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Thermodynamics and dynamics of metallic glass formers: their correlation for the investigation on potential energy landscape.

Great progress has been made in basic features of the potential energy landscape (PEL) theoretically. The present work, however, attempts to cast new light on it from experimental aspects. By a survey of experimental data related to thermodynamics or dynamics of metallic glass-forming liquids, it is found that the increased rate of excitation of vibrational entropy at glass transition tends to increase the rate of generation of configurational part. Although for the type of metallic materials a generally positive relationship exists between the density of the energy minima at glass transition and the liquid fragility strength, just as expected, our main attention is paid to the phenomenon of the scattering of the slopes. Analysis shows that the phenomenon results from the different average height of energy barriers between minima near glass transition. Investigation on the PdNiP metallic system indicates that the mismatch entropy is a dominant factor in the barrier height: a large value of it results in low energy barriers. Our previous work on the AlNiCe system gives the support to this finding.

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Oxygen sites and network coordination in sodium germanate glasses and crystals: high-resolution oxygen-17 and sodium-23 NMR.

Sodium germanate glasses are well-studied materials in which, unlike silicates but analogous to borates, the major structural consequence of alkali addition is generally thought to involve a coordination number increase of the network-forming Ge cations. However, the nature of this change, in particular quantifying fractions of nonbridging oxygens and of five- and/or six-coordinated Ge, has remained unresolved. We present here high-resolution 17O results, including triple-quantum MAS NMR (3QMAS), on a series of crystalline model compounds that allow the definition of ranges of chemical shifts corresponding to oxygens bonded to various coordinations of Ge. These include quartz- and rutile-structured GeO2, Na4Ge9O20, Na2Ge4O9, and Na2GeO3 (germanium dioxide, sodium enneagermanate, sodium tetragermanate, and sodium metagermanate). 3QMAS spectra of Na-germanate glasses ranging from 0% to 27% Na2O clearly show the development of partially resolved peaks as alkali is added, corresponding to signals from nonbridging oxygens (in the highest Na glasses) and to oxygen bridging between one four-coordinated and one higher coordinated Ge. As in conventional models of this system, nonbridging oxygen contents are much lower than in corresponding silicates. Although we do not directly distinguish between five- and six-coordinated Ge, modeling of bridging oxygen populations and comparison with measured speciation suggest that substantial proportions of both species are likely to be present. High-field 23Na MAS NMR shows systematic decreases in mean Na-O bond distance and/or coordination number with increasing alkali content that can be compared with published results for high-temperature liquids. These results, as well as comparison of molar volumes of glasses and high-temperature liquids, suggest the possibility of significant temperature effects on liquid structure.

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Phase transformations in bulk nanostructured potassium niobiosilicate glasses.

In potassium niobiosilicate (KNS) glasses, nanostructuring can be driven and controlled by thermal treatments at the glass transition temperature and/or by modulation of the chemical composition. The tight relationship between nanostructure and nonlinear optical properties suggests these bulk nanomaterials as an appealing route to nanophotonics. The focus of this paper is placed on assessing the phase transformations which occur in these materials upon annealing at the glass transition temperature and subsequent heating. High-temperature resolved X-ray diffraction (HTXRD) and high-resolution transmission electron microscopy (HRTEM) experiments are integrated with previously published results for in-depth insight. It will be shown that nanostructuring evolves from nucleation of niobium-rich nanocrystals, which are up to 20 nm large, uniformly distributed in the matrix bulk, and metastable. Formation kinetics as well as phase transformation of the nanocrystals are determined by the glass composition. Depending on it, nanocrystal nucleation can be preceded or not by phase separation, and the nanocrystals' phase transition can be of first or second order.

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Hydrophobization of glass surface by adsorption of poly(dimethylsiloxane).

