J. P. Parneix

Dielectric relaxation and molecular motion in comb-shaped liquid crystal polymers

Abstract Dielectric studies have been performed on a series of oriented polyacrylates in broad frequency and temperature ranges, 5 Hz to 1 GHz and – 170°C, to + 200°C respectively. In order to assist in the identification of the observed relaxation mechanisms, different molecular structures of the mesogenic pendant group connected to the main chain via a variable intermediate group have been investigated. The influence of the intermediate group (spacer length n), of the central group (O or COO), of the interphenyl group (COO) as well as that of the terminal group (CN or OCH3), upon the nature of the relaxations (α, β, γ or ω) and the critical times of the molecular dynamics in side-chain liquid crystal polymers are discussed separately.

Mesomorphic Properties of Some New Flurinated Liquid Crystals: the 4-n-Alkoxyperfluorobenzoate Series and Trifluoromethylbiphenyl Dervatives

Abstract We describe here the synthesis, the mesomorphic and the dielectric properties of two families of flurinated compounds: the 4-alkoxyperflurobenzoic acid 4′hydroxy-4-cyanobiphenyl esters and the 4-trifluroromethyl-4′ substituted biphenyl derivates. Some compounds exhibit only a wide temperature nematic range, while others have a wide temperature smectic A range with or without a nematic mesophase. These compounds are possible additives to mixtures of interest for displays.

Frequency- and Temperature-Dependent Dielectric Losses in Lightly Doped Conducting Polymers

In this paper, we report the results of an extensive series of measurements concerning the frequency and temperature dependence of the complex conductivity σ* = σ’ + jσ″ of conducting polymers. Numerous data were obtained from a very wide range of compounds such as polyacetylene, polyparaphenylene, polypyrrole, polyaniline and polythiophene.

Study of Reentrant Nematic and Smectic Phases Using Dielectric Relaxation

Abstract Dielectric properties are reported of new pure substances exhibiting reentrant nematic and reentrant smectic phases. The quasistatic dielectric constants (e′‖, e′⊥, e′is) and the dispersion of e*‖, e*⊥ and e*is is the radiofrequency range (5Hz-1GHz) have been investigated. The current experimental dielectric behaviours are examined and discussed using our experimental data.

A.C. complex conductivity of lightly-doped poly-3-methylthiophene

Abstract A.c. complex conductivity measurements of lightly-doped poly-3-methylthiophene are reported. The data have been obtained over a large frequency range (1–2 × 10 10 Hz) between −200 and +200 °C. At low frequencies, the real conductivity remains nearly frequency independent ( σ d.c. conductivity), while at high frequencies, up to the microwave region, the well-known ω s power law is observed. The exponent s is constant at constant temperature. s is about 0.6 at room temperature and decreases as T increases. Various theoretical models are discussed and compared with the experimental results. It is shown that the correlated barrier hopping model accounts best for the data over the investigated frequency and temperature ranges.

Some New Side-Chain Liquid Crystalline Polymers For Non Linear Optics

Two different ways have been investigated to obtain side-chain liquid crystalline polymers suitable for second harmonic generation (SHG). The aim of the first one was to obtain host nematic comb-like homopolymers having a small degree of polar association of their side chains, by using a 3-fluoro-4-cyanophenyl benzoate end group. The second way consisted in synthesizing liquid crystalline polymeric systems where a mesogenic monomer and a monomeric molecule bearing a group exhibiting a large molecular hyperpolarizability p , were copolymerized. In this paper, we report on the synthesis and the phase behaviour of these two families of polymers.

Dynamic Dielectric Anisotropy of Nematics with Respect to Molecular Structure

Abstract The dielectric spectra of several nematics, such as derivatives of cyclohexane and biphenyl, azoxybenzenes, benzoate esters and tolanes are reported. In each case, the complex permittivity measurements are performed for the two main directions of the nematic sample (parallel and perpendicular to the optical axis), up to the microwave region. A comparison of the spectra has led to a partial assignment of the absorptions observed. In fact, the spectra differ significantly with molecular structure. The dielectric dispersion is analyzed and interpreted as a function of the specificity of the different molecular groups, i.e., the bridging groups, the rings, or the terminal substituent groups. The interpretations proposed (reorientation motions, molecular interaction) are discussed.

Frequency and Temperature-Dependent Complex Conductivity of Some Conducting Polymers

In recent years, numerous investigations on the frequency and temperature — dependence of the electrical conductivity in conducting polymers have been reported [1]. Most of these measurements were performed in the radio frequency region. Just a few data have been given in the microwave zone,and they were given only for some frequencies [2]. In this paper, we report the complex conductivity measurements of some conducting polymers, lightly doped so that they are in the semiconducting conductivity region, where some potential applications may occur [3]. Our measurements are performed in the frequency range 5 Hz-1 GHz for all compounds, and up to 20 GHz for some of them. The temperature-dependence of the conductivity is also investigated and discussed.

Complex Conductivity of Lightly Doped Polyparaphenylene (P.P.P.)

Abstract The evolution of the frequency and temperature dependence of the complex conductivity σ*(f, T) of lightly doped polyparaphenylene is reported. The main features of the experimental devices are the folio-wings : a) an automatic determination of σ* within a large frequency range from 1Hz up to the microwave region; b) a cell structure allowing the measurement of films or pellets (thickness <l mm;c) a large temperature range (70K-400K). The d.c. conductivity (σdc) is found to be strongly temperature dependent (our data are consistent with the Kivelsons model: σdc α Tn with n ∼ 11) The a.c. conductivity (σ′ac) is proportional to fs where s ∼ 0.9. The T-dependence of σ′ac slightly decreases as the frequency increases.

Time Domain Spectroscopy of Liquid Crystals

Abstract A new method of dielectric investigation of liquid crystals using a transient excitation is presented. A two channel set-up rapidly gives the response of the sample (for times less than 1 second). A new real time approach gives the dielectric response function directly for the time range 1 μs -30 ps. The method is applied to an original dielectric characterization of cyano-mesogenic series: cyanostilbene, cyanoester and cyanoterphenyl compounds are studied. The data are compared with those of the homologous cyanobiphenyl. The experimental results are discussed in terms of the molecular dynamics.