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A fundamental and much-debated issue in glass science is the existence and nature of liquid–liquid transitions in glass-forming liquids. Here, we report the existence of a novel reentrant structural transition in a S-rich arsenic sulfide liquid of composition As 2.5 S 97.5 . The nature of this transition and its effect on viscosity are investigated in situ using a combination of differential scanning calorimetry and simultaneous Raman spectroscopic and rheometric measurements. The results indicate that, upon heating significantly above its glass transition temperature (261 K), the constituent [Formula: see text] sulfur chains in the structure of the supercooled liquid first undergo a [Formula: see text] chain-to-ring conversion near ∼383 K, which is exothermic in nature. Further heating above 393 K alters the equilibrium to shift in the opposite direction toward an endothermic ring-to-chain conversion characteristic of the well-known λ-transition in pure sulfur liquid. This behavior is attributed to the competing effects of enthalpy of mixing and conformational entropy of ring and chain elements in the liquid. The existence of reentrant structural transitions in glass-forming liquids could provide important insights into the thermodynamics of liquid–liquid transitions and may have important consequences for harnessing novel functionalities of derived glasses. 

The effect of the network-to-molecular structural transformation with increasing phosphorus content in P x Se 100− x (30 ≤ x ≤ 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 ≤ x ≤ 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G′) and loss (G″) shear moduli, and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 ≤ x ≤ 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x ≥ 63, on the other hand, are characterized by a departure from Maxwell behavior as the storage modulus shows a linear frequency scaling G′(ω) ∼ ω over nearly the entire frequency range below the G′–G″ crossover and a nearly constant ratio of G″/G′ in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale.