2010 Volume 67 Issue 11 Pages 626-631
Polystyrene ultra-thin films with a thickness below the diameter of gyration (∼55 nm) were prepared by spincasting of toluene precursor solution; the dependences of their physical properties (density ρ, glass transition temperature Tg, and relaxation time τ) on concentration were examined by means of ellipsometry and X-ray reflectometry. For 22 nm-thickness film, Tg was reduced with decreasing polymer concentration of the precursor below the overlap concentration of polystyrene, whereas ρ at room temperature was increased, in comparison with the bulk state. Evaluating of τ for the un-relaxed film dried at room temperature clearly showed that τ was shortened with decreasing precursor concentration, indicative of an increase of the molecular weight of an entanglement strand. This signifies that the involved local motion of the polymer chain contributes to the reduction of Tg. An important role in the reduction of Tg, specific to ultra-thin films, was found to be played by the interaction between polymer chains in the precursor solution at spincasting.
