In situ study of the temperature activated kinetics of water sorption in an epoxy varnish
Résumé
The dielectric permittivity of a polyepoxy varnish with a glass transition temperature (Tg) of 61 °C is monitored during immersion in a 0.5 M NaCl aqueous solution, by electrochemical impedance spectroscopy, at various temperatures in the range [21; 78] °C. The sorption curves are first analysed with a Fickian approach. An Arrhenius-type dependence of the apparent coefficient of diffusion is observed, with no visible influence of Tg. Below 46 °C, the sorption curves clearly do not obey Fick′s second law of diffusion. On another hand, the Kohlrausch-Williams-Watts function (KWW) satisfactorily fits all the isothermal sorption curves. The KWW time constant has the same temperature activation as the apparent coefficient of diffusion, and likewise is unaffected by Tg. The KWW stretching exponent displays a strong dependence on Tg, correlates with the Fickian behaviour observed in the rubbery state, and is thought to be governed by molecular heterogeneities in the polyepoxy network. A linear rule of mixture is used to estimate the water uptake in the polyepoxy varnish as a function of temperature. The originality of this work lies in the in situ analysis of water uptake kinetics (constant immersion), which is performed above (rubbery state) and below (glassy state) the Tg of the polyepoxy coating.
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