Chemical cross-linking systems

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et al. (Macromolecules, 1983. 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, it appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. 

Corresponding DLS and rheology studies to prove the validity of the Eq. (18) were performed on another thermoreversible system (three mixtures made of xanthan gum and locust bean gum (XG/LBG) (Richter et al. 2004b, 2005) and on the irreversibly radical chemical cross-linking system (W-vinylcaprolactam/ 2-hydroxylethyl- methacrylate/allylmethacrylate) (Richter et al. 2004c). 

In both cases of copolymerization, there is a noticeable decrease in the slope of the modulus curve in the r on of the inflection point. This, in essence, means a decrease in the modulus in the rubbery region. This contrasts with the chemically cross-linked systems where the modulus in the rubbery region shows some increase with increasing temperatures. In the copolymer system, the molecules are inter-coimected by physical cross-hnks due to secondary forces. These cross-hnks can be disrupted reversibly by heating, and this forms the basis of the new class of copolymers referred to as thermoplastic elastomers. 

So far, we have restricted our discussion of supramolecular network materials to chemically cross-linked systems. 

A central criterion for the design of chemically cross-linked systems is the polymerization rate, and this rate correlates directly to the setting time. A fast-curing material is highly desirable in clinics, as it would reduce the working time of the surgeon and the risk of developing infections in the defect. In addition, system viscosity increases... 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

Oil-field chemistry has undergone major changes since the publication of earlier books on this subject Enhanced oil recovery research has shifted from processes in which surfactants and polymers are the primary promoters of increased oil production to processes in which surfactants are additives to improve the incremental oil recovery provided by steam and miscible gas injection fluids. Improved and more cost-effective cross-linked polymer systems have resulted from a better understanding of chemical cross-links in polysaccharides and of the rheological behavior of cross-linked fluids. The thrust of completion and hydraulic fracturing chemical research has shifted somewhat from systems designed for ever deeper, hotter formations to chemicals, particularly polymers, that exhibit improved cost effectiveness at more moderate reservoir conditions. 

A thermally stable NHase from Comamonas testosteroni 5-MGAM-4D (ATCC 55 744) [22] was recombinantly expressed in Escherichia coli, and the resulting transformant cells immobilized in alginate beads that were subsequently chemically cross-linked with glutaraldehyde and polyethylenimine. This immobilized cell catalyst (at 0.5 % dew per reaction volume) was added to an aqueous reaction mixture containing 32wt% 3-cyanopyridine at 25 °C, and a quantitative conversion to nicotinamide was obtained. The versatility of this catalyst system was further illustrated by a systematic study of substrates, which included... 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

Radiation cross-linking of PE requires considerably less overall energy and space, and is faster, more efficient and more environmentally acceptable.93 Chemically cross-linked PE contains chemicals that are byproducts of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable.94... 

Nanoparticles have been prepared from polysaccharides, proteins, and amphiphilic macromolecules by inducing their aggregation followed by stabilization either by heat denaturation or chemical cross-linking. The former can be done by water-in-oil emulsion system or in aqueous environments. The cross-... 

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