Silica or glass particles are introduced in a poly(dimethylsiloxane) (PDMS) matrix for various applications. A particular feature of these systems is that PDMS adsorbs on the surface of the dispersed particles, thus rendering them more hydrophobic with time. The mechanism of this process of in situ hydrophobization is still poorly understood. The major aims of the present study are (1) to quantify the rate of surface hydrophobization by PDMS and, on this basis, to discuss the mechanism of the process; (2) to compare the contact angles of surfaces that are hydrophobized by different procedures and are placed in contact with different fluid interfaces-PDMS-water, hexadecane-water, and air-water; and (3) to check how the type of surfactant affects the contact angles, viz., the effective hydrophobicity of the surface. We present experimental results for the kinetics of hydrophobization of glass surfaces, which are characterized by measuring the three-phase contact angle of glass-surfactant solution-PDMS. The data reveal two consecutive stages in the hydrophobization process: The first stage is relatively fast and the contact angle increases from 0 degrees to about 90 degrees within several minutes. This stage is explained with the physical adsorption of the PDMS chains, as a result of hydrogen-bond formation with the surface silanol groups. The second stage is much slower and hours or days are required at room temperature to reach the final contact angle (typically, 150-160 degrees). This stage is explained as grafting of the PDMS molecules on the surface by chemical reaction with the surface silanol groups. If the glass surface had been pretreated by hexamethyldisilazane (HMDS), so that CH(3) groups had blocked most of the surface silanol groups, the first stage in the hydrophobization process is almost missing-the contact angle slowly changes at room temperature from about 90 degrees up to 120 degrees. The experiments aimed to compare several hydrophobization procedures showed that PDMS ensures larger contact angle (more hydrophobic surface) than grafted alkyl chains. The contact angles at the PDMS-water and hexadecane-water interfaces were found to be very similar to each other, and much larger than that at the air-water interface. Interestingly, we found that the ionic surfactants practically do not affect the contact angle of PDMS-hydrophobized surface, whereas the nonionic surfactants reduce this angle. Similar trends are expected with silica surfaces, as well.

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The effect of strontium oxide in glass-ionomer cements.

The reaction of strontium oxide powder with poly(acrylic acid) has been studied both alone and within glass-ionomer cements. Reaction was found to be slow and the strontium-carboxylate structure was found to be partially covalent in character, as determined by Fourier transform infrared spectroscopy (FTIR). These are similar to the structures formed by calcium in glass-ionomer cements, but are different from typical monomeric strontium carboxylates, which tend to be purely ionic. Strontium oxide powder introduced in two types of glass-ionomer cements, slowed down the setting reaction at both 21 degrees C and 37 degrees C, but at low levels (5 wt %), increased the compressive strength in both cement formulations studied. However, at higher levels, it was found to decrease the compressive strength. This study confirms the view that strontium is a cement-forming ion; but concludes that, except at very low levels, strontium oxide powder does not improve the properties of glass-ionomer cements.

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Bioabsorbable polymer plates coated with bioactive glass spheres.

In order to provide bioabsorbable self-reinforced poly (L/DL) lactide (P(L/DL)LA) 70 : 30 plates with osteoconductive bioactivity, spheres (125-250 microm) of a bioactive glass 13-93 were implanted onto a polymer plate preform by pressing. With appropriate pressing parameters glass spheres were firmly attached onto the polymer plate. The top of the glass spheres remained exposed. The bioactivity of the coated plates was examined in vitro with immersion in phosphate-buffered saline (PBS). Apatite was observed to precipitate on exposed glass sphere surfaces and the whole polymer surface within the first three days.

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Needle-like apatite-leucite glass-ceramic as a base material for the veneering of metal restorations in dentistry.

A needle-like apatite-leucite glass-ceramic was prepared in the SiO2-Al2O3-Na2O-K2O-P2O5-F system. Nucleation and crystallization processes were studied in bulk and powdered samples. The crystallization of leucite follows the mechanism of surface crystallization. After the precipitation of NaCaPO4 crystals and another unknown crystal phase, the formation of needle-like apatite is based on a volume nucleation and crystallization process. The mechanism of the formation of needle-like apatite differs to those of apatite precipitation in glass-ceramics. The morphology of needle-like apatite is comparable to that of apatite in natural teeth. The properties of the glass-ceramic, especially the good chemical durability, the optical properties, as well as mechanical and thermal properties allow glass-ceramic to be used as a main component in a bio-material for the veneering of metal restorations in dentistry.

