Polymer hydration limit

Similar behavior has been observed for noncrystallizing polymers. For example, the diffusivity of water in poly(vinylpyrrolidone) (PVP) (Oksanen and Zografi, 1993) has been shown to increase at water contents beyond the hydration limit. Additional reports have shovm that the hydration limit has physical significance for other polymer excipients. Microcrystalline cellulose and lactose for compression, for example, lose their direct compaction properties at water contents just below (Huettenrauch and Jacob, 1977), and gelatin capsules become brittle as the water content is reduced below Wm (Kontny and Mulski, 1989). Recently, the chemical stability of a model peptide in PVP matrices was shown to improve when the amorphous dispersion was stored below the polymer s hydration limit (Lai et al., 1999a Lai et al., 1999b Lechuga-Ballesteros et al., 2002). 

Penetrant (solvent) diffusion In swellable polymers has been extensively studied and many models have been proposed (see for instance references 31 and 34). Experimental data concerning water penetration in hydrophilic polymers have been mainly concerned with crosslinked polymers with limited swelling. Investigations on linear macromolecules are complicated by concomitant dissolution of individual solvated chains that leads to some erosion of the hydrated polymer mass. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

The plate dryer is limited in its scope of apphcations only in the consistency of the feed material (the products must be friable, free flowing, and not undergo phase changes) and diying temperatures up to 320°C. Applications include speci ty chemicals, pharmaceuticals, foods, polymers, pigments, etc. Initial moisture or volatile level can be as high as 65 percent and the unit is often used as a final dryer to take materials to a bone-dry state, if necessary. The plate dryer can also be used for heat treatment, removal of waters of hydration (bound moisture), solvent removal, and as a product cooler. 

Sulfonated poly(arylene ether)s have shown promise for durability in fuel cell systems, while poly-(styrene)- and poly(imide)-based systems serve as model systems for studying structure-relationship properties in PEMs because their questionable oxidative or hydrolytic stability limits their potential application in real fuel cell systems. Sulfonated high performance polymer backbones, such as poly(phe-nylquinoxaline), poly(phthalazinone ether ketone)s, polybenzimidazole, and other aromatic or heteroaromatic systems, have many of the advantages of poly-(imides) and poly(arylene ether sulfone)s and may offer another route to advanced PEMs. These high performance backbones would increase the hydrated Tg of PEMs while not being as hydrolytically sensitive as poly(imides). The synthetic schemes for these more exotic macromolecules are not as well-known, but the interest in novel PEMs will surely spur developments in this area. 

There is no quantitative model yet describing the observed electro-osmotic drag coefficients as a function of the degree of hydration and temperature. However, the available data provide strong evidence for a mechanism that is (i) hydrodynamic in the high solvation limit, with the dimensions of the solvated hydrophilic domain and the solvent—polymer interaction as the major parameters and (ii) diffusive at low degrees of solvation, where the excess proton essentially drags its primary solvation shell (e.g., H3O+). 

The activity of polymer-supported crown ethers is a function of % RS as shown in Fig. 11 149). Rates for Br-I exchange reactions with catalysts 34, 35, and 41 decreased as % RS increased from 14-17% to 26-34%. Increased % RS increases the hydro-philitity of the catalysts, and the more hydrated active sites are less reactive. Less contribution of intraparticle diffusion to rate limitation was indicated by less particle size dependence of kohMi with the higher % RS catalysts149). 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

At pH 7.0, as temperature increased from 4 to 90°C, solubility increased but water absorption increased then decreased. This suggested that water absorption and solubility may be related to a point, perhaps maximum hydration, at which solubility continues to increase and hydration does not. This appears consistent with the statements of Hermansson ( ) that water absorption is the first step in the solvation of polymers, and swelling may be limited or unlimited. 

A number of polymers exhibit this hydration property. Natural products such as cellulose and starch are or can be made water soluble. Synthetics such as polyvinyl alcohol and polyacrylic acid are also soluble in water. This discussion will be limited to synthetic materials such as polyacrylic acid and its salts, polyvinyl alcohol, polyacrylamide, and polyurethane... 

Inserted L-rhamnopyranosyl units may provide the necessary irregularities (kinks) in the structure required to limit the size of the junction zones and produce a gel. The presence of side chains composed of D-xylosyl units may also be a factor that limits the extent of chain association. Junction zones are formed between regular, unbranched pectin chains when the negative charges on the carboxylate groups are removed (addition of acid), hydration of the molecules is reduced (addition of a cosolute to a solution of HM pectin), and/or pectinic acid polymer chains are bridged by multivalent, eg, calcium, cations. 

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