Dielectric properties and AC conductivity studies of novel NR/PVA full-interpenetrating polymer networks

Fully interpenetrating polymer networks (IPN) based on natural rubber (NR) and polyvinyl alcohol (PVA) were prepared by using Glutaraldehyde as the common crosslinking agent. Crosslinking reactions were confirmed by Fourier Transform Infra-red Spectroscopy. The electrical properties of full-IPN have been studied in the frequency range of 10²–10⁶ Hz. The dielectric constant, dielectric loss and ac conductivity were analyzed as a function of frequency, temperature and blend composition. It was found that the dielectric constant and dielectric loss increased with the addition of PVA into NR. Interpenetrated system showed a significant reduction in dielectric constant and dielectric loss and almost frequency independent. The dielectric constant and dielectric loss were increased with increase in temperature. The change in these parameters with temperature in the IPN system was found too low as compared to the pure blends. The ac conductivity study revealed that the IPN materials exhibited a considerable reduction in conductivity.     

High-frequency dynamics of type B glass formers investigated by broadband dielectric spectroscopy

We present the results of broadband dielectric spectroscopy on two glass formers with strong Johari-Goldstein β-relaxations. In addition to the α- and β-relaxation dynamics, the extension of the spectra up to 1 THz also allows revealing information on the fast β-process in this class of materials. There is clear evidence for a fast process contributing in the region of the high-frequency loss minimum, which is analyzed in terms of the idealized mode-coupling theory.     

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.     

Structure and properties of self-assembled fluorocarbon–silica nanocomposites

The formation of fluorocarbon–silica nanocomposites by the self-assembly of a fluorinated surfactant and aminoalkoxysilane coupling agents was studied by X-ray diffraction, electron microscopy, NMR spectroscopy and thermal analysis. The prepared materials posses a lamellar nanostructure consisting of non-crystalline fluorinated and condensed silica layers, the latter being very thin. The prepared materials show interesting properties for applications, such as hydrophobicity, thermal stability, high content of aminopropyl groups and low dielectric constant (≈2.8), which is almost independent on frequency. Moreover, the dielectric response can be interpreted in the framework of the Maxwell–Wagner model.     

Montmorillonite (MMT) effect on the structure of poly(oxyethylene) (PEO)–MMT nanocomposites and silica–PEO–MMT hybrid materials

In this work we report the effects of incorporation of variable amounts (0.5–25%w/w) of montmorillonite in poly(oxyethylene) based materials in order to decrease the polymer crystallinity. Two different classes of materials were studied: silica-poly(oxyethylene)-montmorillonite hybrids prepared by the sol–gel route and poly(oxyethylene)-montmorillonite nanocomposites prepared by mixing the dry clay or the clay aqueous suspension into the melt poly(oxyethylene). The effects of montemorillonite loading on the poly(oxyethylene) crystallization control and on the nanostructural features were investigated by X-ray powder diffraction, small-angle X-ray scattering and differential scanning calorimetry. Experimental results show that free montmorillonite layers coexist with open aggregates and tactoids in the poly(oxyethylene)-montmorillonite nanocomposites, with different features depending on the filler proportion and preparation route. The intercalation of polymer chains in montmorillonite galleries markedly hinders the crystallization of the poly(oxyethylene) matrix. For hybrids materials the silica phase favors the exfoliation of montmorillonite tactoids, so that samples are predominantly constituted by dispersed platelets.     

Analysis of precursor residues in lithium aluminosilicate gels using XPS and RGA

Lithium aluminosilicate gels of composition 15 mol% lithia-2 mol% alumina-83 mol% silica were prepared by adding nitrates to tetraethyl-orthosilicate (TEOS) and going through the sol-gel process. Samples were prepared in thin film and bulk form. Dried and outgassed samples were studied with X-ray photoelectron spectroscopy (XPS) d residual gas analysis (RGA). XPS spectra show similar species in both thin film and bulk samples, but different relative quantities of each species. Some oxidation of organic groups by nitrate is evident in bulk samples which were heated to 70°C during the drying process. In both thin film and bulk samples, the O 1s spectra indicate oxygens associated with a silicate network and higher binding energy species such as - -O-. The C 1s spectra of bulk gels heated to 140° and 350°C in vacuum show some organics are vaporized at low temperature, while the oxidized-organic residues are decomposed to CO₂ at high temperature. These species are also observed with residual gas analysis at the corresponding temperatures.     

