Free-volume distribution and glass transition of nano-scale polymeric films

Positron annihilation spectroscopy has been used to measure the ortho-positronium lifetime variation with respect to the temperature in an 80-nm polystyrene film on Si in different depths. The surface and interface glass transition temperatures were found to be significantly suppressed by 18 and 12 K at the depth of 5 and 70 nm from the surface, respectively. The observed Tg-depth dependence is interpreted as a different degree of free-volume distributions at the surface and the interface with Si.