Peter Frübing

Ionic-liquid-induced ferroelectric polarization in poly(vinylidene fluoride) thin films

Thin films of ferroelectric β-phase poly(vinylidene fluoride) (PVDF) were spin-coated from a solution that contained small amounts of the ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate. A remanent polarization of 60 mC/m2 and a quasi-static pyroelectric coefficient of 19 μC/m2K at 30 °C were observed in the films. It is suggested that the IL promotes the formation of the β phase through dipolar interactions between PVDF chain-molecules and the IL. The dipolar interactions are identified as Coulomb attraction between hydrogen atoms in PVDF chains and anions in IL. The strong crystallinity increase is probably caused by the same dipolar interaction as well.

Complete relaxation map of polyethylene : filler-induced chemical modifications as dielectric probes

Low-density polyethylene (LDPE) filled with a small amount of titanium dioxide proves to be very suitable for studying the molecular motion of polyethylene by means of dielectric spectroscopy. The titanium dioxide acts as an oxidizing agent that introduces a small concentration of polar carbonyl groups, which does not alter molecular motion, but couples it to the alternating electric field. The frequency and temperature dependences of molecular-motion processes in LDPE are determined from both isochronal and isothermal loss plots. In particular, the temperature dependence of the β process connected with the glass-rubber transition in polyethylene obeys the Vogel-Fulcher-Tammann-Hesse law, which indicates co-operative behaviour. For pure LDPE, a glass-transition temperature of -31 °C is determined dielectrically. It increases with increasing titanium dioxide content.

Relaxation behaviour of thermoplastic polyurethanes with covalently attached nitroaniline dipoles

Abstract Thermoplastic polyurethanes with covalently attached nitroaniline chromophores exhibit a strong high-temperature relaxation associated with a glass transition rather than the typical frequency-independent loss due to the aggregation of polymer chains via hydrogen bonds. This relaxation behaviour suggests the formation of an amorphous structure without separation into soft-segment and hard-segment phases. In such a structure, a field-induced orientation of molecular dipoles is stabilised by the high viscosity of the material below its glass transition. The dipole orientation yields a pyroelectric effect caused by dipole-density changes upon thermal expansion and contraction. The changes in phase separation are investigated by varying the chain length of the soft-phase component and studying its influence on the calorimetric, dynamic-mechanical, and dielectrical properties. Good correlation was found between differential scanning calorimetry, dynamic-mechanical analysis and dielectric spectroscopy as well as measured and calculated pyroelectric coefficients. This leads to a uniform picture of the structural changes caused by the incorporation of nitroaniline chromophores into the polyurethane structure.

Pyroelectric properties and dielectric hysteresis of a poly(vinyl alcohol) with azobenzene-alkoxy side chains

Abstract After thermal poling, pyroelectricity and a pronounced dielectric hysteresis have been observed on films of a poly(vinyl alcohol) with azobenzene-alkoxy side chains (Az-PVA). Based on dielectric spectroscopy data and structural information from differential thermal analysis, the pyroelectricity can be explained with reversible dipole–density changes during thermal expansion, whereas the dielectric hysteresis is proposed to result from the polarisation caused by charge carriers, probably ions. These findings are also significant for the interpretation of dielectric hysteresis measurements on other polymer-electret materials.

Relaxation processes at the glass transition in polyamide 11: From rigidity to viscoelasticity

Relaxation processes associated with the glass transition in nonferroelectric and ferroelectric polyamide (PA) 11 are investigated by means of differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy (DRS) in order to obtain information about the molecular mobility within the amorphous phase. In particular, the effects of melt quenching, cold drawing, and annealing just below the melting region are studied with respect to potential possibilities and limitations for improving the piezoelectric and pyroelectric properties of PA 11. A relaxation map is obtained from DRS that shows especially the crossover region where the cooperative alpha relaxation and the local beta relaxation merge into a single high-temperature process. No fundamental difference between quenched, cold-drawn, and annealed films is found, though in the cold-drawn (ferroelectric) film the alpha relaxation is suppressed and slowed down, but it is at least partly recovered by subsequent annealing. It is concluded that there exists an amorphous phase in all structures, even in the cold-drawn film. The amorphous phase can be more rigid or more viscoelastic depending on preparation. Cold drawing not only leads to crystallization in a ferroelectric form but also to higher rigidity of the remaining amorphous phase. Annealing just below the melting region after cold drawing causes a stronger phase separation between the crystalline phase and a more viscoelastic amorphous phase.