Correlation of ion dynamics and structure in superionic glasses and nanocomposites

Ion dynamics of AgI-doped Ag₂O–TeO₂ glasses and Ag₂S doped glass nanocomposites have been studied using impedance spectroscopy and correlated with their structures investigated using Fourier transform infrared spectroscopy. The composition dependences of the dc conductivity and the activation energy of these glasses and nanocomposites have been compared with those of AgI-doped silver phosphate and borate glasses. We have studied the ion dynamics in the framework of the power-law and the electric modulus formalisms. We have established a correlation between the crossover rate of the mobile silver ions and the rearrangement of the structural units of glassy networks. The scaling of the conductivity spectra has been used to interpret the temperature and composition dependence of the relaxation dynamics. Analysis of the dielectric relaxation in the framework of modulus formalism indicates an increase in ion–ion cooperation in the glass compositions with increasing AgI content.     

Dielectric relaxation of water adsorbed on cellulose

Water is an essential part of the structure of biological materials. To estimate how the physical state of water adsorbed on cellulose affects various properties, the dielectric properties of moist cellulose were investigated. Three dielectric relaxations were obtained. The relaxation with the lowest frequency was due to electrode polarization. The direct-current conduction calculated from the dielectric loss leaving out the relaxations appearing in mid and high frequencies increased rapidly from 7% or more moisture content (MC). Free water, which can dissolve electrolytes, was increased in the adsorbed water of materials with 7% or more MC. The relaxation appearing in the mid-frequency range was due to interfacial polarization in the heterogeneous structure which consists of adsorbed water with large electrical conductivity within the insulating cellulose. The relaxation appearing at higher frequencies was due to the motion of adsorbed water in cellulose. The relaxation was investigated from relationships between activation enthalpy and entropy in the relaxation. The relationships in low MC cellulose were close to those of sugar, while the relationships in high MC cellulose were close to the relationships extrapolated from bulk water.     

Phase transitions and macroscopic properties of NaNO3 embedded into porous glasses

The crystal structure and dielectric response of nanocomposite materials on base of porous glasses with average pore diameters of 320, 46 and 7 nm with embedding sodium nitrate have been studied by neutron diffraction and dielectric spectroscopy in low and high temperature phases up to melting. In porous glasses with 46 and 7 nm pores NaNO₃ forms dendrite nanoclusters with "diffraction" sizes of 50(2.5) and 20(2) nm. Decreasing of particle sizes results in decreasing of T_c (temperature of order-disorder orientational transition) and T_melt and in smearing of structure phase transition. The values of critical exponent β for orientational transition are estimated from temperature dependences of intensities of superstructure elastic peaks for these three types of nanocomposite materials.     

Contributions to the molecular origin of the dielectric relaxation processes in polysaccharides – the high temperature range

For ionic conducting glasses a concept of correlation of electrical conductivity with dielectric relaxation was developed, which was essential for understanding the relaxation processes found for this substance class. This paper describes on attempt to extend the concept for interpreting the so-called σ-relaxation found in all systems of well dried polysaccharides in the solid state at high temperatures. For various cellulose-based materials, starches and other pure polysaccharides it was found that the activation energies of the dc-conductivity and the dielectric σ-relaxations were well correlated and their values were nearly equal (95–110 kJ/mol) for all substances. The shape and the intensity of this relaxation were nearly independent of temperature for one substance. In cellulose derivatives this process can be only observed, if these have sufficient unsubstituted hydroxyl groups. Highly or persubstituted celluloses show no separable σ-relaxation. Two models are discussed for the interpretation of this relaxation process: the first one assumes a local diffusion process of hydrogen ions between high potential barriers in these disordered systems and the second one a conducting pathway through a badly conducting environment in the material.     

