Dielectric properties of polytetrafluoroethylene and tetrafluoroethylene‐hexafluoropropylene copolymer

Precision measurements of dielectric properties of polytetrafluoroethylene and copolymers of tetrafluoroethylene and hexafluoropropylene were made over a temperature range of –40 to +180°C. at frequencies of 10 Hz to 300 kHz. Polytetrafluoroethylene exhibited the typical dielectric behavior of a nonpolar substance: no dielectric loss peak was found over the experimental range of temperature and frequency. The dissipation factor remained constant, being about 2 × 10−5. Copolymers of tetrafluoroethylene and hexafluoropropylene showed two dielectric loss peaks: one occurred at about 100°C. and the other at about −20°C. There was a break point, or an abrupt bend, near 60°C. in the curve of dielectric constant versus temperature, indicating the existence of a transition point. The high‐ and low‐temperature loss peaks correspond to the α and γ relaxations, respectively, which have been found for the mechanical behavior of this copolymer. The dielectric relaxation in the copolymer was attributed to the slightly polar nature of the perfluoromethyl side groups. The activation enthalpy and entropy are 80 kcal./mole and 180 eu for the α relaxation and 12 kcal./mole and 15 eu for the γ relaxation, respectively. The σ and γ relaxations are discussed in connection with the molecular motion of the copolymer chain.