Glass transition at the polystyrene/polyethylene glycol interface observed via contact angle measurements

We investigated the possibility of detecting the interfacial glass transition of a polymer with static contact angle measurements of a liquid polyethylene glycol on the polymer surface. The observed contact angle θ reflects the deviation from an equilibrium state at low temperatures, and the temperature dependence of cos θ changes discontinuously at the interfacial glass transition temperature Tg, manifesting a change in the interfacial entropy. This outcome was demonstrated experimentally for a liquid of polyethylene glycol (PEG) on an atactic polystyrene (PS) surface. The evaluated Tg was ca. 362？K. This value is lower than a calorimetric Tg for a bulk PS. This difference reflects molecular interactions at the interface, indicating that the Tg obtained from the contact angle measurement is sensitive to the dynamics near the polymer/liquid interface