Characterization of organic adsorbates on model glass surfaces

Model silicate glasses in planar thin film structures are prepared using sol/gel techniques. A typical structure consists of 3.0–12.0 nm of glass on vapor deposited Ag. In this study, a pure silica and a 50/50 binary alumina-silica films are examined. Poly(methyl methacrylate) (PMMA) is adsorbed from chlorobenzene solution onto the surfaces. Infrared reflection (IR) spectroscopy, ellipsometry and quartz oscillator microgravimetry (QOM) are applied to characterize the PMMA/silicate glass interfaces formed. The QOM and IR data show that PMMA adsorption on the pure silica surface is irreversible with respect to removal by pure solvent rinse while on the binary glass surface and on a pure alumina surface, solvent rinses remove the initially adsorbed polymer. These results are interpreted in terms of Bronsted acid-base interactions involving the basic properties of the PMMA C=O group, the acidic nature of the silica and the more basic nature of the alumina containing surfaces. Further evidence for this interpretation is given by the IR spectral data which show broadening of the C=O stretching mode to lower frequencies for the irreversible adsorbed polymer on silica as compared with simulated spectra of non-surface bonding PMMA thin films. This spectral perturbation is interpreted as evidence for a hydrogen bonding interaction between OH groups on the silica surface and the C=O groups. The overall conclusion is that the surface of a 50/50 binary alumina-silica composition is dominated by the basic nature of alumina.     

Kovacs effect in PMMA observed by low-frequency Raman scattering (boson peak)

The physical aging of the poly(methyl methacrylate) glass (PMMA) is described by its effects on the refractive index, that reflects the change of mean specific volume, and, on the other hand, on the low-frequency Raman scattering, i.e., on the boson peak. The boson peak depends mainly on the cohesion fluctuations. The memory or so-called Kovacs effect is observed by the appearance of a minimum of refractive index (i.e. a maximum of volume) as a function of the aging time at a higher temperature subsequent to an aging at lower temperature. The minimum of refractive index corresponds to a maximum of the boson peak intensity. However, the cohesion is not directly related to the volume, so that the evolution of the refractive index does not mimic exactly that of the boson peak intensity. Information on the change of cohesion by aging is obtained by enthalpy measurement. The obtained experimental results are discussed in the frame of the heterogeneous cohesion or elasticity at the nanometric scale. This clarifies the phenomenon of physical aging and the consequent Kovacs effect.     

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10 nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3 nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at T_g, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (T_g) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10 K as compared to the pure matrix).     

Molecular mobility in piezoelectric hybrid nanocomposites with 0-3 connectivity: Particles size influence

Polyamide 11/barium titanate nanocomposites have been studied by a combination of dynamic dielectric spectroscopy, thermo stimulated current and differential scanning calorimetry. The correlation between results obtained by dielectric and calorimetric methods allows us to describe the evolution of the physical structure of the hybrid nanocomposites. The molecular mobility of 0-3 connectivity nanocomposites has been explored. The influence of the nanoparticles size is specifically studied. The smaller sized fillers produce a shift of the relaxation modes observed above the glass transition temperature of polyamide 11 towards lower frequency. The increase of the organic/inorganic interface induces an increase of the ratio rigid amorphous phase/soft amorphous phase. The interfaces favour local ordering stabilized by hydrogen bonds at a nanometric scale.     

Glass transition and thermal expansivity in silica-polystyrene nanocomposites

The glass transition temperature T_g and the thermal expansion coefficient have been measured using the capacitive scanning dilatometry for silica-polystyrene (PS) nanocomposites with various silica volume fraction up to 50 vol.%. The glass transition temperatures for silica-PS nancomposites show a deviation from the bulk T_g together with a large scatter. Thermal expansivity decreases with increasing silica fraction both below and above T_g. A clear relaxation peak can be observed in the expansivity-temperature curve for silica-PS nanocomposites. The intensity of the peak decreases with increasing silica fraction. The decrement is larger than the value expected on the assumption that silica particles do not participate in the glass transition.     

Effect of cooling rate on enthalpy and volume relaxation of polystyrene

This paper presents the results of measurements of physical aging on polystyrene with a narrow molecular weight distribution (M_w/M_n = 1.03). The evolution of the aging process was followed by recording the relaxed enthalpy and the accompanying decrease in volume, using differential scanning calorimetry and mercury-in-glass dilatometry, respectively. The measurements were carried out after cooling the sample at constant rate to the aging temperature. The cooling rate varied between 0.0037 and 1 °C/min. The aging data were fitted with the KWW and the TNM functions. The differences in the time scales of enthalpy, h, and volume, v, relaxation were relatively small. From the h(v) graphs the derivative dh/dv was found to amount to about 1.8 GPa, a value significantly exceeding the predictions of the thermodynamic model containing an additional internal variable.     

