Heat treatment effects on dielectric and physico-chemical properties of an epoxy polymer

Infrared (IR) spectroscopy, dielectric spectroscopy (DS), and thermally stimulated depolarization current (TSDC) have been used to study heat treatment effects on an epoxy-based polymer. Variations in physico-chemical and dielectric properties were examined for annealing temperatures between 55 and 170 °C. IR results have shown that heating causes both chain scission and thermal oxidation of the polymer, increasing thus the amount of trapped charges. The complex dielectric permittivity and the dielectric modulus have been analyzed, by means of DS, to highlight and separate charge relaxation phenomena from conduction contributions. Results indicate structural rearrangements, leading to a decrease of dipolar relaxation frequency (from 16 to 13.5 kHz) and an increase of the relaxation strength (around 20%). TSDC measurements have shown a current peak shift towards higher temperatures, and a significant intensity decrease, which is proportional to the quantity of released charges.     

Improvement in characteristics of natural rubber nanocomposite by surface modification of multi-walled carbon nanotubes

We aim to develop high-level applications of NR through the innovative use of multi-walled carbon nanotubes (MWCNTs) to improve reinforcing performance and thermal resistance. In this study, we examined the structures and characteristics of composite materials in which NR was the matrix and MWCNTs were the fillers. We studied the properties of composites containing surface-activated MWCNTs with three different diameters. The results show that the reinforcing performance improves as MWCNT diameter decreases, while thermal resistance improves as we decrease the heat-treatment temperature. The latter occurs because adherence between MWCNTs and NR becomes stronger at lower heat-treatment temperatures. We also found that for practical applications, we need to control active sites on MWCNTs to balance adhesion against thermal resistance.     

Mechanical properties of bulk and nanoscale TiO2 phases

The mechanical properties of bulk and nanoscale TiO₂ phases are examined with a view to assess the available bulk modulus and hardness data, and to understand the size-dependent behaviors. The bulk modulus values of thermodynamically stable bulk TiO₂ phases show a general correlation with Ti-O coordination number. As with the cotunnite-structured (OII) phase, it is likely that the seven-coordinated OI and eight-coordinated fluorite forms of TiO₂ are ultrahard substances. Of the nanoscale phases investigated thus far, nanocrystalline anatase displays the strongest size dependence of bulk modulus values, with possible stiffening behavior effected by incipient grain boundary amorphization under pressure. Nanocrystalline rutile and baddeleyite phases do not show appreciable size dependence in their compression behaviors.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

Electrospun nanofibers of poly(ethylene oxide)/teraamino-phthalocyanine copper(II) hybrids and its photoluminescence properties

Poly(ethylene oxide)/teraamino-phthalocyanine copper (II) (PEO/(NH₂)₄PcCu) hybrid nanofibers with a diameter of 200-300 nm were prepared by electrospinning technique. The hybrid nanofibers membrane was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis), and photoluminescence (PL), respectively. The results indicated that (NH₂)₄PcCu molecule was successfully embedded in the one-dimensional hybrid nanofibers via chemical interaction between PEO and (NH₂)₄PcCu. The PL results showed that the PEO/(NH₂)₄PcCu hybrid nanofibers had an intense emission at about 450 nm. A possible PL mechanism was proposed accordingly.     

Crystallinity and dielectric relaxations in semi-crystalline poly(ether ether ketone)

The Maxwell-Wagner-Sillars (MWS) relaxation is studied for semi-crystalline polymers poly (ether ether ketone) (PEEK), in the range 20 Hz-1 MHz and temperature varying from 80 to 330 °C. The parameter is the crystallization condition in the case of PEEK, which is a semi-crystalline polymer considered as a particulate composite. The relaxation found in the semi-crystalline polymers above the α relaxation of the PEEK is ascribed to the trapping of conductive carriers at the interface between crystalline lamellae and the amorphous matrix. The study of PEEK microstructure is based on differential calorimetry and X-rays diffraction. Two lamellae populations have been detected, that depends on the crystallization temperature and its duration. The crystallinity rate is increasing with crystallization temperature and duration. In dielectric studies, the use of the electric modulus instead of permittivity allows us to minimize the ionic conduction and then leads to the appearance of the interfacial relaxation. According to our measurements, the crystallinity rate is not the main factor of the interfacial relaxation intensity, which also depends on the nature and degree of perfection of the lamellae.     

Characterization of structural modifications in poly-tetra-fluoroethylene induced by electron beam irradiation

The effects of electron beam irradiation doses on the poly-tetra-fluoroethylene (PTFE) have been studied. Several techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), mechanical properties and Fourier transform infrared spectroscopy (FTIR) were applied to characterize the PTFE samples and to study the radiation effects on the crystal structure of the polymer.The irradiation dose up to 150 kGy showed an increase in the crystallinity degree of PTFE, which has been observed and confirmed during the DSC and XRD measurements. The increase in crystallinity was attributed to the scissions of the chain in the amorphous region. Moreover, the number-average molecular weights were estimated from the heat of crystallization measured by DSC technique. The results indicated that the molecular weights were decreased by increasing the heat of crystallization due to irradiation with doses up to 150 kGy. Radiation resistance of the irradiated and non-irradiated PTFE was investigated during its mechanical properties at room temperature. The dose at half value of the elongation at break is about 3.10 kGy while the dose at half value of the tensile strength is about 1.70 kGy.     

