A simple method for measuring water diffusion coefficients in conducting polymers

Abstract

Summary form only given. Some conducting polymers, such as protonated polyaniline (PANI), have higher electrical conductivity in the presence of humid air than as a dried material. One explanation for this observation is that absorbed water enables electron transfer between PANI chains. Water absorption in PANI may be as high as 2 H/sub 2/O molecules per 3 monomer units. Due to the high water uptake the electrical conductivity changes substantially and can be used for estimation of the water diffusion coefficient in PANI. In this work a simple method based on conductivity measurements will be presented and the measurement theory will be discussed. The diffusion coefficient can be estimated from the conductivity data according to equation: ln[/spl Delta//spl sigma/(t)] = constant - /spl alpha/Dt where [/spl Delta//spl sigma/(t), /spl alpha//sup 2/ and D are, respectively, conductivity increase at time t, the first eigenvalue of the diffusion equation depending on the measurement geometry and the water diffusion coefficient. For a normal 4-point measurement using a thin stripe the eigenvalue /spl alpha/ = /spl pi/d where d is the thickness of the sample. For PANI protonated with DBSA or HCI D has a value of ca. 5x10/sup -10/ cm/sup 2//sec at room temperature.