Rheological characterization of complex dynamics in Na–Zn metaphosphate glass-forming liquids

The viscoelastic behavior and shear relaxation in supercooled [NaPO3]x[Zn(PO3)2]1−x metaphosphate liquids with 0.2 ≤ x ≤ 1.0 are investigated using a combination of small amplitude oscillatory and steady shear parallel plate rheometry, resonant ultrasound spectroscopy, and differential scanning calorimetry. The results demonstrate that these liquids are thermorheologically complex with the coexistence of a fast and a slow relaxation process, which could be attributed to the segmental motion of the phosphate chains and the Zn–O bond scission/renewal dynamics, respectively. The segmental motion of the phosphate chains is found to be the dominant process associated with the shear relaxation for all metaphosphate liquids. The compositional evolution of the calorimetric fragility of these liquids is shown to be related to the conformational entropy of the constituent phosphate chains, which is manifested by the width of the relaxation time distribution for the segmental chain motion. This entropy decreases and the temporal coupling between the chain dynamics and Zn–O bond scission-renewal increases with the increasing Zn content as the higher field strength Zn modifier ions provide more effective cross-linking between the phosphate chains.