EVIDENCE FOR A WATER-RICH SURFACE REGION IN POLY(VINYL CHLORIDE)-BASED ION-SELECTIVE ELECTRODE MEMBRANES

Water desorption from ion-selective membranes formulated from poly(vinyl chloride) (PVC) plasticized with 66 % (wt/wt) bis(2-ethylhexyl)adipate (DOA) has been studied by gravimetric and optical methods. The optical method was used to image the distribution of water within the polymer and showed that a water-rich surface layer 20-40 mum thick is formed after several days immersion in solution. Gravimetric H2O desorption data are consistent with these observations. The water concentration in the bulk of the plasticized PVC is about 0.24 M, and after an initial period, water evaporates through the dry surface layer with a diffusion coefficient of (3.0 +/- 0.3) X 10(-6) cm2/s. The average surface region concentration is as high as approximately 17 M, although this may be overestimated. On drying, water diffusion from the wet surface region initially exhibits a diffusion coefficient of approximately 1.8 X 10(-9) cm2/s.