R. Diaz Calleja

A phenomenological study of the structural relaxation of poly (methyl methacrylate)

Abstract The structural relaxation process of poly(methyl methacrylate) has been studied by differential scanning calorimetry. The sample was subjected to different thermal histories with isothermal stages at ageing temperatures of 100 and 80°C for different ageing times. The enthalpy increment suffered by the sample in the isothermal stage of the thermal history (enthalpy loss due to ageing) was calculated as a function of the ageing time and temperature. The specific heat curves measured were fitted to two phenomenological models.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (p) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three subrelaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed.

A.c. conductivity of thermally dedoped polyaniline

Abstract Relation between doping level and a.c. conductivity of the dry emeraldine salt of polyaniline doped in HCl solution (PANIES) is studied using a procedure of in situ thermal dedoping of the polymer during impedance measurements. This allows the content of doping impurity (HCl) to be decreased by more than one order of magnitude and corresponding changes of a.c. conductivity to be observed. The impedance data are interpreted in terms of macroinhomogeneity of the PANI-ES samples: they are assumed to contain clusters of relatively homogeneous composition embedded into disordered polymer network. Accordingly, the a.c. conductivity is ensured by three distinct mechanisms of electron transport: (i) within a chain of polymer; (ii) between neighbouring chains; and (iii) between different clusters.

Structure, electrical conductivity and dielectric relaxation of the phenothiazine-tetracyanoethene 1:1 complex

Abstract Phenothiazine (PTZ) as a donor and tetracyanoethene (TCNE) as an acceptor form the dark blue charge-transfer complex PTZ-TCNE. In the solid state stacks are found in which the donor (D) and acceptor (A) molecules alternate according to the sequence –D—A– –D—A– –D—A–. Within the stacks the donor and acceptor molecules are arranged coplanarly to each other in such a way that an optimum overlap of the corresponding HOMOs and LUMOs is guaranteed, as is shown by MNDO calculations. PTZ-TCNE is a semiconductor with a gap energy of E g = 1.69 eV and a pre-exponential factor of σ o = 575 S cm −1 . Its dielectric relaxation is of the non-Debye type (non-symmetrical Cole-Cole plot, described by a Havriliak-Negami equation) showing a temperature dependence characterized by an Erying equation with an activation enthalpy of ΔH ν = 0.99 eV and entropy ΔS ν ≈ 10 −3 eV K −1 . The temperature dependence was also analysed in terms of an Arrhenius equation leading to E a = 1.05 eV.

Luminescent structures of porous silicon capped by conductive polymers

Abstract Porous silicon (PS) layers capped by conductive polymers (polyaniline (PANI), and polypyrrole (PPY) demonstrate enhanced photoluminescence (PL) properties with respect to that of bare PS layers. PL measurements in the temperature range from 4 to 300 K, show that the maximum luminescence yield is achieved at 100–150 K, whereas it is strongly quenched outside this temperature interval. As regards to the spectral line features of PS-polymer composites, they are similar to those of PS films. The PL lines are asymmetrical and may be decomposed into two Gaussian peaks centred at 1.6 and 1.75 eV. The position of the peaks is not changed with temperature. We assume that the observed PL enhancement is due to interaction of surface silicon atoms with the constituents of the PANI matrix.

Structure and dielectric relaxation of 2,3,7,8-tetramethoxychalcogenanthrene-TCNQ complexes

Abstract 2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-dichalcogena- cyclo -diveratrylenes, Vn 2 EE′) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form charge-transfer (CT) complexes of composition Vn 2 EE′·TCNQ (E = E′ = S, Se; E = S, E′ = Se, Te). The structure of the hitherto unknown compound Vn 2 STe·TCNQ is described. The dielectric relaxation of the whole series of CT complexes is determined and the results are correlated with the structural data.

On splitting of the conductivity contributions from the dielectric loss

Abstract A method is proposed to separate the conductive and interfacial contributions from the loss permittivity curve.

Dynamic mechanical behaviour of poly(benzyl methacrylate) and poly(cyclohexylmethyl methacrylate)

Abstract The dynamic mechanical properties of poly(benzyl methacrylate) (PBzMA), and poly(cyclohexylmethyl methacrylate) (PCHMMA) were studied. In order to show the β relaxation in both polymers and the γ relaxation in PBzMA, the derivative of the real part of the complex modulus d E′ d T was calculated, which is proportional to T−2E″. A detailed study of the γ relaxation spectra of PCHMMA in terms of activation free energy distribution was performed.

Structure, electrical conductivity and dielectric relaxation of a 1,2-dimethoxybenzene-tetracyanoethene 1:1 complex

Abstract 1,2-Dimethoxybenzene (Ver) as a donor and tetracyanoethene (TCNE) as an acceptor from the dark blue charge-transfer complex Ver·TCNE. In the crystal stacks, in which the donor (D) and acceptor (A) molecules alternate according to the sequence …DAD…A…DAD…A…, centro-symmetric DAD units can be distinguished. In these units the molecules are arranged in such a way that an optimum overlap of the corresponding HOMOs and LUMOs is guaranteed, as is shown by MNDO and HAM 3 calculations. Ver·TCNE is a semiconductor with a gap energy of Eg = 0.85 eV and a pre-exponential factor of σo = 0.91 S/cm. The temperature dependence of dielectric relaxation is characterized by an Eyring equation and gives an activation enthalpy of ΔH ‡ = 0.76 eV and entropy ΔS ‡ = 0.95 × 10 −4 eV/K .

Equivalent circuit analysis of the electrical properties of conducting polymers: electrical relaxation mechanisms in polyaniline under dry and wet conditions

Abstract Alternating current impedance measurements were used to characterize the electrical properties of the emeraldine base form of polyaniline. This conjugated polymer presently attracts much attention due to the possibilities of practical applications in sensors. The impedance data were acquired in the range of frequencies from 0.1 to 105 Hz at constant temperature of 30°C. The single surface electrodes covered by the polymer suspension in different solvents (water, N-methyl-pyrrolidinone and m-cresol) were used in the work. The substance was gradually dried during the analysis, being subjected to the continuous flow of dry nitrogen. The results of measurements were interpreted using an electrical equivalent circuit approach. Theoretical curves to fit the experimental data were obtained using ZVIEW program. Corresponding equivalent circuits were found to include a number of capacitors, resistors and constant phase elements with their parameters sensitive to the experimental conditions. Electrical behavior of the samples at different stages of their drying from different types of suspensions was monitored employing this method. The results were interpreted in terms of the involvement of solvent molecules into the mechanism of the electrical conduction of the polymer.