Anthony J. Bur

Dielectric properties of polymers at microwave frequencies: a review

Abstract A review of the dielectric loss spectra of polymers at microwave frequencies has been carried out. While the main focus of attention is the frequency range from 100 MHz to 100 GHz, loss spectra outside this region are also reviewed because variations in temperature can cause a shift of dielectric loss into or out of the microwave range. A large volume of data for low loss polymers (polyethylene, polypropylene and poly(tetrafluoroethylene)), which are used in the communications industry, was available for review. Other polymers, for which data were available and which have significant loss at microwave frequencies are: polystyrene, poly(vinylchloride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(methyl-methacrylate), poly(methyl acrylate), poly(oxymethylene), poly(ethylene oxide), poly(propylene oxide), polyacetylene, and poly(sulphur nitride). Also, the microwave dielectric properties of engineering thermoplastics such as poly(phenylene oxide), polycarbonate and polysulphane have been reviewed. The origins of microwave dielectric loss in polymers are categorized as: (a) dipolar absorption dispersions in both crystalline and amorphous polymers; (b) dipolar losses due to impurities, additives or fillers in a polymer material; (c) microwave absorption in conducting polymers (polyacetylene and poly(sulphur nitride)) for which the current carriers are electrons; and (d) photon-phonon absorption spectra corresponding to the density of states in amorphous regions of a polymer material.

Rodlike and Random‐Coil Behavior of Poly(n‐butyl Isocyanate) in Dilute Solution

The polymer chain configuration, molecular relaxation, and the dipole moment of poly(n‐butyl isocyanate), (–CO–NR–)n, where R = C4H9, have been studied by dielectric measurements of the polymer in dilute solution. The complex dielectric constant, e* = e′ − je″, was measured at room temperature over a frequency range 10−1–1 × 106 Hz for fractionated samples whose molecular weights cover a very broad range, 20 × 103–10 × 106. From observation of the dielectric relaxation time as a function of molecular weight we deduce that the low‐molecular‐weight molecule, Mw   106, the polymer chain configuration can be described as random coil. The relaxation time and dipole‐moment measurements indicate that there is a one‐to‐one correspondence between the end‐to‐end distance and...

Rodlike Behavior of Poly(n‐butyl)isocyanate from Dielectric Measurements

Dielectric measurements have been made on dilute benzene solutions of poly(n‐butyl)isocyanate (  O  = –C–N–       –      R)n, where R = n‐C4H9, at 22.5°C over a frequency range 10−1 to 2×106 Hz. Measurements of relaxation time τ on six samples which have molecular weights in the range 1.4×105 to 2.3×106 showed that τ depends on molecular weight to the 2.7 power. In addition, only one dispersion region was observed for each sample and the corresponding dielectric increments were very large. We conclude from these observations that (1) the poly(n‐butyl)isocyanate molecule is a rigid rod whose relaxation time in dilute solution is associated with the end‐over‐end rotation of the entire rodlike molecule around its minor axis, and (2) the net dipole moments of the molecule have a direction parallel to the major axis of the molecule. Calculation of the length of the molecule from relaxation data indicates that the conformation of the polymeric chain appears to be nonplanar and possibly helical. The dipole momen...

Measurements of the dynamic piezoelectric properties of bone as a function of temperature and humidity

The utilization of piezoelectricity as a clinical or diagnostic tool in medicine and dentistry is dependent upon an adequate quantitative description of the piezoelectric coefficients. Here, measurements of the dynamic piezoelectric d constant, d∗ = d′ − id″, for bovine bone are presented as a function of temperature (20–60°C), relative humidity (r.h.) (33–98%), frequency of applied stress (10−2–102 Hz). and sample orientation. The anisotropic character of cortical bone requires that d be expressed as a matrix containing 18 coefficients. Dispersion phenomena, i.e. d″ ≠ 0, are present in these coefficients for our frequency-temperature-r.h. range. At low r.h. (≤75%), d12 and d13 show little or no dispersion (d″ = 0), whereas varying degrees of relaxation are observed in the other coefficients. The d14 shear term is notable for a large dispersion at f = 40 Hz, T = 38.6°C and 55% r.h., for which d″14 is negative. The negative imaginary coefficient means that energy is being gained in the 14 coefficient. The change in d14, with a change in temperature and/or r.h., is attributed to a change in sample water content. For 98% r.h., the low frequency data (f < 10 Hz) are dominated by a polarization which varies as the reciprocal of the frequency. This polarization is attributed to an interfacial or Maxwell-Wagner polarization which occurs as a result of ionic conduction under the influence of the piezoelectric polarization field.

Temperature Dependence of Fluorescent Probes for Applications to Polymer Materials Processing

We have examined the temperature dependence of fluorescence spectra from dyes that can be used as molecular probes during polymer processing. The dyes, perylene and benzoxazolyl stilbene, are in a class of dyes called band definition dyes, so called because their fluorescence spectra contain distinct intensity peaks at characteristic wavelengths. The dyes were chosen for this study because they are soluble at dopant levels of concentration in organic polymers at elevated temperatures and they survive without degradation at polymer processing temperatures up to 300 °C. Changes induced in the fluorescence spectra over a range of typical processing temperatures were examined using statistical techniques that establish correlations between fluorescence intensity, wavelength, and temperature. The derived correlations are the basis for temperature calibrations that can be applied to process monitoring. A phenomenological model that assumes temperature dependence for both nonradiative and radiative decay modes is developed. A fit of the model parameters to the fluorescence spectra yielded activation energies for the temperature dependence of fluorescence decay rates.

