B. Stoll

Dielectric relaxation of specially prepared polystyrene terminated with rigid polar groups

Two polystyrenes terminated withp-cyanobenzyl andp-cyanobiphenyl groups (=labels) were prepared and their complex dielectric constants were measured in the glass transition region in the frequency range 10−2–106 Hz. The glass temperaturesTg (DSC) were considerably different, 92 and 97.5°C, resp., although their molecular weights were very similar (11 000 and 10 000 g/mol, resp.). Their relaxation behavior showed that the cyano groups relax cooperatively with the polymer segments. The cyanophenyl groups were found to relax with shorter relaxation times than the cyanobiphenyl groups. The measured relaxation strengths showed that there was no association between the dipoles. The relaxation mechanisms of the cyano groups in both labels seemed to be different although the only difference between them was an additional phenyl group in the case of the second label.

Studies on the dielectric behavior of silica-filled butyl rubber vulcanizates after cyclic deformation

Abstract The broadband dielectric spectroscopy (10−3–106 Hz) is used to study the effect of stress-strain cycles on the dielectric properties of butyl rubber vulcanizates filled with silica. The influence of chemical modification of the surface of the silica by silane coupling agent (Si69) also was investigated. In addition Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) were carried out. Dielectric investigations of the samples were done after stress-strain cycles with maximum elongation 26%, 40%, 60%, and 80% from elongation at break, respectively. It was found that the dielectric properties recovered after storage at room temperature for about one year.

Molecular dynamics and alignment studies of silica-filled 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal

A comprehensive study of the dielectric properties of 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal filled with silica particles (particle size 30–80 nm, concentration 2, 3, 5, 10 and 15 wt%). Dielectric spectroscopy in the frequency range 100 to 10 7 Hz was applied to investigate the influence of the filler on the dynamic behaviour of the liquid crystal molecules in both the nematic and isotropic phases. In this frequency range one relaxation process is observed (at f>106 Hz). The dynamical behaviour of the 5CB liquid crystal is described by the Cole-Cole relaxation function. The temperature dependence of the relaxation time obeys the empirical Arrhenius equation. The activation energies are approximately 75 kJ mol1 for the pure 5CB sample in the nematic phase and 50 kJ mol1 for the 5 wt% silica-filled 5CB sample. These values are compared with the corresponding literature values. The reversible electro-mechanical response of these samples under the influence of an applied a.c. electric field is investigated.

Dielektrisches Verhalten polarer Labelmoleküle in Polyisobutylen (PIB) Zusammenhang mit Superstrukturen im Schmelzzustand

ZusammenfassungEs wird über das dielektrische Relaxationsverhalten der beiden Mischungsreihen Tetra- (bzw. Poly-)isobutenylbern-steinsäureanhydrid/PIB im Konzentrationsbereich 0.03 (0.3) bis 6 (30) Gewichtsprozent berichtet. Der polare Mischungspartner TIBBSA (PIBBSA) relaxiert — unabhängig von der Konzentration — um den Faktor 40 (100) langsamer als PIB im Glasprozess. Sein Relaxationsbetrag zeigt im Gebiet um 1,5 (5) Gewichtsprozent eine anomale Konzentrationsabhängigkeit. Diese Mt sich unter der Annahme bevorzugter Einlagerung von Labelmolekiilpaaren in Superstrukturfalten im Rahmen des Mäandermodells quantitativ verstehen.SummaryThe dielectric relaxation of the dipolar label tetraisobutenyl-(and polyisobutenyl-) succinic anhydride has been investigated as well as in mixtures with polyisobutene (PIB), the concentration of the label varying between 0.03 (0.3) and 6 (30) percent by weight. The admixed label TIBBSA (PIBBSA) relaxes — independent of concentration —40 (100) times slower than PIB in the glass process. Its relaxation strength shows an anomalous concentration dependence in the range near 1.5 (5) percent. This finding can be quantitatively explained by the meander model if one assumes that pairs of label molecules preferentially deposit in the meander folds.

Molecular alignment of the 4‐octyl‐4′‐cyanobiphenyl liquid crystal filled with SiO2

The molecular dynamics and molecular alignment of the dispersed 4‐octyl‐4′‐cyanobiphenyl (8CB) liquid crystal with 5 wt % of SiO2 nanosphere particles have been studied using dielectric spectroscopy. The measurements were performed in the frequency range between 102–107 Hz in the SmA, N and I phases. The results show only one Debye relaxation process at high frequency (105–107 Hz). In the bulk material, activation energies of 40, 58 and 63 kJ mol−1 were determined for the SmA, N and the I phases, respectively. In the dispersed sample, the activation energies were found to be 52, 76 and 81 kJ mol−1, respectively. These results are compared with the available data in the literature. The reversible electromechanical response of the dispersed sample under the influence of an applied a.c. electric field was investigated in the SmA, N, and I phases.

Ermittlung von Diffusionskonstanten der Ladungsträger in PVC aus der Frequenzabhängigkeit der komplexen Dielektrizitätskonstante

ZusammenfassungDer Einfluß freier Ionen in einem festen Polymer auf die komplexe Dielektrizitätskonstante im Gebiet sehr tiefer Frequenzen wurde — unter Berücksichtigung der Entladung der Ionen an der Elektrodenoberfläche — auf numerischem Wege berechnet. Durch Vergleich dieser Rechnungen mit Messungen an PVC (1) im Temperaturbereich von 100 °C bis 170 °C konnten Diffusionskonstanten und Konzentrationen der Ionen ermittelt werden.SummaryNumerical calculations showed the influence of free ions in a solid polymer on the complex dielectric constant at very low frequencies. Discharging of the ions at the surface of the electrodes was taken into account. By comparison of these calculations with measurements (1) on PVC in the temperature range 100 °C to 170 °C, diffusion constants and concentrations of the ions could be evaluated.