Sols and Gels

Complex liquids are ubiquitous in materials manufacture. In some cases, they are formed and must be handled at intermediate steps in the manufacture of materials (e.g., sols and gels in the making of ceranucs, mixtures of monomer and polymer in reactive processing of polymers). In other cases (e.g., composite liquids), they are the actual products. Understanding the properties of complex fluids and the imphcations of fluid properties for the design of materials processes or end uses presents a formidable intellectual challenge. 

Bohidar et al. 1998, realized studies of Sol and Gel state properties of aqueous gelatin solutions of concentrations 4%, 6%, 8% and 10% (w/v) were investigated through dielectric relaxation studies done at various temperatures in the range from 20 to 60°C carried out over a frequency range 20Hz-10MHz and no relaxation of any nature was observed. 

Djabourov, M Clark, AH Rowlands, DW Ross-Murphy, SB, Small-Angle X-Ray Scattering Characterization of Agarose Sols and Gels, Macromolecules 22, 180, 1989. 

This paper concerns the main properties of water soluble pectins in sol and gel states. First of all, the methods of purification and characterization are discussed. The method of steric exclusion chromatography equipped with different detectors is demonstrated as the most useful to determine the macromolecular characteristics of these polymers the role of aggregation is pointed out. 

Seeligmann, Torrilhon, and Falconnet, in a treatise on rubber published in France in 1896, recorded interesting early observations on the sol and gel components now known to be present in most specimens of undegraded raw rubber. They referred to them as the two isomeric hydrocarbides of rubber, one being the adhesive principle and the other the nervous principle. It was observed that the latter refused to dissolve on repeated treatment with fresh portions of solvent and that the approximate percentage of this constituent was roughly the same when different solvents were used to extract the soluble portion. The authors called attention also to the enormous swelling capacity of the nervous hydrocarbide. Vulcanization was attributed to a reaction of sulfur with the adhesive principle whereby the adhesive characteristics are suppressed or eliminated. 

It is important to note, however, that beyond a = 1/2 by no means all of the material will be combined into infinite molecules. For example, in spite of the favorable probability of branching, a chain selected at random may be terminated at both ends by unreacted functional groups. Or it may possess a branch at only one end, and both of the succeeding two chains may lead to unreacted dead ends. These and other finite species will coexist with infinite networks as long as l/2[Pg.353]

That the distinction between sol and gel is by no means an arbitrary one is shown by the character of the distribution as depicted in Figs. 69 and 70. The distribution curves in the former figure always are asymptotically zero at large x, even at the gel point. Extremely large (i.e., almost infinite ) molecules, which might be regarded as intermediate between sol and gel, never account for more than an extremely small fraction of the total. The structural distinction between sol and... 

Other descriptions of Pechini-type syntheses presented by various authors are quite similar, and the only difference touches upon the definition of precursors. Some authors prefer to call them polymeric resins (see, for instance, [6, 8, 9]), although the terms sol and gel can be found with even higher probability [4,10-18],... 

The determination of the kinetic regularities for different systems is important in the first turn for understandina of the process of the networks formation and for the study of sol and gel properties. On the other hand, the solution of the kinetic problem is of the great importance from the viewpoint of the further development of the general theory of macromole-cular reactions. 

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

For comparison, a telechelic sulfonated polystyrene with a functionality f = 1.95 was prepared. In cyclohexane the material forms a gel independent of the concentration. At high concentrations the sample swells. When lower concentrations were prepared, separation to a gel and sol phase was observed. Thus, dilution in cyclohexane does not result in dissolution of the gel even at elevated temperatures. Given the high equilibrium constant determined for the association of the mono functional sample, the amount of polymer in the sol phase can be neglected. Hence, the volume fraction of polymer in the gel phase can be calculated from the volume ratio of the sol and gel phases and the total polymer concentration. The plot in Figure 9 shows that the polymer volume fraction in the gel is constant over a wide range of concentrations. 

There is a threshold between the sol and gel states determined by the helix amount. It is situated around X=7%, for a gel of concentration 4.7% g cm 3. The elastic modulus increasing with the number of bonds between chains, we may assume that the junctions are due to the helical sequences. 

Corrected from [2], within which the definition improperly attempts a redefinition of the terms sol and gel. The definition proposed here is recommended for its precision through cross-reference to the properly defined terms. 

The rate of silicate sol and gel formation is pH and water-alcohol-sensitive as is the solubility of the amorphous silica that is formed. Silica networks are based on (Si04) " tetrahedra modified by (O3 Si-O, M+) units and often addition of boron oxide, aluminum oxide, titanium IV oxide, or zirconium IV oxide. 

Farad. Boc. xiv. 10,1921) for the composition of the sols and gels of the inorganic colloidal hydroxides, e.g. zirconia produced by the hydrolysis of zirconium oxychloride. By electropotentiometric measurements of the hydrogen and chlorine ion concentrations of sols formed by hydrolysis as well as freezing point, conductivity and transport number determinations he has shown that a series of salts are formed of the types ... 

Aqueous soap solutions can be obtained in three distinct forms, the sol form containing the ionic micelle, a clear gel, and a white opaque solid the curd. The sol and gel forms of various soaps have been examined by McBain and his co-workers and shown to differ only in elasticity and rigidity, whilst the electrical conductivity, refractive index, concentration of metallic ion and lowering of the vapour pressure are all identical, results to be anticipated on the fibrillar theory. The gel as we have seen is fibrillar in nature and the conversion of a gel into a curd is brought about by the removal of soap fi om solutions in the form of relatively coarse fibres, a process similar to crystallisation. The experiments of Laing and McBain... 

Sols and Gels. The essence of the behavior characteristic of the colloidal state is that double-layer interactions are as significant as bulk interactions. In other words, surface interactions are on a par with volume interactions. This condition can therefore be realized in all systems where the surface-to-volume ratios are high, i.e., at submicroscopic dimensions. 

The statistical treatment of random stepwise crosslinking reactions (e.g. polycondensation) neglecting ring formation originates from Stock-mayer and Flory and is explained in Flory s book (55) on a number of examples. Using simple probability statistics, it is possible to calculate the molecular size distribution in the sol and in the gel, fractions of sol and gel, the crosslinking density and the fraction of free functionalities in... 

Detailed analysis of the results published by Casper and Schulz 2) and measurements with the new chromatograph mentioned above 3) have shown that irrevesible thermodynamics, including two different kinetic effects, has to be applied to explain the resolution of the PDC-column 4 5 9) and to obtain the MWD of narrowly distributed polystyrene samples 6 8). In this way, not only the MWD is obtained, but also kinetic constants and thermodynamic functions of the polymer transfer between sol and gel, as well as hydrodynamic and kinetic spreading parameters of the system investigated, can be calculated from PDC-measurements performed at different constant column temperatures, with the same sample injected. The usual static quantities (such as the exponent of the partition function, ratio of the gel/sol volumes, etc.) proposed by Casper and Schulz can then be obtained by extrapolating the results to the theta temperature of the system. In addition, spreading phenomena alone can directly be... 

Let mp s and mp be the polymer mass of the considered P-mer in the sol and gel, respectively, cp s and cp the corresponding concentrations, u(P) = (dz/dt)p the constant elution rate of the P-mer transported along the z-axis of the vertical PDC-column, and v the mean overall linear rate of the column liquid then, a trivial integration of the chromatographic transport Equation (i.e. the thermodynamically and hydrodynamically defined retention coefficient)... 

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