Cooperative motion and growing length scales in supercooled confined liquids

Using molecular dynamics simulations we investigate the relaxation dynamics of a supercooled liquid close to a rough as well as close to a smooth wall. For the former situation the relaxation times increase strongly with decreasing distance from the wall whereas in the second case they strongly decrease. We use this dependence to extract various dynamical length scales and show that they grow with decreasing temperature. By calculating the frequency dependent average susceptibility of such confined systems we show that the experimental interpretation of such data is very difficult.Comment: 7 pages of Latex, 3 figure

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm–137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition ($\alpha$-relaxation) and a (local) secondary $\beta$-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster ($\le 2$ decades) with decreasing film thickness. This results in a shift of the glass transition temperature $T_{\rm g}$ to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the $\alpha$-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, $T_{\rm g}$ was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of $T_{\rm g}$ with decreasing film thickness. The findings concerning the different thickness dependences of $T_{\rm g}$ are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the $T_{\rm g}$ shifts incorporates recently developed models to describe the glass transition in thin polymer films

Dynamics of H-bonded liquids confined to nanopores

Broad-band dielectric spectroscopy ($\rm 10^{-2}\; Hz$–$\rm 10^7\;Hz$) is employed to study the molecular dynamics of three low-molecular-weight glass-forming H-bonded liquids being confined in (dielectrically inactive) porous glasses with pore sizes of 2.5 nm, 5.0 nm and 7.5 nm. From the relaxation time distribution of the dielectric spectra, a three-layer model is deduced, consisting of molecules having solid-like, interfacial and bulk-like dynamics, respectively. The quantitative analysis of the different contributions shows that the bulk-like fraction scales with the pore size, the interfacial and solid-like layers remaining relatively unchanged. Our results restrict the possible existence of cooperatively rearranging clusters in the system below nanometer scale