Dielectric and electrooptic properties of poly(tetrafluoroethylene) whiskers

The dielectric and electrooptic properties were measured for suspensions of poly(tetrafluoroethylene) (PTFE) microcylinders (20 nm diameter, 0.4 microns length). These suspensions undergo an isotropic to oriented (birefringent) phase transition, analogous to the behavior of liquid crystals. Dielectric dispersion measurements were conducted from 1 kHz - 10MHz. In addition AC electric birefringence measurements were performed using a specially constructed apparatus from 50 Hz - 6 MHz. The permittivity shows a large, broad dispersion centered near 200 Hz, similar to the behavior of latex particle dispersions; in addition a smaller dispersion near 2 MHz was found. No striking change was noted in the dielectric properties at the liquid crystalline phase boundary of the suspensions. The particles exhibit an enormous AC birefringence centered at a frequency of ca. 100 kHz, with the amplitude and peak frequency strongly dependent on the conductivity of the suspension. We interpreted the dielectric data using Schwarz\u27s model of counterion polarization as extended by Takashima, for the case where the major axis of the particle was aligned parallel to the electric field, and Ishiwatari and Schwan, for the case where the minor axis of the particle was aligned parallel to the electric field. The large dielectric dispersions arise from the diffusion of counterions parallel and perpendicular to the long axis of the cylinders. The birefringence measurements were interpreted using theories by Saito and O\u27Konski and Krause. The broad dispersion in the AC birefringence data appears to arise from a combination of the frequency dependent surface conductivity of the particles and a Maxwell-Wagner type polarization. This work shows the usefulness of combined dielectric and AC birefringence measurements for the study of electrical properties of colloidal particles in suspension