The glass transition of organic liquids confined to small pores

The glass-transition temperatures, Tg, of organic liquids confined to small pores were studied by differential scanning calorimetry (DSC). The Tg 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 Tg than in the bulk and the reduction in Tg increased as the pore size decreased. For example, the depression of the glass transition temperature, ΔTg, of benzyl alcohol in 40 Å and 85 Å pores was 7.2 K and 3.1 K, respectively. The magnitude of ΔTg also depends on the material; e.g. for o-terphenyl in the 85 Å pores, ΔTg was 8.8 K versus 3.1 K for benzyl alcohol. In general, it was noted that ΔTg was considerably less than for the depression of the crystalline melting point, ΔTm, studied in related work. For example, for benzyl alcohol in the 85 Å pores, ΔTm was ∼ 25 K and ΔTg was ∼ 3 K.