FURTHER INQUIRY INTO THE CRYOBEHAVIOR OF AQUEOUS-SOLUTIONS OF GLYCEROL

The effects of cooling history as characterized by cooling rate, preglass transition temperature annealing, as well as two-step cooling on the states of solidified aqueous solutions of glycerol were studied by differential scanning calorimetry (DSC). The phase diagrams of various transitions versus cooling rate, annealing temperature, and two-step cooling temperature were constructed for the first time. The state of solidified glycerol solution was classified as either glass homophase or mixture heterophase composed of vitreous material and crystalline on the basis of its thermal behavior during warming. The glass transition temperature of glass residue is approximately -105 ± 1 °C, independent of the initial concentration of the sample and the cooling history. A practical "dangerous temperature zone" associated with vitrification is defined on the basis of experimental results. Cooling rate itself does not seem likely to play a direct part in vitrification, but rather to affect the time lapse or duration in "dangerous temperature zone," which is the direct factor influencing vitrification. A particularly important observation is the apparent existence of a second glass transition observed by DSC, which appears to originate from the concentrated phase. The effect of sample size on various transitions was also studied. The results suggest that the conventionally called ante-melting is a first order transition (either devitrification or melting) that occurs in the vicinity of the interface of glass-crystalline. © 1991.