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Title: Why the glass transition temperature drops in thin polymer films
Plain: 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: 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 $M$.
Settled by: 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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)