Thickness and Composition Dependence of the Glass Transition Temperature in Thin Homogeneous Polymer Blend Films

Abstract

Nowadays there is much interest in thin polymer films due to their importance in science and technology. Studies on Tg in thin polymer films have revealed that it is dependent on film thickness and that such dependence increases with decreasing film thickness. As for the case where the interaction between the polymer and the substrate is not large enough to affect the glass transition, Tg usually decreases with decreasing thickness. On the other hand, Tg of films coated on a strongly favorable substrate increased with decreasing film thickness due to a strong interaction between polymer and substrate. Although many experimental works have been performed on Tg in typical homopolymer thin films, there has been no work dealing with polymer blend systems. The concept of physically blending two or more existing polymers to obtain new products has been attracting widespread interest and commercial utilization.14 This blending method can be used as a good way to control the basic properties of polymers without sacrificing many attractive properties of each polymer. One of the most attractive compatible blends is that of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and polystyrene (PS). Perhaps the most unambiguous criterion of polymer compatibility is the detection of a single glass transition temperature whose temperature is intermediate between those corresponding to the two-component polymers. For the bulk blend systems, PPO and PS are known to be compatible in all compositions. In this study, to find out the thickness dependence of Tg depression in the compatible PPO and PS blend thin films, the Tgs of the films are measured as functions of thickness (200-1200 Å) and composition and compared