Observation of polymer-like flow mechanism in a short-chain phosphate glass-forming liquid.

The dynamics of the phosphate chains and the attendant shear relaxation in a short-chain silver phosphate glass-forming liquid with the composition 51.5%Ag2O-48.5% P2O5 are studied using a combination of high-temperature 31P NMR spectroscopy and parallel plate rheometry. The temperature-dependent evolution of the 31P NMR spectral line shapes indicates that the constituent PO4 tetrahedral chains in this liquid undergo rapid rotational reorientation. The time scale of this dynamics is in complete agreement with that of shear relaxation and, thus, must be responsible for the viscous flow of this liquid. These results demonstrate for the first time that, although the shear relaxation of the network oxide glass-forming liquids is typically controlled by the scission and renewal of bonds between the network-forming cations and oxygen atoms, such a scenario may not always be tenable for liquids with low-dimensional structures consisting of chains.

Observation of a dynamical crossover in the shear relaxation processes in supercooled selenium near the glass transition.

Shear relaxation in supercooled selenium liquid near its glass transition over the viscosity range of 106 Pa s-1012 Pa s is studied using oscillatory parallel plate rheometry. The results demonstrate the presence of a slow, Debye-like relaxation process and a fast, cooperative relaxation process that are attributed, respectively, to the Se-Se bond scission/renewal dynamics and the segmental motion of selenium chains. The slow process displays a nearly-Arrhenius temperature dependence, while the fast process is strongly non-Arrhenius, and their combined contribution to viscosity is estimated using the Maxwell relation. The slow process is found to be coupled to viscous flow over the entire viscosity range. In contrast, the fast process becomes a major contributor to viscosity, and hence, to fragility only near Tg. This dynamical crossover is likely a fundamental characteristic of fragile liquids that represents a temperature dependent evolution of their free energy landscape. The fragility of supercooled selenium liquid appears to be remarkably closely linked to the temperature dependence of the shear modulus of the slow process, thus validating the prediction of the elastic shoving model.

Nature of the floppy-to-rigid transition in chalcogenide glass-forming liquids.

The viscoelastic properties of supercooled AsxSe100-x and GexSe100-x (0 ≤ x ≤ 30) liquids are studied using oscillatory parallel plate rheometry. The liquids with average selenium chain segment length L longer than ∼3 to 5 atoms or average coordination number ⟨r⟩ less than ∼2.2 are characterized by the coexistence of a low-frequency bond scission/renewal based relaxation process as well as high-frequency segmental chain dynamics. The latter process disappears for liquids with higher connectivity, thus implying a dynamical rigidity transition. The temporal decoupling of the high-frequency chain mode from that of the bond scission/renewal process and the shear modulus Gs associated with the low-frequency mode are shown to be unique functions of L or ⟨r⟩ and display strong similarity with the corresponding variation in the fragility m and the conformational entropy of the chain segments. When taken together, these results provide direct experimental support to the entropic rigidity argument originally proposed by Phillips but suggest a floppy-to-rigid transition of the structural network at ⟨r⟩ ∼ 2.2, instead of the conventional rigidity percolation threshold value of 2.4.

Rheology of the λ transition in liquid sulfur: Insights from arsenic sulfide liquids.

The frequency dependence of the storage and loss shear moduli and viscosity of AsxS100-x glass-forming liquids (x = 5, 10, 15, 40) are obtained over a frequency range covering nearly 15 orders of magnitude using parallel plate rheometry. The S-rich (x ≤ 15) liquids are characterized by a ring-to-chain structural transition near Tλ ∼ 120 °C, and their rheological behavior below Tλ strongly resembles that of long-chain and entangled polymers as well as that recently speculated for liquid sulfur above its λ transition. These AsxS100-x liquids are characterized by the coexistence of a slow and a fast relaxation process with similar activation energy. Both processes are coupled to viscosity, but differ in time scale by several orders in magnitude and are assigned, respectively, to the reptation and the Rouse dynamics of the Sn chains in these liquids. Such complex polymer-like rheological behavior disappears in the As40S60 liquid, characterized by corner-shared AsS3/2 pyramids, where a single average shear relaxation time typical of simple liquids instead emerges.

Dynamics at the crystal-melt interface in a supercooled chalcogenide liquid near the glass transition.

Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. These type of processes have been primarily treated as macroscopic phenomena within the framework of phenomenological models and bulk experiments. Here, we report x-ray photon correlation spectroscopy measurements of dynamics at the crystal-melt interface during the radiation induced formation of Se nano-crystallites in pure Se and in binary AsSe4 glass-forming liquids near their glass transition temperature. We observe a heterogeneous dynamical behaviour where the intensity correlation functions g2(q, t) exhibits either a compressed or a stretched exponential decay, depending on the size of the Se nano-crystallites. The corresponding relaxation timescale for the AsSe4 liquid increases as the temperature is raised, which can be attributed to changes in the chemical composition of the melt at the crystal-melt interface with the growth of the Se nano-crystallites.

Structure and Chemical Order in S-Se Binary Glasses.

The compositional evolution of the structure and chemical order in binary S xSe100- x glasses (0 ≤ x ≤ 90) is investigated using a combination of high-resolution 2D 77Se isotropic-anisotropic correlation NMR and Raman spectroscopy. The results indicate that the structure of S-Se glasses consists of two types of topological elements, namely polymeric [Se,S] n chains and eight-membered Se yS8 -y rings (0 ≤ y ≤ 8). The relative concentration of Se atoms monotonically decreases in the chain elements and concomitantly increases in the ring elements with increasing S concentration. Moreover, the Se speciation results are consistent with an average heterocyclic ring composition of Se1S7 at low S content (≤40 at. % S), while the composition shifts to Se1.5S6.5 at higher S content (≥60 at. % S), indicating increasing incorporation of multiple Se atoms in each ring element. The Raman spectra suggest that -Se-Se- association is favored, when more than one Se atom is incorporated in chains and rings. As in their elemental forms, the S and Se atoms retain their preference of forming rings and chains in binary S xSe100- x glasses, which predicts a linear compositional variation in the relative fractions of these topological elements. This structural evolution is consistent with the corresponding variation in the Tg and molar volume, both of which exhibit a linear decrease with increasing S concentration.

Observation of Steady Shear-Induced Nematic Ordering of Selenium Chain Moieties in Arsenic Selenide Liquids.

Structural anisotropy induced by steady shear and its mechanistic relation with shear thinning are investigated in AsxSe100-x glasses (5 ≤ x ≤ 30) quenched from parent liquids subjected to shear rates ranging between 0 and 104 s-1 using polarized Raman spectroscopy and two-dimensional X-ray diffraction. When taken together, the results demonstrate significant shear-induced partial alignment of -Se-Se-Se- chain moieties in the flow direction of the extruded fibers. This alignment is reminiscent of nematic liquid crystals where orientational order exists without positional order. The degree of this structural alignment in quenched glasses appears to be practically independent of the shear rate, although the parent liquids undergo shear thinning at the highest shear rates. It is conjectured that any causal relationship between structural alignment and shear thinning in the liquid may be masked in the glassy state by the postextrusion structural relaxation of the parent liquid.