PolyHEMA phase

In this section, three aspects of the interaction of resin-modified glass-ionomers with water are dealt with, namely water uptake, fluoride release and other ion release and its associated buffering effect. Like conventional glass-ionomer cements, resin-modified glass-ionomers are deployed in an essentially wet environment and their behaviour in terms of each of these features differs more or less from that of conventional glass-ionomers, due to the presence of the polyHEMA phase. These are considered in turn. 

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 ... 

By conditioning of the poIyHEMA layer in a buffer of fixed pH and primary ion concentration prior to use, the activity of the ions (u,) in the polyHEMA layer is constant. This fixes the inner boimdary potential of the ion sensitive membrane Fot an accurate measurement of an unknown activity of the primary ion in the sample solution based on Nemst law, it is essential that the primary ion activity in the membrane phase (a,) remains constant. This can be achieved by the introduction of proper amounts of primary ion-selective receptor molecules (ligands, L) in the membrane. The extraction of a particular ion in the membrane phase is then determined by the product of the partition of the ion over membrane and aqueous phase (/fp) and the association constant of the ion ionophore complex in the membrane (, ). The association constant /I, determines the free cation activity in the membrane ... 

Figure 4 Dependence of donors fluorescence phase angles on pC02. The donor-acceptor pairs were in polyHEMA hydrogel, 25 C, =543...

---