The relation between relaxed enthalpy and volume during physical aging

Several papers have recently presented results of measurements of physical aging by studying the behavior of glassy materials quenched from temperatures above their glass transition temperature T_g. The evolution of the aging process is usually followed by plotting the relaxed enthalpy versus the accompanying decrease in volume. Here, we focus on the slope of such plots, which are found to be similar to the inverse value of the isothermal compressibility close to T_g. An explanation of this empirical result is attempted in the frame of a model that interconnects the defect Gibbs energy with properties of the bulk material.     

Aging of a low molecular weight poly(methyl methacrylate)

Dielectric relaxation of a low molecular weight PMMA was measured in both the equilibrium and glassy states. The intense β process, due to motion of the pendant ester moiety, exhibits a change in T-dependence of both its relaxation time and dielectric strength at T_g, reflecting the coupling of this high frequency process to the backbone conformational changes underlying the α dynamics. Consequently, we can exploit the change in the properties of the β process during physical aging to quantify the α relaxation time and its T-dependence for glassy PMMA. The obtained values, τ_α > 10⁴, are too long for direct measurement.     

Elastic anomalous behavior of silica glass under high-pressure: In-situ Raman study

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. III. The driving forces of clustering

Molecular dynamics simulations have been carried out to determine the mean force and the potential of mean force of two La ions in a soda silicate glass. The reaction coordinate starts from a well separated state where the network accommodates the high-field strength cations and then brings them together to form an La-O-La linkage that can lead to the formation of free O anions. It is found that clustering of La ions is favored, at least for dimer states. Similar calculations for the ion pairs of La-La, Na-Na and La-Na in pure silica reveal that the local molecular environments of La and Na ions are quite distinct.     

Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates

In this study, an easy method to increase hydrophobicity of the polymer/silica hybrid coating was demonstrated. UV-curable nano-sized colloidal silica was synthesized and surface-modified both by a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA), and a capping agent, trimethyethoxysiliane (TMES). The formed particles were introduced into the poly(2-hydroxyethyl methacrylate) (PHEMA) matrix to yield PHEMA/silica hybrid hard coatings on plastic substrates via a UV-curing process. Differential scanning calorimetric (DSC) analyses of the hybrids indicated increases of the glass transition temperature (T_g) with increasing silica content in the hybrids; in general, an increase of 23 °C could be achieved for hybrids doped with 15 wt.% silica. Thermal decomposition temperature (T_d), as measured by the thermal gravimetric analyzer (TGA), was found to depend on the silica content in a trend similar to that on T_g. Specifically, a large increase of 25 °C was observed when the sample contained 15 wt.% silica. The pencil hardness of the PHEMA/silica hybrids coated on poly(methyl methacrylate) (PMMA) substrates can reach 5H, in comparison with 2H for pure PHEMA coating. Abrasion resistance was enhanced when silica nanoparticles were incorporated. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably as the TMES content was increased. In the extreme case, a hard surface with a water contact angle (92°) has been obtained.     

A Simple Synthesis,Characterization, and Properties of Poly(methyl methacrylate) Grafted CdTe Nanocrystals

A facile chemical synthesis of poly(methyl methacrylate) (PMMA) grafted CdTe nanocrystals (CdTe NCs) has been realized. Initially, CdTe NCs were prepared in a controlled manner using 2-mercaptoehanol (ME) as a capping agent. Then, 3-(trimethoxysilyl) propyl methacrylate having C?C double bonds were anchored with ME capped CdTe NCs through condensation reaction. Subsequently, AIBN initiated in situ free radical polymerization provided PMMA-g-CdTe nanohybrids. The FT-IR investigation suggested the formation of robust covalent bonding between CdTe NCs and the organic PMMA segment. XPS analysis also confirmed PMMA-g-CdTe nanohybrids. The physical structure and morphology of the as-prepared nanohybrids were studied by XRD and TEM. The thermal stability of the hybrids was enhanced in comparison with pure PMMA as indicated by TGA analysis. The UV-Vis absorption and photoluminescence measurements of the PMMA-g-CdTe nanohybrids showed their potential optical properties.     

Structural study of fractal silica humid gels

The structures of gels of silica obtained from solutions of TMOS, methanol and water have been studied by small angle X-ray scattering (SAXS) using synchrotron radiation. The SAXS results indicate that all the gels studied exhibit fractal structures. It is inferred that gels obtained under basic conditions are volume fractals built up by surface and volume fractal units for high and low water content, respectively. The gels obtained under acidic conditions are all volume fractals having structural units much smaller than those of basic gels. The influence of aging on the structure of basic gels has also been established.     

