Reduction of the Glass Transition Temperature in Polymer Films: A Molecular-Dynamics Study

We present results of molecular dynamics (MD) simulations for a non-entangled polymer melt confined between two completely smooth and repulsive walls, interacting with inner particles via the potential $U_{\rm wall}\myeq (\sigma/z)^9$, where $z \myeq |z_{\rm particle}-z_{\rm wall}|$ and $\sigma$ is (roughly) the monomer diameter. The influence of this confinement on the dynamic behavior of the melt is studied for various film thicknesses (wall-to-wall separations) $D$, ranging from about 3 to about 14 times the bulk radius of gyration. A comparison of the mean-square displacements in the film and in the bulk shows an acceleration of the dynamics due to the presence of the walls. %Consistent with this result is the observation that, within %the film, regions closer to the walls have higher mobility than those further %away towards the film center. This leads to a reduction of the critical temperature, $T_{\rm{c}}$, of mode-coupling theory with decreasing film thickness. Analyzing the same data by the Vogel-Fulcher-Tammann equation, we also estimate the VFT-temperature $T_{\rm{0}}(D)$. The ratio $T_{\mr{0}}(D)/T^{\mr{bulk}}_{\mr{0}}$ decreases for smaller $D$ similarly to $T_{\mr{c}}(D)/T^{\mr{bulk}}_{\mr{c}}$. These results are in qualitative agreement with that of the glass transition temperature observed in some experiments on supported polymer films.