Fluorescence anisotropy sensor and its application to polymer processing and characterization

An optical sensor containing polarizing optical components has been constructed to monitor fluorescence anisotropy during polymer processing and to carry out remote sensing of polymer products doped with fluorescent dyes. The sensor is a compact unit that is used to polarize incident excitation light as well as to analyze the polarization of generated fluorescent light. Optical fibers are used to carry light between the sensor head and the light source and detecting equipment. The anisotropy measurement yields information about the orientation of a fluorescent dye molecule that has been doped into polymer matrix. Fluorescent dyes that have geometrical asymmetry in their molecular structure are used. Experiments are described for which the sensor is positioned in line during extrusion, during specimen extension, and where the sensor is used to carry out area scans of films and sheets. Measurements were made on polyethylene, polyethylene terephthalate, and polybutadiene resins that contained a low concentra...

A study of thermal depolarization of polyvinylidene fluoride using x-ray pole-figure observations

Measurements of piezoelectric and pyroelectric activity, density, and x‐ray pole figures were used to study the effect of thermal aging on the state of polarization in polyvinylidene fluoride. A rolled and poled  β‐phase specimen of polyvinylidene fluoride was subjected to thermal aging which consisted of temperature cycling between room temperature and successively higher maximum temperatures, TMAX, where TMAX ranged from room temperature to 164 °C. We found that the room temperature piezo and pyroelectric activity decreased linearly as a function of TMAX from 75 °C to 164 °C at which temperature the specimen had 30% of its original activity; a linear extrapolation of these data to zero activity yielded a temperature Tc=207 °C. From density measurements at room temperature, crystallinity was calculated and found to remain constant during thermal cycling. X‐ray pole‐figure observations of the (200) (110) composite diffraction of the  β‐phase crystal showed single‐crystal texture of the rolled specimen and...

In-line dielectric monitoring during extrusion of filled polymers

Real-time monitoring of the dielectric properties of polymer melts and filled polymer melts has been carried out during extrusion. The measurements are obtained using a dielectric cell that is placed directly in line with the extruder machine. The dielectric cell consists of interdigitating electrodes that are deposited on the inside of a ceramic ring that is electrically insulated and temperature controlled to the set point of extrusion. As the processed resin passes through the ring, its permittivity and conductivity are measured. The spatial sensitivity of the cell was determined experimentally and was biased to the resin flowing near the electrodes. Using the spatial sensitivity function, we examined the time profile of the transition from one composition to another during extrusion. We demonstrate the operation of the cell during the processing of polystyrene filled with aluminum oxide and calcium carbonate and of polyethylene-ethyl vinyl acetate copolymer filled with montmorillonite clay.

A dielectric slit die for in-line monitoring of polymer compounding

The dielectric slit die is an instrument that is designed to measure electrical, rheological, ultrasonics, optical, and other properties of a flowing liquid. In one application, it is connected to the exit of an extruder, pump or mixing machine that passes liquefied material such as molten plastic, solvents, slurries, colloidal suspensions, and foodstuffs into the sensing region of the slit-shaped die. Dielectric sensing is the primary element of the slit die, but in addition to the dielectric sensor, the die contains other sensing devices such as pressure, optical fiber, and ultrasonic sensors that simultaneously yield an array of materials property data. The slit die has a flexible design that permits interchangeability among sensors and sensor positions. The design also allows for the placement of additional sensors and instrumentation ports that expand the potential data package obtained. To demonstrate sensor operation, we present data from the extrusion and compounding of a polymer/clay nanocomposit...

Extrapolation to Single‐Relaxation‐Time Behavior in Solutions of Poly(n‐butylisocyanate)

The dielectric relaxation time of rodlike poly(n‐butylisocyanate) (PBIC), (–CO–NR–)n, where R = C4H9, has been observed for dilute solutions as a function of the polydispersity in the PBIC molecular weight. The mode of relaxation is rotation about the minor axis of the rodlike molecule (end‐over‐end rotation). The dielectric relaxation time for this mode is a sensitive function of molecular weight, and a distribution of relaxation times Φ(τ) is observed whenever a distribution of molecular weights ψ(M) is present. A one‐to‐one correspondence between Φ(τ) and ψ(M) is established by observing the half‐width W / 2 of the dielectric dispersion on a log frequency plot as a function of the ratio Mw / Mn, where Mn and Mw are the number and weight averages of ψ(M). Extrapolation of the W / 2 vs Mw / Mn data to the monodispersed case (Mw / Mn = 1.0) is carried out in order to determine whether or not a monodispersed sample has a single relaxation time. The accuracy of the extrapolated half‐width suffers because th...