Magnetic and dielectric properties of low vanadium doped nickel-zinc-copper ferrites

The new spinel-type of general formula Ni_0.6+xZn_0.2Cu_0.2V_xFe_2−2xO₄ with 0.0≤x≤0.25 was synthesized by the usual ceramic method. Structure of the prepared ferrites was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Room temperature magnetic hysteresis loops were measured using magnetic field strength up to 6 kOe. Saturation magnetization (M_s) increased with vanadium content up to x=0.05 and then decreased. Variation of (M_s) as a function of x is explained in terms of cation redistribution between A and B sublattices. Coercive force (H_c), remanent induction (B_r) and squareness of the hysteresis loop (B_r/M_s) as functions of x are presented. Dielectric permittivity (ε′, ε″) and dielectric loss tangent (tan δ) were measured as functions of frequency and temperature. These parameters were found to be strongly dependent on V₂O₅ concentration. The variation of dielectric loss tangent with frequency at different temperatures shows abnormal behavior, where more than one relaxation peaks were observed at low and high temperatures. This behavior could be attributed to the collective contribution of two types of carriers (p and n) to polarization.     

Electrospun nanofibers of poly(acrylonitrile)/Eu and their photoluminescence properties

Nanofibers of poly(acrylonitrile)/Eu³⁺ were prepared by using sol-gel processing and electrospinning technique. The diameter of the nanofibers in the non-woven membranes was about 70-100 nm. The nanofibers were characterized by scanning electron microscopy (SEM), fourier transform infrared (FT-IR), and photoluminescence (PL). In our work, the PAN/Eu³⁺ hybrid nanofibers showed excellent photoluminescence properties. A possible PL mechanism was proposed accordingly. It is expected that these kind of materials would be applied in equipments such as optoelectronic nanodevices in the future.     

Influence of morphology on the emissive properties of dye-doped PVP nanofibers produced by electrospinning

Dye-doped polymer micro- and nanofibers with tailored light emission properties have great potential for applications in optical, optoelectronic, or photonic devices. In this study, these types of structures were obtained by electrospinning rhodamine 6 G-doped polyvinylpyrrolidone (PVP) using a polymer solution of 10% (mass) concentration in ethanol. Polymer nanofibers with different morphologies (smooth and beaded) and diameters of about 500 nm were obtained using different electrospinning conditions with the same solutions. Fluorescence optical microscopy observations showed that the dye was distributed uniformly in the doped PVP nanofibers. Different shifts were observed when we compared the wavelength of the dye emission band peak of the smooth nanofibers (566 nm) and the wavelength of the dye emission band peak of the beaded fibers (561.5 nm) produced by electrospinning in different conditions with the wavelength of the emission band peak for transparent thin films produced by spin coating (558 nm) using the same polymer solution. This demonstrates that it is possible to tune the optical properties of electrospun dye-doped polymer nanofibers simply by modifying the morphology of the material, i.e., the parameters of the electrospinning process.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Study of dielectric behaviour of Mn-Zn nano ferrites

Dielectric properties of Mn_0.4Zn_0.6Fe₂O₄ ferrites synthesised by co-precipitation method have been investigated as a function of frequency (up to 30 MHz) at different temperatures. Dispersion in dielectric constant has been observed between temperatures 450-500 K. DC resistivity was found to increase up to 100 times greater than those for the samples prepared by the conventional ceramic methods. Resistivity variation with temperature is also reported in the present work. The particle size is calculated using Scherrer equation for Lorentzian peak, which comes out between 9 and 19 nm. Possible mechanisms contributing to these processes have been discussed.     

Dielectric properties and fluctuating relaxation processes of poly(methyl methacrylate) based polymeric nanocomposite electrolytes

Solid polymer nanocomposite electrolytes (SPNEs) consisted of poly(methyl methacrylate) (PMMA) and lithium perchlorate (LiClO₄) of molar ratio C=O:Li⁺=4:1 with varying concentration of montmorillonite (MMT) clay as nanofiller have been prepared by classical solution casting and high intensity ultrasonic assisted solution casting methods. The dielectric/electrical dispersion behaviour of these electrolytes was studied by dielectric relaxation spectroscopy at ambient temperature. The dielectric loss tangent and electric modulus spectra have been analyzed for relaxation processes corresponding to the side groups rotation and the segmental motion of PMMA chain, which confirm their fluctuating behaviour with the sample preparation methods and also with change of MMT concentration. The feasibility of these relaxation fluctuations has been explained using a transient complex structural model based on Lewis acid–base interactions. The low permittivity and moderate dc ionic conductivity at ambient temperature suggest the suitability of these electrolytes in fabrication of ion conducting electrochromic devices and lithium ion batteries. The amorphous behaviour and the exfoliated/intercalated MMT structures of these nanocomposite electrolytes were confirmed by X-ray diffraction measurements.     

