Polymer Engineering and Science, Vol.35, No.12, 1053-1060, 1995
Measuring the Viscoelastic Properties of an Ethylene-Tetrafluoroethylene Copolymer at Ultrasonic Frequencies
Since poly(tetrafluoroethylene) (PTFE) and its copolymers are soluble only at high temperatures in primarily perfluorocarbon solvents, direct measurement of their molecular weight distribution by conventional dilute solution techniques is not possible. Instead, the structure must be inferred from the linear viscoelastic properties of the melt. To make this inference, the spectrum must be determined over a wide enough range of relaxation times to include the plateau modulus, G(N)(o). This is accomplished by time-temperature superposition for most polymers. For PTFE and its copolymers, the range between the melting point and the degradation temperature is too narrow for time-temperature superposition to be used reliably for determination of the high frequency response. Hence, as they do not exist in the melt state over a wide enough temperature range for superposition, their viscoelastic properties at high frequencies must be measured directly. A new technique employing a piezoelectric ultrasonic composite oscillator has been developed. The viscoelastic properties, G’(omega) and G"( omega), of Tefzel 280 (a high molecular weight ethylene-tetrafluoroethylene copolymer) have been measured at ultrasonic frequencies (40 to 200 kHz).