Martin S. Beevers

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.

Dielectric and dynamic Kerr effect studies of poly(n-butyl isocyanate) and poly(n-octyl isocyanate) in solution

Abstract Static and dynamic Kerr effect studies on poly(n-butyl isocyanate) (PBIC) and poly(n-octyl isocynate) (POIC) in CCl4 solution are reported and are compared with dielectric studies. The variation of dipole moment μ and Kerr constant factors θ1 and θ2 with temperature suggest the chains for MW ∼ 105 depart from a rod-like conformation, and this is also indicated by a detailed analysis of the dynamic Kerr effect and dielectric data. It is shown that the ratio of areas for the rise and decay transients for the Kerr effect are inconsistent with the simple model for rotational diffusion. In addition the averaging process involved in the apparent dielectric relaxation time 〈τD〉 and the Kerr effect decay average relaxation time 〈τK,d〉 is examined and it is shown that for rigid-rod, coil and Kratky-Porod behaviour that 〈τ D 〉 〈τ K,d 〉 ⋍ 〈 A D A K 〉 a factor which would be 3 for rotational diffusion, and is found to be ∼1 for PBIC and POIC again indicating the departure from simple rotational diffusion for the present samples.

Dielectric and optical studies of a nematogen (4,4-n-heptyl-cyanobiphenyl)

The permittivity, refractive index, molecular dipole moment, dielectric relaxation parameters and molecular Kerr constant are reported for the nematogen 4,4-n-heptyl-cyanobiphenyl (HCB) in the nematic phase, in the pure isotropic phase and in benzene solutions. The anisotropy of molecular polarizability is less than in benzene. Kirkwood–Bordewijk g-factors show significant dipole antiparallelism in the isotropic and nematic phases: in the latter it is much larger parallel to the director axis than perpendicular to it. The two major (orthogonal) component activation enthalpy and entropy terms for dipole reorientation (Bauer) in the nematic phase (including benzene solutions) are compared with those in the isotropic media: in the pure nematic ΔHB(‖)= 60 ± 8 kJ mol–1; ΔHB(⊥)= 6 ± 2 kJ mol–1. The current theoretical representations of the nematic phase are examined using the experimental data. The occurrence of pre-transitional molecular interaction in benzene solutions (above 3 mol dm–3) and in the pure isotropic phase can be detected.

Consideration of dielectric relaxation and the Kerr-effect relaxation in relation to the reorientational motions of molecules

Relations between field-free orientational time-correlation functions and dielectric and Kerr-effect responses to step-on and step-off electric fields are considered. Whereas the dielectric and Kerr-effect decay transients are simply related to 〈P1(u, t)〉 and 〈P2(u, t)〉, where u= cos θ, the rise transients are not, in general, related to these quantities; assumptions and specific reorientation mechanisms are required in order to understand the rise transients. The dielectric and Kerr-effect relaxations are considered for the models of rotational diffusion and “fluctuation relaxation”. Experimental data for poly-n-butylisocyanate in solution and tritolyl phosphate are discussed in terms of the theory and mechanisms for motion.

Studies of Cholesteryl Oleyl Carbonate in its Isotropic and Homeotropic States Using Optical Rotation, Kerr-Effect and Light-Scattering Techniques

Abstract The temperature dependence of optical rotation, static and dynamic Kerr-effect, and polarized and depolarized dynamic light-scattering for cholesteryl oleyl carbonate has been studied in a range including the isotropic liquid, homeotropic, and supercooled homeotropic states. Above the isotropic-cholesteric liquid crystal transition, marked pre-transitional effects are observed and are interpreted in terms of the build-up of angular correlations between molecules. On supercooling through this transition the observed magnitudes and correlation times are continuous well into the supercooled region, but show a smaller variation with temperature than that observed in the pre-transition range due, apparently, to a tendency for the angular correlation factors to saturate at the lowest temperatures. These data are discussed in relation to mean-field theory for mesogens and to the nature of the phase transformations between isotropic liquid, homeotropic liquid, and cholesteric focal-conic liquid crystal s...

