Generic α relaxation in a strong GeO_{2} glass melt.

The viscoelastic α relaxation in glass-forming GeO_{2} was measured over a range of temperatures near the glass transition using photon correlation spectroscopy. The relaxation in this "strong" glass former exhibits a nonexponential decay identical to that found in a great many simple organic "fragile" liquids. This finding contradicts the longstanding conjecture that nonexponentiality of viscous relaxations near the glass transition are correlated to the liquid's fragility. Instead, the findings offer support for a recent proposal that the nonexponentiality parameter of the α-relaxation in supercooled liquids displays a universal value ß(T_{g})=1/2 near the glass transition.

Erratum: Fragility of network-forming glasses: A universal dependence on the topological connectivity [Phys. Rev. E 92, 062804 (2015)].

This corrects the article DOI: 10.1103/PhysRevE.92.062804.

Assessing the network connectivity of modifier ions in metaphosphate glass melts: A dynamic light scattering study of Na-Zn mixtures.

Photon correlation spectroscopy conducted on polymeric metaphosphate melts [NaPO3]1-y[Zn(PO3)2]y shows a systematic decrease in glass fragility as the more strongly bonding Zn cation replaces the more weakly bonding Na cation as a crosslinking agent between PO3 chains. This decrease is similar to that observed previously in Na-Al melts and the decrease in fragility for both systems is shown to be fully consistent with a recently reported universal pattern of fragility in network forming glasses as a function of network connectivity. Unique to the Na-Zn system is the appearance of an ultraslow relaxation in the dynamic structure factor (slower than the viscoelastic decay) that is not present in either Na-Al or Na-Li metaphosphate mixtures. This relaxation appears to originate from the diffusion of the Zn cation within the melt which is partially coupled to the oxide network. Taken together, these results underscore the need to distinguish between network-forming cations of high ionic bond strength that contribute to the connectivity of the oxide network and those of lower bond strength that do not contribute.

Fragility of network-forming glasses: A universal dependence on the topological connectivity.

The fragilities of over 150 different network-forming glass melts are shown to conform to a common dependence on just one parameter: the connectivity of the weakest network structure present in the associated glass solid. This includes both nonoxide network-forming chalcogenide melts as well as a variety of alkali oxide glasses, and spans a broad range of connectivity, ϕ, from polymeric structures (ϕ=2) to overconstrained random networks with connectivities well in excess of the rigidity threshold (ϕ(C)=2.4). A theoretical framework for the origin of this universal pattern is offered within the context of entropic models of the glass transition.

Universal patterns of equilibrium cluster growth in aqueous sugars observed by dynamic light scattering.

Dynamic light scattering performed on aqueous solutions of three sugars (glucose, maltose and sucrose) reveal a common pattern of sugar cluster formation with a narrow cluster size distribution. In each case, equilibrium clusters form whose size increases with increasing sugar content in an identical power law manner in advance of a common, critical-like, percolation threshold near 83 wt % sugar. The critical exponent of the power law divergence of the cluster size varies with temperature, increasing with decreasing temperature, due to changes in the strength of the intermolecular hydrogen bond and appears to vanish for temperatures in excess of 90 °C. Detailed analysis of the cluster growth process suggests a two-stage process: an initial cluster phase formed at low volume fractions, ϕ, consisting of noninteracting, monodisperse sugar clusters whose size increases ϕ(1/3) followed by an aggregation stage, active at concentrations above about ϕ=40%, where cluster-cluster contact first occurs.

Universal Patterns of Equilibrium Cluster Growth in Aqueous Sugars Observed by Dynamic Light Scattering.

Dynamic light scattering performed on aqueous solutions of three sugars (glucose, maltose and sucrose) reveal a common pattern of sugar cluster formation with a narrow cluster size distribution. In each case, equilibrium clusters form whose size increases with increasing sugar content in an identical power law manner in advance of a common, critical-like, percolation threshold near 83 wt% sugar. The critical exponent of the power law divergence of the cluster size varies with temperature, increasing with decreasing temperature, due to changes in the strength of the intermolecular hydrogen bond and appears to vanish for temperatures in excess of 90 °C. Detailed analysis of the cluster growth process suggests a two-stage process: an initial cluster phase formed at low volume fractions, φ, consisting of non-interacting, monodisperse sugar clusters whose size increases φ(1/3) followed by an aggregation stage, active at concentrations above about φ = 40%, where cluster-cluster contact first occurs.

Ultraslow relaxation of hydrogen-bonded dynamic clusters in glass-forming aqueous glucose solutions: A light scattering study.

We report static and dynamic light-scattering measurements of aqueous glucose solutions near their glass transition. Photon correlation spectroscopy reveals two relaxation processes present in the supercooled liquid: a nonexponential and nonhydrodynamic, alpha -relaxation occurring at short times and an exponential and hydrodynamic relaxation occurring at longer times. The slow relaxation is seen only in the polarized scattering geometry and is in many ways identical to the "ultraslow" mode recently observed by others in specially annealed molecular glass-forming liquids and attributed to the formation of long-range density correlations or "dynamic clusters." Static light scattering confirms the existence of excess scattering in our glucose solutions that is consistent with clusters in a size range between 30 and 60nm . The size of the clusters varies with the water content and the clustering appears to be associated with the percolation of a hydrogen-bonded glucose network.

Dynamic light scattering in mixed alkali metaphosphate glass forming liquids.

We report the first ever photon correlation spectroscopy performed on single alkali and mixed alkali metaphosphate glasses at refractory temperatures above the glass transition. We find not only a significant decrease in the glass transition temperature but also a decrease in fragility for the mixed alkali composition as compared with the single akali glasses. We argue that structural relaxation in these polymeric oxide glasses is largely controlled by the cross linking cations and that the changes in fragility that we observed are a reflection of changes in the cooperativity of structural relaxation wrought by the substantial decrease in the ion mobility that accompanies the mixing of alkali ions.

Distinguishing two contributions to the nearly constant loss in ion-conducting glasses.

In an effort to understand the origins of the nearly constant loss in disordered materials, we report dielectric studies of a series of sodium germanate glasses, (Na2O)(x)(GeO2)(1-x) for x=0, 0.003, 0.01, 0.03, and 0.1 at temperatures between 85 and 700 K. Analysis of the conductivity scaling for these glasses demonstrates the existence of two contributions in the near constant loss; one due to mobile cations that conforms to the same scaling properties found for ion hopping at high temperatures and the other due to the glass network which dominates at low temperatures and low ion densities.