Influence of the structure of sulfonated polyimide membranes on transport properties

Sulfonated polyimides (SPIs) are block copolymers composed of hydrophilic and hydrophobic sequences. These polymers of vitreous nature have the capacity of absorbing large water quantity. Water uptake and ionic conduction properties have been studied for (SPI) membranes varying the ionic content and distribution along the polymer chain. From volume and weight increase measurements on membranes soaked in water, an amount of porosity has been estimated. It results that for membranes synthesized with longer block sequence length at constant equivalent weight, the ionic conductivity is smaller whereas the water uptake and the amount of porosity are higher. Water sorption isotherms for SPIs varying the block sequence length have been compared. The sorption isotherm curvature differences for SPI membranes could be linked to the difference in the amount of porosity inside membranes. The effect of the membrane preparation procedure on water sorption has also been investigated. The porosity appears to be created during membrane preparation in boiling methanol when the residual solvent is extracted from this vitreous polymer structure. (C) 2001 Elsevier Science B.V. All rights reserved.