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

Abstract

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 Tg for the adsorbed layer. It gradually reduces to the bulk Tg 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 Tg and an Arrhenius dependence for temperatures below the bulk Tg. 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 Tg and contracts for lower and higher temperatures. At bulk Tg, the estimated length of the interfacial width, computed via the various definitions, ranges between 1.0 and 2.7 nm.

Fast Processing Affects the Relaxation of Polymers: The Slow Arrhenius Process Can Probe Stress at the Molecular Level

“…34 In addition to dielectric spectroscopy, 40 the SAP can also be studied via measurements of the elastic modulus 41 and molecular dynamics simulations. 42 Thanks to its weaker temperature dependence, the SAP is capable of efficiently driving glasses toward less energetic states with time scales much shorter 18 than those otherwise necessary for macroscopic relaxations mediated by the α-modes. Although no systematic change of the SAP peak position is observed upon reduction of the polymer concentration, 39 a strong decrease of its intensity with h can be observed already at 700 nm.…”

“…Differently than other processes slower than the α-relaxation at T > T g , − the thermal barrier of the SAP is temperature-independent and this quantity is sufficient to uniquely determine the experimental time scale of the process . In addition to dielectric spectroscopy, the SAP can also be studied via measurements of the elastic modulus and molecular dynamics simulations . Thanks to its weaker temperature dependence, the SAP is capable of efficiently driving glasses toward less energetic states with time scales much shorter than those otherwise necessary for macroscopic relaxations mediated by the α-modes.…”