Temperature dependence of the dynamics and interfacial width in nanoconfined polymers via atomistic simulations

We present a detailed computational study on the temperature effect of the dynamics and the interfacial width of unentangled cis-1,4 polybutadiene linear chains confined between strongly attractive alumina layers via long, several μs, atomistic molecular dynamics simulations for a wide range of temperatures (143–473 K). We examine the spatial gradient of the translational segmental dynamics and of an effective local glass temperature $(TgL)$⁠. The latter is found to be much higher than the bulk T_g for the adsorbed layer. It gradually reduces to the bulk T_g at about 2 nm away from the substrate. For distant regions (more than $≈1.2nm$⁠), a bulk-like behavior is observed; relaxation times follow a typical Vogel–Fulcher–Tammann dependence for temperatures higher than T_g and an Arrhenius dependence for temperatures below the bulk T_g. On the contrary, the polymer chains at the vicinity of the substrate follow piecewise Arrhenius processes. For temperatures below about the adsorbed layer’s $TgL$⁠, the translational dynamics follows a bulk-like (same activation energy) Arrhenius process. At higher temperatures, there is a low activation energy Arrhenius process, caused by high interfacial friction forces. Finally, we compute the interfacial width, based on both structural and dynamical definitions, as a function of temperature. The absolute value of the interfacial width depends on the actual definition, but, regardless, the qualitative behavior is consistent. The interfacial width peaks around the bulk T_g and contracts for lower and higher temperatures. At bulk T_g, the estimated length of the interfacial width, computed via the various definitions, ranges between 1.0 and 2.7 nm.