1,720,964 research outputs found
Inter-chain and intra-chain hopping transport in conducting polymers
No full textThe d.c. conductivity sigma(dc) and the a.c. electric response of poly(3-decylpyrrole) prepared with different dopants and synthesis conditions were studied over a broad temperature (75-300 K) and frequency (100 Hz-40 MHz) range, both in bulk state (films) and in solution. Concerning films, the temperature dependence Of sigma(dc) followed Mott's law, with the hopping parameters strongly doping dependent. A.c. conductivity sigma(ac) was strictly coupled to d.c. transport, so that a master curve resulted from a plot of normalised complex permittivity versus a critical frequency proportional to sigma(dc). Similar results to the bulk case were observed for non-dilute solutions, whereas for very dilute solutions sigma(dc) scaled with temperature as the reciprocal of solvent viscosity. Moreover, a steeper bilogarithmic slope Of sigma(ac) versus frequency was found. Inter-chain charge transport in dilute solutions can be attributed to dissociated counterions diffusing through the solvent, whereas intra-chain hopping can eventually contribute only at high frequencies
ELECTROHYDRODYNAMIC DOMAIN PATTERNS IN FREELY SUSPENDED LAYERS OF NEMATIC LIQUID-CRYSTALS WITH NEGATIVE DIELECTRIC ANISOTROPY
No full textStatic and Dynamic behaviour of the vortex-electrohydrodynamic instability in freely suspended layers of nematic Liquid Crystals
No full textTemperature and pressure dependences of the relaxation dynamics of supercooled systems explored by dielectric spectroscopy RID A-8503-2012
No full textA wide-band (10(2)-2 x 10(10) Hz) dielectric study of epoxy compounds was carried out under isobaric conditions (atmospheric pressure) by changing the temperature down to the supercooled and glassy phases: One of these systems (diglycidyl ether of bisphenol A (DGEBA)) was also measured under isothermal conditions at 293 K by changing the pressure from 0.1 up to 235 MPa. The analysis of variable-temperature measurements gave evidence of a connection between the alpha, beta-splitting phenomenon, the breakdown of the Debye-Stokes-Einstein (DSE) relation which turns into a fractional DSE law, and the transition of the alpha-relaxation dynamics between two different temperature regimes. The variable-pressure measurements revealed that the pressure dependence of the alpha-relaxation time in DGEBA is better described by a second order polynomial function rather than a Vogel-Fulcher-like function. The perfect scaling observed between couples of isobaric and isothermal spectra with the same value of the alpha-relaxation time suggests that the dielectric response is controlled in the same way by both temperature and pressure. From the comparison of the density dependence of isobaric and isothermal relaxation times it turns out that the glass transition is controlled not by only the volume but by both the temperature and the volume. The relative influences of these two variables on the relaxation dynamics of DGEBA at 293 K and atmospheric pressure were also evaluated
STRUCTURAL MACROMOLECULAR PARAMETERS DETERMINING ELECTROCONDUCTIVITY OF OXIDIZED POLY-N-VINYLPYRROLE
No full textThe dependence of electroconductivity on conjugation length as well as the related basic mechanism was investigated by chemical (FeCl3) oxidation of insulating polymers from N-vinylpyrrole (NVP) starting with different Fe/NVP ratios and different molecular weights. The results allow to establish that oxidation occurs mainly intrachain and that electroconductivity has a sharp increase for a conjugation length higher than approximately 15 NVP-units
Study of the relaxation behaviour of a tri-epoxy compound in the supercooled and glassy state by broadband dielectric
No full textInfluence of the end groups on dynamics of propylene glycol oligomers studied by wideband dielectric spectroscopy
No full textThe relaxational dynamics of poly(propyleneglycol) (PPG) and poly(propyleneglycol)-diglycidylether (PPGDE) compounds, the latter with different average number of repeating units, was studied by broadband dielectric spectroscopy (10 mHz to 10 GHz) from above to below the glass transition temperature in the range 115-350 K. The dielectric response of all systems showed a high temperature single relaxation while below a crossover temperature T-B it split in a main (alpha-) and a secondary (beta-) relaxation. The effect on the main and secondary relaxation of different groups terminating the polymer chain was analyzed: in particular, a decrease of the secondary relaxation time was observed for PPGDE with respect to PPG with the same molecular weight. Moreover the change of the Vogel temperature T-0 paralleled what previously found for similar systems. The relationship between main and secondary relaxation and the microscopic nature of this latter were analyzed in the framework of the coupling model. (C) 2002 Elsevier Science B.V. All rights reserved
Hopping charge transport in conducting polymers studied by d.c. conduction and dielectric response analysis.
No full textThe conducting polymer poly(3n-decylpyrrole) (P3DP) has a promising chemical stability and processability. The long alkylic chains makes P3DP soluble in common organic solvents even if they partially hinder a regular molecular arrangement. On account of structural disorder, the material has a moderate conductivity despite the high doping level (one counterion each 4-5 pyrrole rings). The d.c. conductivity and the dielectric response (100 Hz - 40 MHz) of P3DP films with different dopants and synthesis conditions were measured at temperatures between 80 and 300 K. The d.c, conductivity of all the samples was well described by the variable range hopping model; the hopping parameters were found to be much affected by the synthesis and doping conditions. The electrical response exhibited a well-defined relaxation peak, visible only after deducting the d.c. conductivity contribution from the loss factor. The temperature behavior of the loss peak frequency paralleled that of the d.c. conductivity. The Barton-Nakajima-Namikawa equation, relating d.c. conductivity, relaxation time and relaxation strength, was verified. The relaxation strength, too large for being connected with a dipolar relaxation, was ascribed to the displacement of hopping charges. The conclusion of the analysis has been that the electrical response of the system was dominated by the hopping charge transport
Dielectric relaxation phenomena in polymers: Recent findings by chemical vitrification experiments
No full textA novel approach to the dielectric analysis of relaxation phenomena in glass-forming liquids is discussed. Instead of cooling, the system is polymerized so that the glass transition is approached by a chemical vitrification process. Measurements were carried out in a wide frequency interval covering 7 decades, from 10(3) to 10(10) Hz, on different polymerizing systems, namely two epoxy systems, the butyl acrylate and an unsaturated polyester. The relaxation function adopted for extracting the dielectric parameters is validated through a careful verification of the chemical and physical meaning of the changes observed in the dielectric susceptibility as the reaction gores on. The evolution of the shape of the relaxation function is discussed within the percolation theories developed for undercooled liquids and a satisfactory agreement between experimental results and theoretical predictions was found. The scaling behaviour vs. conversion of relaxation times of both main and secondary processes closely parallels those observed in many glass-formers vs. temperature and it can well be represented by Vogel Fulcher-like and Arrhenius-like equations, respectively
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