Graham Williams

Conference on Electrical Insulation and Dielectric Phenomena

Whilst the discovery and development of manmade polymer materials dates from the pioneering works of Goodyear (vulcanized rubber) and Hyatt (celluloid plastics) in the mid-nienteenth century, and to Baekeland (Phenol-formaldehyde resins) at the beginning of this century, the remarkable growth of the synthetic fiber, rubber and plastics industries followed the preparative achievements of the I.C.I. group in Cheshire (polyethylene), Carothers at Du Pont, Wilmington (linear polyesters and nylons) and the German chemists (polyvinyl halides) in the 1930s, and the U.S. Government Synthetic Rubber Program during World War II. The ability to vary chemical structure and composition (e.g. for copolymers) and physical structure (by fillers, plasticizers, thermal and mechanical treatments, processing methods) made it possible to create materials which could be tailor-made for a particular application. Thus the industries grew to the dominant positions they hold today.

Molecular dynamics of liquid crystalline side-chain polymers: The dielectric relaxation behaviour of a siloxan polymer in the nematic and isotropic phases

Abstract The dielectric relaxation behaviour of a liquid crystalline side-chain polymer having a siloxan backbone to which is attached an aromatic ester group via a C 5 methylene spacer has been studied in the nematic and isotropic phases. One process was observed in the isotropic phase and was characteristic of that of a glass-forming liquid. A composite process was observed in the liquid crystalline phase. In the transition range the dielectric behaviour changed continuously from that of the liquid crystal to the isotropic material. The characteristic features of the relaxations are described and their mechanisms are discussed in relation to earlier dielectric data for small molecule nematogens and for liquid crystalline side-chain polymers based on acrylate and methacrylate monomers.

Dipole relaxation in polyethyl methacrylate and polyethyl acrylate as a function of frequency, temperature and pressure. The α, β and αβ relaxations

The dielectric relaxation in polyethyl methacrylate has been studied over a wide range of frequency, temperature and pressure (up to 3000 atm). Three relaxation regions α, β and (αβ) were observed. The α relaxation moved rapidly to lower frequencies with increased pressure, whereas the β relaxation was quite insensitive in location to this variable. The (αβ) relaxation was split into α and β processes with increased pressure. The α relaxation in polyethyl acrylate moved rapidly and continuously to lower frequencies as pressure was increased. The α, β and (αβ) relaxations are discussed in terms of molecular mechanisms. A comparison of dipole relaxations in the alkyl methacrylate series must be made with caution in view of the α and (αβ) relaxations which occur in these polymers.

An appreciation of experimental factors in time-domain spectroscopy

Abstract A systematic account is given of the experimental factors involved in the determination of the complex dielectric permittivity for the frequency range

Molecular dynamics of a smectic liquid-crystalline side-chain polymer in its unaligned and aligned states as studied by dielectric relaxation spectroscopy

Abstract Dielectric relaxation data are presented for unaligned, partially aligned and aligned thin films of a smectic liquid-crystalline side-chain polymer in the ranges 12 to 105Hz and 299 to 343 K. The unaligned material exhibits a complicated dielectric absorption which has been resolved into two broad components (δ and α processes). The dielectric absorption changes remarkably on alignment of the material such that the δ process is enhanced by a factor of three and the α process is considerably diminished. A consideration is given of the molecular theory for dielectric relaxation in these materials and the variation in dielectric properties is shown to depend on the dipole components μml∥ and μml⊢, the order parameter S for the mesogenic head groups and the order parameter S d for the director alignment.

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 motion in polystyrene-plasticizer systems as studied by dielectric relaxation

Abstract The dielectric relaxation behaviour of mixtures of polystyrene with the glass-forming liquids di-n-butyl phthalate, tri-tolyl phosphate and di-octyl phthalate, has been studied over a wide range of compositions. Three distinct dipole relaxation processes are observed, the α process being due to the reorientation of solvent molecules as modified by the polymer molecules in the environment, the α″ process is due to the motion of solvent molecules in cooperation with, and largely determined by, the motion of the polymer molecules, and the α′ process is due to the partial re-orientation of solvent molecules. A comparison is made between the behaviour of these systems and of pure solid dipolar polymers and of supercooled molecular liquids.

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.

Molecular motion in melt samples of poly(propylene glycol) studied using dielectric and Kerr effect relaxation techniques

Abstract Dielectric relaxation times and electro-optical Kerr-effect relaxation times have been measured for melt samples of pure poly(propylene glycol) of nominal molecular weight 1025, 2025 and 4000, over the temperature range 209K–255K. Kerr-effect relaxation curves were analysed in terms of two main components: a ‘fast’ primary process (A), associated with a negative optical birefringence; and a ‘slow’ secondary process (B), associated with a positive optical birefringence. Measurement of dielectric loss at different frequencies for poly(propylene glycol) of molecular weight 2025 and 4000 indicated two main relaxation processes: a ‘fast’ process (the normal α-process), which carried with it most of the dielectric loss; and a ‘slow’ secondary process of much smaller amplitude. The close correspondence between dielectric and Kerr-effect relaxation times, for the primary and secondary relaxation processes, indicated that these techniques are probing effectively different aspects of the same molecular motion. Dielectric and Kerr-effect relaxation times for the secondary process depended greatly on the molecular weight of the polymer and were compared with the predictions of the models for the reptational motions of chains in the bulk polymer, as proposed by De Gennes and by Doi and Edwards.

The effect of a Gaussian distribution of chain-lengths on the phase-behaviour of a model system of rod-like macromolecules in solution

Abstract The calculations of Flory and co-workers for the formation and composition of isotropic, bi-phasic and anisotropic systems composed of rod-like macromolecules in solution have been extended to include a Gaussian distribution of rod-lengths. The critical concentrations for the beginning and end of the bi-phasic range v o ∗ p and v o ∗∗ p respectively, the volume fraction of anisotropic phase Φ A , the compositions and average molecular weights of isotropic and anisotropic phases, and the order parameter S of the anisotropic phase are calculated as a function of polymer concentration v o p for rod-length x o in the range 30 to 70 and for a Gaussian distribution half-width Δ 1 2 in the range 10 to 70. The extent of the bi-phasic range and the composition of the two components is found to be strongly dependent on the breadth of the distribution and average rod-length. The applicability of the calculations to data for poly(alkylisocyanates) in solution is briefly considered.