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Effects of glass ionomer cements on bone tissue.

In vivo biocompatibility of glass ionomer cements (GICs) was evaluated for use in orthopaedic surgery using a rat model and compared with conventional bone cement, Polymethyl methacrylate, PMMA. The unset GICs and PMMA were inserted into the marrow cavities of rat femora and retained in situ for various periods of time. The PMMA bone cement showed complete biocompatibility with no interference with reparative bone. The conventional GIC with smaller glass particles and lower powder/liquid ratio showed an initial minor toxic effect on rat bone tissue with later disturbance of adjacent bone formation. The conventional GIC with larger-size glass particles and higher powder/liquid ratio and resin-modified GIC showed more severe toxic effect on rat tissue with the resin-modified GIC affecting the rat bone tissue later. The causes of toxicity associated with the conventional GIC with larger glass particles and higher powder/liquid ration and the resin-modified GIC are thought to be related with the unreacted acid component of both materials and longer ongoing metallic ion release.

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Mechanical properties of biodegradable polymer sutures coated with bioactive glass.

Combining commercially available Polyglactin 910 (Vicryl) sutures with bioactive glass powder offers new possibilities for application of composite materials in tissue engineering. Commercial bioactive glass (45S5 Bioglass) powder was used to coat Vicryl sutures and the tensile strength of the sutures was tested before and after immersion in simulated body fluid (SBF) as a means to assess the effect of the bioactive glass coating on suture degradation. Different gauge lengths (126.6 and 111.6 mm) and strain rates (2.54, 11.4 and 25.4 mm/min) were tested. The tensile strength of composite sutures was slightly lower than that of as-received Vicryl sutures (404 MPa versus 463 MPa). However after 28 days immersion in SBF the residual tensile strength of the coated sutures was significantly higher, indicating a protective function of the Bioglass coating. The tensile strength results were similar for the different gauge lengths and strain rates investigated. A qualitative explanation for the effect of bioactive glass coating on polymer degradation is offered.

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Post-irradiation hardness of resin-modified glass ionomer cements and a polyacid-modified composite resin.

This study examined the post-irradiation hardness of resin-modified glass ionomer cements and a polyacid-modified composite resin using a digital microhardness tester. Change in hardness of these materials over a period of 6 months was compared to that of conventional glass ionomer cements and a composite resin. With the exception of the composite resin, all materials showed a significant increase in hardness over 24 h after their initial set. Dual-cure resin-modified glass ionomer cements showed decreased hardness with increased storage time in saline at 37 degrees C. Results suggest that the addition of resins to glass ionomer cements does not improve initial hardness and does not negate the acid-base reaction of conventional cements. Resin addition may, however, lead to increased water sorption and decreased hardness.

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Glass-ionomer dental restorative: part I: a structural study.

A structural study of glass-ionomer cement (GIC) dental restoratives has been completed. Transmission electron microscopy, selected area electron diffraction, and X-ray diffraction studies indicate domain-like microstructure in a new experimental material, whereas a featureless amorphous gel-like microstructure exists in the conventional GIC. Nuclear magnetic resonance studies were also conducted. The new experimental GIC contains domains of (i) bonelike material (apatite), (ii) mesoporous material and (iii) other framework structures (aluminium phosphate in the high cristobalite structure), with its setting chemistry a restructuring of the aluminosilicate glass around the template of poly(acrylic acid). Conventional glass-ionomer cement may set by a similar but slower process. Leaching properties of glass-ionomer cements are also explained.

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Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses.

