Sol-gel phase transitions

Irradiation at X>460 nm gave a fast cis-trans isomerization, with a concomitant increase in Tg. The sol-gel phase transition could be controlled in this way by light, while read-out of the states could be performed by measuring the modulation of the transmission or CD. For instance, the traras-azobenzene gelators displayed a CD effect, presumably due to the formation of helical aggregates, whereas the cis-isomers did not. 

The formation of a three dimensional infinite network in a polymer by a chemical or physical process leads to a gel. Polymer gels are classified into two types, irreversible and reversible. The latter are formed by cross-linking due to physical interaction between certain points on different polymer chains, and the sol-gel phase transition is induced by a change in temperature. At temperatures below Tgeb... 

The sol-gel transition (Winter and Mours 1997 Stauffer 1998 Stauffer et al. 1982 Adam and Lairez 1996) of polymers is a very important branch of critical phenomena. Its kinetics characterizes the chemistry and physics of gelation. The determination of the gelation threshold is an important parameter to control the mechanical properties of a gelling system in polymer industry. In the sol-gel phase transition, an infinitely large macromolecule is formed, which can only swell but cannot be dissolved in a solvent. Critical phenomena are those which occur exactly in the phase transition or asymptotically close to it. 

Murata, K. Aoki,M. Suzuki, T. Harada, T. Kawabata. H. Komori, T. Ohseto. F. Ueda, K. Shinkai. S. Thermal and light control of the sol-gel phase-transition in cholesterol-based organic gels—Novel helical aggregation modes as detected by circular-dichroism and electron-microscopic observation. J. Am. Chem. Soc. 1994, 116, 6664-6676. 

The reversible nature of DNA hybridization in response to an external stimulus causes this type of DNA-polymer hydrogels to have special properties, such as sol-gel phase transition and responsive releasing capability [93]. Thus, potential applications of these materials as label-free DNA sensing device or for controlled drug delivery have been discussed. Just a small portion of DNA is needed to achieve hydrogel responsiveness. 

Finally it was observed that UV Ught irradiation of a (S)-29Me gel in 1-butanol induced the trans-to-cis isomerization in the azobenzene unit, thereby causing the gel to turn into the solution. The gel could be re-formed by visible light irradiation. The sol-gel phase transition could be controlled reversibly by light and monitored efficiently by CD spectroscopy [44,46]. 

For the free ligand, the fibers were observed to have a left-handed heUcal shape. Remarkably, both color change and sol-gel phase transition could be reversibly controlled by the redox state of the metal in the complex. It is worth mentioning that metal coordination was also useful to control the selfassembling properties of cholic amide-phenanthroline gelating agents [86]. 

S.-i. Kawano, N. Fujita and S. Shinkai, Quater-, quinque- and sexithiophene organogelators nnique ther-mochromism and heating-free sol-gel phase transition, Chem. Eur. J., 11, 4735-4742 (2005). 

FIGURE 2.3 Thermal stimuli-responsive sol-gel phase transition of self-assembled supramolecular nanofibers. Reprinted with permission from Reference 30. Copyright 2007 John Wiley Sons, Inc. For a color version of this figure, see the color plate section. 

James et al. synthesized a new gelator by combining a boronic acid moiety with the cholesterol skeleton (cholesterylphenylboronic acid 1). It was found that saccharide complexes of 1 efficiently gelatinize several organic solvents. The gelation properties such as the sol-gel phase transition... 

Injectable hydrogels undergo a sol-gel phase transition in response to external stimuli, typically a temperature change. At room temperature the polymer is in solution and can be injected in the body to fill in a defect. When... 

Table 2 Influence of the added [60]fullerene on (Tgei) of 2a/toluene and 2c/toluene gels the sol-gel phase transition temperatures...

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