Enrique Sánchez Martínez

Dielectric relaxation in chlorinated polyethylene-polypropylene copolymers

Dielectric relaxation measurements were carried out on eight chlorinated polyethylene-polypropylene (PEPP) copolymers in the range of temperatures covering the main dielectric absorption. Chlorination of PEPP is expected to change the dynamic dielectric properties gradually with increasing amount of chlorine in the polymer chains. Thus, in the present study, increasing degrees of chlorination give a clear shift of the glass transition temperature towards higher values, except in the range between 40 and 51% chlorine, where an anomalous behaviour was observed. The same tendency is also observed in the relaxation strength (Δe). The value of Δe has been estimated by using a non-linear squares regression program (LEVM6) to calculate the parameters of the Havriliak-Negami empirical equation. It appears reasonable to assume that the anomalous behaviour observed can be attributed to a compensation of the dipolar moments of chlorine groups in the macromolecules.

Structure, electrical conductivity and dielectric relaxation of the 1,2-bis(methylthio)benzene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1 : 1 charge-transfer complex

1,2-Bis(methylthio)benzene forms a 1 : 1 charge-transfer (CT) complex with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). X-Ray analysis shows the compound to have a columnar structure with stacks in which the equidistantly and coplanarly arranged donor and accepter molecules alternate. By the relative orientations of the successive donor and accepter molecules in the stacks an optimum overlap of the relevant HOMOs and LUMOs, which are of is-type according to MNDO calculations, is guaranteed.The electrical conductivity of the CT complex is relatively low (as a consequence of the crystal structure), but shows a temperature dependence typical of semiconductors. The dielectric behaviour of the compound has also been studied. The results can be represented by an electrical model circuit of the ladder type, indicating that the following contributions are of similar importance: (i) hopping conduction into the bulk of the polycrystalline sample, (ii) interfacial impedance of the polycrystals, and (iii) electrode adsorption-diffusion phenomena.

Dielectric relaxation peaks in the low-frequency/high-temperature range of the loss permittivity curve studied by the thermostimulated depolarization current (TSDC) technique in 2,3,7,8-tetramethoxychalcogenanthrenes–TCNQ charge-transfer complexes

The dielectric behaviour of the 1 : 1 charge-transfer complexes of the 2,3,7,8-tetramethoxychalcogenanthrenes (5,10-dichalcogena-cyclo-diveratrylenes) Vn2EE′(E = E′= S, Se; E = S, E′= Se) with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) is studied by the thermostimulated depolarization current (TSDC) technique as an alternative experimental method to the more conventional a.c. measurements. In the low-frequency side of the dielectric loss curves (Iµ″vs. frequency plots) the relaxation peaks of the compounds are hidden by a continous increase in dielectric loss. By a transformation of complex permittivity Iµ*=Iµ′–iIµ″ into complex polarizability α*=α′–iα″ loss peaks can be observed in the α″vs. frequency plots (E. Sanchez Martinez, R. Diaz Calleja, P. Berges, J. Kudning and G. Klar, Synth. Met., 1989, 30, 67). The existence of these relaxations is now proven by the TSDC method.

Dielectric relaxation in hydroxypolyesters

Dielectric spectra of a hydroxypolyester (I) and an imide-modified hydroxypolyester (II) are presented. In both cases two clearly defined zones of relaxation are shown. At the high-temperature side of the spectrum the α relaxation zone is produced by micro-Brownian motion of the main backbone and by free charges, and below room temperature, a complex β secondary relaxation zone, showing indications of structure, is detected. Detailed analysis of the subglass relaxation zone is given and activation energies from an Arrhenius plot (In f vs. T–1) are calculated for both polymers. By application to the low-frequence (high-temperature) side of the α relaxation of a transformation which converts permittivity Iµ* into polarizability α* a new relaxation peak appears; this is attributed to the conductivity in the liquid state.

Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-tetramethoxychalcogenanthrene–2,3-dichloro-5,6-dicyano-l,4-benzoquinone 1 : 1 charge-transfer complexes