Dielectric relaxation of short-chain alkyl bromides

Abstract The dielectric relaxation of Br(CH 2 ) N −1 Br, with N = 4 and N = 5, of Br(CH 2 ) N −1 Br with N = 3 and N = 4 and for 1,2-dichloroethane is deduced using the Jernigan method, where the internal conformational changes are governed by the rotational isomeric state model of Flory. It is found that the dipole moment correlation function for internal conformational changes depends upon the choice of reference coordinate system, and that the dipole moment correlation function cannot be generalized so as to include both overall and internal motions of the chain. An alternative approach, which involves the rotational isomeric state model but conserves angular momentum during an elementary conformational change, is suggested. The available microwave dielectric data on such compounds are considered in relation to the results of the theory.

Dynamic Kerr-effect and dielectric relaxation studies of a poly(methylphenyl siloxane)

Dynamic Kerr-effect and dielectric relaxation studies have been for a poly(methylphenyl siloxane) fluid in the region of its α relaxation process. The relaxations are broader than a single relaxation-time process and we find τK,r < τK,d < τϵ and βK,r ≅ βK,d < βϵ, where K, r; K, d and ϵ refer to Kerr-effect rise, decay and dielectric, respectively; τ and β are the relaxation time and the Williams-Watts relaxation parameter, respectively. It is suggested that the observed Kerr-effect relaxation is largely due to the motions of the phenyl group and that a substantial part of this process is due to motions about the bond joining the silicon atom to the aromatic ring.

The Electro-Optical Kerr Effect of Optically Active Liquids

Abstract The electro-optical Kerr effect of optically active liquids has been studied theoretically, applying the lamellar model of Jones, and experimentally, by measurement of the solution Kerr constant B12 of a series of solutions of d-and dl-camphor in carbon tetrachloride. Equations are presented which allow the intrinsic phase retardation, δ, associated with the Kerr effect to be determined using the Senarmont method in conjunction with either dc or pulsed electric fields. The method of determining δ, using pulsed electric fields is particularly useful for solutions having an appreciable electrical conductivity. The time-dependent behaviour of a relaxing system, measured using quadratic detection of the optical transient resulting from the application of a pulsed rectangular electric field, is predicted to be essentially unaffected by optical activity over a wide range of δ and optical rotation.

Molecular dynamics of viscous liquids. A comparison of dielectric and Kerr-effect relaxation for tritolyl phosphate, ortho-terphenyl and their mixtures

Low frequency dielectric (10–2 to 105 Hz) and Kerr-effect (10–5 to 102 s) relaxation behaviour of pure liquid tritolylphosphate, super-cooled o-terphenyl and three tritolylphosphate +o-terphenyl mixtures have been examined. The Kerr-effect rise and decay transients for each of these systems at a given temperature, are characterized by the same relaxation time τK,r and τK,d respectively. For both pure liquids τK,r and τK,d are essentially the same as the dielectric relaxation time τD, and the dielectric loss curves and Kerr-effect transients are adequately described by the William–Watts empirical relaxation function. These results suggest that reorientational motions in highly viscous liquids do not occur by rotational diffusion, but that they are entirely consistent with a “fluctuation relaxation” mechanism, and they are discussed in terms of the Ivanov and Anderson models of rotational Brownian motion. For some o-terphenyl + tritolylphosphate mixtures τK,r=τK,d > τD and possible explanations of this result are considered.

Correlations between theoretical and experimental Kerr constants of n-alkyl amides in solution in dioxan and water

The theoretical molecular Kerr constants of eleven alkyl amides (related to formamide, acetamide and propionamide), calculated using bond polarisabilities and solution dipole moments, are compared with infinite dilution molecular Kerr constants determined in 1,4-dioxan at 298 K. Kerr effect data are also presented for several alkyl amides in solution in benzene, cyclohexane and water at 298 K. Theoretical molecular Kerr constants were found to be directly proportional to the corresponding experimental molecular Kerr constants measured in 1,4-dioxan. The optical polarisability tensors, associated with carbon–carbon and carbon–nitrogen single bonds and with carbon–hydrogen bonds of the amide alkyl groups, are not significantly changed in going from a dioxan to a water environment.