Classes of materials

Another important class of materials which can be successfiilly described by mesoscopic and contimiiim models are amphiphilic systems. Amphiphilic molecules consist of two distinct entities that like different enviromnents. Lipid molecules, for instance, comprise a polar head that likes an aqueous enviromnent and one or two hydrocarbon tails that are strongly hydrophobic. Since the two entities are chemically joined together they cannot separate into macroscopically large phases. If these amphiphiles are added to a binary mixture (say, water and oil) they greatly promote the dispersion of one component into the other. At low amphiphile... 

Semiconductors are a class of materials whose conductivity, while highly pure, varies witli temperature as exp (-Ag//cg7), where is tlie size of a forbidden energy gap. The conductivity of semiconductors can be made to vary over orders of magnitude by doping, tlie intentional introduction of appropriate impurities. The range in which tlie conductivity of Si can be made to vary is compared to tliat of typical insulators and metals in figure C2.16.1. 

This review has covered many of the essential features of the physical chemistry of nanocrystals. Rather than provide a detailed description of the latest and most detailed results concerning this broad class of materials, we have instead outlined the fundamental concepts which serve as departure points for the most recent research. This necessarily limited us to a discussion of topics that have a long history in the community, leaving out some of the new and emerging areas, most notably nonlinear optical studies [152] and magnetic nanocrystals [227]. Also, the... 

It is the third of these criteria that offers the most powerful insight into the nature of the polymerization process for this important class of materials. We shall frequently use the terms step-growth and condensation polymers as synonyms, although by the end of the chapter it will be apparent that step-growth polymerization encompasses a wider range of reactions and products than either criteria (1) or (2) above would indicate. 

In the discussion of these combined topics, we use statistics extensively because the description of microstructure requires this kind of approach. This is the basis for merging a discussion of copolymers and stereoregular polymers into a single chapter. In other respects these two classes of materials and the processes which produce them are very different and their description leads us into some rather diverse areas. 

A new class of materials called smart tagged composites has been developed for stmctural health monitoring appHcations. These composites consist of PZT-5A particles embedded into the matrix resin (unsaturated polyester) of the composite (16). 

Perfluoroepoxid.es were first prepared ia the late 1950s by Du Pont Co. Subsequent work on these compounds has taken place throughout the world and is the subject of a number of reviews (1 5). The main use of these epoxides is as intermediates in the preparation of other fluorinated monomers. Although the polymerisation of the epoxides has been described (6—12), the resulting homopolymers and their derivatives are not significant commercial products. Almost all the work on perfluoroepoxides has been with three compounds tetrafluoroethylene oxide (TFEO), hexafluoropropylene oxide (HFPO), and perfluoroisobutylene oxide (PIBO). Most of this work has dealt with HFPO, the most versatile and by far the most valuable of this class of materials (4). 

Antiwear Compounds. Additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, ie, when fuU lubricating films cannot be maintained. Two general classes of materials are used to prevent metallic contact. 

A variety of additives are used to control the properties of wetting and dispersion of pigments, flow, Hthography, and mb-off of inks. These additives belong to classes of materials such as surfactants, bentonite clays, alkyds, functional resins, polymers, etc. 

There are many types of lasers, having a wide variety of methods of constmction and based on many different classes of materials. The properties of some commercially available lasers are summarized in Table 1. Typical available characteristics are given. More detailed compilations of the properties of commercially available lasers are available (20,21). 

The cermet class of materials contains a large number of compositions (57). Most cermets are carbide-based, eg, WC and titanium carbide [12070-08-5] TiC. Cemented tungsten carbides are widely used for cutting tools and car parts. 

Early marketing efforts for spunbonded fabrics centered on their substitution for existing, ie, woven, textile fabrics. Generally, success was achieved ia areas where only fiinctionahty was important. Extremely slow progress has occurred ia areas where textile-like aesthetics are required. Nevertheless, spunbonded fabrics are recognized as a unique class of materials within the general category of nonwoven fabrics (see Nonwoven fabrics, staple fibers). 

