Surface glass transition of polymer films supported by a weakly interacting substrate*

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and contact angle measurements were used to measure the surface glass transition temperature, T_g^surf, of polystyrene (PS) films with molecular weights from 12k to 1119 kg/mol. For the films supported by silica, as the film thickness, h, was decreased, T_g^surf decreased in a manner similar to » T_g^film – 20 K (where T_g^film is the T_g of the films), supporting correlation between T_g^surf and T_g^film. At the same time, the width of the surface glass transition, W, broadened noticeably when h fell below » R_g + 10 nm, where R_g is the radius of gyration the polymer. We have also studied T_g^surf of PS films supported by polydimethylsiloxane (PDMS) - a substrate with weaker surface interactions with PS than silica does. A similar but bigger reduction in T_g^surf and a similar broadening in W were observed with decreasing h. We tentatively interpret these results based on de Genne’s sliding chain motion.