Network sohds

Molecules attract each other by weak intermolecular forces. In a covalent network sohd, each atom is covalently bonded to many other atoms. 

Silicon is the fundamental component of integrated circuits. Si has the same structure as diamond. Is Si a molecular, metal-he, ionic, or covalent-network sohd ... 

Most metals are malleable, which means that they can be hammered into thin sheets, and ductile, which means that they can be drawn into wires ( Figure 12.10). These properties indicate that the atoms are capable of sHpping past one another. Ionic and covalent-network soHds do not exhibit such behavior they are typically brittle. 

Amorphous solid Crystalline solid Materials science Polycrystalline sohd Solid state Section 15.8 Covalent network sohd Ionic crystal Metallic crystal Molecular crystal... 

Fig. 4. (a) Process streams ia heat-exchange network where A and B represent hot streams, C and D cold streams, (b) The soHd lines represent superstreams, composites constmcted from the process streams of (a). The dashed line represents the hot stream horizontally repositioned to generate a... 

ISI is available in hard copy and electronically at EPA s headquarters and regional Hbraries, and through the National Technical Information Service (NTIS). The electronic form may be installed on IBM PC-compatible computers or placed on local area networks, and mn under Microsoft WINDOWS or WordPerfect s Library program. The Macintosh version is no longer available. The 1993 update will include the ISI hardcopy, PC disks, and the PC system user manual. EPA also pubHshes ACCESS EPA, which provides sources of information, databases, and pubHcations within the EPA. Chapter 5 of that pubhcation includes important environmental databases in air and soHd waste, pesticides and toxic substances, water, and cross-program (110). EPA also provides databases accessible through EPA Hbraries, which describe the private EPA and commercial databases available to Hbrary users (111). 

As the length and frequency of branches increase, they may ultimately reach from chain to chain. If all the chains are coimected together, a cross-linked or network polymer is formed. Cross-links may be built in during the polymerisation reaction by incorporation of sufficient tri- or higher functional monomers, or may be created chemically or by radiation between previously formed linear or branched molecules (curing or vulcanisation). Eor example, a Hquid epoxy (Table 1) oligomer (low molecular weight polymer) with a 6-8 is cured to a cross-linked soHd by reaction of the hydroxyl and... 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

If the dispersion particles are attracted to each other, they tend to flocculate and form a stmcture. At low concentrations the particles form open aggregates, which give a fractal stmcture (93,94). At higher concentrations a network stmcture results, which can be so pronounced that the mixture has a yield point and behaves like a soHd when at rest. Shearing breaks up this stmcture, and viscosity decreases. 

The dissolution of soluble sihcates is of considerable commercial importance. Its rate depends on the glass ratio, sohds concentration, temperature, pressure, and glass particle size. Commercially, glasses are dissolved in either batch atmospheric or pressure dissolvers or continuous atmospheric processes. Dissolution of sodium sihcate glass proceeds through a two-step mechanism that involves ion exchange (qv) and network breakdown (18). 

In addition to the above techniques, inverse gas chromatography, swelling experiments, tensile tests, mechanical analyses, and small-angle neutron scattering have been used to determine the cross-link density of cured networks (240—245). Si soHd-state nmr and chemical degradation methods have been used to characterize cured networks stmcturaHy (246). H- and H-nmr and spin echo experiments have been used to study the dynamics of cured sihcone networks (247—250). 

Gels are viscoelastic bodies that have intercoimected pores of submicrometric dimensions. A gel typically consists of at least two phases, a soHd network that entraps a Hquid phase. The term gel embraces numerous combinations of substances, which can be classified into the following categories (2) (/) weU-ordered lamellar stmctures (2) covalent polymeric networks that are completely disordered (2) polymer networks formed through physical aggregation that are predominantly disordered and (4) particular disordered stmctures. 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Dehydration or Chemical Stabilization. The removal of surface silanol (Si—OH) bonds from the pore network results in a chemically stable ultraporous soHd (step F, Fig. 1). Porous gel—siHca made in this manner by method 3 is optically transparent, having both interconnected porosity and sufficient strength to be used as unique optical components when impregnated with optically active polymers, such as fiuors, wavelength shifters, dyes, or nonlinear polymers (3,23). 

Propenylphenoxy compounds have attracted much research. BMI—propenylphenoxy copolymer properties can be tailored through modification of the backbone chemistry of the propenylphenoxy comonomer. Epoxy resins may react with propenylphenol (47,48) to provide functionalized epoxies that may be low or high molecular weight, Hquid or soHd, depending on the epoxy resin employed. Bis[3-(2-propenylphenoxy)phthalimides] have been synthesized from bis(3-rutrophthalimides) and o-propenylphenol sodium involving a nucleophilic nitro displacement reaction (49). They copolymerize with bismaleimide via Diels-Alder and provide temperature-resistant networks. 

It was shown, that the conception of reactive medium heterogeneity is connected with free volume representations, that it was to be expected for diffusion-controlled sohd phase reactions. If free volume microvoids were not connected with one another, then medium is heterogeneous, and in case of formation of percolation network of such microvoids - homogeneous. To obtain such definition is possible only within the framework of the fractal free volume conception. 

One crucial and hence central step in the design, fabrication and operation of DNA chips, DNA microarrays, genosensors and further DNA-based systems described here (e.g. nanometer-sized DNA crafted beads in microfluidic networks) is the immobilization of DNA on different soHd supports. Therefore, the main focus of these two volumes is on the immobilization chemistry, considering the various aspects of the immobihzation process itself, since different types of nucleic acids, support materials, surface activation chemistries and patterning tools are of key concern. 

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