Swollen gels

Membranes Ion-exchange membranes are highly swollen gels containing polymers with a fixed ionic charge. In the interstices of the polymer are mobile counterions. A schematic diagram of a cation-exchange membrane is depicted in Fig. 22-57. 

The gel content (toluene insoluble per cent) of the rubber and the swelling index of the gel (the ratio of the volume of a swollen gel to its unswollen volume). The former is a measure of the amount of cross-linked material and the second a measure of the intensity of cross-linking. It has been found that a sample of medium gel content (5-20%) and a medium swelling index (10-20) gives the best impact strength in the blend. 

Besides a direct addition to growth media, there are other ways of utilizing SAH for example, coating of seeds and bare roots, drilling of germinated seeds in a swollen gel ( fluid drilling [14, 15]), plant nutrient media for hydroponics, etc. 

The parameters which characterize the thermodynamic equilibrium of the gel, viz. the swelling degree, swelling pressure, as well as other characteristics of the gel like the elastic modulus, can be substantially changed due to changes in external conditions, i.e., temperature, composition of the solution, pressure and some other factors. The changes in the state of the gel which are visually observed as volume changes can be both continuous and discontinuous [96], In principle, the latter is a transition between the phases of different concentration of the network polymer one of which corresponds to the swollen gel and the other to the collapsed one. 

Taking into account the usual conditions of the hydrogel application, we shall limit our discussion to that part of the phase diagram which depicts the swollen gel and touch upon the collapse only in passing since a detailed analysis of this phenomenon is given by Tanaka [96, 97], Ilavsky [19, 98], and Khokhlov [99],... 

The traditional methods for measuring the swelling degree [100] are, as a rule, limited due to the difficulties in quantitative separation of the swollen gel from the outer solution because of extremely low strength of the former. These difficulties can be avoided by measuring the dimensions of a regular shape sample directly in an excess of liquid [19, 101,102], The other example is the modified volumetric method recently developed by us especially for SAH [103],... 

Elasticity is another important property of SAH which distinguishes the swollen gel from a viscous (but capable of flowing) solution of the non-crosslinked macromolecules. 

Dissolution of polymers is a very slow process it can take days or even weeks for particularly high relative molar mass substances. Two stages are discernible during the process of dissolution. Firstly, a swollen gel is produced by solvent molecules gradually diffusing into the polymer. Secondly, this gel gradually disintegrates as yet more solvent enters the... 

FIGURE 3.24 Plots of volume fraction of the filled rubber in the swollen gel (Vrf) against aging temperature for acrylic rubber (ACM)-silica and epoxidized natural rubber (ENR)-silica hybrid nanocomposites. (From Bandyopadhyay, A. and Bhowmick, A.K., Plastic Rubber Comp. Macromol. Eng., 35, 210, 2006. Courtesy of Money Publishers.)... 

A close analogy exists between swelling equilibrium and osmotic equilibrium. The elastic reaction of the network structure may be interpreted as a pressure acting on the solution, or swollen gel. In the equilibrium state this pressure is sufficient to increase the chemical potential of the solvent in the solution so that it equals that of the excess solvent surrounding the swollen gel. Thus the network structure performs the multiple role of solute, osmotic membrane, and pressure-generating device. 

Donnan-Type Equilibria in Polyelectrolyte Gels.—In a somewhat more rigorous fashion we consider the reduction of the chemical potential of the solvent in the swollen gel to be separable into three terms which severally represent the changes due to the mixing of polymer and solvent, to the mixing with the mobile ionic constituents, and to the elastic deformation of the network. Symbolically... 

The Sephadex was soaked for 2k hours in the experimental solvent, and the fines decanted before being packed in the columns by pumping the swollen gel from a packing reservoir with the chromatographic eluent at 2 ml/min. Calibration standards of each of the Br AMP, OH-AMP and NH2 AMP derivatives were obtained. Solutions of each were made up in the experimental buffer, at concentrations between 50 and 150 ppm, and chromatographed. The calibration curves are shown in Figure 2. 

The reduction of palladium(II) with an alcoholic solution of NaBH4 [101] or by treatment in situ of the methanol-swollen material under hydrogen [129] yielded a supported palladium catalyst, referred to as self supported by the authors [101,129]. The same co-polymerization reaction was carried out inside the nanopores of a DMF-swollen gel-type resin made by DMA and MBAA (crosslinker, 4% mol) [101,129], thus obtaining a sequential IPN [131]. Also this material was transformed into a... 

The problem of transport of molecules through swollen gels is of general interest. It not only pertains to catalysis, but also to the field of chromatographic separations over polymeric stationary phases, where the partition of a solute between the mobile phase (liquid phase) and a swollen polymeric stationary phase (gel phase) is a process of the utmost importance. As with all the chemical and physicochemical processes, the thermodynamic and the kinetic aspect must be distinguished also in partition between phases. 

The model, therefore, predicts the elution behavior of solutes during a chromatographic process over a swollen gel as the stationary phase as a function of solute size and of the gel nanomorphology. On the reverse, from the elution behavior of solutes of known molecular size it is possible to extract the polymer chain concentration from chromatographic experiments, where an unknown swollen gel is the stationary phase. This is the basis of the ISEC, which is so often mentioned through this chapter [16,17,105,106]. 

The polymer chain concentration and the polymer chain radius were employed to assess the rotational mobility of a molecule within a swollen gel [14]. To this purpose the gel is considered a thick, viscous polymer suspension. Its viscosity can be evaluated with the following equation, proposed by Nicodemo and Nicolais for concentrated suspensions of polymeric fibers [147] ... 

To summarize, there is a sizable and self-consistent body of data indicating that rotational and translational mobility of molecules inside swollen gel-type CFPs are interrelated and controlled mainly by viscosity. Accordingly, T, self-diffusion and diffusion coefficients bear the same information (at least for comparative purposes) concerning diffusion rates within swollen gel phases. However, the measurement of r is by far the most simple (it requires only the collection of a single spectrum). For this reason, only r values have been used so far in the interpretation of diffusion phenomena in swollen heterogeneous metal catalysts supported on CFPs [81,82]. 

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