Water vapor absorption in the polyamide active layer of reverse osmosis (RO) membranes

Many of the basic thermodynamic and physical properties of the active layers of interfacially polymerized membranes are essentially unknown. In this work, we use combined measurements of water uptake and biaxial stress as a function of relative humidity to gain insights on the interactions of water with a commercial reverse osmosis (RO) membrane, FT-30 manufactured by Dow Liquid Separation (Midland, Ml). The active layer of the commercial RO membranes is attached to either a quartz crystal microbalance or a glass cover-slip using cured polyimide as an adhesion layer. The polysulfone support layer of the membrane is then dissolved in DMF (N,N-dimethylformamide). We use a laser-scanning optical system to measure the changes in curvature of the glass cover-slip and in this way determine the changes in the biaxial stress of the polyamide layer created by swelling of the polyamide layer by water absorption. Both the microbalance sample and the stress sample are placed in a sealed box in which the humidity can be controlled. The mass change of the polyamide layer is 1.2 μg cm^-2 when the humidity changes from 0% to 90% and the corresponding biaxial stress change is -35 MPa (compressive); this water uptake corresponds to 6% of the mass of the polyamide layer. The changes in water absorption with partial pressure of water vapor more closely resemble a Langmuir adsorption isotherm than the Flory-Huggins theory of polymer solutions. If we nevertheless use the Flory-Huggins theory to analyze the data, the interaction parameter at high humidity is χ^≈ 2, which is comparable to the value of χ for nylon. Comparisons of water uptake and biaxial stress indicate that the partial specific volume of water is larger at high humidity than at low humidity.