Decoupling between birefringence decay, enthalpy relaxation and viscous flow in calcium boroalumosilicate glasses

The relaxation kinetics at the glass transition and in the sub-Tg range in a calcium boro-alumosilicate glass (E-glass composition) was studied from viscosity, birefringence and calorimetric measurements to clarify the thermal dependence of structural relaxation in hyper-quenched glass fibres. A decoupling of birefringence relaxation and viscous relaxation, i.e., a large discrepancy between the activation energies for optical recovery ΔHΔn = 25 ± 4 kJ mol− 1, and viscous flow at the glass transition Eη = 399 ± 8 kJ mol− 1 is evident from fibre annealing and viscometric experiments. The decoupling ratio of ΔHΔn/Eη = 0.06 is found to be close to that of γ-relaxation (internal friction) to α-relaxation in sodium trisilicate glass when plotted in a reduced Tg/T-scaled form. In contrast to the birefringence, the enthalpy relaxation is found to be related to the isostructural dependence of viscosity. These observations imply that the thermal dependence of the optical anisotropy relaxation is related, as the alkali mobility, to local structural arrangements, whereas the enthalpy relaxation seems to be dominated by relaxation of the silicate melt network [Yue, Y.Z., Christiansen, J., de, C., Jensen, S.L., 2002. Determination of the fictive temperature for a hyperquenched glass. Chemical Physics Letters 357, 20–24], including cooperative motion of larger structural ranges.