Positron annihilation study of pore size in ordered SBA-15

Ordered mesoporous materials such as SBA-15 possess a network of channels and pores with a well-defined size in the nanoscale range (2–50 nm). This particular pore architecture makes them suitable candidates for a variety of applications. Different techniques have been used to measure pore diameters. PALS (positron annihilation lifetime spectroscopy) nanoprobe has been used to investigate free volume in several materials, including mesoporous silica. PALS can be used to find out if the micropore and mesopore structures of samples prepared under different experimental conditions are different. Indeed, considering that the pores present a cylindrical shape, an equation was developed that uses specific pore volumes, theoretical density, and specific surface measurements to evaluate structural connectivity. Our goal is to determine the influence of aging temperature on the porous structure of SBA-15 samples. The structural evolution was studied by PALS, small-angle X-ray (SAXS), N₂ adsorption desorption isotherms and computational modeling to evaluate connectivity. The variation of aging temperature changes the pore structure, indicating the presence of micropores and connections between mesopores. Materials aged at high temperatures present the lowest microporosity.     

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu₂O₃) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.     

γ-Fe2O3/SiO2 nanocomposites for magneto-optical applications: Nanostructural and magnetic properties

Monolithic nanocomposites consisting in γ-Fe₂O₃ (maghemite) nanoparticles embedded in a silica gel have been prepared as potential magneto-optic materials, suitable for magnetic field sensing. The effects of several processing parameters on the structural and magnetic properties of the samples were evaluated to optimize their performance. Their structural properties have been studied by means of X-ray diffraction, thermogravimetric analysis and transmission electron microscopy, while the magnetic behavior has been characterized by Mössbauer spectroscopy and Faraday magnetometry. Our results indicate that maghemite crystallization conditions must be carefully controlled to obtain nanoparticles of an adequate size and to avoid subsequent evolution to other undesired ferric oxide phases.     

Synthesis by co-sputtering of Au–Cu alloy nanoclusters in silica

In this work we investigated the Au–Cu binary nanophase diagram by means of structural and compositional characterization of alloy nanoclusters synthesized in silica by means of rf-cosputtering technique. In order to obtain the formation of metallic nanoclusters, as deposited samples have been annealed in reducing atmosphere at 900 °C. A good agreement with the bulk alloy phase diagram has been found: in particular the fcc cubic alloy solid solution is present in all the investigated compositions from pure Au to pure Cu, with a lattice parameter following the modified Vegard’s law typical of the bulk alloy. Three ordered phases are also present corresponding to the Au:Cu atomic ratios of 3:1, 1:1 and 1:3. The linear and non-linear optical properties of the nanocomposites have been measured: the surface plasma resonance red-shifts with increasing the Cu content in the alloy and it spans the entire region from that of pure Au (530 nm) to that of pure Cu (570 nm).     

The influence of titania content on structure and properties of spherical mixed silica/titania particles

Spherical mixed silica/titania particles are prepared from silica/titania sol by an ultrasonic vibrator. The titania content of the samples varies from pure silica to a titania mole fraction of 0.31. Narrow size distributions with most frequent particle diameter of about 1 μm are obtained. Specific surface area and pore volume, mean pore radius as well as the meso pore size distribution are influenced by titania content. The spheres are composed of both silica and titania homogeneous distributed as it is indicated by scanning electron microscopy. X-ray diffraction studies show that the particles must be considered amorphous. Structural modifications of the framework are detected. Samples with varying refractive index may be prepared.     

Adsorption of alcohol vapors on alkoxide-derived silica gels

The adsorption behavior of alcohol vapors on acid-catalysed silica gels was investigated. The adsorption isotherms determined at two ambient temperatures were type IV or pseudo type I of BET classification, and were all interpreted by the duplicated processes of multilayer adsorption and capillary condensation. Some of the isotherms obtained gave both the monolayer capacity and the pore volume, which made it possible to calculate the mean pore radii and the fractal dimensions of the pore surfaces. The fractal dimensions obtained were close to 2, indicating the essentially smooth surface on the molecular scale. The gels prepared from the solutions containing smaller alkoxy groups were found to retain larger pore volume, which was explained in terms of the aging effect under acidic conditions.     

Synthesis of sodium silicate-based hydrophilic silica aerogel beads with superior properties: Effect of heat-treatment

This work demonstrates the synthesis of hydrophilic and hydrophobic high surface area silica aerogel beads with a large pore volume. Wet gel silica beads were modified and heat-treated under atmospheric pressure after modification of the surface by trimethychlorosilane (TMCS). The effects of heat treatment on the physical (hydrophobicity) and textural properties (specific surface area, pore volume, and pore size) of silica aerogel beads were investigated. The results indicated that hydrophobicity of the silica aerogel beads can be maintained up to 400 °C. The hydrophobicity of the silica aerogel beads decreased with increasing temperature in the range of 200-500 °C, and the beads became completely hydrophilic after heat treatment at 500 °C. The specific surface area, cumulative pore volume, and pore size of the silica aerogel beads increased with increasing temperature. Heating the TMCS modified bead gel at 400 °C for 1 h resulted in silica aerogel beads with high surface area (769 m²/g), and large cumulative pore volume (3.10 cm³/g). The effects of heat treatment on the physical and textural properties of silica aerogel beads were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric and differential analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), and Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption and desorption methods.