Shear relaxation, fragility, and dynamical heterogeneity of sodium borosilicate liquids

Abstract The viscosity, fragility index, and shear relaxation dynamics of a range of sodium borosilicate (NBS) liquids have been studied using a combination of differential scanning calorimetry, beam‐bending viscometry, and shear‐mechanical spectroscopy (SMS). Our results show a non‐monotonic compositional variation of the isokom temperature T 12 corresponding to a viscosity of 10 12 Pa s and the fragility index m , consistent with the known structural evolution involving boron coordination change and non‐bridging oxygen formation. A negative correlation is observed between T 12 and m . On the other hand, the quantity ( T g − T 12 ), where T g is the glass transition temperature, becomes increasingly positive as m decreases below ∼50, which suggests an increase in the temporal decoupling between enthalpy and shear relaxation with decreasing fragility in these NBS liquids. SMS measurements, which involve frequency‐ and temperature‐dependent scans of storage and loss moduli, are shown to yield shear relaxation timescales that are consistent with those derived from viscosity via the Maxwell model. Furthermore, the dynamical heterogeneity associated with the shear relaxation of these liquids as estimated from the Kohlrausch–Williams–Watts stretching exponent β , remains temperature‐independent near T g in fragile NBS liquids but increases with temperature for the strongest liquid. This observation supports recent findings of distinct β ( T ) behavior in strong versus fragile liquids. Similarly, consistent with the recent results for organic liquids, no correlation is observed between m and β near T g for NBS liquids.