IR Spectroscopic Ellipsometry to Characterize Microfiltration Membranes

The porosity and thickness of microfiltration membranes are critical characteristics to understand the performance and properties. Here, we demonstrate the nondestructive characterization of two commercial microfiltration membranes using IR ellipsometry. The porous nature of these membranes effectively scatters visible light, but the longer IR wavelengths decrease depolarization and enable ellipsometric characterization of freestanding membranes with useful ellipsometric data primarily in the fingerprint region. Fits of the ellipsometric data provided the membrane thickness and both local and average porosity from the dielectric constant. The thicknesses of the membranes from ellipsometry agreed well with the measured thickness from SEM cross sections, while the average porosity estimated from the density of the membrane was slightly higher than the ellipsometry analysis, which can be accounted for by considering the incomplete densification of the membrane used to determine the refractive index of the polymer matrix. Moreover, the fit of the ellipsometric data indicated that the membranes are not uniform but rather have a measurable gradient in porosity that is difficult to detect from SEM alone. This work demonstrates the characterization of freestanding microfiltration membranes with IR ellipsometry, overcoming the limitation of visible light ellipsometry, and points toward potential for nondestructive determination of chemical and porosity gradients through the thickness of microfiltration membranes.