Dielectric and Dilatometric Studies of Glass Transitions in Thin Polymer Films

Dielectric relaxation and thermal expansion spectroscopy were made for thin polystyrene films in order to measure the temperature Tα corresponding to the peak in the loss component of susceptibility due to the α-process and the α-relaxation time τ as functions of film thickness d. While the glass transition temperature Tg decreases with decreasing film thickness, Tα and τ were found to remain almost constant for d > dc and decrease drastically for d < dc for high temperatures. Here, dc is a critical thickness. Near the glass transition temperature, the thickness dependence of Tα and τ is more prominent. The relation between the fragility index and non-exponentiallity is discussed for thin films of polystyrene.