The effect of the composition and bonding configuration of the bioactive silica-based glasses on the initial stage in vitro bioactivity is presented. Information of the IR active Si-O groups of glass in the system SiO(2)-P(2)O(5)-CaO-Na(2)O-K(2)O-MgO-B(2)O(3) was obtained by fourier transform Infrared (FTIR) spectroscopy. Two different bands associated to non-bridging oxygen stretching vibrations (Si-O-1NBO and Si-O-2NBO) and a gradual shifting of the bridging oxygen stretching vibration (Si-O) have been observed and evaluated. Both effects are attributed to a decrease of the local symmetry originating from the incorporation of alkali ions into the vitreous silica network. The Si-O-NBO(s)/Si-O(s) absorbance intensity ratio increases with a gradual incorporation of the alkali ions (diminution of SiO(2) content) following a linear dependence up to values close to 50 wt % of SiO(2). In vitro test analysis by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) showed a correlation between the amount and type of the non-bridging oxygen functional groups and the growth of the silica-rich and CaP layers. It was found that a minimum concentration of Si-O-NBO bonds in the glass network is required in order to have an efficient ion exchange and dissolution of the silica network. Finally, the bioactivity of the glass is favored by the presence of the Si-O-2NBO groups in the glassy network. The role of these functional groups in the dissolution of the silica network through the formation of silanol groups and the adsorption of water is discussed.

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Investigation of the mechanical performance of young glass-ionomer cement using dynamic mechanical analysis.

Glass-ionomer (or more correctly, glass polyalkenoate) cements have wide applications in dentistry. This paper reports an investigation using dynamic mechanical analysis (DMA) on the setting of typical conventional glass-ionomer cements of varying age. Rectangular section cement samples were stored for four weeks in distilled water at 37 degrees C before being tested. The experimental procedure involved the clamping of the sample in tensile mode and heating through a 37-95 degrees C temperature range in water. A general behavioral trend was followed where all the samples showed increased flexibility with rise in temperature until a "threshold" temperature was reached, whereupon sudden tensile stiffening was observed. The temperature at which the stiffening took place was dependent on the age of the cement, and was interpreted in terms of the secondary cement forming reactions of silica and phosphate. The younger samples stiffened at significantly lower temperatures than the older ones. The activated nature of glass-ionomer setting chemistry meant that younger cements could be prematurely aged through heating. Differential scanning calorimetry was used to study the effect of heating on the distribution of loosely bound water in the cements that had, and had not been exposed to a DMA cycle. Most notably, it was seen that the DMA process did not affect the water in the matrix. This leant further credence to the hypothesis that the stiffening observed during the DMA heating process was caused by accelerated network formation.

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Investigations into the stabilization of drugs by sugar glasses: III. The influence of various high-pH buffers.

PURPOSE: To study the effect of the high-pH buffers ammediol, borax, CHES, TRIS, and Tricine on the glass transition temperature of the freeze concentrated fraction (Tg') of trehalose/buffer and inulin/buffer solutions at pH 6.0 and pH 9.8. Also, the glass transition temperature (Tg) of sugar glasses obtained after freeze drying of these solutions was elucidated. Additionally, the effect occurring during the freezing process on the pH of the various buffers was investigated. Furthermore, the stability of alkaline phosphatase (AP) incorporated in these sugar glasses prepared from solutions at pH 9.8 was evaluated. METHODS: The Tg' and Tg were measured using differential scanning calorimetry (DSC), and the change of pH during freezing was estimated by using an indicator solution added to the respective solutions. The enzymatic activity of AP after freeze drying and storage at 60 degrees C was evaluated by an enzymatic activity assay. RESULTS: It was found that the Tg' and Tg of the samples investigated are strongly influenced by the presence of the buffer. On freezing, only minor changes of the pH were observed. The samples with the lowest Tg and the samples containing buffers that formed complexes with the sugars showed the poorest stability of the AP. CONCLUSIONS: The stabilizing capacities of sugars that are currently recognized as excellent stabilizers for proteins during drying and storage can be completely lost if certain high-pH buffers such as ammediol, borax, and TRIS are used at high concentrations. Loss of stabilizing capacities can be ascribed to strong depression of the Tg' and Tg or to complex formation.

