Volume phase transition phenomena

Many recent studies on polymer gels are related to volume phase transition phenomena of poly(acrylamide) PAAm gel [7] and poly(N-isopropyl acrylamide) PNIPAAm gel [8, 9]. The volume phase transition in gels was extensively studied by Tanaka and his coworkers [10, 11]. 

Since volume phase transition of a gel is thought to bring about dramatic changes in physical properties, this phenomenon is expected to be applied to the creation of new types of materials with switching ability. One application is the synthesis of switching-functional membranes. 

It has been established that the volume phase transition of gels is an universal phenomenon [17]. Dynamic light scattering studies indicate that the dynamic fluctuations of the density correlation diverges in the vicinity of the volume phase transition point of the gel. It has also been shown that the time scale of the density fluctuations become slow in the vicinity of the volume phase transition... 

In spite of the constant density of the gel, the friction of the poly(N-isopropylacrylamide) gel reversibly decreases by three orders of magnitude and appears to diminish as the gel approaches a certain temperature. This phenomenon should be universal and may be observed in any gel under optimal experimental conditions of the solvent composition and the temperature because the unique parameter describing the friction is the correlation length which tends to diverge in the vicinity of the volume phase transition point of gels. The exponent v for the correlation length obtained from the frictional experiment is far from the theoretical value. It will, therefore, be important to study a poly(N-isopropylacrylamide) gel prepared at the critical isochore where the frictional property of gel may be governed by the critical density fluctuations of the gel. 

As discussed in the previous section, the molecular interactions rule the mam)-scopic size and shape of gels. Since these interactions are functions of temperature, polymer concentration, solvent composition (if a mixture of solvents is used), and pH and salt concentration (for geb capable erf ionization), the volume phase transition can be induced by controlling one or some of these parameters. Before the phase transition was found in gels, various researchers had developed gels that change their degree of swelling when a stimulus b applied to them. This article, however, will describe only the systems that use the phase transition phenomenon. 

In presence of certain substances, the phase transition temperature of thermo-sensitive hydrogels is altered. This phenomenon is used to design electrothermi-cally adjustable hydrodynamic microtransistors, which are also called chemostat microvalves (Richter et al. 2007a). The valve seat of the device (Fig. 14a) is tempered by a heater and an integrated temperature sensor is used for a closed-loop control. The volume phase transition temperature of PNIPAAm decreases with increasing alcohol content in water (Fig. 14b, solid symbols). Therefore, each critical alcohol concentration or volume phase transition correlates with one characteristic isotherm. Tempered at a particular isotherm the valve switches at a certain concentration (Fig. 14b, open symbols). 

D. Patterson in 1968 based on an analysis of Flory-Rehner theory. It took ten years for the phenomenon to be experimentally observed after prediction. It was found by T. Tanaka that, when a critical amount of an organic solvent was added to a water-swollen poly(acrylamide) gel, the gel collapses. Many gels of synthetic and natural polymers have been studied. Subsequent experiments showed that a volume phase transition (swelling/collapse) could also be brought about by changes in other environmental parameters such as pH, ionic strength, and temperature. 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

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