Gel determination

As described in Chapters 1, 2 and 4, gels are present in many gum rubbers, exerting a critical influence on their processability. For gel-containing rubbers, the dilute solution methods are not applicable, because gels are removed by filtration. Therefore, the determination of gel content is the first order of the business in the characterisation of gum rubbers. 

ASTM D3616-95 [1] describes a method for determining of gel, swelling index and dilute solution viscosity (DSV). The method has been developed for E-SBR and NBR, and toluene and methyl ethyl ketone, respectively, are used as solvents. The method is for the determination of macrogel content hut it may also be used for microgel, a particle 

The gel content is determined indirectly from the amount of soluble fraction and directly by weighing the dried gel. Examples of the reproducibility of the data by round-robin tests are given for two SBR copolymers and three NBRs of 0 to 85 % gel content. For the zero sample the standard deviation is as high as 100% indicating that a very small amount of gel may be distributed non-uniformly. The standard deviations for 50-85% gel samples are at most a few per cent. 

Sometimes a gel is generated in the post-polymerisation process it may come from hung up material somewhere in the reactor, dryer or in the passage-way. They may be hard or soft. They may appear sporadically and are difficult to catch. 

---

Values of specific surface of alumina gels determined by nitrogen adsorption and by mercury porosimetry ... 

It is seen that at high concentrations (a) becomes unity and the surface is completely covered with the more strongly adsorbed solvent. The adsorption isotherm of chloroform on silica gel, determined by Scott and Kucera (5) is shown in figure 1. It is seen that the monolayer of chloroform collects on the surface continuously until the chloroform content of the mobile phase is about 50%. At this concentration the monolayer appears complete. Thus, between 0 and 50% chloroform in the n-heptane, the interactions between the solute and the chloroform in the mobile phase are continuously increasing. 

Rudzinski and coworkers (35,36) first used the second and third gas-solid virial coefficients obtained from GC data to estimate surface areas. The surface area of silica gel determined using virial expansion data was greater than that obtained using the BET method (Section 11.1). The discrepancy was explained by noting the BET method does not take the lateral interactions into account. These interactions have an effect of decreasing the effective area of the adsorbent, thus making the calculated BET area less than it should be (Table 11.7). 

Table 1 gives some representative synthesis conditions, gel times, and architectural features of the resulting gels determined by small-angle X-ray scattering and nitrogen sorption measurements. 

Multiple field data can be used to determine the dominant interaction and to provide an estimate of the quadrupole interaction even when a distinct quadrupole lineshape cannot be discerned. As an example, the linewidths of the Ti-O-Si fragments in the above amorphous gel determined at 5.6 and 14.1 T are shown in Table 6.1. 

The structural properties of SiC>2 gel determined from gas adsorption analyses are summarized in Table 5.6. The data in Table 5.6 indicate that the stacking density of the primary Si02 colloidal particles in SiC>2 gel is usually about 0.331 0.522. The most compact arrangement of the primary SiC>2 colloidal particles happened in sample 2, which gave a coordination number of 6, while the loosest arrangement happened in sample 1, which gave a stacking density of 0.331 and coordination number of 4. 

Casting-solution and environmental variables permit far greater control over the ultimate structure and performance of phase inversion membranes than does the modification of a primary gel into a secondary gel by postformation treatments. Because the properties of the primary gel determine to a large extent those of its secondary counterpart, the former should be considered as more fundamental and important in discussing the effects of fabrication parameters such as casting-solution composition, upon performance. Once a primary gel has been formed, it may be utilized as such (particularly for low-pressure applications) or it may be subjected to various physical and/or chemical treatments for conversion into a more pressure-resistant secondary gel. 

Unless otherwise noted, the reaction was carried out using 1.0 mmol of Michael acceptors and donors (1 1) in 1.0 mL of solvent. The molar ratio of acceptor donor ruthenium is 50 50 1 (S/C = 50). Isolated yield after flash chromatography on the silica gel. Determined by HPLC analysis. dS/C = 100. eNot determined. -Determined by 13C NMR of the ketals derived from the products and (2R,3 )-butanediol. 

