Origin of the conflicting confinement effects

among different dynamic measures

The findings that both the diffusion coefficient, D, and glass transition temperature, T_g, of polystyrene films supported by silicon (PS-SiO_x) decreases with decreasing film thickness, h, had been one of the biggest puzzles about the effect of confinement on polymer dynamics for a long time. Our subsequent finding that the effective viscosity, ƞ_eff, of PS-SiO_x exhibits the same h-dependence as D (and T_g) substantiated the puzzle.^{3, 12} By collaborating with Ellison’s group in UT Austin (Now in Minnesota), we simultaneously examined the T_g-, ƞ_eff– and D– confinement effects of polyisobutyl methacrylate (PiBMA) supported by silica. ¹² The result shows that the measurements of T_g, ƞ_eff and D are due to different averages of gradients of the respective dynamic measure in the film. As a result, different dynamic measures may exhibit different confinement effects as found in the literature.

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