Size and confinement effects on the glass transition behavior of polystyrene/o-terphenyl polymer solutions

Polystyrene $\left(\mathrm{PS}\right)/o-\mathrm{terphenyl}\hspace{0.5em}\left(o\mathrm{TP}\right)$ solutions confined to nanometer scale pores were studied by differential scanning calorimetry to investigate size and confinement effects on the glass transition. We observed two glass transitions $T_g$ in all thermograms for materials confined in the controlled pore glasses. One was at a lower temperature than the bulk state $T_g$ and the other was at a higher temperature. The lower transition temperature decreases with decreasing pore size, which is consistent with previous reports from this laboratory on small molecule glass formers and some other reports in similar systems. Although $o\mathrm{TP}$ and $o\mathrm{T}\mathrm{P}/\mathrm{P}\mathrm{S}$ are not hydrogen bonding materials, we interpret the higher temperature transition as due to the existence of an interacting layer at the pore surface. A two-layer model in which there exists a “core” liquid in the center surrounded by the interacting layer at the pore surface is consistent with our observations.