Volume phase transition temperature

The volume phase transition temperatures Tc for collapsing and Ts for swelling changed depending on the activity of the entrapped enzyme. When the enzyme was active in the presence of 47.3 mM of ethyl butyrate, Tc = 29.8 °C and Ts = 29.1 °C, while Tc = 28.4 °C and Tg = 27.6 °C with inactivated enzyme. The change in the phase transition temperature was presumably caused by the change in the substrate and product composition induced by enzymatic reaction within the gel phase. 

Fig. 4 (a) Hydrodynamic radii of open circles 1, filled circles 5, open squares 10, filled squares 20, and triangles 40 mol% TBAm copolymer microgels as a function of temperature at pH 3.5. (b) Dependence of the volume phase transition temperature on the mol% of TBAm in copolymer microgels at pH 3.5 (circles) and pH 8 (squares). Reprinted from [73] with permission. Copyright 2003 American Chemical Society... 

The volume phase transition temperature of thermally sensitive particles can be determined using various methods and techniques fluorescence study, light scattering, differential scanning calorimetry, and turbidity measurement. 

The adsorption process can more likely be attributed to electrostatic interaction. In fact, the increase in temperature raises the surface charge density on the thermally sensitive particle, as evidenced by electrophoretic mobility vs. temperature. In addition, the amount of water is at least close to 30% above the volume phase transition temperature. This adsorption profile (reduced adsorbed amount vs. temperature, as reported in Figure 12.22) is generally observed when the adsorption temperature is well controlled in the case of attractive electrostatic interactions and only the plateau is drastically affected by the pH, salinity, and surface charge density. 

In accordance with the reversibility of the colloidal properties of thermally sensitive particles, the adsorption of proteins is also found to be reversible in the same cases. In fact, 90% of adsorbed protein can be desorbed just by lowering the temperature (i.e., from above to below the volume phase transition temperature). The hydration processes of the particles lead to a reduction in adsorption afhnity, which favors the desorption process (Figure 12.25). Furthermore, the desorbed... 

Such behavior can be explained as follows when batch adsorption is performed above the volume phase transition temperature (1) the mechanical entrapment of protein molecules in the interfacial shell layer due to the poly(NlPAM) tentacles (octopus-like adsorption process) and... 

FIG. 7 Dependence of volume phase transition temperature (Tv) of NIPA gel (cy-lindrically shaped wet sample with d0 = 0.7 mm) on initial surfactant concentration. (From Ref. 26.)... 

Fig. 12 shows micro-gel particles cross-linked by irradiation of a phase- separated solution in the two differently swollen states, at 25°C in a highly-swollen state and at 40°C at low degree of swelling. For details see (Amdt et al. 2001a). Irradiation of a high concentrated solution results in a bulky hydrogel with typical dimension in the cm-range. The sponge-like stmcture of the formed PVME hydrogel (irradiation of a PVME-solution in the phase-separated state) at different temperatures (swollen above and below the volume phase transition temperature) is shown in Fig. 13. The irradiation dose was 50 kGy.

Fig. 13 Secondary electron micrographs of PVME hydrogel in different states at different magnifications. (a, b) Gel at high degree of swelling (25°C, temperature below volume phase transition temperature) (c, d) Gel at small degree of swelling (40°C, temperature above the volume phase transition). Reprinted from Arndt et al. (2001b), p. 321. Copyright Wiley-VCH Co. KGaA. Reproduced with permission...

Zhou SQ, Wu C (1996) In-situ interferometry studies of the drying and swelling kinetics of an ultrathin poly(W-isopropylacrylamide) gel film below and above its volume phase transition temperature. Macromolecules 29 4998-5001... 

Strong intensity of the vibration mode Volume phase transition temperature Very strong intensity of the vibration mode Weak intensity of the vibration mode... 

Volume phase transition temperature (critical temperature) Time... 

Fig.l6 Gel volume phase transition temperature in solutions with different content of salts and metal ions. Reprinted from (Guenther et al. 2007b) with kind permission from Elsevier... 

Temperature-sensitive hydrogels with actuator properties show aLCST behaviour. They are swollen at low temperatures and shrink by exceeding of the volume phase transition temperature T. The best known hydrogel with LCST behaviour is PNIPAAm (Fig. 9). 

The probably simplest controllable basic functionality consists of a thermo-sensitve hydrogel and a directly attached resistive heater (Fig. 10a) (Richter et al. 2003 Arndt et al. 2000). As a function of the electrical power per Joule heat the hydrogel can be heated above its volume phase transition temperature. 

Microvalves are the simplest hydrogel-based components. The gel actuator is directly placed within a valve chamber (Fig. 11a). The thermo-sensitive PNIPAAm is swollen at room temperature and closes the valve. For opening the attached resistive heater has to be activated. Exceeding the volume phase transition temperature of approximately 34 °C the gel shrinks and opens the valve seat. 

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). 

Fig. 14 Electronically controllable hydrodynamic microtransistor, (a) Schematic set-up. (b) Volume phase transition temperature of microgel determined by DSC measurements (solid symbols) as well as operating point of the transistor device (open symbols) for different alcohol concentrations in water. Reproduced with permission from (Richter et al. 2007a) p. 1109-1110, copyright Wiley-VCH Verlag GmbH Co. KGaA.

Debord, J. D. and L. A. Lyon (2003). Synthesis and characterization of pH-responsive copolymer microgels with tunable volume phase transition temperatures. Langmuir 19(18) 7662-7664. 

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