Advanced Materials Forum III

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Authors: Paulo Inácio, José N. Marat-Mendes, C.J. Dias
Abstract: Most piezoelectric biosensors normally use crystals, as in the quartz crystal microbalances or surface acoustic wave devices. A new system is described in which piezoelectric polymer films (made of polyvinylidene fluoride, PVDF, and Immobilon, a porous type of PVDF) are used to produce a mass sensitive oscillatory resonant device. Recent results demonstrated that this system successfully detects the binding between bovine IgG (immunoglobulin G) and anti bovine IgG. In order to improve the performance of the biosensor the electromechanical behaviour of such film-membrane is being studied. Miniaturization is also a desirable trend which will be pursued. Thus reports on the dependence on the electrical, mechanical and biological properties with the size of the film will be presented.
Authors: Jivago Serrado-Nunes, Vitor Sencadas, Ai Ying Wu, Paula M. Vilarinho, Senentxu Lanceros-Méndez
Abstract: Chain reorientation may be induced in polyvinylidene fluoride (PVDF) in its β-phase by applying a deformation perpendicular to the pre-oriented polymeric chains. This reorientation begins right after the yielding point and seems to be completed when the stress-strain curve stabilizes. As the deformation process plays an important role in the processing and optimisation properties of the material for practical applications, different deformation stress was applied to the PVDF lamellas and their topographic change and piezoelectric response were studied by means of scanning force microscopy in a piezo-response mode. The experimental results confirm the previously observed chain reorientation that occurs right after the yielding point and that is completed when the yielding region is passed. This reorientation is accompanied by a stretching of the granular structures observed in the topographical images and variations in the domain response. The observed results help to explain the variations in the macroscopic response of the material.
Authors: Eugen R. Neagu, José N. Marat-Mendes
Abstract: The electric conductivity σ in highly insulating materials is determined by the equilibrium thermally generated carriers and by the injected carriers. The injected excess electrons will dominate the thermally generated electrons when the total number of injected electrons substantially exceeds the total number of initially empty electron traps existing in the material. Under these circumstances the electrical charge transport mechanism is no longer ohmic. In order to analyze the dependence of σ upon injected/trapped charge, isothermal and non-isothermal currents in Teflon FEP have been investigated at various temperatures, field strengths, in a vacuum or in ambient air conditions. At temperatures below 413 K, for charging times longer than about 10 s but shorter than about 600 s, the electric conductivity is almost electrical field strengths independent proving that the injected charge plays a minor role. For these conditions the charge is mostly trapped in superficial traps. At higher temperatures σ is field dependent. The final thermally stimulate discharge current has a peak around 500 K with a mean apparent activation energy around 1.35 eV. For a well conditioned sample the peak current is strongly dependent on the charging electric field and on the mean trapping depth of the injected charge. The relaxation time of the trapped charge is around 106 s at 523 K, proving that the injected charge is very stable, a fact of significant importance for applications.
Authors: José N. Marat-Mendes, Eugen R. Neagu
Abstract: Thermoelectrets of poly(acrylonitrile) have been prepared for various electric field polarization strengths. The thermally stimulated discharge current technique has been used to study the remanent charge (dipolar and/or trapped charge) stability. The measurements were carried out in a vacuum. Two overlapping peaks and a significant increase of the current at high temperature have been observed. The first peak is centered around 372 K and the second one is centered around the polarization temperature. When the polarizing field increases from 0.47 to 1.2 MV m-1 the apparent activation energy varies from 1.08 to 1.40 eV, revealing a weak polarization field strength dependence, while the relaxation time at infinite temperature τ0 decreases from 2.05 x10 –15 to 1.05 x 10-18 s. The relaxation time at 300 K increases from 3.48 x 106 to 1.7 x 109 s indicating that the trapped electric charge may be stable at room temperature for the given polarization conditions.
Authors: José N. Marat-Mendes, Eugen R. Neagu
Abstract: The direct current (dc) electric measurements in dielectrics, especially for highly insulating materials, are strongly influenced by the ambient air, mainly by the water vapors contained by the ambient (humid) air which determine a significant increase of the electric conductivity. Beside this, there is a second effect related to the interaction of the absorbed and adsorbed gas molecules with the polymer matrix that modify both the surface and the bulk conductivities. The aim of the paper is to present the influence of the absorbed and/or adsorbed gas molecules on the electric charge trapping and/or detrapping in polyamide 11. The measurements were carried out under ambient (humid) air, in a vacuum at 8 x 10-3 Torr and in dry nitrogen. Because the trapping/detrapping processes and the molecule diffusion process in polymeric materials are slow processes, a variant of the thermally stimulated discharge current method, namely the final thermally stimulated discharge current method and the final isothermal discharge current method have been used in the temperature range from 20 to 190 0C. The absorbed/adsorbed gas molecules into polymer matrix results in a perturbation of the thermodynamic equilibrium and consequently a charge transfer could occur speeding up the deep level charge detrapping.
