Influence of physical ageing on the excessive heat capacity of hyperquenched silicate glass fibers

Two types of hyperquenched continuous glass fibers, basaltic glass fibers and soda-lime silicate glass fibers, are thermally aged within a temperature range from 0.55Tg to 0.95Tg, where Tg is the glass transition temperature. The results show the complexity of enthalpy relaxation of the aged fibers. During up-scanning of the aged fibers in a differential scanning calorimeter, a relaxation exotherm occurs above a certain temperature, which is due to the release of the excess energy relative to that of a glass cooled at the rate 0.33 K/s. At a certain ageing temperature, an endotherm occurs prior to the appearance of the exotherm. There is a smooth crossover between the endotherm and the exotherm. The gradual shift of the crossover from lower to higher temperature with increasing ageing temperature is not due to the distribution of fictive temperatures, but to a distribution of the excess potential energy over all configurational coordinates. A phenomenological equation has been proposed to describe the decrease of the remaining energy with increasing ageing temperature.