Polymer-hydration

It was outlined in the previous section that the thermal analysis of polymers is not without problems caused by moisture. Pharmaceutically, the presence of moisture may be desirable, unavoidable or part of the functional aspects of the polymer. Thus the presence of low levels of moisture improves the compaction of HPMC by plasticisation [123], increases tablet strength by hydrogen bonding, is part of the function by which hydrophilic polymers perform in matrix tablets [123], or are used as water absorbents in wound dressings [125]. Finally, gels containing water, formed at the surface of a matrix tablet, or as gels for topical delivery contain defined quantities of water. Whichever system is examined, their use is controlled by their properties, some of which can be assessed by thermal analysis. 

In theory, water interacts with polymers in at least three ways. (1) Water may be tightly bound to the polymer (and is therefore incapable of freezing, certainly at temperatures -50°C). (2) Water may be loosely bound to the polymer, but if the temperature is low enough will freeze. This loosely bound water is generally assumed to freeze in the range -50° to -20°C. (3) Thirdly water may be present that has not interacted with the polymer. This behaves and freezes as bulk water. 

Alternatively the polymers may be exposed to different relative humidities and the samples scanned after storage. This requires careful treatment of samples since adsorbed water may be easily lost on transfer of samples from the humidity cabinet to the encapsulation device. Weight gain may be assessed by weight loss on TG analysis or by the enthalpy of dehydration during the DSC scan [125]. The technique has been used to study water distribution for HPMC [128] and in gelatin, pectin and sodium carboxymethylcellulose samples [125]. 

Clearly, the techniques of thermal analysis may also be used for systems in the gel from when the polymers may be regarded as already hydrated. Since these are effectively dissolved polymers, the thermal scans will be dominated by the endotherm corresponding to the melting of free water. However, other thermal events may be detected to the low temperature side of these major endotherms. These may be due to melting of different classes of water, or even a glass transition, followed by recrystallisation of phase-concentrated water [25]. Differentiation of the events by dielectric spectroscopy has at least allowed partial confirmation of the thermal data [ 130]. 

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Other techniques to promote complete polymer hydration include vigorous mixing and slow addition of the polysaccharide. Specially designed mixing devices have been used to promote rapid particle dispersion ( 1). Adding already prepared dispersions of guar, HPG, and HEC in nonaqueous media is another means of promoting rapid... 

Various particle surface treatments have been used to delay polymer hydration until polymer particles have been thoroughly dispersed. These include guar treatment with borax (2,94) and HEC treatment with glyoxal (95). 

Hydrated Acidic Polymers. Hydrated acidic polymers are, by far, the most commonly used separator materials for low-temperature fuel cells. Their typical nanoseparation (also see Section 1) leads to the formation of interpenetrating hydrophobic and hydrophilic domains the hydrophobic domain gives the membrane its morphological stability, whereas the hydrated hydrophilic domain facilitates the conduction of protons. Over the past few years, the understanding of the microstructure of these materials has been continuously growing, and this has been crucial for the improved understanding of the mechanism of proton conduction and the observed dependence of the conductivity on solvent (water and methanol) content and temperature. 

Acetylcholinesterase was immobilised by entrapment into a PVA-SbQ matrix (see experimental details in Refs. [88,95]). The need of polymer hydration slightly increases the response times, when compared to other immobilisation techniques. Nevertheless, the entrapment presents the advantage of providing biosensors with longer lifetimes due to the protective effect of the polymer matrix. 

For hydrophilic water-soluble polymers, hydration is the first step of dissolution in aqueous solutions, followed by dissolution of the hydrated phase. The latter step involves disentanglement of polymer molecules. In general, the dissolution kinetics follow Eq. (5.2), suggesting that the solubility of polymers and the viscosity of the hydrated phase are the major variables affecting the dissolution rate. Diffusion of dissolved drug molecules through the hydrated polymer layer also may contribute to the overall release kinetics. 

Smart, J. D. The role of water movement and polymer hydration in mucoadhesion, in Bioadhesive Drug Delivery Systems Fundamentals, Novel Approaches, and Development, New York Marcel Dekker, 1999. 

Polymer hydration causes disruption of primary and secondary structures. 

Yandrasits, M.A. et al.. Dynamics of PFSA polymer hydration measured in situ by SAXS, ECS Trans., 3, 915, 2006. 

China used these polymers in designed controlled release tablets of nifedipine and nimodipine. Using the drug solubility, polymer hydration rate, quantity of polymer, and quantity of an added surfactant as the inputs, they were able to use neural networks to predict drug release successfully. 

The mechanism of drug release from hydrophilic matrix tablets after ingestion is complex but it is based on diffusion of the drug through, and erosion of, the outer hydrated polymer on the surface of the matrix. Typically, when the matrix tablet is exposed to an aqueous solution or gastrointestinal fluids, the surface of the tablet is wetted and the polymer hydrates to form a gelly-like structure around the matrix, which is referred to as the gel layer . This process is also termed as the... 

As discnssed previously, HPMC is a non-ionic polymer and hence the polymer hydration and gel formation of its matrix is essentially independent of pH of a typical dissolntion media nsed. However, when drngs with pH-dependent aqneous... 

Using the natural polymer, hydrated cellulose, alumina nanocrystalline materials were successfully fabricated. The structure simulates the complex architecture of the origin biological matrix. They were composed of oxide nanograins, mesopores, and the branched system of capillars. These nanostructured alumina fibrous material possess the high absorption ability and may be used as drainage of purulent wound in medicine. 

Bond distributions can be analyzed for any molecular architecture. The notion of bonds can be taken in its widest meaning to comprise weakly bound solvation networks, molecular meshes, multiple entangled chains (including double-stranded DNA), and cross-linked polymers. Hydration clusters... 

Hailwood and Horrobin ( ) develqped an equation for water sorption of cellulose based on a solution theory. It permits the calculation of the fraction of the san )le inaccessible to water vapor. However, Hailwood and Horrobin assumed in the development of their equation that an ideal solid solution of polymer, hydrated polymer and water is formed. This assutrption has laeen questioned in the general discussion following the presentation of their paper and also by McLaren and Rcwen (29). 

Polymer hydration results in the relaxation of stretched, entangled, or twisted macromolecules, exposing the adhesive sites. Furthermore, chain interdiffusion is favored by polymer-water interactions dominating the corresponding polymer-polymer interactions. ... 

Cline ET, Cramer FB, DuPont de Nemours El, Process for melt spinning acrylonitrile polymer hydrates, U.S. Pat., 4,238,442, 9 Dec 1980. 

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