Effective Tg of confined polymer-polymer mixtures.: influence of molecular size

Polystyrene (PS) and poly(tetramethyl bisphenol polycarbonate) (TMPC) form thermodynamically compatible mixtures below a lower critical solution temperature. In this regime, the PS-component in thin PS-TMPC films, supported by SiOx/Si substrates, preferentially enriches the free surface, forming a wetting layer, whereas the TMPC component enriches to the polymer-substrate interfacial region. We examined the dependence of the effective glass transition temperature, T-g, on film thickness, h, for 50/50 wt % mixtures on SiOx/Si substrates. In these experiments, the molecular weight, M-PS, of the PS component was varied by over 2 orders of magnitude (4 kg/mol < M-PS < 900 kg/mol) while the TMPC molecular weight remained fixed. The glass transition temperature of the M-PS = 4 kg/mol sample is approximately 25 degreesC lower than that of the other PS samples. We show that the effective glass transition temperature decreased with decreasing film thickness, DeltaT(g) < 0, where Tg(infinity) is the glass transition temperature at large h. Moreover, despite the variation in the glass transition temperatures associated with the PS components in the mixtures, DeltaT(g)(h) was, within experimental error, independent of M-PS. T-g(infinity), on the other hand, was sensitive to the DeltaT(g) of the PS component. The implications of these findings are discussed.