Freezing hysteresis

When we compared the viscosities of solutions of natural rubber and of guttapercha and of other elastomers and later of polyethylene vs.(poly)cis-butadiene, with such bulk properties as moduli, densities, X-ray structures, and adhesiveness, we were greatly helped in understanding these behavioral differences by the studies of Wood (6) on the temperature and stress dependent, melting and freezing,hysteresis of natural rubber, and by the work of Treloar (7) and of Flory (8) on the elasticity and crystallinity of elastomers on stretching. Molecular symmetry and stiffness among closely similar chemical structures, as they affect the enthalpy, the entropy, and phase transitions (perhaps best expressed by AHm and by Clapeyron s... 

One study specifically designed for PEFC was reported by Thompson et al.8 They used a direct current to measure the proton conductivity at low temperature. In conjunction with the DSC data, they found the dependency of crossover temperature (temperature where the activation energy changes) on water content and hysteresis between freezing and melting. 

The seasonal variation in thermal hysteresis found in D. canadensis is due to more than shifts in concentrations of THPs, however. Unlike the freezing-avoidance strategy of Antarctic and Arctic fishes, in which the amount of thermal hysteresis appears to be due strictly to the concentration of THPs in the body fluids, freeze-avoiding insects employ a complex set of mechanisms to achieve the high levels of thermal hysteresis found for hemolymph in winter-collected specimens. 

Low-temperature calorimetry (S83,B112) has been used to study coarse porosity. The method is based on the fact that water in pores freezes at a lower temperature than water in bulk. The ice forms through the advance of a front, analogous to the intrusion of mercury or the desorption of water. Hysteresis effects indicated the existence of necks in the pores, and the occurrence of up to three distinct peaks on curves of apparent heat capacity against temperature was interpreted as indicating maxima in the pore size distribution. Coarsening of the pore structure on drying was confirmed. 

Figure 1. (a) Relative permittivity, k, as a function of temperature, showing melting and freezing along with the hysteresis, (b) Shift in the melting temperature ATm as a function of l/H for nitrobenzene in CPG. The linear behavior is consistent with the Gibbs-Thomson equation. 

The comparison of the hysteresis behavior in simulation and experiment, shows that the hysteresis is mainly due to the existence of metastable states rather than due to kinetic effects. The asymmetry in the freezing and melting branches of the adsorption curve is explained based on the Landau free energy surfaces. The Landau free energy approach is a powerful tool in determining the freezing temperature, nature of the phase transition, structure of the confined phases, existence of metastable states and origin of the hysteresis behavior. 

The use of frontal chromatography would appear to avoid the errors produced by hysteresis in that the sorption and desorption processes can be separated in time. An extensive study of this method for water sorption, using freeze-dried coffee as a... 

Pore size distributions are determined from the hysteresis loop in gas adsorption/desorption isotherms and from calorimetric measurements by the shift in the melting (or freezing) peak for a phase transition of water inside the pores. The determination of the fractional rejection properties is done by permeation experiments of a macromolecular solute with a broad molecular weight distribution (MWD). The MWD of permeate and feed are compared and translated into a fractional rejection curve. The comparison of results obtained from these three independent methods for some characteristic membranes gives an indication of the strength and weakness of each of the methods studied. 

Figure 63.1a shows the equilibrium moisture content (dry basis) at the different a for the desorption experiments (obtained from samples dried by hot air) and also for the adsorption experiments (obtained from the freeze-dried samples). Both isotherms are very similar and no hysteresis phenomenon was observed. Hysteresis is justified by assuming that during... 

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