Authors: M. Carmo Lança, Eugen R. Neagu, Len A. Dissado, José N. Marat-Mendes
Abstract: Cross-linked polyethylene (XLPE) peelings from aged power cables from three different sources were studied using a combined procedure of isothermal and thermo-stimulated current measurements. Different parameters, such as electric field, temperature, charging/discharging times, can be selected in order to make an analysis of the space charge characteristics (such as, relaxation times and activation energies). Three different cables peelings were analyzed: A – electrically aged in the laboratory at high temperature, B – service aged for 18 years and C – thermally aged in the laboratory at high temperature. The results were compared for the different types of samples and also with previous results on laboratory aged and produced films of low-density polyethylene (LDPE) and XLPE.
Authors: M. Carmo Lança, Eugen R. Neagu, Paulo C. Silva, Luís Gil, José N. Marat-Mendes
Abstract: Electrical properties of natural cork, commercial cork agglomerates (for floor and wall coverings) and a recently developed composite of cork/TetraPak® were studied. Measurements of isothermal charge and discharge currents were made for natural cork samples in different directions (axial, radial and tangential cuts). The isothermal current characteristics and the samples conductivity were investigated under different conditions (electric field, temperature and environmental conditions: in air at ambient relative humidity (RH), dry air and vacuum), also the samples could be or not conditioned (dried in vacuum or in a P2O5 atmosphere at room temperature). From these results the influence of water on the electrical properties of natural cork could be seen. In order to compare the three different cork materials a preliminary study was made. Isothermal charge and discharge currents and conductivity after 1h charging were measured and compared for different electric fields and temperature in air at ambient RH.
Authors: Jivago Serrado-Nunes, A.Z. Kouvatov, Volkmar Müller, Horst Beige, Pedro M. Brandão, Michael Belsley, M. Vitor Moreira, Senentxu Lanceros-Méndez
Abstract: The phase diagram of the poly(vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) copolymer system shows for VDF contents of 50...85 mol% a ferroelectric (FE)-paraelectric (PE) phase transition below melting temperature. Investigations on P(VDF-TrFE) 75/25 samples revealed a slight anisotropic behaviour, which leads to a strongly anisotropic stretching effect both on the phase transition and on the amount and nature of the FE phase in samples subjected to mechanical stretching along the main directions of the film. In this work, both the refractive index n1,2 and the piezoelectric coefficient d33 of mechanically stretched P(VDF-TrFE) have been measured for samples with different levels of permanent deformation. These parameters are found to reflect the anisotropy of the permanently deformed samples. The stretching effect is most pronounced (n1,2) or limited (d33) to the vicinity of the yielding point of the material. Above the yielding point, almost the piezoelectric d33 coefficient of the non-deformed sample is observed for samples with large permanent deformation.
Authors: Carlos M. Costa, Vitor Sencadas, João F. Mano, Senentxu Lanceros-Méndez
Abstract: In this work, mechanical and thermal experimental techniques have been applied in order to relate the mechanical response with the microscopic variations of the material. Stress-strain results along the main directions of β-poly(vinylidene fluoride), β-PVDF, in poled and non-poled samples enables to investigate the influence of the poling process on the mechanical response of the material. Further, differential scanning calorimetry experiments allow the investigation of the effect of poling in the degree of crystallinity of the material as well as on the stability of the crystalline phase. Thermogravimetric analysis was used to investigate the kinetics of the thermal degradation of poled and non-poled β-PVDF samples. The differences observed between the two materials suggest that the poling affects the mechanical properties of the material especially in the direction parallel to the polymeric chains and originates changes at a molecular level that remain beyond the melting of the material.
Authors: Anca Stanculescu, Florin Stanculescu, Laura Tugulea, Marcela Socol
Abstract: The main purpose of this paper is to investigate the optical properties of PTCDA and Alq3 films, prepared by two steps, vacuum evaporation and deposition processes on platelets of glass, quartz, and indium-tin-oxide (ITO) coated glass. We have emphasised the bands structure of the absorption spectra with peaks situated at 358 nm, 374 nm, 475 nm and 552 nm in PTCDA, respectively 232 nm, 261 nm and 380 nm in Alq3 that confirms the dominant presence of Alq3 meridianal molecular isomer. For PTCDA films deposited on glass coated with ITO, the structure of the weak double peak at low wavelength is partially modified, but the positions of the two important absorption peaks situated at 2.25 eV and 2.61 eV are unchanged. The two different luminescence emission peaks obtained in Alq3 for different excitation wavelengths (λ=360 nm and λ=520 nm) suggest the existence of the facial isomer beside the meridianal one. We have evidenced a significant Stocks shift in the spectra (EPTCDA=0.40 eV; EAlq3=0.9 eV) and a large Frank- Condon shift (0.40-2.3 eV), suggesting important effect associated respectively with the solid state structure and important conformational differences between the ground and excited state.

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