Alkaline Phosphatase↗

Macroporous glass-ceramic materials with bioactive properties.

In the present research work, glass powders and three different organic starches were used to realize macroporous glass-ceramic scaffolds for bone substitutions. For this purpose, bioactive glass powders belonging to the system SiO2-CaO-Na2O-MgO were mixed in a liquid medium with the desired amount of the selected organic phase. Afterwards, by progressively raising the temperature, the water uptake of starches occurred and led to the gelling of the whole system. The resultant gel underwent two thermal treatments in order to eliminate the organic phase and to allow the sintering of the glassy phase. In this way, macroporous glass-ceramic scaffolds were successfully prepared. The samples were characterized by means of optical and scanning electron microscopy with compositional analysis. The volume and mean size of the obtained porosity were investigated by means of mercury intrusion porosimetry, whereas its morphology was assessed by means of microscopic observations. The structure of the original and the resultant materials were investigated by X-ray diffraction. In order to study the reactivity of the scaffolds towards physiological media, the samples were soaked in a simulated body fluid for various times. On their soaked surfaces, scanning electron microscopy and compositional analysis were carried out in order to assess their bioactivity.

Body Fluids↗

Fixation of distal femoral osteotomies with self-reinforced poly(L/DL)lactide 70:30/bioactive glass composite rods. An experimental study on rats.

Self-reinforced poly(L/DL)lactide 70:30/bioactive glass [SR-P(L/DL)LA/bioactive glass] composite rods, 2 mm in diameter and 36 mm in length, were implanted into the dorsal subcutaneous tissue of 16 rats. Osteotomies of the distal femur were fixed with these rods (2 x 15 mm) in 64 other rats. The follow-up times varied from one week to one year. After sacrifice, three-point bending and shear tests, and molecular weight measurements were performed for subcutaneously placed rods. Radiological, histological, histomorphometrical, microradiographic, and oxytetracycline-fluorescence studies of the osteotomized and intact control femora were performed. At 24 weeks the mechanical properties had decreased significantly. Thirty-nine osteotomies healed uneventfully. One of the 64 evaluated osteotomies showed signs of infection at six weeks, and there were 19 non-unions and six delayed unions. In 20 operations the fixation was loose and out of these 14 non-unions were observed. No gross signs of inflammatory or foreign-body reactions were observed. The present investigation showed that the mechanical strength and fixation properties of SR-P(L/DL)LA/bioactive glass composite rods are suitable for fixation of cancellous bone osteotomies in rats as long as the operative technique is correct. The present article is the first report on the application of SR-P(L/DL)LA/bioactive glass composite rods for fixation of cancellous bone osteotomies.

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Absence of thermodynamic phase transition in a model glass former

The glass transition can be viewed simply as the point at which the viscosity of a structurally disordered liquid reaches a universal threshold value. But this is an operational definition that circumvents fundamental issues, such as whether the glass transition is a purely dynamical phenomenon. If so, ergodicity gets broken (the system becomes confined to some part of its phase space), but the thermodynamic properties of the liquid remain unchanged across the transition, provided they are determined as thermodynamic equilibrium averages over the whole phase space. The opposite view claims that an underlying thermodynamic phase transition is responsible for the pronounced slow-down in the dynamics at the liquid-glass boundary. Such a phase transition would trigger the dynamic standstill, and then be masked by it. Here we perform Monte Carlo simulations of a two-dimensional system of polydisperse hard disks far within its glassy phase. The approach allows for non-local moves in a way that preserves micro-reversibility. We find no evidence for a thermodynamic phase transition up to very high densities; the glass is thus indistinguishable from the liquid on purely thermodynamic grounds.

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Supercooled liquids and the glass transition.

Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state-supercooling-has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.

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