Volume phase transition systems

The scattered intensity increases with increasing temperature as shown in Fig. 10. The scattered intensity diverges at the spinodal temperature, Ts in this particular case Ts = ca. 34.6 °C. Experimentally such a divergence cannot be expected because a macroscopic phase separation occurs and the scattered intensity remains finite. It is worthy to note that the difference in the scattered intensities between at 34.6 °C and at 35.0 °C clearly indicates that the system undergoes a transition. The critical phenomena of the volume phase transition of non-ionic gels with respect to temperature will be discussed in Sect 5.4. 

Several attempts have been made to construct stimuli-responsive polymer solution and gel systems which undergo isothermal phase transitions by external stimulation, such as photons or chemicals. Aqueous solutions of poly(A -isopropylacrylamide) having photoisomerizable chromophores or host molecules in the pendant groups showed reversible phase separations by photoirradiation or by the addition of specific metal or ammonium ions. The gels made of the polymers also underwent photostimulated or chemical-induced volume phase transitions. 

The phase transitions, such as a phase separation of polymer solutions, a sol-gel transition, or a volume phase transition of gels, are always accompanied by conformation changes of polymers. Therefore, when the phase transitions are induced isothermally by external stimulation, the transitions cause efficient conformation changes. This contribution describes how such efficient stimuli-responsive polymer systems can be constructed. 

As described above, volume-phase transitions in gels with immobilized enzymes are available for the biochemical creation of mechanical energies when coupled with enzymatic changes within the gel phase. In the design of such immobilized enzyme systems, the concept of controlling the phase transition threshold by... 

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. 

One can see in the figure that for both gel systems the temperature dependence of volume change is not continuous an abrupt change occurs when the temperature exceeds 30 °C. It is also seen that within the experimental accuracy no difference was observed between PNIPA and mPNIPA gel beads. The presence of magnetic nanoparticles influences neither the measure of volume change nor the collapse transition temperature (abbreviated as 7c). A careful analysis based on derivatives of the qr-T curves has shown that for both kinds of PNIPA beads, Tq was found to be 32 °C. We have also studied the effect of cross-linking density on the volume phase transition. Figure 39 shows these results. 

The diffusion of a low-molecular- weight compound into a network is of great importance for the application of hydrogels, e.g., as dmg release system or as sensor material. The volume phase transition can be induced by a change of the composition of the swelling agent at constant temperature. Figure 26 shows a... 

---