EFFECT OF ABSORBED WATER ON THE THERMAL RELAXATION OF BIAXIALLY STRETCHED CROSSLINKED POLY(METHYL METHACRYLATE)

For three samples of the title materials the thermal relaxation characteristics were determined in rising temperature experiments and found to be dependent on water content. On heating, the polymers revert to the unstretched state and the onset temperature of this relaxation decreases with increasing water content. The most highly crosslinked polymers have the highest equilibrium water content at room temperature, the lowest relaxation onset temperature when saturated, and the highest relaxation onset temperature when dry. The effect of water on thermal relaxation is consistent with the observed depression of glass transition temperature. The relaxation of one of the materials was also studied isothermally. The effect of water is a temperature shift in the curve of log (maximum relaxation rate) against temperature. For a water content of 3.4 wt % this temperature shift is 25K, the same as the observed reduction in Tg. The effect of temperature on the maximum relaxation rate is of the form of the WLF equation. The apparent activation energy for thermal relaxation is lower in the saturated polymer. © 1979.