Dielectric and optical properties of polymer-liquid crystal composite

Dielectric properties of polymer-liquid crystal mixture, having constituent polymer, poly-butyl methacrylate (PBMA) and liquid crystal, cholesteryl nonanoate, are reported as a function of frequency and temperature. The measurement has been done in a temperature range of 300-375 K and frequency range of 100 Hz-10 MHz. The dielectric permittivity and dielectric loss shows significant changes with the addition of polymer molecules in liquid crystal. The significant feature of composite formation is that the pure liquid crystal and polymer do not show dielectric relaxation in the frequency range covered, while the composite shows relaxation peak at a particular frequency. The optical transmittance of pure liquid crystal and composite has also been measured and compared.     

Sign inversion of dielectric anisotropy in nematic liquid crystal by dye doping

The present paper reports sign inversion in dielectric anisotropy of a nematic liquid crystal, i.e. 5CB, as an effect of doping dye (solvent green 3) in small amount. It is the result of strong variation of the parallel component of dielectric permittivity with temperature for a dye doped sample. This behavior is attributed to the interaction taking place between the nematic liquid crystal molecule and the dye molecule. This behavior of dielectric anisotropy has been explained on the basis of interaction between the dye (guest) and the liquid crystal molecules (host).     

Nanocomposites of poly(l-lactide) and surface modified magnesia nanoparticles: Fabrication, mechanical property and biodegradability

A novel nanocomposite based on biodegradable poly(l-lactide) (PLLA) was prepared by the incorporation of surface modified magnesia (g-MgO) nanoparticles using a solution casting method. The mechanical properties, biodegradable properties and protein adhesion behavior of the g-MgO/PLLA nanocomposites were investigated. Scanning electron microscopy (SEM) results showed that g-MgO nanoparticles could comparatively uniformly disperse in PLLA matrix. The addition of g-MgO nanoparticles to PLLA matrix improved the tensile strength and elastic modulus, whereas reduced the elongation at break. The mass loss results showed that the nanocomposites with higher g-MgO content had faster degradation rates. The in vitro pH value determination results indicated that the g-MgO nanoparticles could neutralize effectively the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited enhanced protein adsorption, i.e., with the increase of g-MgO content, the amount of protein adsorption increased. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more protein than the pure PLLA.     

Electrical transport and optical properties of vanadyl phosphate—polyaniline nanocomposites

The nanocomposites of conducting polyaniline and layered vanadyl phosphate, VOPO₄·2H₂O are synthesized by redox intercalation method. Water content decreases with insertion of polyaniline molecules. In scanning electron micrographs plate like structures are observed for both VOPO₄·2H₂O and intercalated nanocomposites. Protonation of polyaniline and interaction with vanadyl phosphate are observed in infrared and UV absorption spectroscopy. Intercalation improves conductivity of pristine vanadyl phosphate. Thermally activated electrical dc conductivity at low temperature shows two distinct slopes around 210 K for both the nanocomposites. The optical band gap of vanadyl phosphate decreases from 4.0 to 3.7 eV due to insertion of polyaniline.     

Stability of anti-reflection coatings via the self-assembly encapsulation of silica nanoparticles by diazo-resins

A modified silica nanoparticle (MSNP) solution was formed by the encapsulation of negatively charged silica nanoparticles by the UV-crosslinkable polycation oligomer diazo-resin (DAR). Appropriate DAR encapsulation concentrations were determined by use of zeta-potential and dynamic light scattering measurements. The MSNPs were used in conjunction with poly(styrene sulfonate) (PSS) to grow homogenous ionic self-assembled multilayer anti-reflection coatings. Stability was induced within the films by the exposure of UV-irradiation that allowed for crosslinking of the DAR and PSS. The films were characterized by UV/vis/IR spectroscopy and field emission scanning electron microscopy. The transmission and reflection levels were >98.5% and <0.05%, respectively. The refractive indices resided in the 1.25–1.26 range. The solvent stability was tested by sonication of the films in a ternary solvent (H₂O/DMF/ZnCl₂ 3:5:2 w/w/w).     

Production of Sr-deficient bismuth tantalates from microwave-hydrothermal derived precursors: Structural and dielectric properties

Strontium bismuth tantalates were produced for the first time from microwave-hydrothermal precursors at , for 2 h. Structural and dielectric properties were investigated by X-ray diffraction and complex impedance spectroscopy. A high ferroelectric-paraelectric transition temperature of was observed, together with two different dielectric regimes for the ac electrical conductivity below T_c. The activation energies were calculated as 0.155 and 0.531 eV, and are related to conduction by oxygen vacancies. It was concluded that the low activation energies showed by these materials could contribute to their fatigue-free nature.