Filled hydrogels

Fig. 15 FESEM micrographs of PVME filled with (a, b) ferroelectric BaTiOs particles, (c, d) ferromagnetic nickel particles and (e, f) ferroelectric poly(vinylidene fluoride). The figures show the filled hydrogel in the swollen state (a, c, d) low temperature,) and shrunken state (b, d, f) high temperature). The bars correspond to (a-d) 1 pm and (e, f) 500 nm. Reprinted from Theiss et al. (2005), p. 2262. Copyright John Wiley Sons Inc. Reproduced with permission...

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

Medicine. The polymethacrylates have been used for many years in the manufacture of dentures, teeth, denture bases, and filling materials (116,117) (see Dental materials). In the orthodontics market, methacrylates have found acceptance as sealants, or pit and fissure resin sealants which are painted over teeth and act as a barrier to tooth decay. The dimensional behavior of curing bone-cement masses has been reported (118), as has the characterization of the microstmcture of a cold-cured acryUc resin (119). Polymethacrylates are used to prepare both soft and hard contact lenses (120,121). Hydrogels based on 2-hydroxyethyl methacrylate are used in soft contact lenses and other biomedical appHcations (122,123) (see Contactlenses). 

When the hydrosol ceases to flow like a Hquid (the gel time), it is termed a hydrogel (Fig. 11a). As formed, the pores are filled with the medium (usually water) in which the gel is prepared. The hydrogel may be washed to remove the by-product salt and sold in that form, in which case it may consist of up to 70% water. Because the water is trapped in the pores, the final product can stiU be a relatively free-flowing powder. 

Properties. SUica gel (see Eig. 8) is a coherent, rigid, continuous three-dimensional network of spherical particles of coUoidal sUica. Both sUoxane, —Si—O—Si—, and sUanol, —Si—O—H, bonds are present in the gel stmcture. The pores are intercoimected and fUled with water and/or alcohol from the hydrolysis and condensation reactions (40). A hydrogel is a gel in which the pores are filled with water. A xerogel is a gel from which the hquid medium... 

PMBV/PVA hydrogel (filled circles) and on TCPS (circles)... 

Hydrogels are simply water-filled gels. They are characterized by being hydrophilic yet not completely soluble in water. Those hydrogels that are able to absorb large amounts of water are referred to as superwater adsorbents. 

In a second series of experiments of type 1, the influence of the nature of the diamine on the rate of ZSM-48 crystallization has been examined (Table 2). Compared with the hydrogel involving 1,6 diaminohexane, ZSM-48 crystallizes more rapidly when 1,8 diaminooctane is present in the hydrogel. Probably the lenght of the diaminooctane chain is better accomodated into the channels of ZSM-48 zeolite as to achieve a more complete pore volume filling. Indeed, the channel length per unit cell of... 

Figure 11.6 Dialysis capillary setup that could be used to employ the SWNT sensing system in vivo. The dextran-SWNT and Con A mixture is retained in the capillary while glucose is free to diffuse across the membrane. A biocompatible hydrogel, filled with VEGF, can be used to coat the capillary. Such a system could be implanted beneath the skin, with SWNT excitation from a laser photodiode and fluorescence detection from a CCD camera. Adapted with permission from Ref. 17.

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