Class II Materials

formamide, and guanidinium salts are believed to increase the CMC of surfactants in aqueous solution, especially polyoxyethy-lenated nonionics because of their disruption of the water structure (Schick, 1965). This may increase the degree of hydration of the hydrophilic group, and since hydration of the hydrophilic group opposes micellization, may cause an increase in the CMC. These water structure breakers may also increase the CMC by decreasing the entropy effect 

Materials that promote water structure, such as xylose or fructose (Schwuger, 1971), for similar reasons decrease the CMC of the surfactant. 

Dioxane, ethylene glycol, water-soluble esters, and short-chain alcohols at high bulk phase concentrations may increase the CMC because they decrease the cohesive energy density, or solubility parameter, of the water, thus increasing the solubility of the monomeric form of the surfactant and hence the CMC (Schick, 1965). An alternative explanation for the action of these compounds in the case of ionic surfactants is based on the reduction of the dielectric constant of the aqueous phase that they produce (Herzfeld, 1950). This would cause increased mutual repulsion of the ionic heads in the micelle, thus opposing micellization and increasing the CMC. 

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FPN No. 1) For additional information on group classification of Class II materials, see Manual for Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991. 

The two matrices in these cements are of a different nature an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant -COOH groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula ... 

Both class I and class II resin glass polyalkenoate cements are claimed to bond to dentine. This can be accepted. But that the bond is stronger and develops more rapidly than that of the conventional glass polyalkenoate cement, as is claimed for class II materials (Minnesota Mining Manufacturing Company, 1989) requires confirmation. 

There is bound to be one problem with resin glass polyalkenoate cement. Because the matrix is a mixture of hydrogel salt and polymer, lightscattering is bound to be greater than in the conventional material. Moreover, the zinc oxide-containing glass of class II materials is bound to be opaque. This makes it difficult to formulate a translucent material and is the reason why their use is restricted to that of a liner or base. However, the class II material cited will be radio-opaque because it uses strontium and zinc, rather than calcium, in the glass. 

All chemicals tested were classified according to the scheme shewn in Table l. Class I compounds are inactive while Class II materials are good toxicants but do not have the required delayed toxicity. Class III compounds have delayed action, but the concentration range of their activity is too narrow. The type of activity we are looking for in a toxicant is exemplified by a Class IV or V response, i.e., it exhibits delayed toxicity over a wide range of concentrations. 

To understand the effects produced, it is necessary to distinguish between two classes of organic materials that markedly affect the CMCs of aqueous solutions surfactants class I, materials that affect the CMC by being incorporated into the micelle and class II, materials that change the CMC by modifying solvent-micelle or solvent-surfactant interactions. 

For the class I materials, the salt (ES-I) is the primary form, obtained from the polymerization. EB-1 is prepared indirectly by de-doping the polymerization product. The class II materials are approached via the base form EB-II, which is obtained from EB-I either by dissolution in NMP or DMSO and casting, or by an... 

RDFs for class II materials have been obtained by Laridjani el al. [306]. The authors have compared the experimental results with calculated distribution functions, involving both intrachain and interchain contributions. For the latter they consider the crystalline lattices described previously as a starting point. The main conclusion is that there is considerable similarity of local order in crystalline and amorphous regions, for both F.B-ll and ES-II. In the amorphous phase, the range of this order is about 5 A, an order of magnitude smaller than the size of the crystalline domains. 

The magnetic susceptibility was first measured as a function of protonation for the Class I family of materials [12]. Results showed a quasi-linear increase in Pauli susceptibility, which led to the proposal of the formation of metalHc islands of the emeraldine salt in the emeraldine base form with protonation. X-ray diffraction studies of the ES-I series, as a function of protonation, gives a quasi-linear increase in the crystalline salt fraction with protonation, supporting the formation of crystalline metallic islands with increasing protonation level. Electron spin resonance studies of the Class II materials, as a function of protonation level, show a dramatically different behavior [20]. Initial protonation of the base EB-II leads to a spinless material, until compositions are achieved, such that essentially all of the amorphous regions are fully doped. At that point, there is an increase in the Pauli susceptibility, corresponding to the formation of crystalline ES-II (Fig. 7). Preliminary studies show that the Pauli susceptibility is essentially the same for the ES- and ES-II structures. 

Typical Class II materials are ferrodielectric ceramics and polymer electrolytes with high dielectric constants. They are used to develop high capacitance capacitors. For example, metalized polymer foils can achieve a few thousand picofarads to a few microfarads an electrolytic capacitor can offer a capacitance in the range of a few to several thousand microfarads some... 

In Class-II materials components are chemically linked by strong covalent or iono-covalent bonds. The molecules used as starting building blocks possess two distinct functionalities alkoxy groups (R-O-M bonds) and metal-carbon (M-C) links. The alkoxy groups can be formed into an oxo-polymer network by hydrolysis-polycondensation reactions in a sol-gel. Hybrids can be obtained from organically modified silicon alkoxides such as polyfunctional or polymer functionalised alkoxysilanes. The network-forming functionalities can be covalently connected in a sol-gel in several ways ... 

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