Water uptake mechanism

Chemically modified with glycidyl methacrylate (GMA) pH-sensitive hydrogel pH range studies showed that the water uptake mechanism of the AG-MA hydrogel was controlled by Fickian diffusion and polymer relaxation. At high pH values, the water uptake profile became more dependent on polymer relaxation. [125]... 

Mathew et al. [196] developed cross-Hnked bionanocomposites using chitosan reinforced with chitin nanocrystals and gluteraldehyde as the cross-linker. These composites were characterized by FTIR, XRD, and atomic force microscopy (AFM). The authors found that cross-hnking and chitin whiskers content were both found to impact the water uptake mechanism. Cross-hnking provided dimensional stabihty in addic medium and significantly decreased the equihbrium water uptake. Moreover, incorporation of chitin nanocrystals provided increased permeation selectivity to chitosan in neutral and addic medium. 

To solve this problem, we recently developed a lab scale solution film casting process to fabricate novel poly(arylene ether sulfone) copolymer membranes in continuous way. A small reverse roll casting machine was built and used to cast membranes with vmiform, defect free membranes with thickness about 25 to 50 microns. Efforts were also be made to evaluate the properties of the films, including proton conductivity, water uptake, mechanical properties and morphology of the membranes, and compare them with that of hand cast films cast with the same or similar polymer samples. The preliminary results obtained were reported here. 

Another major drawback of polysaccharides is their hydrophilic nature leading to low degrees of adhesion between fiber and matrix [11]. Moisture absorption takes place by three types of mechanisms namely diffusion, capillarity, and transport via micro cracks [2]. Among the three, diffusion is considered to be the major mechanism. Water absorption largely depends on the water-soluble or hygroscopic components embedded in the matrix, which acts as a semipermeable membrane. While, fiber/matrix adhesion and fiber architecture also affect the moisture absorption. The results of the water sorption experiment showed an interesting trend. The extent of water uptake was not very significant and also did not increase linearly with amount of filler (Table-2). 

In contrast to TMC-DLLA, the mechanical properties at equilibrium water uptake of TMC-CL copolymers are not significantly different from those in the dry state as shown in Table 8.1. 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

Water uptake has been implicated as a mechanism of action for tablet disintegrants. Khan and Rhodes studied the adsorption and absorption properties of various disintegrants [56]. They concluded that the... 

Some approach to tailor a new natural anion exchanger from ODA-modified clinoptilolite was studied. We verified repeatedly clinoptilolite enhanced oxyanion uptake mechanism after regeneration and so far confirmed the feasibility at laboratory level. To set this process for water treatment in recycling operation, a pilot plant research is necessary to accomplish. In contrast to clay s properties, which surface have been usually... 

In summary, there have been several physical/mechanical means developed to improve the functionality, safety, and sensory properties of psyllium. These previous investigations have indicated the possibility to improve the physicochemical, sensory, biological properties of psyllium for its optimal applications in foods. However, none of them could sufficiently solve the strong gelling and extreme water-uptake problems of psyllium. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

The study of the dynamical behavior of water molecules and protons as a function of the state of hydration is of great importance for understanding the mechanisms of proton and water transport and their coupling. Such studies can rationalize the influence of the random self-organized polymer morphology and water uptake on effective physicochemical properties (i.e., proton conductivity, water permeation rates, and electro-osmotic drag coefficients). 

In this model, proton transport in the membrane is mapped on a percolation problem, wherein randomly distributed sites represent pores of variable sizes and fhus variable conductance. The distinction of pores of differenf color (red or blue) corresponds to interfacial or by bulk-like proton transport. Water uptake by wet pores controls the transition between these mechanisms. The chemical structure of the membrane is factored in at the subordinate structural levels, as discussed in the previous subsections. 

Notwithstanding any particular structural model, water transport in PEMs, in general, should be considered a superposition of diffusion in gradients of activity or concentration and hydraulic permeation in gradients of liquid or capillary pressure. Hydraulic permeation is the predominant mechanism xmder conditions for which water uptake is controlled by capillary condensation, whereas diffusion contributes significantly if water strongly interacts with the polymeric host. The molar flux of liquid water in the membrane, N, is thus given by... 

Caramella, C., Colombo, P., Conte, U. Ferrari, F., and La Marma, A., Water uptake and disintegrating force measurements Towards a general understanding of disintegration mechanisms. Drug Dev. Ind. Pharm., 12 1749-1766 (1986). 

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