Abstract
Changes in the dielectric properties during isothermal ageing of a network-structure glass have been measured. The height of the sub-Tg relaxation peak initially increases on ageing, reaches a maximum value and then decreases. The initial increase is caused by the chemical reactions that increase the glass transition temperature towards a limiting value and produce an -OH group in a relatively loosely packed region of the amorphous matrix. When the matrix becomes almost completely crosslinked or when no further reaction occurs, spontaneous densification causes a decrease in the height of the β-relaxation peak. Thus the effects of isothermal ageing involves two process: chemical, that dominates during the early period, and physical, that ultimately determines the dielectric behavior. The increase in the dielectric permittivity as a result of chemical reaction calculated from the increase in Tg and the configurational entropy theory is one third the measured value. This indicates that the molecular rotational diffusion in local regions of the amorphous matrix involves also groups other than those produced by the chemical reaction. The stretched exponential decay parameter was calculated from the limited data here and using a procedure given earlier. Its value of 0.11 is independent of the ageing time.