R. E. Barker

Time domain spectroscopic measurements of the dielectric properties of heterogeneous materials

In recent years the powerful new techniques of time domain dielectric spectroscopy (TDDS) have revolutionized the field of high frequency dielectric measurements. However, due to the relative newness of the method many questions remain unresolved concerning the applicability of TDDS and the interpretations and limitations of the measurements. In previous and continuing work we have attempted to use TDDS to detect and follow phase changes when a homogeneous single phase liquid transforms into a two phase system consisting of a dispersion of small particles of the new second phase in the remaining original phase.[1] Although the TDDS system provides a good way to detect the phase changes as a function of transition temperature and composition, questions remain about the meaning of the calculated dielectric permittivity (ε and ε″) of the two phase system. The present investigation was undertaken to obtain experimental results for two phase systems with separately known dielectric properties for the somewhat more interesting case of solid polymer particles in a dielectric fluid.

Electrical conductivity of anisotropic poly (paraphenylene-benzobisthiazole)

Since the quantitative characterization of nonohmic behavior of the polymer would aid in understanding the conduction mechanism we made a series of measurements at different voltages (7). For a fiber (PPBT 28555-19-2) with an effective length of 200 µm, the axial current was quite linear up to 0.12 MV/m (the highest field available on this sample because of a design feature of the Keithley 642 electrometer which limits the maximum voltage that can be used). In future experiments still higher fields will be tried.

Considerations in charge transfer by transient contact of small particles

The author addresses the transfer of charge during collisions between small particles, idealized as spheres, in terms of classical electrostatics. The topics considered include the distribution of charge between spheres of different radii left in contact for a long time and the rate of transfer through finite resistances during transient contact. The model has implications for a variety of phenomena from the charging of hail stones to triboelectric effects in dry powders such as sugar, flour, and xerographic toner particles. An attempt is made to develop a simple two-body model for charge transfer between colliding spheres which will provide a framework to answer questions about equilibrium charging and rate of exchange if only classical electrostatics were applicable. This sets the stage for inferences about many-body processes and the modifications that quantum mechanics might impose.<<ETX>>

Dielectric properties to 3 GHz of liquid crystal mixtures and several alcohols by time domain reflectometry

Studies have been made of the complex dielectric permittivities of mixtures of cholesteric liquid crystals and of a homologous series of normal alcohols. The dielectric spectroscopy was done by the method of time-domain reflectometry (TDR). The liquid crystals were cholesteryl alkanoates with cholesteric-isotropic transition temperatures of 19 degrees C and 72 degrees C. The 50 weight percent mixture has T/sub CI/=32 degrees C. This transition was investigated by TDR. In the case of the alcohols a simple quantum model has been developed to explain a systematic variation of the dielectric loss peaks versus molecular length. A comparison with experimental data indicates that all of the alcohols studied (up to C/sub 12/H/sub 25/OH, dodecanol) behave as fully extended molecules at the temperature at which librational motion goes over into rotation.<<ETX>>

The Influence Of Ionic And Non-ionic Additives On The Electric Potential Distribution In Dielectric Liquids

Modifications of the electrostatic probe techniques of Forster [1,2] and of Bloor and Morant [ 3 ] have been used to study the influence of additives, including extended chain ions such as sodium dodecyl sulfate ( S D S ) and dissolved polymers, on the distribution of electric potential C#J(X) across low conductivity liquids between parallel electrodes. The systems studied include toluene with dissolved polystyrene and n-octanol with dissolved SDS. It is thought that these s stems reflect some of the conditions relevant to transrormer fluids and also may be of significance in biophysical considerations [ 4 ] . The distribution of space charge p(x) is estimated by the application of Poissons equation to the measured potential function @(x). From such data implications can be made about the effects of molecular interactions.

Models for the dependence of switching time on electric field in ferroelectric polymers

The change in macroscopic polarization of ferroelectric polymers, when the applied field E is increased, is considered to be due to the growth of certain saturated domains at the expense of less favored adjacent domains with different directions of polarization. The rate of switching from one state of polarization to another can be viewed as a thermally activated process where the activation energy depends approximately linearly, with a negative slope, on E. Some data for PVDF (polyvinylidene fluoride) are analyzed to demonstrate this fact. Molecular models, based on statistical thermodynamical considerations of polarization domains of cylindrical shape, are discussed.<<ETX>>

Time domain dielectric spectroscopy as a tool to study phase nucleation processes

Conventional (frequency domain) dielectric spectroscopy (FDDS) is very useful for studying molecular interactions in systems which are in equilibrium. However, it is not usually convenient, or in some cases feasible, to investigate transient processes by FDDS. A major advantage of time domain dielectric spectroscopy (TDDS) is that many types of transient phenomena can be studied. As a point in question, the use of TDDS for the investigation of the kinetic processes associated with the nucleation and growth of a second phase in an initial phase will be discussed. The use of the technique for the study of phase separation in polymer blends will also be discussed and preliminary results for a model system, phenol-water, will be presented.

Ionic transients in an extended chain aromatic hetero-cyclic polymer

In an effort to better understand the process of charge transport in extended chain aromatic hetero-cyclic polymers we have made detailed studies of d.c. conduction in one such rigid rod polymer, viz. poly(p-phenylene benzobisthiazole) or PPBT *** Two types of transient currents have been measured. One of these is the gradual decay of current over a long period of time in a PPBT sample exposed to a step function voltage. Typically the current I drops by about three orders of magnitude over a period of six hours and, when plotted on a log-log graph, exhibits a series of linear segments I ∝ t−n. Such measurements were made on samples doped with LiCl, NaCl, CaCl2, LaCl3, etc. The other type of transient, measured for the same samples, resulted when the humidity was suddenly increased from dry conditions to some finite value on a specimen in a quasi-steady state of conduction. A large increase of relative current (ΔI/Imin) occurs, followed by a gradual decrease over a long period of time. Efforts to relate these effects to the nature of the conduction mechanisms and diffusion of water are discussed.

Problems in the determination of anisotropic transport properties in very thin polymer fibers

The technological need often arises to know the conductivity of a thin fiber. There are numerous references in the literature to the conductivity of fibers but when one looks in detail at these references it is found that the measurements have been made on woven fabrics or mats of fibers.

Ions at interfaces in solid polymers

When one tries to decide whether a given electrical phenomenon in a solid polymer is due to ionic or to electronic effects it often appears that Nature has chosen to be evasive on the issue. Thus, whereas one well done scientific paper may claim ionic effects predominate in a given polymer, the conclusion of another equally competent research report may be that electronic effects are the most important in the same polymer. Sometimes these conclusions are based on solid state theories which have been developed for crystalline materials with relatively well understood defects and then carried over qualitatively to polymers which are structurally much more complicated and which always contain morphologically significant amounts of non-crystalline material. Thus, it is appropriate to investigate models which attempt to account for the inherent inhomogenities of crystalline polymers. This paper summarizes some simple considerations involving the use of electrostatic image theory in conjunction with the Clausius-Mossotti relation to approximate the interaction of ions with the interfaces between crystalline and amorphous regions in polymers.