Transport and dielectric properties of poly(ethylene terephthalate) as determined via electrochemical techniques

The electrochemical impedance spectroscopy (EIS) technique was used to evaluate the water transport (diffusion and equilibrium water uptake) and the dielectric properties of free-standing poly(ethylene terephthalate) (PET) membranes at 40°C. Permeability and diffusion coefficients were also obtained using the Payne cup method and the MacBain quartz spring balance to assess the reliability of the EIS method when compared to other techniques. In addition, an electromigration (dc) technique was used to estimate the NaCl diffusion coefficient across PET films. Results obtained indicate that PET is highly permeable to water and much less permeable to salt. The water diffusion coefficient, D, varies from 2.11 × 10-9 to 9.97 × 10-9 cm2 s-1 for thicknesses between 22 and 205 μm, whereas the equilibrium water uptake, W, varies from 0.54 to 0.95 wt% for the same given range of thicknesses. The average calculated dielectric constant of the free-standing PET films is 3.6. An estimate of the NaCl diffusion coefficient, Ds, is 9.34 × 10-14 cm2 s-1. Transport properties results obtained via the electrochemical technique are in reasonable agreement with those obtained with the classical gravimetric method.