIEW E, 98, 062502 (2018).

DOI: 10.1103/PhysRevE.98.062502

Using molecular dynamics simulation of a polymer glass model we investigate free-standing polymer films focusing on the in-plane shear modulus mu, defined by means of the stress-fluctuation formula, as a function of temperature T, film thickness H (tuned by means of the lateral box size L), and sampling time Delta t. Various observables are seen to vary linearly with 1/H, demonstrating thus the (to leading order) linear superposition of bulk and surface properties. Confirming the time-translational invariance of our systems, mu(Delta t) is shown to be numerically equivalent to a second integral over the shear-stress relaxation modulus G(t). It is thus a natural smoothing function statistically better behaved as G(t). As shown from the standard deviations delta mu. and delta G, this is especially important for large times and for temperatures around the glass transition. mu and G are found to decrease continuously with T and a jump-singularity is not observed. Using the Einstein-Helfand relation for mu(Delta t) and the successful time-temperature superposition scaling of mu(Delta t) and G(t), the shear viscosity eta(T) can be estimated for a broad range of temperatures.

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