The Formation of Gels

One method of achieving a polymerised polymer-network is by the use of two distinct types of compounds, a monomer and a crosslinker. A simple example of this combination is styrene as a monomer and divinyl 

The term S represents the strength of the network. The power law exponent m was found to depend on the stochiometric ratio r of crosslinker to sites. When they were in balance, i.e. r = 1, then m - 1/2. From Equations (5.140) and (5.141) this is the only condition where G (co) = G (cd) over all frequencies where the power law equation applies. If the stochiometry was varied the gel point was frequency dependent. This was also found to be the case for poly(urethane) networks. A microstructural origin has been suggested by both Cates and Muthumkumar38 in terms of a fractal cluster with dimension D (Section 6.3.5). The complex viscosity was found to depend as  

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Torkelson and coworkers [274,275] have developed kinetic models to describe the formation of gels in free-radical pol5nnerization. They have incorporated diffusion limitations into the kinetic coefficient for radical termination and have compared their simulations to experimental results on methyl methacrylate polymerization. A basic kinetic model with initiation, propagation, and termination steps, including the diffusion hmitations, was found to describe the gelation effect, or time for gel formation, of several samples sets of experimental data. 

The formation of gels can be achieved by various chemical principles ... 

A. Parker. Process for delaying and controlling the formation of gels or precipitates derived from aluminum and corresponding compositions together with its applications particularly those concerning the operation of oil wells. Patent EP 266808,1988. 

The original test for the presence of pectinesterase was the formation of gel after an enzymic de-esterification of pectin in the presence of Ca2+ ions. This behavior was observed by Fremy107 more than 100 years ago, after he found the presence of pectase in carrots. 

A small fraction of POM is created via abiotic processes, all of which involve transformation of DOM into the particulate phase. As already noted, destabilization of colloidal DOM can lead to the formation of gels. Increasing salinity destabilizes colloids, so flocculation of DOM is common in estuaries. 

Evidence for the formation of gels from aPS systems is obtained from simple mechanical, (1.4.5) viscoelastic, (7.8) thermodynamic (1.6) and spectroscopic ( ) techniques. Simple tube tilting, falling ball methods and differential scanning calorimetry have been used to determine the phase diagrams for a number of systems. Viscoelastic measurements on the aPS-carbon disulfide system show that the low frequency response indicative of a... 

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

Ferric chloride is sometimes an important constituent of the hydrolyzing mixture. The formation of gels during polymerization can be controlled. Patents are in existence covering the hydrolysis of chlorosilanes by pouring their solutions onto the surface of a swirling solution of the active electrolyte. 

Figure 3 shows that the delta Mooney indexes show an abrupt change from negative to positive values in the same region. Thus, there is a clear relationship between the onset of dominant crosslinking and processability. The deterioration in physical characteristics is not coincidental with the formation of gel since, in both of these examples, the gel point occurs at conversions greater than 60%. 

Pronounced self-assembly of 281 and 282 in nonpolar solvents (e.g., aliphatic and aromatic hydrocarbons) takes place as a result of hydrogen bonding between amide groups to result in the formation of gels above a critical gelation concentration. 

In the polymerization of BD by Ti-, Co- and Ni-based catalyst systems the polymerization has to be shortstopped at a specific monomer conversion in order to avoid the formation of gel. In contrast, polymerization catalysis by Nd catalysts does not need control of monomer conversion. As gel formation is particularly low with Nd catalysts full monomer conversion can be accomplished [427,428]. 

Contrary to the production of EP-rubbers, from a statistical point of view the formation of gel is much more pronounced in BD polymerization. This as-... 

The oxides are produced from tertiary amines by adding the amine to 35% solutions of hydrogen peroxide over a period of 1 h at 60°C with very good mixing. During the addition period, small amounts of water must be added to prevent the formation of gel [38-40]. The... 

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