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

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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 wall (σ/z)⁹, where z |z particle-z wall| and σ 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, Tc, of mode-coupling theory with decreasing film thickness. Analyzing the same data by the Vogel-Fulcher-Tammann equation, we also estimate the VFT-temperature T0(D). The ratio T0(D)/Tbulk0 decreases for smaller D similarly to Tc(D)/Tbulkc. These results are in qualitative agreement with that of the glass transition temperature observed in some experiments on supported polymer films.