Mohamed Tabellout

Dielectric, ultrasonic and carbon-13 nuclear magnetic resonance relaxation measurements of t-butyl alcohol–water mixtures

Dielectric, ultrasonic and 13C nuclear magnetic resonance relaxation measurements are reported on mixtures of t-butyl alcohol and water over the temperature range 278–308 K. Supporting measurements of the refractive index, density and viscosity are also presented. Deviations from ideal mixing behaviour are observed in the volumes of mixing, permittivities and adiabatic compressibilities of the mixtures. The location of the minima in the excess occurring at different compositions depend upon the method of measurement. Similarly the relaxation behaviour, as observed using different techniques, reveals different aspects of the motion of the molecules in the fluid. The various effects observed are rationalized in terms of a model in which it is assumed that t-butyl alcohol dimers and trimers and various hydrated forms play an important role in determining the structure of these mixtures.

The poly(ethylene terephthalate)/polyaniline composite: AFM, DRS and EPR investigations of some doping effects

A composite based on PET as a matrix and a thin layer of PANI produced by chemical polymerisation of aniline in the surface layer of the PET is investigated by means of four probe conductivity method, standard and conducting AFM, DRS and EPR techniques. Conducting AFM shows that PANI forms conducting clusters. Standard AFM topographical images prove that the undoped form of the composite has a flat surface, whereas the doped one exhibits mountainous features. DRS spectra revealed that the transition from the undoped form to the doped one is accompanied with an increase of the high frequency peak associated to conductivity of the clusters leading to the appearance of two low frequency relaxation processes connected with interfacial polarization phenomena. It is found that the relaxation behaviors of the composite doped by HClO4 and HCl acids are similar. The EPR spectra obtained in both doped and undoped forms of the composite are induced by two kinds of paramagnetic centres (PC). Irrespective to the dopant, anomalous thermal behaviors of the PC spin relaxation times and susceptibilities are evidenced above 180 K. These effects are connected with changes in the electronic transport properties.

Real-time dielectric studies of network formation in thermally activated epoxy-amine and isocyanate-triol systems

Abstract Dielectric measurements are reported on the changes that occur in the nature of the dipole relaxation processes during cure of an epoxy-amine and a diisocyanate-triol system. The epoxy resin forms a vitrified solid in the final cured state, whereas the urethane retains its elastomeric properties. The initial behaviour for the epoxy resin is dominated by ionic conduction processes. Subtraction of the conductivity contribution reveals a dipolar relaxation process, which is analysed using the Havriliak-Negami (HN) equation. The characteristic exponent 1 - n of the HN equation changes in a similar manner to that found with other epoxy-amine systems. However, the dipolar process in the urethane occurs at very high frequency and a different form of the 1 - n dependence is observed. Sensitivity of the dielectric method for the detection of vitrification and gelation is critically assessed. If the vitrification is allowed to occur, then detection of a gel point may not necessarily be inferred from the dielectric data.

Dielectric relaxation properties of poly(ethylene-terephthalate)–polyaniline composite films

Abstract The dielectric relaxation measurements on poly(ethylene-terephthalate) (PET) containing adsorbed aniline and polyaniline (PAni), prepared by polymerization into the PET matrix, have been performed between 173 and 403 K in the frequency range 1 to 10 7 Hz. The influence of aniline and PAni on the β -relaxation of PET and the effect of the presence of PAni on the α -relaxation of PET have been studied. For the films PET–PAni doped with HCl, two relaxation processes around 0.5–5 kHz and 0.2–1 MHz were observed.

Application of the Maxwell–Wagner–Hanai effective medium theory to the analysis of the interfacial polarization relaxations in conducting composite films

A new approach to explain the interfacial polarization phenomenon in conducting composite films is proposed. HCl-doped poly(ethylene terephthalate) (PET) and polyamide-6 (PA-6) matrices with embedded polyaniline (PANI) particles as filler were investigated and analysed, combining dielectric spectroscopy and AFM electrical images with the effective medium theory analysis. Up to three relaxation peaks attributed to the interfacial polarization phenomena were detected in the studied frequency range (0.1 Hz–1 MHz). The AFM electrical images revealed that the doped PA-6/PANI composite can be modelled as a single-type particle medium and the PET/PANI one as a two-type particle medium. A simple dielectric loss expression was derived from the Maxwell–Wagner–Hanai mixture equation and was applied to the experimental data to identify the interfaces involved in each of the relaxation peaks. The parameter values (permittivity, conductivity, volume fraction of the PANI particles) were found to agree well with the measured one, hence validating the models.

