Gel-sol transformations

Thixotropy is a phenomenon that occurs frequently in dispersed systems. It is defined as a reversible, time-dependent decrease in viscosity at a constant shear rate. Generally, a dispersion that shows an isothermal gel-sol-gel transformation is a thixotropic material. The mechanism of thixotropy is the breakdown and reforming of the gel structure. 

The gel of gelatine differs from that of silica in that in the former the sol-gel transformation is reversible and not in the latter. The coalescence of the amicrons of gelatine to form streptococcic fibrils occurs at low temperatures, a process readily reversed by elevation of the temperature which decreases the amount of adsorbed water, increases the mobility of the fibrils, and permits of their rupture into smaller units. Gelatine is thus readily rendered more disperse by water. 

Copolymer of m-acrylamidophenylboronic Sol-gel transformation Insulin Plasma glucose levels 40... 

Although a sol-gel transformation is present in the synthesis of any thermosetting polymer, in some fields it is used in a rather restrictive way. For example, ceramists associate sol-gel processes with the hydrolytic con-... 

Problem 8 Give an elementary idea about sol-gel transformation. Mention the physical changes occurring therein. What is the effect of dissolved substance on sol-gel transformation ... 

This sol-gel transformation proceeds perfectly continuously, because there are no singular points or rapid changes in direction in the temperaturetime curve. 

TV) Viscosity Viscosity is the property which changes remarkably during the sol-gel transformation. The relationships are clear and well defined in the case of glycerosols of gelatin as studied and examined by Arsiz. 

Samples of mesoporous titanium dioxide were synthesized via sol-gel hydrolysis of Ti(IV) tetrabutoxide in 1-butanol by atmospheric moisture in the presence of a structure-directing template dibenzo-18-crown-6 ether. After completion of the sol-gel transformation and ageing of the precipitate, the mother liquor with the precipitate was exposed to hydrothermal treatment (HTT) at 100 - 175 °C for 24 h. Then, the precipitate was separated, dried, and calcinated on air at 350 - 500 °C for 4 hours. 

Synthesis of mesoporous TiO/M composites and their photocatalytic activity in hydrogen evolution. Hydrolysis of Ti(TV) tetrabutoxide with subsequent sol-gel transformation in the presence of dibenzo-18-crown-6 ether as a template yields amorphous titanium dioxide (Fig. 1). Calcination of the parental amorphous sample at 500 °C causes crystallization of Ti02 and formation of a porous material with high specific surface area and a narrow pore size distribution with the average pore diameter 5.0 nm (Table 1, sample No. 2). Combination of the hydrothermal treatment (HTT) of Ti02 samples at 100 - 175 °C with their subsequent calcination... 

Thixotropy can be defined as an isothermal, reversible, sol-gel transformation and is a behavior common to many foods. Thixotropy is an effect brought about by mechanical action, and it results in a lowered apparent viscosity. When the body is allowed sufficient time, the apparent viscosity will return to its original value. Such behavior would result in a shear stress-rate-of-shear diagram, as given in Figure 8-19. Increasing shear rate results in increased shear stress up to a maximum after the maximum is reached, decreasing shear rates will result in substantially lower shear stress. 

The purpose of selecting XMou-SiOj as the inorganic precursor is that they are chemically active toward siloxanes. At first the hybrid XMo,-Si02 sols were prepared via hydrolysis of TEOS in the presence of XMo,under acidic conditions. A little amount of nitric acid was added to the solution in order to increase the solubility of XMo, and still remained the mono lacunary structures. Second, the sols of the hybrid materials were introduced into the spaces between the templating PS sphere arrays (with suction applied) subsequently, a solid hybrid material skeleton was constructed via in situ sol-gel transformation. The PS templates were removed by extraction with toluene solution. 

The over-all sol-gel transformation is reversible the rigidity, levorotation, volume, and heat content changing in imison through a fairly well-defined temperature range (Smith, 1919 Pleass, 1930 Katz, 1933 Holleman, et al., 1934 Derksen, 1935 Ferry, 1948b Ferry and Eldridge, 1949). 

Solutions are stable at pH 3-11. Increasing temperature reduces the viscosity of solutions. Hypromellose undergoes a reversible sol-gel transformation upon heating and cooling, respectively. The gel point is 50-90°C, depending upon the grade and concentration of material. 

Samples of mesoporous WO3/Z1O2 were obtained by the method described in [7] including formation of zirconia-tungstate sol in the presence of aqueous polyvinylalcohol (PVA) solution with subsequent sol-gel transformation. Xerogel containing polymeric template was carbonized in nitrogen stream and carbon formed was then burned out in air. Surface area of samples after calcining at 700°C was 150 - 200 m g. 

Plastisol Adhesive consisting of polymers (e.g., polyvinyl chloride) in plasticizers, solidifying to an adhesive or sealing layer by (physical) entrapment of the polymer in the plasticizers when heated (sol-gel transformation). 

Stilbene and Related compounds. The influence of sol-gel transformations on the emission spectra of cis and trans-stilbene has been assessed. The results of INDO/S calculations on stilbene has estimated the influence of the excited states involved in isomerization. The photostability of solutions of trans-stilbene to high-energy excitation with a XeCl laser has been studied. Geometric isomerization of both cis- and trans-stilbene is the result of irradiation of the compounds in NaY zeolites. A photostationary state composed of 65% cis and 35% trans is... 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

Thixotropic Pseudoplastic flow that is time-dependent. At constant applied shear rate, viscosity gradually decreases, and in a flow curve hysteresis occurs. That is, after a given shear rate is applied and then reduced, it takes some time for the original dispersed species alignments to be restored. Thixotropy in gels is sometimes termed reversible sol-gel transformation. 

Hysteresis of sol-gel transformation has been studied (5, 24), as have gelation as a function of age (2, 7), the precipitant action of quaternary ammonium com-... 

The most successful metal precursors are alkoxides of Ti and Zr, because their reactivity is easily controllable. They are liquids at room temperature and can be used without a solvent, and the metal concentration is high. After infiltration into the voids of templates, they can be solidified via a sol-gel transformation. Therefore, well-ordered macroporous crystalline TiOi and ZrOi can often be prepared. 

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