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In the literature: contested
Why the glass transition temperature drops in thin polymer films
In plain words
Very thin polymer films soften at temperatures tens of kelvin lower than the bulk material. Why, and why the effect depends on chain length in free-standing films, is debated.
Precise statement
Free-standing polystyrene films thinner than about 8e-6 cm (800 Angstrom) show $T_g$ reductions of tens of kelvin relative to bulk, with a dependence on molecular weight not seen in supported films. Determine whether the reduction arises from a mobile surface layer propagating into the film, from chain confinement or from another mechanism, and predict $T_g(h, M)$ for film thickness h and molecular weight $M$.
What would settle it
A theory that predicts $T_{g}(h, M)$ for free-standing and supported films and agrees with measurements of local mobility profiles across the film.