Crosslinking aqueous systems

Similarly to the phospholipid polymers, the MPC polymers show excellent biocompatibility and blood compatibility [43—48]. These properties are based on the bioinert character of the MPC polymers, i.e., inhibition of specific interaction with biomolecules [49, 50]. Recently, the MPC polymers have been applied to various medical and pharmaceutical applications [44-47, 51-55]. The crosslinked MPC polymers provide good hydrogels and they have been used in the manufacture of soft contact lenses. We have applied the MPC polymer hydrogel as a cell-encapsulation matrix due to its excellent cytocompatibility. At the same time, to prepare a spontaneously forming reversible hydrogel, we focused on the reversible covalent bonding formed between phenylboronic acid and polyol in an aqueous system. 

Rohm and Flaas Company has developed Amberlite polymeric adsorbent resins that can be used to remove organic compounds from contaminated groundwater, aqueous wastes, and vapor-phase wastes. Amberlite XAD-4 is a crosslinked polystyrene-type polymer. It is hydrophobic and has no ionic functional groups incorporated into its resin structure. The material is most useful in removing low-molecular-weight organic substances from aqueous systems. Amberlite XAD-4 has been commercially available since the 1970s. 

Latex IPNs. Latex IPNs are the third type of IPNs and are manufactured according to the general schematic illustrated in Figure 3. Latex IPN synthesis involves the initial synthesis of a crosslinked seed polymer, usually in the form of an aqueous latex. The seed latex is then swollen with a second monomer/crosslinker/initiator system which is then polymerized "in situ" to form an aqueous IPN emulsion. Materials of this type are best suited to coating applications as illustrated by the development of "Silent Paint" by Sperling et al ( ). However, latex IPNs are limited to water emulsifiable monomer/polymer systems, most of which have fairly low service temperatures, less than 150 C. 

One of the initial disadvantages of the Sephadex polymers was being limited to use with aqueous systems. In 1964, John Moore of Dow Chemical patented a polymet that could be used for size exclusion separations involving organic solvents. These initially were highly crosslinked styrene-divinyl benzene (Styragel, Poragel, Bio-gel-S). 

Recent research effort (28-29) made It possible to take advantage of the ease of water base processing techniques. An Interesting facet of these silicone elastomer aqueous systems is the chemistry Involved. The colloidal silica used as the means of reinforcement also takes a part in the crosslinking of these materials. 

Figure 5 Selection of common cross-linkers used in molecular imprinting protocols. Both ethyleneglycol dimethacrylate (EDMA) and divinylbenzene (DVB) are very common crosslinkers in molecular imprinting. Other acrylate-based cross-linking monomers conunonly used include the branched cross-linker trimethylolpropane trimethacrylate (TRIM)-[24]. Among the water-soluble cross-linkers, there are phenylene-diacrylamide, V,V-methylene diacrylamide [22], and Z w-acryloylpiperazine [92], which have been used in aqueous systems for the imprinting of, e.g., enzymes.

Understanding of sur ctant sorption onto soil is needed to assess surfiictant mobility in soil and sur ctant-6cilitated transport of hydrophobic pollutants in soil/aqueous systems. Micdle-like amphiphilic nano-sized polyurethane (APU) particles synth ized fiom amphiphilic urethaiK acrylate ankmomers could solubilize a model hydrophobic pollutant, phenanthrene wittin their hydrophobic interiors. Batch experiments were conducted with soil slurries to compare APU Sodium Dodecyl Sulfiife (SDSX anionic surftctant fi>r the sorption onto soil. APU particles (Kn,f=0.2 mL/g) were weakly adsorbed onto the sandy soil compared to SDS (Knnf l.3 mL/g), due to their chemically crosslinked structure. Compared with SDS, APU particles exhibited the hi ier extraction efRciency to remove phenanttnene fiom the contaminated sandy soil. 

The adsorption of surface-active agents onto nonpolar surfaces from non-aqueous solvents has been much less intensively studied than aqueous systems. Such studies have generally been limited to carbon black and crosslinked polymer dispersions in hydrocarbon solvents. The orientation of the adsorbed molecules in those cases appears to remain more or less parallel to the surface, although the exact details will depend on the history of the carbon surface (e.g., the presence of oxide layers, or charges) and the mode of preparation of the polymer dispersion (i.e., polar or ionic catalyst residues in the surface). 

Polsoirea, 99 Polsoirethane, 99 Poly(vinyl acetate), 199, 200 Poly(vinyl alcohol) (PVA), 9,20,94-5, 108, 110, 200, 202, 204, 258-9 analysis of dynamics, 261-2 aqueous system, 58 C CP/MAS NMR spectrum, 259-61 crosslink structure, 259-61 Poly(vinyl alcohol) (PVA)/sodium borate, self-correlation function, 205 Polyfvinyl alcohol) (PVA)-water gel, 57 Polyfvinyl chloride) (PVC), 109, 324 Poly(vinyl methylether) (PVME), 108, 245 Porod law, 212 Porod region, 196 Proportionality equations, 127 Protein gels, 288-90 Proteins, 282... 

Siddall and Johnson [24] prepared absorbent polymers from neutralized acrylic acid and TMPTA in an aqueous medium, containing a polymeric dispersant such as carboxymethylcellulose or polyvinylalcohol to aid In uniform crosslinking when an insoluble crosslinker was used. Monomers were polymerized at 20-60 percent concentration with conventional free radical initiators for aqueous systems, at temperatures in the range of 20 to 100°C, which depended in part on the specific initiators used. The resulting gel intermediate was dried and pulverized to a granular product. Saline swelling capacities were in the range of 20 to 70 g/g. 

Swelling experiments have been used to determine the crosslink density of water swollen poly(vinyl alcohol) and poly(2-hydroxyethylmethacrylate) gels [28]. However, this method Is less valuable for water absorbent polymers than It is for organic systems. First of all, obtaining a meaningful value of aqueous systems is difficult due to weaknesses in the theory for systems with extensive hydrogen bonding. If Xi is used... 

The non-thermoplastic casein binder ensures processability in the wiping process by reducing sensitivity to heat and improving hot rub resistance. Useful top coats include a nitrocellulose lacquer or an aqueous system with a very hard and hence plating-fast polyurethane dispersion. If necessary, the latter can be crosslinked to improve the rubfastness. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Some terpolymers contain an additional cure site monomer, for example, bromotetrafluorobutene, to permit crosslinking with peroxides. Peroxide curing gives vulcanisates more resistance to amine stabilisers in motor oils, more resistance to methanol containing motor fluids. Resistance to acids, aqueous media and steam is also improved. Compression set and heat resistance are slightly inferior to bisphenol A cure systems. 

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