Measurement of water vapor diffusion in nanoscale polymer films by frequency-domain probe beam deflection

We developed an optical pump-probe technique, frequency-domain probe beam deflection (FD-PBD) to measure the diffusion of water vapor in nanoscale polymer films with microsecond temporal resolution and micrometer spatial resolution. We describe the quantitative model of beam deflection for multilayer structures, the experimental approach, and the application of FD-PBD to measure the diffusivity (D _m) of saturated water vapor in various glassy polymer films including polyimide (PI, D _m = 7.0 × 10^-13 m² s^-1), poly(methyl methacrylate) (PMMA, D _m = 1.2 × 10^-12 m² s^-1), poly-(vinylpyrrolidone) (PVP, D _m = 1.7 × 10^-12 m² s^-1), and cellulose acetate (CA, D _m = 2.6 × 10^-11 m² s^-1), and the piperazine/trimesoyl chloride (PIP/TMC, D _m = 9 × 10^-11 m² s^-1) nanofiltration membrane synthesized by interfacial polymerization. The uncertainty of the measurements is typically ≈8%.