Fiber. High molecular weight linear PPS is weU-suited for fiber applications. The inherent properties of PPS (flame resistance, chemical resistance, and thermal stability) make PPS fiber highly desirable ia textile applications (128). PPS fiber has been designated by the U.S. Federal Trade Commission as a new generic class of materials called sulfar. Typical fiber properties are listed ia Table 7 (see High performance fibers). 

The term amino resin is usually appHed to the broad class of materials regardless of appHcation, whereas the term aminoplast or sometimes amino plastic is more commonly appHed to thermosetting molding compounds based on amino resins. Amino plastics and resins have been in use since the 1920s. Compared to other segments of the plastics industry, they are mature products, and their growth rate is only about half of that of the plastics industry as a whole. They account for about 3% of the United States plastics and resins production. 

Aluminosilicate Fibers. Vitreous alurninosihcate fibers, more commonly known as refractory ceramic fibers (RCF), belong to a class of materials known as synthetic vitreous fibers. Fiber glass and mineral wool are also classified as synthetic vitreous fibers, and together represent 98% of this product group. RCFs were discovered in 1942 (18) but were not used commercially until 1953. Typical chemical and physical properties of these materials are shown in Table 3. 

Retarders. The purpose of vulcanization retarders is to delay the initial onset of cure in order to guarantee sufficient time to process the unvulcanized mbber. Three main classes of materials are used commercially, including organic acids and anhydrides, cyclohexylthiophthalimide (Santogard PVI or CTP), and a sulfenamide material (Vulkalent E). 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

The most commonly used scale inhibitors are low molecular weight acrylate polymers and organophosphoms compounds (phosphonates). Both classes of materials function as threshold inhibitors however, the polymeric materials are more effective dispersants. Selection of a scale control agent depends on the precipitating species and its degree of supersaturation. The most effective scale control programs use both a precipitation inhibitor and a dispersant. In some cases this can be achieved with a single component (eg, polymers used to inhibit calcium phosphate at near neutral pH). 

Oil Repellent. Fluorochemicals are the only class of material that can provide oil repeUency without altering the porosity of the paper or paperboard. Physical barriers to oil penetration are used primarily for their moisture- or gas-barrier properties, with retarded oil penetration as a secondary benefit. The most common od-repeUent additives are long-chain perfluoroalkyl phosphate salts of ammonia or diethanol amine. Commercial sources include Scotchban (3M), Zonyl (DuPont), and Lodyne (Ciba Specialties). There are also a fluorochemical carboxylate salt, Lodyne (Ciba Specialties), and fluorochemical copolymers, eg, Scotchban (3M). The widest range of oily fluid holdout is provided by the fluorochemical copolymers. 

Carbon Composites. In this class of materials, carbon or graphite fibers are embedded in a carbon or graphite matrix. The matrix can be formed by two methods chemical vapor deposition (CVD) and coking. In the case of chemical vapor deposition (see Film deposition techniques) a hydrocarbon gas is introduced into a reaction chamber in which carbon formed from the decomposition of the gas condenses on the surface of carbon fibers. An alternative method is to mold a carbon fiber—resin mixture into shape and coke the resin precursor at high temperatures and then foUow with CVD. In both methods the process has to be repeated until a desired density is obtained. 

Pyroelectrics. Pyroelectric ceramics are materials that possess a uoique polar axis and are spontaneously polarized ia the abseace of an electric field. Pyroelectrics are also a subset of piezoelectric materials. Ten of the 20 crystal classes of materials that display the piezoelectric effect also possess a unique polar axis, and thus exhibit pyroelectricity. In addition to the iaduced charge resultiag from the direct pyroelectric effect, a change ia temperature also iaduces a surface charge (polarizatioa) from the piezoelectric aature of the material, and the strain resultiag from thermal expansioa. 

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