Electro‐optic and Rheological Behaviour of Aqueous Dispersions of Polytetrafluorethylene (PTFE) Fibrilles

Abstract

Col/aides / Electric Birefringence / Flow Birefringence / Rods / ViscosityElectro-optic and rheological measurements were carried out on PTFE-fibrilles. The crystalline fibrilles had an axial ratio of about 100 and a thickness of 200 A.The dispersions were stabilized by ionic Perl1uorosurfactants and could be kept for months without precipitation.-The rotational diffusion constant of the fibrilles was determined as a function of the volume fraction. Both electric birefringence and rheological measurements gave identical results for the orientation times. In the dilute and semidilute concentration range the rotational diffusion constant was the same and did not change at the overlap concentration. The shear modulus was found to increase in the semidilute concentration with the square of the volume fraction of the PTFE fibrilles. -Electric birefringence measurements with ACpulses showed a remarkable result. The stationary birefringence changed sign as a function of the frequency. It is concluded that this phenomenon is related to the adsorption/desorption equilibrium of the surfactant ions on the PTFE fibrilles. The equilibrium is shifted by the electric field. These conclusions could be confirmed by light scattering measurements in AC-fields.Ber. Bunsenges. Phys. Chern. 92, 10-16 (1988) -

Electric birefringence measurements in aqueous polyelectrolyte solutions

“…Figure6. Stationary birefringence An¡t in DEB measurements as a function of the frequency for different concentrations.…”

Electric birefringence anomaly of solutions of ionically charged anisometric particles

“…Electric birefringence measurements have been made on different colloidal systems like on polymer and polyelectrolyte solutions, on micellar and clay solutions, on microemulsions and on fibre dispersions [5][6][7][8][9][10] and on virus solutions [11]. When the structures in the investigated solutions were ionically charged, usually a second effect with opposite sign, and with a rotation time that is a factor of three to ten longer than the first one appeared.…”

Transient SANS Studies on p‐Tetrafluorethylen (p‐TFE)

“…The experiments are sensitive to the shape and to the distribution function J'of the axis of the fibrilles. The fibrilles had been examined first by electron microscopy [1,2] and the micrographs had indicated some polydispersity in the diameter and the length distribution (see Fig. 1).…”

“…The amplitudes of these fluctuations are small. Previous measurements have shown that the orientation of the fibrilles in an electric field give a birefringence An which changes sign with the frequency of an applied sinusoidal field [2]. In an optical experiment the distribution function fof the rod axis and the polarisibilities related to f and the internal structure of the rods have to be taken into account [4].…”