Hydration polymer solutions

Phase diagram of the hydrated polymer solutions. The number n of repeat units is varied from curve to curve. Binodals (solid lines) and spinodals (broken hnes) are drawn. The critical solution points are indicated by the open circles, (a) Xq = 0.002, (b) Xq = 0.003, (c) Xp=0.005. (Reprinted with permission from Ref. [29].)... 

In addition to an array of experimental methods, we also consider a more diverse assortment of polymeric systems than has been true in other chapters. Besides synthetic polymer solutions, we also consider aqueous protein solutions. The former polymers are well represented by the random coil model the latter are approximated by rigid ellipsoids or spheres. For random coils changes in the goodness of the solvent affects coil dimensions. For aqueous proteins the solvent-solute interaction results in various degrees of hydration, which also changes the size of the molecules. Hence the methods we discuss are all potential sources of information about these interactions between polymers and their solvent environments. 

We used modifications of the standard solid-state CP-MAS (cross-polarisation, magic-angle spinning) experiment to allow the proton relaxation characteristics to be measured for each peak in the C spectrum. It is known that highly mobile, hydrated polymers can not be seen using either usual CP-MAS C spectrum or solution NMR (6). We found, however, that by a combination of a long-contact experiment and a delayed-contact experiment we could reconstruct a C spectrum of the cell-wall components that are normally too mobile to be visible. With these techniques we were able to determine the mobility of pectins and their approximate spatial location in comparison to cellulose. 

H Yasuda, CE Lamaze, LD Ikenberry. Permeability of solutes through hydrated polymer membranes I. Diffusion of sodium chloride. Makromol Chem 118 196-206, 1968. 

H Yasuda, A Peterlin, CK Colton, KA Smith, EW Merrill. Permeability of solutes through hydrated polymer membranes HI. Theoretical background for the selectivity of dialysis membranes. Makromol Chem 126 177-186, 1969. 

W Brown, K Chitumbo. Solute diffusion in hydrated polymer networks. Chem Soc Faraday Trans I 71 1-11, 1975. 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

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. 

Soluble matrix systems. The third matrix system is based on hydrophilic polymers that are soluble in water. For these types of matrix systems, water-soluble hydrophilic polymers are mixed with drugs and other excipients and compressed into tablets. On contact with aqueous solutions, water will penetrate toward the inside of the matrix, converting the hydrated polymer from a glassy state (or crystalline phase) to a rubbery state. The hydrated layer will swell and form a gel, and the drug in the gel layer will dissolve and diffuse out of the matrix. At the same time, the polymer matrix also will dissolve by slow disentanglement of the polymer chains. This occurs only for un-cross-linked hydrophilic polymer matrices. In these systems, as shown in Fig. 5.3, three fronts are formed during dissolution9-11 ... 

Polyethylene glycol also has a long history of use as an agent for protein precipitation (4). It shares some of the positive attributes of ammonium sulphate in having a low heat of solution and not promoting denaturation of proteins. It appears that after the addition of polyethylene glycol, proteins are excluded from the space occupied by the hydrated polymer, and their effective concentration is increased to a level incompatible with solubility. It is less effective in the purification of IgG but is useful for the isolation of the larger IgM. 

This simplification was used by Ottewill and Walker (7) in their study of the adsorption of a nonionic surfactant onto polystyrene latex in aqueous sodium chloride. In the case of carboxylated emulsion polymers, evidence from conductometric titrations suggests that the carboxyl groups are generally concentrated near the particle surface. The resultant model of an expanded particle is that of a hydrated acid-rich shell surrounding a compact polymer core. The hydrated shell may be viewed as a dilute polymer solution where the density is close to that of water, i.e., Pe= P0. With this assumption, Equation 1 reduces to the form ... 

Non-Ionics of the C E -type have a very typical solubility behaviour, which is related to the EO-water interaction, hydration for short. First, poly(ethylene oxide), (PEO)jj is fairly soluble in water at room temperature, but polylpropylene oxide) (PPO) is not (as expected), and neither is poly(methylene oxide) (PMO), (unexpected). This irregular trend reminds us that solubility is not only determined by hydration in solution, but also by the Gibbs energy in the crystalline phase, which will be related to the molecular packing therein. Based on this difference in solubility, and hence in adsorbability, surface active polymers of the PEO-PPO type have been synthesized [Pluronics]-, they have a wide scope of application. 

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... 

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