  The force-level Elastically Collective Nonlinear Langevin Equation theory of
activated relaxation in glass-forming free-standing thin films is re-visited to
improve its treatment of collective elasticity effects. The naive cut off of
the isotropic bulk displacement field approximation is improved to explicitly
include spatial anisotropy with a modified boundary condition consistent with a
step function liquid-vapor interface. The consequences of this improvement on
dynamical predictions are quantitative but of significant magnitude and in the
direction of further speeding up dynamics and further suppressing Tg. The
theory is applied to thin films and also thick films to address new questions
for three different polymers of different dynamic fragility. Variation of the
vitrification time scale criterion over many orders of magnitude is found to
have a minor effect on changes of the film-averaged Tg relative to its bulk
value. The mobile layer length scale grows strongly with cooling and correlates
in a nearly linear manner with the effective barrier deduced from the
corresponding bulk isotropic liquid alpha relaxation time. The theory predicts
a new type of spatially inhomogeneous "dynamic decoupling" corresponding to an
effective factorization of the total barrier into its bulk
temperature-dependent value multiplied by a function that only depends on
location in the film. The effective decoupling exponent grows as the vapor
surface is approached. Larger reductions of the absolute value of Tg shifts in
thin polymer films are predicted for longer time vitrification criteria and
more fragile polymers. Quantitative no-fit-parameter comparisons with
experiment and simulation for film-thickness-dependent Tg shifts of PS and PC
are in reasonable accord with the theory, including a nearly 100 K suppression
of Tg in 4 nm PC films. Predictions are made for polyisobutylene thin films.
