Gelation solvent effects

The thermodynamic excluded volume effect should be also reflected as a solvent effect on gelation. That is, a more delayed gelation is expected in a good solvent than in a poor solvent this was the case in the solution polymerization of DAP in various solvents [75,76]. In this connection, Walling [3] has reported the solvent effect on the gelation in the copolymerization of MMA with EDMA, but the result was contrary to our expectations. So, we will discuss this subject in more detail later. 

A similar solvent effect was also observed in the gelation behavior of the copolymerization of MMA with nonaethylene glycol dimethacrylate [104] The delay between the actual gel point and the theoretical one was much larger in... 

Yan, N. et al., Pyrenyl-linker-glucono gelators. Correlations of gel properties with gelator structures and characterization of solvent effects. Langmuir, 2013. 29(2) 793-805. 

Scheme 13 Azobenzene-appended cholesterol LMOGs bearing pendant ammonium or diamine units as effective gelators of polar organic solvents...

Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

The chlorosilanes are dissolved in a suitable solvent system and then blended with the water which may contain additives to control the reaction. In the case of methylsilicone resin the overall reaction is highly exothermic and care must be taken to avoid overheating which can lead to gelation. When substantial quantities of chlorophenylsilanes are present, however, it is often necessary to raise the temperature to 70-75°C to effect a satisfactory degree of hydrolysis. 

In the foregoing considerations, formation of elastically inactive cycles and their effect have not been considered. For epoxy networks, the formation of EIC was very low due to the stiffness of units and could not been detected experimentally the gel point conversion did not depend on dilution in the range 0-60% solvent therefore, the wastage of bonds in EIC was neglected. For polyurethanes, the extent of cyclization was determined from the dependence on dilution of the critical molar ratio [OH] /[NCO] necessary for gelation (25) and this value was used for the statistical calculation of the fraction of EIC and its effect on Ve as described in (16). The calculation has shown that the fraction of bonds wasted in EIC was 2-2.5% and 1.5-2% for network from LHT-240 and LG-56 triols, respectively. 

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