Dielectric relaxation in piezo-, pyro- and ferroelectric polyamide 11

Ferroelectric polyamide 11 films were prepared by melt-quenching, cold-drawing and electrical poling. Their ferroelectricity was studied by means of dielectric-hysteresis measurements. A remnant polarisation of up to 35 mC/m/sup 2/ and a coercive field of 75 MV/m were obtained. The piezoelectric d/sub 33/ coefficient and the pyroelectric coefficient of the films are reduced by annealing just below the melting region, but remain at about 3 pC/N and 8 /spl mu/C/(m/sup 2/K), respectively, during further heat treatment. Differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation (TSD) were applied for investigating the conformational changes induced by melt-quenching, cold-drawing and annealing. The results indicate that the cold-drawn film mainly consists of a rigid amorphous phase which exhibits considerably lower conductivity, no glass transition and consequently no dielectric /spl alpha/ relaxation. Instead, an /spl alpha//sub r/ relaxation is found, which is related to chain motions in regions of the rigid amorphous phase where the amide-group dipoles are not perfectly ordered. Annealing removes imperfectly ordered structures, but does not affect the ferroelectric polarisation. Therefore, it may be concluded that essentially the /spl alpha//sub r/ relaxation causes the thermally nonstable part of the piezo- and pyroelectricity in polyamide 11.

Origin of temperature dependent conductivity of α-polyvinylidene fluoride

The conductivity of α-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer.

Dielectric relaxation behaviour of nematic liquid crystals dispersed in poly(vinylidene fluoride-trifluoroethylene)

Polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and a nematic liquid-crystal (LC). The anchoring effect was studied using dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a space-charge relaxation. In PDLC films containing 10 wt% of LC, an additional, bias field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the hindered movement of the LC molecules, this relaxation process is considerably slowed down, compared with the related process in the pure LC.

Influence of uniaxial stretching rate and electric poling on crystalline phase transitions in poly(vinylidene fluoride) films

In this work, the phase transition from the paraelectric α phase to the ferroelectric β phase during uniaxial stretching of PVDF films at rates between 1 and 1000 mm/min was investigated. In our experiments, the final stretching ratio and the temperature during stretching were 4.5 and 110 °C, respectively. The comparison of Infra Red (IR) spectra obtained on unstretched and stretched films shows that the β phase content is strongly determined by stretching rates, i.e. increases during stretching at high rates. An almost pure β PVDF film was achieved by stretching at rates above 600 mm/min. X-Ray Diffraction (XRD) in stretched PVDF films reveals that the size of the β-phase crystallites is smaller after faster stretching. Increasingly, the a-phase crystallites are destroyed and can no longer be found after very fast stretching at 1000 mm/min. An electric poling at electric fields of up to 140 MV/m also results in a phase transition from the α to the β phase in stretched PVDF films according to the observed XRD spectra. Poling leads to a further growth of the β phase crystallites and, consequently, to an increase of the coercive field in ferroelectric-hysteresis measurements.

Surface-charging behavior of plasma-treated polymer films

Thin films of the Teflon/sup TM/ homo- and copolymers TFE, FEP, PFA, MFA, AF, and PVDF-TFE (VF/sub 2/-VF/sub 4/) were treated with a microwave plasma in hydrogen or argon, and charged with a point-to-grid corona. It was found that a hydrogen-plasma treatment has a greater influence on charge stability than an argon-plasma process and that the stability of plasma-treated and negatively charged fluoropolymers is lower than that of untreated samples, while the stability of plasma-treated and positively charged fluoropolymers is higher. X-ray photoelectron-spectroscopy showed ablation of fluorine and incorporation of oxygen- and nitrogen-containing groups into the surface. Furthermore, Teflon-PFA and polyethylene terephthalate (PETP) samples were charged in a radio-frequency argon plasma by way of self-biasing. In this case, the charge level increases with increasing bias and charging time. However, enhanced sample heating must be taken into account at higher plasma power. With a plasma, PFA can be charged to higher initial values, but PETP shows better charge stability.