Influence of the structure of sulfonated polyimide membranes on transport properties

Sulfonated polyimides (SPIs) are block copolymers composed of hydrophilic a nd hydrophobic sequences. These polymers of vitreous nature have the capaci ty of absorbing large water quantity. Water uptake and ionic conduction pro perties have been studied for (SPI) membranes varying the ionic content and distribution along the polymer chain. From volume and weight increase meas urements on membranes soaked in water, an amount of porosity has been estim ated. It results that for membranes synthesized with longer block sequence length at constant equivalent weight, the ionic conductivity is smaller whe reas the water uptake and the amount of porosity are higher. Water sorption isotherms for SPIs varying the block sequence length have been compared. T he sorption isotherm curvature differences for SPI membranes could be linke d to the difference in the amount of porosity inside membranes. The effect of the membrane preparation procedure on water sorption has also been inves tigated. 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.