Networks superabsorbent

Undoubtedly, the properties of superabsorbent hydrogels occupy the key position in the problem under consideration. Being directly connected with the network formation reaction, they provide all necessary information about the details of this process. Also, the SAH properties are found to be the most reliable basis for understanding and predicting their behavior in real systems, i.e. in the soil, in contact with plants, in physiological media, etc. 

Another very interesting class of crosslinked polyelectrolytes are the so-called superabsorbents. They predominantly consist of crosslinked and (partially) neutralized poly(acrylic acid) and, hence, represent a network of flexible polymer chains that carry dissociated, ionic groups. Due to this structure they can function as water-swellable gels. Although they are hard, sandy powders in a dry... 

Fig. Z22 View of superabsorbent polymer network with cross-linked sites (SAP)...

In this section, we want to present some recent results which have been obtained by B. Mann using ESPResSo, and explain, how this has been done [41,43,47]. The system under investigation is a polyelectrolyte network, i.e. a gel of cross-linked charged pol uners. One of the most prominent features of such a network is that it is able to absorb large amounts of the solvent, up to several hundred times its dry mass. Due to its remarkable properties, this hydrogel has many industrial applications, e.g. as superabsorbants in diapers,... 

The superabsorbent composites containing SH show release of the fertilizer over 10—40 days, depending on the SH content (5 wt% to 30 wt%). The release rates into water in the initial period are higher since the SH existing on the surface or freely incorporated in the composite network are dissolved more readily in water. The SH bonded with the polymeric network needs more time to diffuse from the hydrogel granule and dissolve in water. Figure 5.72 shows schematic structures of a PAA-co-AAm/SH... 

FIGURE 5.72 Schematic structures of a PAA-AM/SH superabsorbent composite (see text) (a) in dry state (b) in swollen state (c) after release of SH which is on the surface or freely incorporated in the polymer network and (d) after release of SH that is bonded with the polymer network. (After Zhang, J., Liu, R., Li, A., and Wang, A. 2006. Ind. Eng. Chem. Res., 45, 48. With permission.)... 

The placement of fixed charges in a swellable polymer network can endow the network with a variety of useful properties, perhaps the most important of which is a discontinuous swelling transition (262). For example, the volume of a polyelectrolyte gel may abruptly expand by several hundred times as the electrolyte concentration of a bathing solution decreases below a critical value (263). This property underlies the superabsorbency of many polyelectrol)d e gels. A discontinuous swelling transition can be imderstood in terms of concepts already discussed, with attention focused on coimterions and their impact on osmotic pressure. 

The elastic modulus also influences the swelling kinetics of the superabsorbent polymer in the blend (11). For a cross-linked polymer, different shaped particles have different swelling kinetics. This results from the coupling between the diffusion of water into the polymer network through the surfaces of the particles, and the relaxation of the cross-linked polymer chains in the presence of the swelling agent. Of the simple shapes, spherical particles have the fastest kinetics whereas flat disks have the slowest kinetics. Spherical particles that are deformed into a disk-like shape swell more slowly, and therefore the more deformable swollen particles (lower elastic modulus) will be slower than similar sized particles with higher modulus that are less deformable. 

Hydrogels are networks of polymer chains that are water insoluble and superabsorbent, with a degree of flexibility... 

If repulsive forces dominate in the interaaions between polymer chain units, the gel exhibits strong swelling and behaves as a superabsorbent. Should the attractive forces (in addition to the repulsive ones) contribute to the interaction between network units, the gel can undergo a collapse. 

Cao, D. and Heimann, R.B. (1993) Structure and mechanical properties of superabsorbent poly(acrylamide)-montmorillonite composite hydrogels. Polym. Gels Networks, 1, 225-246. 

However, when superabsorbent powders incorporated with other nonwoven materials, the blend materials have special porous structure in wet and swelling properties under pressure. The capacity of superabsorbent materials to hold absorbed water inside the network under certain external stress on the material were defined as absorbency under load, " " which simulate the performance of superabsorbent materials in various applications, such as baby diapers and sanitary napkins. 

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