Class I hybrids

In more complex forms of this resin hybrid, other dimethacrylates may be present, such as the ethylene glycol dimethacrylates, and bis-GMA, when HEMA acts as a co-solvent for water and bis-GMA (Antonucd, McKinney Stansbury, 1988). The general composition of these materials, which we term class I hybrids, is summarized below  

Other difunctional hydroxy dimethacrylates, e.g. the ethylene glycol dimethacrylates 

In chemically-cured materials, one example of an initiator/activator system is hydrogen peroxide as initiator, ascorbic acid as activator and cupric sulphate as co-activator. In light-cured materials, camphorquinone is used as a visible-light photochemical initiator, sodium p-toluene-sulphinate as activator and ethyl 4-dimethylaminobenzoate as photoaccelerator. 

If there is too little water in a composition then the add-base reaction 

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  

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The final materials are Class I hybrid materials where PVAL tailors the hybrid degradation, that is the rate at which the hybrid material is dissolved by the physiological fluids, being replaced by biological newly formed bone. 

Classifying particles, in filtration, 11 326 Class I hybrids, 13 536, 543, 544 Class II hybrids, 13 536, 543 Clastogenesis, 25 206 Clathrate hydrates, 14 170—171 Clathrate receptor chemistry, 16 797 Clathrates, 12 374 14 159, 170-182 formation of, 10 633-635 26 869 Hofmann- and Werner-type, 14 171-172 phenol-type, 14 180 tri-o-thymotide, 14 179 Claus catalysts... 

The ability to form hybrid silica glasses under aqueous, room-temperature conditions (at which proteins and cells are active) opens up the possibility to extend sol-gel processing to the encapsulation of biologicals. However, as mentioned earlier, the traditional sol-gel route using silicon alkoxides has to be adapted to avoid excess alcohol as well as low pH conditions (see section 3.2.1). Moreover, because biospecies could be denatured by covalent bonding, most bioorganic-inorganic composites are Class I hybrids. 

Inorganic/Organic Interpenetrating Networks (Class I Hybrids)... 

Poly(vinyl-acetate), the polymer chosen for this study, is functionally incapable of covalently bonding with the polymerizing silicon alkoxide, TEOS. Therefore, the Si02/PVAc composites may be considered class I hybrid materials. Reasons for interest in mixing PVAc with silica include its excellent optical transparency, a refractive index close to that of silica, and its solubility in ethanol-water mixtures. The solubility of PVAc in ethanol-water mixtures allows the polymer to be added directly to a typical alkoxide sol-gel formulation of alkoxide, alcohol, water, and catalyst, followed by tiie polymerization of the inorganic network around the organic polymer domains. 

Figure 43.7 Schematic of the interpenetrating inorganic and organic networks of a class I hybrid material.

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