Dielectric low-k composite films based on PMMA,PVC and methylsiloxane-silica: Synthesis,characterization and electrical properties

This report describes the preparation of low-k inorganic–organic hybrid dielectric films, based on a polymethylmethacrylate–polyvinylchloride (PMMA–PVC) blend and a silica powder functionalized on the surface with methylsiloxane groups (m-SiO₂). By dispersing m-SiO₂ into a [(PMMA)_x(PVC)_y] 50/50 (x/y) wt% polymer blend, six [(PMMA)_x(PVC)_y]/(m-SiO₂)_z hybrid inorganic–organic materials were obtained, with z ranging from 0 to 38.3 wt% and x = y = (100 − z)/2. The transparent, homogeneous, crack-free films were obtained by a solvent casting process from a THF solution. The morphology, thermal stability and transitions of hybrid materials were studied by environmental scanning electron microscopy (ESEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). ESEM revealed that hybrid dielectric films are very homogeneous materials. The electrical response of the dielectric films was studied by detailed broadband dielectric spectroscopy (BDS). BDS measurements were performed at frequencies of 40 Hz to 10 MHz and a temperature range of 0–130°C. In these temperature and frequency ranges the proposed materials have a dielectric constant of <3.5 and a tan δ of <0.05. BDS also revealed molecular relaxation events in [(PMMA)_x(PVC)_y]/(m-SiO₂)_z materials as a function of temperature and sample composition. Results showed that these films with z in the range 25–35 wt% are very promising low-k dielectrics for applications in organic thin film transistor (OTFT) devices.     

The structure of simple silicate glasses in the light of Middle Infrared spectroscopy studies

The aim of the work is to determine the internal structure of simple silicate glasses based on structural studies. Due to the absence of the long-range order, the X-ray methods usually applied in the studies of crystalline materials are of low applicability in the investigations of glasses. Therefore, spectroscopic methods, such as Middle Infrared (MIR), which make it possible to ‘see’ the short- and the middle-range orders are extremely suitable in their studies. MIR investigations have shown that the glasses studied exhibit domain composition, which corresponds to the order of certain crystalline phases. Analysis of the MIR spectra of simple silicate glasses and their mathematical decomposition allowed us to identify the bands characteristic for ring systems as well as those originating from Si-O⁻, Si = O defects. Appearance of the bands characteristic for pseudolattice ring vibrations (740-600 cm^− 1) in the MIR spectra of glasses is an evidence of the existence of over-tetrahedral order.     

蓝宝石衬底上GaN薄膜的结构和光学特性表征

采用低压金属有机化学气相沉积(LPMOCVD)法,成功地在(0001)晶向的蓝宝石(Al2O3)衬底上制备了高质量的GaN薄膜.并利用X射线衍射(XRD)谱和椭圆偏振光谱(SE)对其结构和光学特性作了表征.XRD谱中,在34.5°和72.9°附近出现了两个尖锐的衍射峰,分析表明这两个衍射峰分别对应纤锌矿(Wurtzite) 结构GaN薄膜的(0002)和(0004)晶向.其中GaN (0002)晶向衍射峰的半高宽(FWHM)很窄,只有0.1°左右,并且GaN(0004)晶向衍射峰强度很强,二者均证实了采用LPMOCVD法制备的GaN薄膜具有高的质量.在介电函数和反射谱中,GaN高的透明性(＜3.44eV)诱导了强的干涉振荡.室温下拟合出的表征带间跃迁的光学带隙约为3.44eV.     

Optical properties of thermally evaporated CdS thin films

CdS thin films of varying thicknesses were deposited on cleaned glass substrates at room temperature by thermal evaporation technique in a vacuum of about 2 x 10^‐5 torr. UV‐VIS spectra of the films were studied using the optical transmittance measurements which were taken in the spectral region from 300 nm to 1100 nm. The absorbance and reflectance spectra of the films in the UV‐VIS region were also studied. Optical constants such as optical band gap, extinction coefficient, refractive index, optical conductivity and complex dielectric constant were evaluated from these spectra. All the films were found to exhibit high transmittance (∼ 60 ‐ 93 %), low absorbance and low reflectance in the visible/near infrared region from ∼ 500 nm to 1100 nm. The optical band gap energy was found to be in the range 2.28 – 2.53 eV. All the films annealed at 300°C for 4 hours in vacuum (∼ 10^‐2 torr) showed a decrease in the optical transmittance with its absorption edge shifted towards the longer wavelength, leading to the result that the optical band gap decreases on annealing the films. Also, on annealing crystallinity of the films improves, resulting in decrease in the optical transmittance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract

In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm^?1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ^?1.     

