Rheological behavior of molecular vs network chalcogenide supercooled liquids

The viscoelastic behavior of supercooled glass-forming liquids along the binary join As₄S₃-GeS₂ with As₄S₃ contents varying from 81.25 to 9 mol. % and correspondingly with structures varying from predominantly molecular to a three-dimensional tetrahedral network is studied by small-amplitude oscillatory shear parallel plate rheometry. The storage shear modulus G' shows a scaling behavior of G'(ω) ∼ ωⁿ in the terminal (low-frequency) regime, where n varies between 1 and 2 and shows an increasingly anomalous departure from the expected value of 2 (Maxwell scaling) with increasing molecule content. A concomitant departure from the Maxwell scaling is also observed for the loss modulus G″ at frequencies above the G'-G″ crossover. On the other hand, the variation in the phase angle δ with the complex modulus indicates that the molecular liquid does not display a purely viscous response even at the lowest frequencies. These results, combined with an analysis of the relaxation spectra of these liquids, suggest that the anomalous behavior of molecular liquids may be linked to their rather broad relaxation spectrum and the presence of slow relaxation processes associated with molecular clusters. Additionally, these liquids are also characterized by a wide high-frequency plateau in the relaxation spectral density that can be linked to the rotational dynamics of the constituent molecules. Such fundamental differences between the rheological behavior of molecular and network liquids may explain the significantly higher fragility of the former.