2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-chalcogena-cyclo-diveratrylenes, ‘Vn2E2’, E = S, Se) form isotypical 1 : 1 charge-transfer (CT) complexes with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). X-ray analysis of Vn2S2·DDQ shows the compound to have a columnar structure with segregated stacks of donors and acceptors. The donors are virtually planar in accordance with a formulation of [Vn2E2]+[DDQ]–. Donor cations and acceptor anions are equidistant in their respective stacks, but in each case they inclined to the stacking axis, nevertheless guaranteeing an optimum overlap of the half-filled frontier orbitals which are of π-type character according to MNDO calculations. Dielectric ac measurements of permittivity Iµ′ and loss factor Iµ″ clearly reveal two processes, a dielectric one at low temperatures and a conductive one at high temperatures. The dielectric process can be described by the Havriliak–Negami (HN) and the Kohlrausch–Williams–Watts (KWW) model, and the conductive process by a Debye-type plot. Using these methods, the relevant parameters are evaluated. The dc conductivities of polycrystalline samples moulded at 108 Pa show a temperature dependence in the plots of ln σ vs. T–1, which is typical of semiconductors. Two slopes are found; that in the low-temperature region (< 285 K) is explained by an easy-path model (intragrain conductivity with low activation energies), whereas in the high-temperature region conduction across the grain boundaries (with higher activation energies) is becoming predominant. The activation energies for the intrinsic conductivities obtained by the ac and dc measurements are similar. Despite the columnar structure with segregated stacks, due to stoichiometric oxidation states of the components, the absolute values of conductivity are low (ca. 10 –6 S cm–1 at 293 K), though higher (by a factor of ca. 103) than those of compounds like Vn2E22·TCNQ with stacks in which donor and acceptor molecules alternate.

Structure, electrical conductivity and dielectric relaxation of charge-transfer complexes with 2,4,6,8-tetramethoxydibenzoselenophene as a donor

2,4,6,8-Tetramethoxydibenzoselenophene (m-Se) forms 1 : 1 charge-transfer (CT) complexes with the acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyano-1,4-quinone (DCQ) and is oxidized by iodine to [m-Se]I3. X-Ray analyses of m-Se–TCNQ and m-Se–TCNE show both CT complexes to have columnar structures with stacks in which the coplanarly arranged donor and acceptor molecules alternate.The electrical conductivities of the CT complexes with TCNQ, TCNE and DCQ and of the compound [m-Se]I3 were relatively low (as a consequence of the crystal structures), however, and show the typical temperature dependences of semiconductors. The dielectric behaviour of the three CT complexes have also been studied, by conventional ac. measurements and by the thermostimulated depolarisation current (TSDC) technique. Relaxation peaks can be observed after a transformation of the complex permittivity Iµ* to the complex polarisability α*. The existence of these relaxations is confirmed by the TSDC measurements. By thermal sampling a nearly constant distribution of activation energies is obtained.

Dielectric relaxation in 2 : 1 charge-transfer complexes of 2,3,7,8-tetramethoxychalcogenanthrenes and tetracyanoethene

The dielectric behaviour of 2 : 1 charge-transfer complexes of the 2,3,7,8-tetramethoxychalcogenanthenes (5,10-dichalcogenacyclodiveratrylenes) Vn2EE′(E = E′= S, Se; E = S, E′= Se) with tetracyanoethene (TCNE) is studied by conventional a.c. measurements and by the thermostimulated depolarization current (TSCD) technique as an alternative experimental method. In the low-frequency side of the dielectric loss curves (Iµ″vs. frequency plots) the relaxation peaks of the compounds are hidden by a continuous increase in dielectric loss. By a transformation of complex permittivity Iµ*=Iµ′–Iµ″ into complex polarizability α*=α′– iα″ the loss peaks can be seen in the α″vs. frequency plots. The existence of these relaxations is confirmed by TSDC measurements.

Halbleiter-Komposite aus [(Me3Sn)3FexCo1-x(CN)6]-Aggregaten (0 l x ≤ 1) und Polypyrrol / Semiconducting Composites Involving [(Me3Sn)3FexCo1-x(CN)6]-Assemblies and Polypyrrole

Pyrrole has been incorporated into the channel-like cavities of polymers of the type: [(Me3Sn)3FeIIIxCoIII1-x(CN)6] with 0 < x ≤ 1. The resulting black composites involving polypyrrole (Ppy) and Fe(II) display electrical conductivities σ between ca. 10-5 and 10-1 S · cm-1. Usually, composites prepared in the presence of small amounts of H2O exceed those in conductivity resulting under strictly anhydrous conditions owing to notably lower thermal activation energies (0.04 to 0.08 eV). An oxidative polymerization mechanism differing from that assumed for “classical” modes of Ppy generation is likely, since the N(pyrrole)/Fe(II) ratio turns always out to be ≥ 1. Exposure of a composite with x = 1 to an atmosphere of NH3 causes a decrease of σ from 1.5 · .10-1 to 2.7 · .10-6 S · cm-1; this NH3 uptake, and the concomitant change of σ, appear to be partially reversible. 1-Methylpyrrole, 2,5-dimethylpyrrole and aniline are oxidized by, and incorporated into, the organotin polymer, too, but the resulting σ-values do not exceed ∼10-9 S · cm-1.