Zhaohua Chang

Further inquiry into the cryobehavior of aqueous solutions of glycerol

Abstract 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.

Physical aging of the glassy state: sub-Tg ice nucleation in aqueous sorbitol systems

Abstract The effects of isothermal and isochronal annealing below the glass transition on the devitrification temperature, Td, and excess enthalpy, ΔH, of the hydrated sorbitol glass have been investigated by DSC. Following isothermal annealing, both Td and ΔH vary with annealing time in a curvilinear manner. Both curves of Td and ΔH versus isochronal annealing temperature appear in a cupola shape, peaking at approximately the same annealing temperature corresponding to the annealing time duration. The Td shift and ΔH variation coincide with each other and are probably of the same origin in association with the glass relaxation. Td shift can be used as a measure of the extent of physical aging in the glasses of cryoprotective agents. High density of ice nuclei evidently occurs at temperatures near glass transition and thus the temperature of cryopreservation for biological systems should be well removed from the glass transition range.

Ultra-rapid freezing by spraying/plunging: pre-cooling in the cold gaseous layer

A thermophysical model is established to analyse the influence of pre‐cooling of a biological specimen in the cold gas layer associated with spray‐freezing techniques. The basic principles governing the process of pre‐cooling are provided. It is concluded that pre‐cooling is one of the major limiting steps in attaining an overall ultra‐rapid cooling rate. Pre‐cooling has a substantial influence on the nature of the final frozen specimens. In order completely to avoid crystallization before entry into the liquid cryogen and maximize the overall cooling rate of the specimen, precautions should be taken to control the height of the gaseous layer and the size of the specimen. The probability of the specimen being frozen in the cold gaseous layer is reduced by increasing the entry speed. The effectiveness, however, becomes less marked at speeds greater than 10 m/s. In order to minimize the risk of misinterpreting the measured cooling rate, it is necessary to specify the pre‐cooling conditions. The pre‐cooling effect is much more evident in liquid helium than in cryogens such as propane, ethane, Freon 12, 13 and 22.