Three-dimensional polymeric networks

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

II. Structure of Three-dimensional Polymeric Networks as Biomaterials... 

As long as tn processes of this type only bifunctional monomers are used, the resulting macromolecules are linear and, as a consequence, are of the soluble and fusible type. They can be used as fiber formers, rubbers or thermoplastic resins. If. however, some of the monomers are Ui- or tetra-inethylolnrea, the reaction leads to three-dimensional polymeric networks which are hard and bntde thermosetting resins, such as Bakelite or Glyptal. ... 

The crystal structure of methyllithium is not built of discrete (CH3Li)4 molecules. Such tetrameric units are connected together to give a three-dimensional polymeric network, as illustrated in Fig. 9.6.28. The Li-C bond length within a tetrameric unit is 231 pm, which is comparable to the intertetramer Li-C bond distance of 236 pm. 

Trichloroborazene reacts with bis(trimethylsilyl)carbodiimide to form non-oxide B/C7N gels. These gels consist of three-dimensional polymeric networks linked by carbodiimide groups." Similar polycarbodiimide hydrid gels are prepared from tris-s-triazines and bis(trimethylsilyl)carbodiimide." The viscoelastic properties of a gel system based on methyltrichlorosilane and bis(trimethylsilyl)carbodiimide have been reported." ... 

Matsumoto and coworkers incorporated poly(A -(hydroxyphenyl)maleimides) into composite materials with phenol-formaldehyde resins. Poly(iV-(4-hydroxyphenyl)malei-mide) has been shown to form miscible blends with phenolic resins and, after crosslinking, produces composites with good thermal and chemical stability. The hardening or crosslinking agent most commonly used is hexamethylenetetramine (HMTA), to form an insolnble and infusible three-dimensional polymeric network (Scheme 29). 

Lydrogels are water-swellable, three-dimensional polymeric networks. Interest in hydrogels has increased in the past several years as new applications have been discovered (i). The capacity of the hydrogels to absorb water is enormous and can be as much as 1000 times the weight of the polymer. For this reason, hydrogels are used in many fields. Some of the applications of hydrogels are... 

The formation of such a space charge layer is intuitively appealing, although it is unlikely to extend very far. If protons were to play a role in the buildup of a large three-dimensional polymerous network of the kind shown in Table 6, then necessarily some negative counter ions would have to be built into such a layer. However, a more serious flaw of this model is that it cannot account for the migration of two or three valent metal ions through such a polymer layer. 

Swelling is one of the functional properties used to characterize biopolymers required for modified or controlled drug delivery systems. The biopolymers employed for modified drug delivery applications are hydrogels that form three-dimensional polymeric networks when they come into contact with water, they absorb many times their weight of water but they do not dissolve. Swelling of the ceUulosics in different biorelevant media must be established. 

Upon mixing and subsequent hardening a three-dimensional polymeric network develops within the material, which is intimately combined with the three-dimensional stracture of the hardened cement paste. A variety of polymer dispersions may be combined with inorganic cements, as long as the polymeric material is sufficiently resistant to sustain the high-pH enviromnent of the cement paste. These may be thermoplasts, such as polyvinyl acetate, polyvirtyl chloride or polyacrylate thermosets, such as epoxides, polyesters, or polyurethanes and also elastomers, such as natural rabber latex or a butadiene-styrene copolymer. Polymer additions between 5% and 20% may be considered typical. 

The factor of two in this equation arises because there are two distinguishable reactions, one for each end of the polymer. Additional equations can be written to find the rate of change in the concentration of polymers of each chain length (Dotson et al, 1996). This model has didactic value, but most geochemical polymers are three-dimensional so they have more than two attachment points. Models of three-dimensional polymerization networks, such as occur with silica polymerization (Jin et al, 2011), can become quite complex. 

The above simple description of sol-gel chemistry identifies two key ideas. First, a gel forms because of the condensation of partially hydrolyzed species into a three-dimensional polymeric network. With time the colloidal particles and condensed species link together to become a 3-D network. The physical characteristics of the gel network depend greatly upon the size of the particles and extent of cross-linking prior to gelation. At gelation, viscosity increases sharply, and solid object results in the shape of the mold. 

Figure 9.2 Formation of a three-dimensional polymeric network through esterification between boronic acid-appended polymers and hydrated polymers.

We have seen that molecules must provide two reaction sites to produce a condensation polymer. If more than two sites per molecule are available, then the chain may branch and a three-dimensional polymeric network structure may be formed, Chains connected to each other by occasional bridges are called CROSS-LINKED POLYMERS. 

Linseed oil, tung oil, and other highly unsaturated oils are used as the basis for the oil-based paints. They dry (polymerize) by an oxidative mechanism, forming ether bonds between the triglyceride molecules, and through a series of oxidatively initiated free radical reactions attacking the double bonds (47,48). Since multiple points of oxidation are present, a three-dimensional polymeric network results. 

The work describes applications of functional polymers for separations of bio-cultivated substances combining primary data with review of previously pubhshed works. An attention is paid to relationships between properties of the selected polymer, target product(s), and contaminants. Exploitation of these relationships for benefits of the separation efficiency is described. Specific phenomena and interactions taking place in phase of the polymer are discussed as well as effect of these phenomena on the separation processes. This includes specific interactions with functional groups and three-dimensional polymeric networks, transformations of substances in the polymer phase, dimerization, ion exchange isothermal supersaturation, etc. 

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