The glass transition of organic liquids confined to small pores

The glass-transition temperatures, T_g, of organic liquids confined to small pores were studied by differential scanning calorimetry (DSC). The T_g was measured as a function of pore size in controlled pore glasses (CPG) having pore diameters in the range of 40-730 Å. The surface of the glass was treated with hexamethyldisilazane to promote wetting by the organic liquids studied (o-terphenyl and benzyl alcohol). Glasses formed in the pores had a lower T_g than in the bulk and the reduction in T_g increased as the pore size decreased. For example, the depression of the glass transition temperature, ΔT_g, of benzyl alcohol in 40 Å and 85 Å pores was 7.2 K and 3.1 K, respectively. The magnitude of ΔT_g also depends on the material; e.g. for o-terphenyl in the 85 Å pores, ΔT_g was 8.8 K versus 3.1 K for benzyl alcohol. In general, it was noted that ΔT_g was considerably less than for the depression of the crystalline melting point, ΔT_m, studied in related work. For example, for benzyl alcohol in the 85 Å pores, ΔT_m was ∼ 25 K and ΔT_g was ∼ 3 K.