Water-swellable polymers

The development of water-swellable polymers depends on aspects of their synthesis, properties evaluation, optimization and correlation of these properties with synthesis conditions. Obviously, studying the behavior of SAH in contact with liquid and solid phases of the soil as well as with plants requires developing physical models and algorithms suitable for the prediction of SAH efficiency. 

Zweigle, M.L. and Lamphere, J.C. "Crosslinked, Water-Swellable Polymer Microgels," US Patent 4,172,066(1979). 

An intermediate layer acting as a control element, which separates the drag layers. This control element, made from a mixture of water-swellable polymers, would act as a barrier in order to delay the release of the dose of the drag present in the second layer. 

Operating as described, cylindrical three-layer tablets were prepared, weighing about 900 mg and having two layers of active substance separated by a barrier layer of water swellable polymer mixture (granulate B). 

Superabsorbents. Water-swellable polymers are used extensively in consumer articles and for industrial applications. Most of these polymers are cross-linked acrylic copolymers of metal salts of acrylic acid and acrylamide or other monomers such as 2-acrylamido-2-methylpropanesulfonic acid. These hydrogel forming systems can have high gel strength as measured by the shear modulus (134). Sometimes inorganic water-insoluble powder is blended with the polymer to increase gel strength (135). Patents describe processes for making cross-linked polyurethane foams which contain superabsorbent polymers (136,137). 

Cylindrical samples of a water-swellable polymer were immersed in water for different times. The profile of the water front moving from the surface to the inner core of the sample is visualized (Ghi et al. 1997) and compared with model calculations of the water transport. The Fickian nature of the diffusion process is proved. As a result of stress formation during the diffusion processes, cracks were formed. From analysis of T2 the presence of two types of water within the polymer follows a less mobile phase of water that is interacting strongly with the polymer matrix and a more mobile phase within the cracks. 

The aim of the current studies was to investigate an approach to the preparation of very high strength water-swellable polymers that would provide processability and control of the extent of water swell. The approach centered upon... 

FIGURE 54.23 A schematic representation of inverse miniemulsion or microemulsion polymerization for the preparation of nanometer-sized particles of water-soluble and water-swellable polymers as well as cross-linked particles in the presence of cross-linkers. (Reprinted from Polymer, 50(19), Oh, J.K., Bencherif, S.A., and Matyjaszewski, K., Atom transfer radical polymerization in inverse miniemulsion A versatile route toward preparation and functionalization of microgels/nanogels for targeted drug delivery applications, 4407-4423. Copyright 2009, with permission from Elsevier.)... 

The diverse class of water-soluble and water-swellable polymers comprising polyacrylamides contains some of the most important synthetic polymeric materials used to improve the quality of life in our modem society. Acrylamide-containing pol5nners fall into three main categories nonionic, anionic, and cationic. The projected annual sales growth rate of polyacrylamides between 1999 and 2002 is 4-7% (1). The multi-billion-dollar global market value of this class of materials makes it an economically important segment of the chemical industry. 

Water permeability data measured for a number of polymers as a function of water vapor pressure are given in Figure 3. Water permeability is essentially independent of water vapor pressure for hydrophobic polymers such as silicone, polysulfone etc. Water swellable polymers (e.g., ionomers such as Nafion and PFSA show increasing water permeability with increasing water vapor pressure. Cellulosic oolymers also show a similar effect, albeit to a lesser degree. 

Many creative methods for modifying polyacrylamides formed by inverse emulsion polymerization for enhanced activity in target applications are found in the patent literature. Flesher reported the synthesis of a cross-linked, water swellable polymer, useful for flocculation in high shear dewatering devices, by adding a small amount of a divinylic cross-linking agent [10]. 

H. Brehm, Process for the agglomeration of water-swellable polymers by means of sinter granulation, US Patent 5 248 709, assigned to Chemische Fabrik Stockhausen GmbH (Krefeld, DE), September 28, 1993. 

Amongst the various water-soluble or water-swellable polymers with high molecular weight used in hydrophilic matrices, hypromellose (hydroxypropylmethylcellulose [HPMC]) is the most commonly used polymer [5,13,37-40]. Other polymers have been studied and used on their own or in combination with HPMC to successfully modulate drug release. Examples include polyethylene oxide (PEO), with a recent review looking at its application in controlled release tablet systems [41]. Typical water-soluble hydrophilic polymers used may be classified based on their chemistry as follows ... 

The absorption rate behavior of crosslinked,water swellable polymers is explained in terms of diffusion of water into the polymer network. The absorption kinetics can be studied over time using a viscosity versus time measurement. Alternately, a one point determination of sorption time can be used to compare various polymers. The limitation of this method is that the polymers tested must have a sufficiently high swelling capacity so that the simplifying equations hold. This limitation can be eliminated by a proper choice of polymer/sallne solution ratio in the vortex test. The variables which control the absorption rate of ail the samples studied are swelling capacity, mesh size of powder particles and the distribution of particle sizes, the specific surface area of the product, and the apparent density of the polymer. 

Ezrin M, Cassidy HG. Electron exchange polymers. 6. Preparation of water-soluble and water-swellable polymers. J Am Chem Soc 1956 78 2525-2526. 

Hydrogels have been prepared from many water swellable polymers or water soluble ones after cross-linking. However, it is often very difficult to predict the permanency of a hydrogel vivo. 

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