Rigid networks

Silica, Si02, is a hard, rigid network solid that is insoluble in water. It occurs naturally as quartz and as sand, which consists of small fragments of quartz, usually colored golden brown by iron oxide impurities. Some precious and semiprecious stones are impure silica (Fig. 14.36). Flint is silica colored black by carbon impurities. 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

Moreover, the curves of 2.0 and 5.0 phr TAC, have a more hnear relationship compared with those crosslinked with DCP alone or in the presence of 0.5 phr TAC. This strongly suggested that at higher TAC concentrations, a more rigid network was present which restricted the ability of samples to swell. The results also give further evidence that gel content is not really a suitable parameter to evaluate solid properties when using TAC in the crosslinking system. 

The crosslinking reaction is an extremely important one from the commercial standpoint. Crosslinked plastics are increasingly used as engineering materials because of their excellent stability toward elevated temperatures and physical stress. They are dimensionally stable under a wide variety of conditions due to their rigid network structure. Such polymers will not flow when heated and are termed thermosetting polymers or simply thermosets. More than 10 billion pounds of thermosets are produced annually in the United States. Plastics that soften and flow when heated, that is, uncrosslinked plastics, are called thermoplastics. Most of the polymers produced by chain polymerization are thermoplastics. Elastomers are a category of polymers produced by chain polymerization that are crosslinked (Sec. 1-3), but the crosslinking reactions are different from those described here (Sec. 9-2). 

This last explanation appears to be the most feasible since we did not observe deviations from the expected half-order kinetic law when those multiacrylate monomers were polymerized in dilute solution where no rigid network is formed (30). A further feature which corroborates this conclusion is... 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

The increases in both emission intensity and emission lifetime of fac-ClRe(CO)3(4,7-Ph2-phen) are again related to a reduction in the nonradiative decay pathways as the epoxy material polymerizes and forms a rigid network. Significantly, though, the observed rigidochromic shifts in the epoxy-based net-... 

An increase in heat deflection temperature of some thermoplastic polymers can be achieved by the addition of polyfunctional aromatic cyanates (BPA/DC in particular) and trimerization catalysts. A rigid network is formed as a resul t of the cyanate trimerization. The polymer material consists of a linear polymer and a crosslinked network and belongs to the class of semi-IPNs (semi-interpenetrating Polymer Networks) the corresponding classification is given in [34-37]. 

Thus, in all discussed oxides, the structural transformations under pressure can be described as a multiple set of transformations. First, changes occur in the intermediate-range order with a slight increase in the coordination number and a transition to the rigid network state then, a coordination transformation with the increase of the first coordination number follows. According to the computer simulation data, additional buildup of pressure to the megabar range causes a subsequent complementary increase in the coordination number. 

The earliest routes for forming ceramics from sol-gel solutions involved the precipitation of metal oxide particles from solutions. These form a true colloidal suspension a sol. Upon destabilization of this sol, aggregation takes place and a rigid network is formed a gel. A gel is intermediate between a solid and a liquid. The term sol—gel has since been used by the materials science community to describe, albeit erroneously, virtually all chemical processing of ceramics from solutions (e.g., metal oxide particle precipitation or metalorganic decomposition). This discussion focuses on the gel aspects of sol—gel synthesis and not on the sol aspects, which are treated separately in this book. 

Figure 9 The crystal structure of LaBe. The horon atoms (small circles) form a three-dimensional rigid network with 1A shorter bonds between the octahedra than within the octahedra...

The changes in polymer appearance after exposure to aqueous media were evaluated using light and SEM microscopy. Before exposure to buffer, the polymer is transparent. After exposure to buffer the polymer became opaque, and when it was cut, two different regions were found the outer region which is gel and the core, which appears as a soft solid matrix. SEM analysis of the interface of the polymer exposed to buffer showed that a kind of a rigid network was formed across the polymer sample. This network causes the polymer drop to keep its shape in water. 

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