Reduction of the glass transition temperature in polymer films: a molecular-dynamics study

We present results of molecular-dynamics simulations for a nonentangled polymer melt confined between two completely smooth and repulsive walls, interacting with inner particles via the potential U(wall)=(sigma/z)(9), where z=/z(particle)-z(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. This leads to a reduction of the critical temperature T(c) of the mode coupling theory with decreasing film thickness. Analyzing the same data by the Vogel-Fulcher-Tammann (VFT) equation, we also estimate the VFT temperature T0(D). The ratio T0(D)/T(bulk)(0) decreases for smaller D similarly to T(c)(D)/T(bulk)(c). These results are in qualitative agreement with that of the glass transition temperature observed in some experiments on supported polymer films.