IR Spectroscopic Ellipsometry to Characterize Microfiltration Membranes

The porosity and thickness of microfiltration membranesare criticalcharacteristics to understand the performance and properties. Here,we demonstrate the nondestructive characterization of two commercialmicrofiltration membranes using IR ellipsometry. The porous natureof these membranes effectively scatters visible light, but the longerIR wavelengths decrease depolarization and enable ellipsometric characterizationof freestanding membranes with useful ellipsometric data primarilyin the fingerprint region. Fits of the ellipsometric data providedthe membrane thickness and both local and average porosity from thedielectric constant. The thicknesses of the membranes from ellipsometryagreed well with the measured thickness from SEM cross sections, whilethe average porosity estimated from the density of the membrane wasslightly higher than the ellipsometry analysis, which can be accountedfor by considering the incomplete densification of the membrane usedto determine the refractive index of the polymer matrix. Moreover,the fit of the ellipsometric data indicated that the membranes arenot uniform but rather have a measurable gradient in porosity thatis difficult to detect from SEM alone. This work demonstrates thecharacterization of freestanding microfiltration membranes with IRellipsometry, overcoming the limitation of visible light ellipsometry,and points toward potential for nondestructive determination of chemicaland porosity gradients through the thickness of microfiltration membranes.