Volumetric Phase Transition

Volumetric phase transition is the phenomenon in which the equilibrium degree of swelling or volume shows large discontinuous change in response to the external conditions, such as temperature or solvent composition. Although it was theoretically predicted many years ago, Tanaka et al. discovered it experimentally for the first time on acrylamide 

It should be noted that only when the volume is discontinuous should the term phase transition be used. Also, the i ion where the change is from continuous to discontinuous is the critical phase transition. In principle, one cannot claim phase transition without first experimentally proving discontinuity, such as coexistence of two phases. Although it is not an easy task to show critical phase transitions or even a sufficiently close-to-phase transition, the majority of the literature on gels is vague on these issues and the terms phase transition or critical point are often used without detailed evaluation. 

For this problem it is also easier to replace the stability of gels with the mechanical stability of networks. The requirement for mechanical stability of a homogeneous, isotropic material is given by bulk modulus 0, and the corresponding stability requirement of networks is given 

This graph expresses both the volume ratio (V /V o) and the volume fraction of the network (( i).The broken line indicates the primary phase transition. Here binodality is observed. 

If the equilibrium swelling line is (a), phase transfer will not take place. Volumetric phase transition occurs in the case of (c) where the gel 

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The hatched area indicates an unstable region, the broken line is the spinodal line, and the dotted line is the volumetric phase transition. 

Volumetric phase transition typically occurs when the concentration dependence of x is large. 

Polyelectrolyte gels exhibit various anomalous phenomena, including volumetric phase transitional electrical shrinkage, and nonfrozen water [29-32]. Of these, the electrical properties of ionic gels (e.g., electrical shrinkage) have been extensively studied because of their important role in response and control by electricity. They can also be involved in the information transfer of biomaterials. The sol-gel transition using direct... 

Fig.1 Surfactant concentration dependence on volumetric phase transition temperature of a NIPAAm gel in SDS. DTAC and NODE solutions [36].

Figure 7 shows the temperature dependence of the activity of a fiiee enzyme and the enzyme when included as part of PNIPAAm and PVME gels. The activity of the free enzyme in the temperature range 20-50°C increases as temperature increases. In contrast, the enzyme included in the PNIPAAm gel drastically reduces its activity 30°C and there is almost no activity above the volumetric phase transition temperature. Park and Hoffman reported that there is little difference in the activity at a certain constant temperature above and below the transition when the gel is subjected to higher and lower temperatures, leading to an increase in apparent activity as a result of the acceleration of the supply of the matrix... 

Figure 8 shows the time variation of the glucose concentration produced during repeated temperature changes of above and below the volumetric phase transition temperature. In the case of PNIPAAm, there is no enzyme activity above the phase transition temperature at 37°C. 

The amount of glucose passing through a PVME gel shows the same trend as for the PNIPAAm gel when temperature dependence is considered. Below the volumetric phase transition temperature of the PVME gel at 38°C, glucose diffusion is high. Around the transition temperature, it decreases and almost no diffusion takes place above the transition temperature. 

Fig. 1 Local volumetric phase transition of a dye-modified polymer gel by visible light irradiation.

In this section, the volumetric phase transition of the dye-modified, thermoresponsive polymer gel by irradiation of light has been reviewed. Using this principle, the amount of transmitted light can be reduced discontinuously by irradiating any gel made of any polymer that will shrink when heat is used. Furthermore, it is also possible to develop a gel that swells by irradiation of light and increases the intensity of the transmitted light discontinuously. In diis case, it is necessary for the gel to swell discontinuously when light is not irradiated. 

There are many natural hydrogels such as konnyaku, agar, and kamaboko. As polymer chemistry develops, synthetic polymer gels are increasingly used for separation, and as medical materials and sealants. However, these gels do not respond to external stimuli. Since the discovery and theoretical development of the volumetric phase transition by Tanaka (MIT) [9] many stimuli-responsive gels have been synthesized and their stimuli responsive behavior has been studied. 

A thermoresponsive polymer hydrogel can be synthesized by copolymerizing these acrylamide-type monomers and a difunctional monomer such as methylenebisacrylamide. Hirokawa and other researchers studied the temperature dependence of the degree of swelling of a thermoresponsive poly(N-isopropylacrylamide) gel in pure water [18]. It was observed that a discontinuous volumetric phase transition occurred around the LCST of the polymer and the volume change was 8 times. 

C. It shows high solubility to polar solvents other than water-like alcohol. If it is pure, then it can be stored for several months in a dark place without stabilizing agents. The polymer s concentrated aqueous solutions show gel-like properties. Partially hydrolyzed gel will exhibit volumetric phase transition by changing the temperature, pH, and solvent composition. 

At this temperature, a drastie volumetric phase transition can be seen for this gel. 

When doing this in water, the gel will have a volumetric phase transition if temperature is >32°C, so polymerize it at a low temperature using a polymerization accelerator. 

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