Water in polymer membranes. Part I : Water sorption and refractive index of cellulose acetate

The dependence of water absorption of 2 to 4 μm thick membranes of cellulose acetate on relative humidity was determined by measuring small changes in their asymmetric waveguide properties in the visible spectral region. Simultaneous measurements of changes in film thickness provide a direct method for obtaining the change in film volume as a function of water concentration and a new measure of polymer porosity. Data are presented for a typical film fabricated from cellulose acetate (CA398-30) which illustrate the usefulness of studying water absorption by integrated optics techniques.