Reduction in permittivity of poly(tetrafluoroethylene) due to tensile strain

Permittivity of poly(tetrafluoroethylene) (PTFE) was measured while subjected to tensile stress. It was observed that the permittivity of PTFE decreases with increasing tensile strain; by approximately 10 % at 140 % longitudinal strain. Comparing porosity values calculated from a) measurements of sample density, and from b) measured permittivity by means of a simple mixture formula, it is shown that the observed reduction in permittivity with increase in tensile strain is not only due to the increased porosity of the strained samples but includes other contributions as well.