Joaquim J. Moura-Ramos

Dipolar relaxation mechanisms in the vitreous state, in the glass transition region and in the mesophase, of a side chain polysiloxane liquid crystal

Abstract The dipolar relaxation mechanisms in a side chain liquid crystalline polysiloxane have been studied by Thermally Stimulated Discharge Currents (t.s.d.c.) and by Dielectric Relaxation Spectroscopy (d.r.s.). The study was carried out in a wide temperature range covering the vitreous phase, the glass transition region and the liquid crystalline phase. Different discharges were observed in the t.s.d.c. spectrum of this polymer which were attributed, in the order of increasing temperature, to local non-cooperative motions probably involving internal rotations in the spacer and in the alkyl group of the mesogenic moiety, to the Brownian motions of the main chain associated with the glass transition and to motions involving reorientations of the components of the dipole moment of the mesogenic side group in the liquid crystalline phase. The dielectric relaxation spectrum, on the other hand, is dominated by two relaxation processes both of which are above the measured glass transition temperature and sho...

Molecular dynamics and macroscopic alignment properties of thermotropic liquid-crystalline side chain polymers as studied by dielectric relaxation spectroscopy

Abstract It is shown that dielectric relaxation spectroscopy provides a convenient means of studying the anisotropic reorientational dynamics of the mesogenic head groups in thermotropic liquid-crystalline side chain polymers. Their alignment behaviour in directing a.c. electric fields of different amplitudes and frequencies is examined, and samples having a macroscopic alignment which is fully homeotropic, fully planar or any desired intermediate alignment have been prepared. The nature and extent of alignment in such samples has been determined by dielectric spectroscopy. In addition both the temperature and pressure variations of the average dielectric relaxation times for certain relaxation processes have been determined and a bulk alignment phenomenon in the absence of a directing electric field is reported.

Fragility in side-chain liquid crystalline polymers : the TSDC contribution

In the present work a procedure is proposed to obtain the fragility of a glass from Thermally Stimulated Depolarisation Currents (TSDC) data. From previous TSDC studies on wide series side-chain liquid crystalline polymers, and particularly from a careful thermal sampling analysis of their glass transition relaxation, the fragility parameter was obtained. The proposed fragility scale is based on the concept of deviation from the zero entropy prediction, and is such that zero is the limit of fragility for infinitely strong glasses. The fragility of 10 side-chain liquid crystalline polymers is discussed in terms of their molecular structure.

Dielectric relaxation in poly(n-alkyl methacrylate)s and their mixtures with p-nitroaniline

In the work described in this paper the dielectric properties of solid solutions of p-nitroaniline in poly(methyl methacrylate), poly(ethyl methacrylate) and poly(n-butyl methacrylate) were measured in the frequency range from 20 to 106 Hz and the results obtained were compared with those of the pure polymers. It is shown that the presence of the solute has a strong influence on the relaxation process of poly(methyl methacrylate), and this is ascribed to the formation of hydrogen bonds between the amino group of the solute and the side groups of the polymer. In poly(ethyl methacrylate) this effect is less pronounced and it is absent in the case of poly(n-butyl methacrylate), suggesting that the increasing size of the n-alkyl group prevents hydrogen bond formation between the solute and the polymer.

The Deborah number, relaxation phenomena and thermally stimulated currents

In the field of the science of flow and deformation of matter (rheology), the so-called Deborah number plays an important role, since it describes the influence of time on the observed flow properties. The technique of thermally stimulated depolarisation currents (TSDC) constitutes a dielectric technique whose experimental result is an electric current flow that is the manifestation of a dipole relaxation. In the present work we use the concept of the Deborah number to describe and interpret a TSDC experiment. We show that, in the linear heating ramp of a TSDC experiment, the Deborah number decreases as the temperature increases, in such a way that it reaches a value of unity at the temperature of maximum intensity of the experimental peak. This particular feature is the origin of a new procedure, proposed here, for evaluating the fragility of a glass-forming liquid.

Molecular motions in solid chloropentamethylbenzene: A thermally stimulated depolarisation currents study

The dipolar relaxation mechanisms present in solid chloropentamethylbenzene have been studied using the technique of thermally stimulated discharge currents (TSDC). A relaxation peak observed at ca. −150 °C has been attributed to the onset of the reorientational freedom in the crystal and is believed to correspond to the transition between an ordered phase and a rotationally disordered phase. At higher temperatures, ca. 10 °C, another relaxation peak appears in the TSDC spectrum of chloropentamethylbenzene, whose assignment at the molecular level is not yet completely elucidated. Taking into account other results from the literature, it is suggested that this peak could arise from self-diffusional motions in the solid, which probably involve out-of-plane molecular tumbling as well as in-plane molecular motions. The results obtained in the present work are compared with those obtained, by the same technique, on other hexasubstituted benzenes. In this context, some general features of the TSDC technique are discussed, namely the existence of a general relationship between the activation parameters associated with the different individual components of the relaxations in the TSDC spectrum, independent of the materials under study and of the nature of the observed relaxations. A reformulation of the so-called ‘zero entropy condition’ is also presented.

Molecular motions in poly(vinyl acetate) and in poly(vinyl acetate)p-nitroaniline mixtures

Abstract In the present paper we examine the dielectric properties of solid solutions of p -nitroaniline in a poly(vinyl acetate) matrix in the frequency range from 16 to 10 5 Hz and the results are compared with those for pure poly(vinyl acetate). It is shown that the presence of the solute increases the activation energy of the α relaxation process of the polymer and broadens the distribution of relaxation times for this process. It is suggested that these observations can be interpreted in terms of the formation of hydrogen bonds between the solute and the side groups of the polymer.

Effect of pressure on the dielectric-relaxation behaviour of a liquid-crystalline side-chain polymer

Abstract The dielectric properties of a siloxane liquid crystal polymer have been studied over a range of frequencies (10-10 5 Hz), temperatures (50–75°C) and applied hydrostatic pressures (up to 1.5 kbar). The relaxation is due to the anisotropic motions of the dipolar mesogenic groups and its rate is strongly dependent upon the sample temperature and applied pressure. The δ- and α-relaxations are observed and are assigned to specific modes of motion of the dipolar groups, and their behaviour, with respect to temperature and pressure variations, are compared and contrasted with the relaxation of chain segments in amorphous solid polymers.

The cavity models and the curvature dependence of the surface tension. Spherical cavities in anisotropic solvents

In the present work the solution process of globular solutes (xenon, cyclohexane, cyclooctane and adamantane) in n-alkane solvents is analyzed. New experimental data on solution enthalpies of adamantane in those solvents are presented. The cavity model previously proposed is corrected with respect to the dependence of the surface tension on the curvature radius of the microscopic cavity and a new equation is proposed to describe this dependence.