Electronic Processes and Energy Storage in Inorganic/Organic Nanohybrids

Three types of nanohybridized structures such as organic/dielectric, organic/semiconductor, and organic/graphen-like ones are investigated. The effect of ultrasonic exposure on the material structure and its influence on the dielectric permeability and on the dielectric loss tangent are studied for the first type of nanostructures. The magnetoresistance of a structure at room temperature is studied in the second type of materials. The change of the kinetic parameters of the intercalation process is shown for the third type of nanohybrid structures.     

The structural transformation and properties of spin-on poly(silsesquioxane) films by thermal curing

In this study, the structural transformation and properties of five commercially available poly(silsesquioxanes) by thermal curing were investigated, including poly(hydrogen silsesquioxanes) (HSQ and T12), and poly(methylsilsesquioxanes) (MSQ, T7 and T9). These materials with a different cage/network ratio and side groups (Si-H and Si-CH₃). The FTIR spectra show that the poly(silsesquioxane) films have different contents of the Si-O-Si cage and network structures, which significantly affects the refractive index and dielectric constant. The shifting of the Si-O-Si network band in the FTIR spectra can be correlated with their molecular structures. The refractive indices and dielectric constants of the studied poly(silsesquioxane) films increase with increasing the Si-O-Si network content. The retention of the Si-H or Si-CH₃ side group suggests the existence of the cage structures in the poly(silsesquioxane) films. The Si-O-Si cage structure results in a larger free volume than the Si-O-Si network structure in the poly(silsesquioxane) films and thus reduces the refractive index and dielectric constant. It is supported by the porosity result. The order of the refractive index in the studied poly(silsesquioxanes) films is T12>HSQ for the Si-H side group and T7>T9>MSQ with the Si-CH₃ side group, which can be correlated with the Si-O-Si network content. The poly(silsesquioxane) film with the Si-CH₃ side group has a lower refractive index than the Si-H side group at the same Si-O-Si network content, which is probably due to the steric hindrance effect of the CH₃ group.     

Interaction of Se0 nanoparticles stabilized by poly(vinylpyrrolidone) with gel films of cellulose Acetobacter xylinum

The sorption and desorption of poly(vinylpyrrolidone)-Se⁰ (PVP-Se⁰) nanoparticles on gel films of cellulose Acetobacter xylinum (CAX) are investigated. It is revealed that the hydrodynamic radius R _h of PVP-Se⁰ nanoparticles decreases from 57 nm in the initial solution (without CAX gel films) to 25 nm after the sorption of nanostructures on gel films and then increases to approximately 100 nm after the desorption of nanoparticles with water from dry samples of the CAX gel film-PVP-Se⁰ nanocomposite. It is found that selenium atoms do not penetrate into crystallites of the cellulose nanofibrils and replace water molecules sorbed by the primary hydroxyl groups of their walls. Poly(vinylpyrrolidone)-Se⁰ nanoclusters differ in the number and size upon their sorption inside the cellulose gel film and on the film surface.     

Changes in molecular dynamics of the tri-ethylene glycol dimethacrylate monomer upon polymerization

The dielectric behavior of the tri-ethylene glycol dimethacrylate monomer was investigated by dielectric relaxation spectroscopy after isothermal polymerization at different temperatures and compared with the unreacted monomer. The relaxation process associated with the monomer glass transition slightly deviates to lower frequencies/higher temperatures in partial polymerized samples. Additionally, it is stronger depleted the higher the polymerization temperature, vanishing after a temperature where the polymerization reached completion. A secondary relaxation process related with localized mobility with the appearance of an excess wing in the bulk monomer previously studied in a narrower frequency range, becomes better defined after partial polymerization due to the depletion of the main relaxation process. Its location and activation energy is independent of the polymerization temperature. Whereas its intensity does not change significantly upon polymerization, the decrease of the intensity of the glass transition process could be taken as a degree of conversion. A αβ splitting scenario was drawn that maintains its pattern in the partial polymerized systems relatively to the bulk monomer. The fragility parameter of the bulk monomer was recalculated (m = 85), slightly increasing when the monomer left unreacted co-exists with the newly formed polymer.