Microporous membrane filters: a static light scattering study

We studied some of the more employed microporous membrane filters via small-angle static light scattering under quasi-index matching conditions. We report data from both acetate of cellulose (AC) and mixed-esters of cellulose (MEC) membranes of various pore sizes ps. On short length scales, the membranes have fractal morphology. While AC filters may be described as surface fractals, MEC membranes turn out to be mass fractals. Furthermore, the scattered intensity distributions exhibit a peak at a finite wave vector qm, thus indicating the presence of a certain degree of spatial order over a characteristic length Λ = 2π/qm. Λ is found to increase with ps, as expected, although it is always larger than ps and is not trivially related to it. We propose a description of the membrane structure which accounts for the experimental results and is further corroborated by mass density measurements.