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The force-level Elastically Collective Nonlinear Langevin Equation theory ofactivated relaxation in glass-forming free-standing thin films is re-visited toimprove its treatment of collective elasticity effects. The naive cut off ofthe isotropic bulk displacement field approximation is improved to explicitlyinclude spatial anisotropy with a modified boundary condition consistent with astep function liquid-vapor interface. The consequences of this improvement ondynamical predictions are quantitative but of significant magnitude and in thedirection of further speeding up dynamics and further suppressing Tg. Thetheory is applied to thin films and also thick films to address new questionsfor three different polymers of different dynamic fragility. Variation of thevitrification time scale criterion over many orders of magnitude is found tohave a minor effect on changes of the film-averaged Tg relative to its bulkvalue. The mobile layer length scale grows strongly with cooling and correlatesin a nearly linear manner with the effective barrier deduced from thecorresponding bulk isotropic liquid alpha relaxation time. The theory predictsa new type of spatially inhomogeneous "dynamic decoupling" corresponding to aneffective factorization of the total barrier into its bulktemperature-dependent value multiplied by a function that only depends onlocation in the film. The effective decoupling exponent grows as the vaporsurface is approached. Larger reductions of the absolute value of Tg shifts inthin polymer films are predicted for longer time vitrification criteria andmore fragile polymers. Quantitative no-fit-parameter comparisons withexperiment and simulation for film-thickness-dependent Tg shifts of PS and PCare in reasonable accord with the theory, including a nearly 100 K suppressionof Tg in 4 nm PC films. Predictions are made for polyisobutylene thin films.

Dynamic Gradients, Mobile Layers, Tg Shifts, Role of Vitrification Criterion and Inhomogeneous Decoupling in Free-Standing Polymer Films