Epoxy resins, cross-linked glassy

In order to decrease the overpotential for cysteine oxidation in cysteine-containing peptides, graphite-epoxy resin or carbon paste electrodes with immobilized cobalt phthalocyanine have been employed in HPLC systems [53,87-89]. Glassy carbon electrodes modified with a mixed-valence ruthenium(I V,III) oxide film stabilized by cyano cross-links [61,62], indium ferricyanide [63], or a Prussian blue film [64], and a bismuth(V)-doped lead dioxide Pb02 film on gold [65] have also been used for detection following HPLC separation. In addition, a carbon fiber modified with a nfixed-valence ruthenium(IV,III) oxide film stabilized by cyano cross-links has been used in CE [46]. 

Tg especially wl en deformed under the influence of an overall hydrostatic compressive stress. This behaviour is illustrated in Fig. 5.37 where true stress-strain curves are given for an epoxy resin tested in uniaxial tension and compression at room temperature. The Tg of the resin is 100°C and such cross-linked polymers are found to be brittle when tested in tension at room temperature. In contrast they can show considerable ductility in compression and undergo shear yielding. Another important aspect of the deformation is that glassy polymers tend to show strain softening . The true stress drops after yield, not because of necking which cannot occur in compression, but because there is an inherent softening of the material. 

Materials in the glassy state are in general stable, and neither react or decompose very quickly because the molecules are immobile over short time periods. This feature is used to advantage in freeze-drying where materials are specifically produced in a glassy state to render them stable and yet easily used see Section 1.5.8. Other materials such as epoxy resins need to be heated above their 7 in order for them to react and cross-link. 

Figure 7. Apparent activation energies vs. temperature in glassy region of Un-aged and Aged Loosely Cross-linked DGEBA/DADPS Epoxy Resins.

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