α-melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol

Abstract

The α-melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol (POS) has been investigated by differential scanning calorimetry (DSC), combined with polarized-light microscopy. Starting from a completely liquid state, the melt was first cooled down and maintained at a temperature, T1 , during a time, t1, where the α-phase formed. Then it was heated to a temperature, T2, above the melting point of α for isothermal solidification into a solid phase, which was identified as δ. Based upon DSC solidification peaks, the time-temperature-transformation (TTT) diagram of POS was constructed for these solidification conditions and was compared with the TTT diagram of direct crystallization from the melt. The α-melt-mediated solidification showed accelerated kinetics of the δ-phase. The effects of T1 and t1 were also studied: at short t1, crystallization was faster with a decreasing value of T1, whereas the opposite trend was observed for a longer plateau at T1. These tendencies were interpreted in terms of three competing phenomena: the density of δ-nuclei that can form during the plateau at T1, α-δ solid-state transformation, and memory effects of molecule arrangements in the α-remelted phase.