Randall E. Youngman

Topological Principles of Borosilicate Glass Chemistry

Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between the formation of nonbridging oxygen and the conversion of boron from trigonal to tetrahedral configuration. Using this model, we derive a detailed topological representation of alkali-alkaline earth-borosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid fragility, and hardness. The modeling approach enables an understanding of the microscopic mechanisms governing macroscopic properties. The implications of the glass topology are discussed in terms of both the temperature and thermal history dependence of the atomic bond constraints and the influence on relaxation behavior. We also observe a nonlinear evolution of the jump in isobaric heat capacity at the glass transition when substituting SiO(2) for B(2)O(3), which can be accurately predicted using a combined topological and thermodynamic modeling approach.

Self-assembly and hydrogelation promoted by F5-phenylalanine

Phenylalanine (Phe)-derived molecules have been exploited as low molecular weight hydrogelators. Perturbing the hydrophobic and π–π interactions that promote self-assembly and hydrogelation of these derivatives will facilitate improved understanding of hydrogelation phenomena and the design of small molecule hydrogelators with novel properties. The efficient self-assembly and hydrogelation of Fmoc-protected pentafluorophenylalanine (Fmoc-F5-Phe) are reported herein. Suspensions of Fmoc-F5-Phe in water undergo rapid self-assembly to entangled fibrillar structures within minutes, giving rise to rigid supramolecular gels. Self-assembly occurs at concentrations as low as 2 mM (0.1 wt%). Variation of the fluorinated aromatic side chain or N-terminal functionalization perturbs hydrogelation, implicating fluorous and π–π interactions as the primary determinants for molecular recognition and self-assembly. The hydrophobic and electronic properties of F5-Phe provide remarkable potential for functional self-assembly in a minimal amino acid scaffold.

Irreversibility of Pressure Induced Boron Speciation Change in Glass

It is known that the coordination number (CN) of atoms or ions in many materials increases through application of sufficiently high pressure. This also applies to glassy materials. In boron-containing glasses, trigonal BO3 units can be transformed into tetrahedral BO4 under pressure. However, one of the key questions is whether the pressure-quenched CN change in glass is reversible upon annealing below the ambient glass transition temperature (Tg). Here we address this issue by performing 11B NMR measurements on a soda lime borate glass that has been pressure-quenched at ~0.6 GPa near Tg. The results show a remarkable phenomenon, i.e., upon annealing at 0.9Tg the pressure-induced change in CN remains unchanged, while the pressurised values of macroscopic properties such as density, refractive index, and hardness are relaxing. This suggests that the pressure-induced changes in macroscopic properties of soda lime borate glasses compressed up to ~0.6 GPa are not attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures.

Structure of glasses in the pseudobinary system Ga(2)Se(3)-GeSe(2): violation of chemical order and 8-N coordination rule.

Structure of glasses in the pseudobinary system Ga2Se3-GeSe2 with Ga2Se3 content ranging from 6.3 to 30 mol % is investigated using a combination of Raman and multinuclear ((71)Ga, (77)Se) solid state nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the structure of these glasses consists primarily of a corner sharing network of (Ge/Ga)Se4 tetrahedra with some fraction of edge-sharing GeSe4 tetrahedra and of ethane-like (Se3)Ge-Ge(Se3) units, in which the Ga, Ge, and Se atoms adopt coordination numbers of 4, 4, and 2, respectively. As expected, the concentration of metal-metal bonds increases with addition of Ga2Se3 as the glass structure becomes too deficient in Se to satisfy the tetrahedral coordination of both Ga and Ge by Se atoms alone. These metal-metal bonds are mostly limited to Ge-Ge homopolar bonds, indicating a violation of chemical order. At relatively high degrees of Se-deficiency, however, spectroscopic evidence suggests the formation of triply coordinated Se atoms as an alternate mechanism to accommodate the tetrahedral coordination of Ga and Ge atoms. This observation indicates a violation of the 8-N coordination rule and is reminiscent of oxygen triclusters in isoelectronic Al2O3-SiO2 glasses. Compositional variation of physical properties such as density, molar volume, optical band gap, glass transition temperature, and fragility are shown to be consistent with the proposed structural model.

Structural and topological aspects of borophosphate glasses and their relation to physical properties

We establish a topological model of alkali borophosphate and calcium borophosphate glasses, which describes the effect of both the network formers and network modifiers on physical properties. We show that the glass transition temperature (Tg), Vickers hardness (HV), liquid fragility (m), and isobaric heat capacity jump at Tg (ΔCp) of these glasses are related to the network topology, which is determined by structure of the glass. Therefore, we also demonstrate that the temperature dependent constraint theory can quantitatively explain the mixed network former effect in borophosphate glasses. The origin of the effect of the type of network modifying oxide on Tg, HV, m, and ΔCp of calcium borophosphate glasses is revealed in terms of the modifying ion sub-network. The same topological principles quantitatively explain the significant differences in physical properties between the alkali and the calcium borophosphate glasses. This work has implications for quantifying structure-property relations in complex glass forming systems containing several types of network forming and modifying oxides.