The vio for all particles that fall on a curve such as curve 1 in Figure 1 is the p in the first-dimension gel determined for particles that form the portion of curve 1 that is parallel to the dotted line (i.e., the smaller particles). Accuracy as high as 2% in measuring uo s can be achieved with previously-described (22) procedures of control for voltage gradient and temperature. In case R is constant and po varies, all bands are found on one size line. If discrete bands are not resolved, a single ellipsoidal band with its longer axis coincident with the size line is formed (see refs. 19, 23). 

Zeolite beta was synthesized with different silicon sources. The dissolution of the silicon sources and the reaction temperature obviously determined the crystallization rate of the zeolite. Silica sol reacted rapidly at 110, 140, and 170°C, while fumed silica and TEOS only reacted readily at 170°C. The aluminum content in the reaction gel determined the termination of the crystal growth, after all aluminum was consumed, the crystal growth stopped. The acidic properties of the obtained materials were very similar, but a significant influence of the silicon source on the particle size of the resulting zeolite was observed. 

The polymerisation of polyacrylamide gels within the pores of Kieselguhrii or polyhipe-structures (PS-DVB-copolymer with 90% pore volume) 20 allows the synthesis of composite resins, which due to their low compressability are well suited for continous flow reactors. The Kieselguhr and polyhipe matrices serve thereby as skeletons whereas the polyacrylamide gels determine the chemical and swelling properties of the reactive matrix. 

At the sol-gel transition, the molecular structure of the gel determines the probability that a gel will dry in one piece or will break into fragments. It is convenient to think of this transition as the formation of the last bond needed to create an infinite molecule. However, this transition has not been defined in terms of thermodynamics, so it may be misleading to call it the sol-gel transition at all. Yet, in practice the transition is determined by qualitative inspection when an abrupt Increase in viscosity occurs. The goal of this work with TEOS is to find the optimum combination of variables which gives a structure at the sol-gel transition which can be processed to form variously fibers, beads, frits, microballoons, shapes, seals, coatings or inorganic sponges. 

The self-diffusion coefficients of toluene in polystyrene gels are approximately the same as in solutions of the same volume fraction lymer, according to pulsed field gradient NMR experiments (2fl). Toluene in a 10% cross-linked polystyrene swollen to 0.55 volume fraction polymer has a self-diffusion coefficient about 0.08 times that of bulk liquid toluene. Rates of rotational diffusion (molecular Brownian motion) determined from NMR spin-lattice relaxation times of toluene in 2% cross-linked ((polystytyl)methyl)tri-/t-butylphosphonium ion phase transfer catalysts arc reduced by factors of 3 to 20 compai with bulk liquid toluene (21). Rates of rotational diffusion of a soluble nitroxide in polystyrene gels, determined from ESR linewidths, decrease as the degree of swelling of the polymer decreases (321. 

All packing materials are subject to the development of backpressure under flow conditions. The mechanical stability of the gel determines its maximum allowable flow rate in operation. The pressure and flow characteristics, as illustrated... 

Kanaya T, Takeshita H, Nishikoji Y, Ohkxira M, Nishida K, Kaji K (1998) Mioo- and mesoscopic stmctme of poly(vinyl alcohol) gels determined by neutron and light scattraing. Supramol Sci 5 215... 

Palladlum-dlmethylglyoxime complex on sihca gel Determination of palladium at trace levels by atomic absraption spectrometry [68]... 

Domjan, A., Geissler, E., and Laszl6, K. 2010. Phenol-polymer proximity in a thermoresponsive gel determined by solid-state H- H CRAMPS NMR spectroscopy. Soft Matter 6 247-249. 

How is this equilibrium state of the gel determined solvent filled networks are generally addressed as gels, water filled networks often also more specifically as hydrogels Equilibrium conditions have to be established for the solvent molecules, because they can move freely from one part of the system, the gel, to the other part, the surroundings with the pure solvent. The requirement is the equality of the (molar) chemical potential of the solvent (subscript s) in the two phases (superscripts g and 0)... 

For chromium speciation with ICP-DRC-MS analysis, ammonia was selected as the reaction gas. Airborne Cr(VI) was separated from Cr(III) using a silica gel determination of ultratrace quantities of Cr(VI) was possible using preconcentration via complexation and ICP-DRC-MS. For aqueous samples, ion pair reversed phase HPLC was applied. ... 

---