Thermal Phenomena Study Of Linear Polyalcohols Aqueous Solutions At Low Temperature

Linear polyalcohols, such as ethylene glycol (EG), glycerol, propylene glycol, are commonly used cryoprotectants. These compounds can penetrate into cells and embryos rapidly and osmotic shock can be reduced dramatically. During heating and (or) cooling processes, the cryoprotectant aqueous solutions show many thermal events such as crystallization, melting, glass transition, devitrification (cold crystallization) and enthalpy relaxation. It is very important to study these phenomena since it is useful for understanding the cryoprotective mechanism, establishing appropriate heating and (or) cooling protocols and designing of vitrification solution. Only when nucleation mechanism and supercooling behaviors be understood first, the correct cooling protocols can be determined for special cryoprotective system. In general, cryoprotective effect is related to hydration capacity of solutes used as cryoprotective agents. So it is important to study hydration properties for choosing cryoprotectants. Glass transition temperature is key parameter for glass forming system. In many case, devitrification can cause badly hurt to cells and tissues, so how to avoid devitrification upon heating process is also a important subject needed to be studied. Glass state is non-equilibrium state, and will undergo enthalpy relaxation process below glass transition temperature. Since enthalpy relaxation can heighten devitrification temperature, so it is necessary to study enthalpy relaxation behaviors below glass transition temperature.Four topics are involved in this paper, list below:1. Supercooling phenomenon in linear polyalcohols aqueous solutionsNucleation temperature of aqueous solutions with different solutes are measured by differential scanning calorimetry (DSC). The possible similarity on supercooling behaviors of linear polyalcohols aqueous solutions at the same concentration are examined.2. Hydration properties of linear polyalcoholsHydration properties of linear polyalcohols are studied by measuring melting enthalpy during heating and crystallization enthalpy during cooling. The unfrozen water (UFW) content in the solutions is calculated and compared from the molecular level. The different hydration behaviors between the compounds with hydroxy group or with methyl group are also discussed.3. Glass transition and enthalpy relaxationGlycerol/water binary system with high solute concentrations are chosen as model system. The glass transition temperature are measured and the enthalpy relaxation parameters of TNM model with the changing concentrations are calculated. The relationship between the dynamic fragility and the thermodynamic fragility, varying with concentration, are studied. The 50% EG (by mass) aqueous solution is also studied with the emphasis on the effect of crystallinity on the glass transition and the enthalpy relaxation behaviors.4. Kinetics of devitrificationSeveral different kinetic methods, such as isothermal crystallization kinetics, multiple nonisothermal (scanning) kinetics and one nonisothermal kinetics, are used to test for EG and glycerol aqueous solution systems. The crystallization kinetic parameters of these solutions with different consideration are calculated.Some new phenomena found are list below:1.Generally, the heterogeneous supercooling points (SCP) have no obvious relation with concentration for most aqueous solutions. But it has been found that the heterogeneous supercooling temperature has apparent relation with concentration for EG and glycerol aqueous solutions. (1) At low concentration region (0%~5%), supercooling points decreased (from 0% to ~2%) and then increased with increasing concentration. At moderated concentration region (5%~25%), results showed that supercooling behaviors became disorderly and unsystematic. At high concentration region (>25%), supercooling points decreased with the increasing solutes contents.2.The experiments of low concentrations show that the unfreezable water contents of EG aqueous solutions are almost the same as that of glycerol aqueous solutions for the same concentrations. This implicates that the solutions with same number of hydroxyl group have the same hydration capacity.3. For high concentrations of these two aqueous solutions,"exponential decay"of the melting enthalpy were found as the concentration increasing, since the opportunity of interaction between solutes molecules increased. But for those methyl-group compounds such as MeSO2 , the melting enthalpy of its aqueous solutions decay linearly.4. Our experiments showed that the Gordon-Taylor equation can not be used for glycerol aqueous solution. Furthermore, Gordon-Taylor relationship is no meaning if the scanning rates and Tg reading method is not given.5. The specific heat changes during glass transition processes (ΔCp) were relative not only to water content but also to heating rates for glycerol/water mixtures.6.With the increasing of water contents in glycerol aqueous solutions, the structure relaxation activation energies and dynamic fragilities were decreased. Since the thermodynamic fragilities (ΔCp) were increased with the increasing of water content, so the dynamic fragility and thermodynamic fragility change inversely if the water contents were changed in glycerol/water mixtures. 7.The DSC thermograms upon warming show that the pure EG has a single glass transition, while the 50%EG solution has two (Tg1 and Tg2) if the solution crystallized partially. Tg1 was equal to Tg of 50%EG solution in purely amorphous phase, while Tg2 was 6K higher than it. It is believed that the lower temperature transition represents the glass transition of bulk amorphous phase of EG aqueous solution glass state, while the higher one is related to ice inclusions, whose mobility is restricted by ice crystal. Cryomicroscopic observation, specific heat measurement and enthalpy relaxation experiments are also clarified the presence double glass transition processes.8. Our study showed that"one non-isothermal"kinetics can not give correct result if the crystallization peak is not symmetrical. A improved methods was given using the half-peak data which could give good results.