Pressure-Induced Changes in Interdiffusivity and Compressive Stress in Chemically Strengthened Glass

Glass exhibits a significant change in properties when subjected to high pressure because the short- and intermediate-range atomic structures of glass are tunable through compression. Understanding the link between the atomic structure and macroscopic properties of glass under high pressure is an important scientific problem because the glass structures obtained via quenching from elevated pressure may give rise to properties unattainable under standard ambient pressure conditions. In particular, the chemical strengthening of glass through K(+)-for-Na(+) ion exchange is currently receiving significant interest due to the increasing demand for stronger and more damage-resistant glass. However, the interplay among isostatic compression, pressure-induced changes in alkali diffusivity, compressive stress generated through ion exchange, and the resulting mechanical properties are poorly understood. In this work, we employ a specially designed gas pressure chamber to compress bulk glass samples isostatically up to 1 GPa at elevated temperature before or after the ion exchange treatment of a commercial sodium-magnesium aluminosilicate glass. Compression of the samples prior to ion exchange leads to a decreased Na(+)-K(+) interdiffusivity, increased compressive stress, and slightly increased hardness. Compression after the ion exchange treatment changes the shape of the potassium-sodium diffusion profiles and significantly increases glass hardness. We discuss these results in terms of the underlying structural changes in network-modifier environments and overall network densification.

Influence of Aluminum Speciation on the Stability of Aluminosilicate Glasses against Crystallization

In this letter, we investigate the correlation between glass microstructure and glass stability (GS) in soda lime aluminosilicates. We find a loss of GS (i.e., an onset of crystallization) in the glasses above a critical concentration of Al2O3 when heating at the standard rate of 20 K/min. This loss in GS may be attributed to formation of five-fold coordinated Al species when [Al2O3]/[Na2O] > 1. The primary crystalline phase is identified as nepheline, in which Al exists in four-fold coordination. This implies that the five-fold coordinated Al is energetically less stable compared to Al in a tetrahedral environment.

Structure-topology-property correlations of sodium phosphosilicate glasses

In this work, we investigate the correlations among structure, topology, and properties in a series of sodium phosphosilicate glasses with [SiO2]/[SiO2 + P2O5] ranging from 0 to 1. The network structure is characterized by (29)Si and (31)P magic-angle spinning nuclear magnetic resonance and Raman spectroscopy. The results show the formation of six-fold coordinated silicon species in phosphorous-rich glasses. Based on the structural data, we propose a formation mechanism of the six-fold coordinated silicon, which is used to develop a quantitative structural model for predicting the speciation of the network forming units as a function of chemical composition. The structural model is then used to establish a temperature-dependent constraint description of phosphosilicate glass topology that enables prediction of glass transition temperature, liquid fragility, and indentation hardness. The topological constraint model provides insight into structural origin of the mixed network former effect in phosphosilicate glasses.

Temperature-dependent densification of sodium borosilicate glass

Densified glasses recovered from a high-pressure state are of potential technological interest due to their modified physical and chemical properties. Here we investigate the temperature-dependent densification behavior of a sodium borosilicate glass in a gas pressure chamber at 1 GPa. The temperature is varied for a 30 min treatment between 0.6Tg and 1.15Tg, where Tg is the glass transition temperature, and the treatment duration is varied between 10 and 10 000 min for compression at 0.9Tg. Permanent densification occurs for temperatures above 0.7Tg and the degree of densification increases with increasing compression temperature and time, until attaining an approximately constant value for temperatures above Tg. The same temperature and time dependence is also found for the glass mechanical properties (hardness and brittleness) and the network structure, i.e., fraction of three-fold versus four-fold coordinated boron atoms and ring versus non-ring trigonal boron atoms, and the extent of mixing of Si and B. The results provide insights into the temperature-dependence of the network densification and the relative roles of viscous flow and more localized rearrangements.

Volume and structural relaxation in compressed sodium borate glass

The structure and properties of glass can be modified through compression near the glass transition temperature (Tg), and such modified structure and properties can be maintained at ambient temperature and pressure. However, once the compressed glass undergoes annealing near Tg at ambient pressure, the modified structure and properties will relax. The challenging question is how the property relaxation is correlated with both the local and the medium-range structural relaxation. In this paper, we answer this question by studying the volume (density) and structural relaxation of a sodium borate glass that has first been pressure-quenched from its Tg at 1 GPa, and then annealed at ambient pressure under different temperature-time conditions. Using 11B MAS NMR and Raman spectroscopy, we find that the pressure-induced densification of the glass is accompanied by a conversion of six-membered rings into non-ring trigonal boron (BIII) units, i.e. a structural change in medium-range order, and an increase in the fraction of tetrahedral boron (BIV), i.e. a structural change in short-range order. These pressure-induced structural conversions are reversible during ambient pressure annealing near Tg, but exhibit a dependence on the annealing temperature, e.g. the ring/non-ring BIII ratio stabilizes at different values depending on the applied annealing temperature. We find that conversions between structural units cannot account for the pressure-induced densification, and instead we suggest the packing of structural units as the main densification mechanism.