Investigation of PEG crystallization in frozen PEG-sucrose-water solutions. I. characterization of the nonequilibrium behavior during freeze-thawing

Our objective was to characterize the nonequilibrium thermal behavior of frozen aqueous solutions containing PEG and sucrose. Aqueous solutions of (i) sucrose (10%, w/v) with different concentrations of PEG (1-20%, w/v), and (ii) PEG (10%, w/v) with different concentrations of sucrose (2-20%, w/v), were cooled to -70°C at 5°C/min and heated to 25°C at 2°C/min in a differential scanning calorimeter. Annealing was performed at temperatures ranging from -50 to -20°C for 2 or 6 h. Similar experiments were also performed in the low-temperature stage of a powder X-ray diffractometer. A limited number of additional DSC experiments were performed wherein the samples were cooled to -100°C. In unannealed systems with a fixed sucrose concentration (10%, w/v), the T′_g decreased from -35 to -48°C when PEG concentration was increased from 1% to 20% (w/v). On annealing at -25°C, PEG crystallized. This was evident from the increase in T′_g and the appearance of a secondary melting endotherm in the DSC. Low-temperature XRD provided direct evidence of PEG crystallization. Annealing at temperatures ≤40°C did not result in crystallization and a devitrification event was observed above the T′_g. In unannealed systems with a fixed PEG concentration (10%, w/v), the T′_g increased from -50 to -40°C when sucrose concentration was increased from 5% to 50%, w/v. As the annealing time increased (at -25°C), the T′_g approached that of a sucrose-water system, reflecting progressive PEG crystallization. A second glass transition at ∼ -65°C was evident in unannealed systems [10%, w/v sucrose and 10 (or 20%), w/v PEG] cooled to -100°C. Investigation of the nonequilibrium behavior of frozen PEG-sucrose-water ternary system revealed phase separation in the freeze-concentrate. Annealing facilitated PEG crystallization.