Dielectric relaxation properties of the polymer coating during its exposition to water

Abstract Dielectric relaxation spectroscopy is applied for the study of α- and Maxwell–Wagner (MW) relaxations of polyurethane (PU) coating on steel and gold substrates during a water exposition. The evolution of the α-relaxation time of the PU coating in contact with water can be characterized by four stages: (1) Relaxation time increases with starting of the water penetration. The presence of water molecules in the coating leads to an increase in charge carriers at the metal/polymer double layer which influences the cooperative molecular motions in the polymer coating. (2) Relaxation time decreases because of the plasticizing effect of the water in the polymer. (3) Decreasing of the relaxation time and ionic conductivity associated with formation and growing of blisters. Appearance of blisters gives way to an interfacial dielectric relaxation (MW) in the low frequency region. (4) Overlapping of the α-relaxation peak by MW effect when the coating is completely displaced with water. The role of the metal/polymer interaction on these stages is discussed.

Ultrasonic studies of polyurethane network formation

Abstract The formation of polyurethane networks has been followed by measuring ultrasonic absorption and velocity as a function of time for discrete frequencies. In order to determine the relaxation spectra in the pre-gel phase, ultrasonic measurements were performed on quenched samples over the frequency range 5–1000 MHz. The data observed are discussed in terms of molecular motions and interactions.

Polyamide‐12/Polyaniline Layered Composite Films: Specificity of the Formation and Raman Spectroscopy Investigation

Composite polyamide‐12/polyaniline (PA‐12/PANI) films were prepared by a treatment of PA‐12 films swelled in aniline with a water solution of ammonium persulfate (APS) and HCl. The existence of a very thin (2 µm) subsurface PANI containing layer in the composite film was shown by confocal micro‐Raman resonance spectrometry. A combination of the micro‐Raman and UV–Vis measurements allowed determining the PANI concentration in this conductive layer. This approach allowed studying the kinetics of the matrix polymerization process. It is shown that this solid phase process has the strong kinetic specificity as compared with known solution based systems. It is suggested that this is due to not only the physical‐chemical interactions of aniline and formed PANI with amide groups of the matrix but also to a physical state of aniline localized in voids of the free volume of the matrix and to formation of the additional oxidant HOCl in the oxidizing solution.

Application of dielectric relaxation spectroscopy to study metal/polymer interfacial interaction

Abstract Dielectric Relaxation Spectroscopy (DRS) has been applied for the investigation of the influence of the metal/polymer interfacial interaction on the dielectric relaxation properties of polyurethane (PU), in the frequency range from 0,1 Hz to 10 MHz at temperatures from 223 to 423 K in the dependence of thickness of the PU layer (from 10 to 150 μm) and electrode material (gold and steel). Strong electrostatic interaction between steel substrate and the PU results in changing the dielectric relaxation properties of the latter, comparatively with gold substrate. The influence of metal/polymer electric double layer becomes significant at thickness less than 60 μm for the steel substrate. For the gold electrodes, the relaxation characteristics of the PU hardly depend on thickness.

Flexoelectric response in soft polyurethane films and their use for large curvature sensing

The flexoelectric effect is simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It may be seen as an alternative transduction mechanism to the piezoelectric effect to directly sense the curvature of bent flexible thin structures. In the case of large curvatures, flexible and compliant sensors are required and soft polar elastomers may be suitable for curvature sensing. In this study, we report the flexoelectric characterization of soft semi-crystalline polyurethane (PU) films with thicknesses ranging from 1.7 μm to 350 μm. Dynamic bending experiments have been performed on PU films deposited onto rigid steel substrates in the vicinity of the mechanical resonance frequency of the cantilever beams. Quasi-static flexoelectric coefficients of PU films could be obtained by using a classical oscillating model. A global large increase of μ 12 ′ with the decreasing film thickness was found, especially for thicknesses lower than 25 μm. The variation of μ 12 ′ is explained by the presence of a Youngs Modulus gradient through the thickness of PU films. Besides, a concomitant uncommon dramatic decrease in the dielectric constant is observed. The combination of these two effects contributes to enhancing the flexocoupling “F” constant with the decreasing thickness. At last, the potential use of a 6.6 μm-thick soft PU film as a large curvature sensor has been experimentally demonstrated by subjecting a flexible Aluminum foil/Polyethylene terephthalate bilayered cantilever to large deflections. A curvature of about 80 m−1 (radius of curvature of ∼1.2 cm) could be sensed under low frequency (3 Hz) bending motion. These results may pave the way for the development of low cost and easy to implement soft flexoelectric elastomer-based large curvature sensors on highly flexible metallic structures.The flexoelectric effect is simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It may be seen as an alternative transduction mechanism to the piezoelectric effect to directly sense the curvature of bent flexible thin structures. In the case of large curvatures, flexible and compliant sensors are required and soft polar elastomers may be suitable for curvature sensing. In this study, we report the flexoelectric characterization of soft semi-crystalline polyurethane (PU) films with thicknesses ranging from 1.7 μm to 350 μm. Dynamic bending experiments have been performed on PU films deposited onto rigid steel substrates in the vicinity of the mechanical resonance frequency of the cantilever beams. Quasi-static flexoelectric coefficients of PU films could be obtained by using a classical oscillating model. A global large increase of μ 12 ′ with the decreasing film thickness was found, especially for thicknesses lower than 25 μm. The variation of μ 12 ...