synopsisThe cure behavior and thermal degradation of high Tg epoxy systems have been investigated by comparing their isothermal time-temperature-transformation (TIT) diagrams. The formulations were prepared from di-and trifunctional epoxy resins, and their mixtures, with stoichiometric amounts of a tetrafunctional aromatic diamine. The maximum glass transition temperatures (TgJ were 229°C and > 324°C for the fully cured di-and trifunctional epoxy materials, respectively. Increasing functionality of the reactants decreases the times to gelation and to vitrification, and increases the difference between Tg after prolonged isothermal cure and the temperature of cure. At high temperatures, there is competition between cure and thermal degradation. The latter was characterized by two main processes which involved devitrification (decrease of modulus and T,) and revitrification (char formation). The experimentally inaccessible Tcm (352T) for the trifunctional epoxy material was obtained by extrapolation from the values of Tgm of the less highly crosslinked systems using a relationship between the glass transition temperature, crosslink density, and chemical structure.
The elastoviscous nature of the human trilaminar tear film has been studied extensively. Similar rheological studies of artificial tear preparations should facilitate design of more efficacious tear substitutes.
Commercial artificial tears can be measured rheologically and thereby subdivided into non-Newtonian, moderately non-Newtonian, and Newtonian groups with respect to their viscous and elastic behaviors, which should reflect their performance.
SynopsisThe addition of a brominated epoxy resin as a chain extender to di-, tri-, and tetrafunctional epoxy systems cured with diaminodiphenylsulfone has been investigated. The effect of increased BER content on the glass transition temperature of the cross-linked systems is discussed. The Tg of these systems is in the order T ' (tri) > T ' (tetra) > Tg (di). The contributions of the chemical structure and the cross-link density to the glass transition temperature are calculated. The kinetics of the reaction was followed dynamically by DSC. The overall order of the reaction and activation energies decrease with increased BER content. The thermal stability of these systems is also discussed.
EXPERIMENTAL
MaterialsDiglycidyl ether of bisphenol A, DR (DER 332), and triglycidyl ether of tris (hydroxyphenyl) methane, XD (XD 7342), both from the Dow Chemical Company, and tetraglycidyl diaminodiphenylmethane, MY (MY720), Ciba-Geigy, were the epoxy resins. A brominated epoxy resin, BER (F2001P), Makhbhim Chemical Works, was the chain extender. Diaminodiphenylsul-*Incumbent of the Steinfeld Career Development Chair.
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