Fractal morphology, spatial order, and pore structure in microporous membrane filters

We present a small-angle static light scattering study of some of the more commonly employed microporous membrane filters under quasi index matching conditions. We have investigated filters of various pore sizes and of two different chemical compositions, namely, acetate of cellulose (AC) and mixed esters of cellulose (MEC). The scattering data reveal that on short length scales the membranes have fractal morphology, a feature already reported in the literature for porous glasses. Membrane filters however exhibit a wider spectrum of morphologies. In fact, while AC filters may be described as surface fractals (like porous glasses), MEC membranes turn out to be mass fractals. Furthermore, for all the samples the scattered intensity distribution is peaked at a finite wave vector qm, a feature typical of systems with a spatial quasi periodicity Λ = 2π/qm. The relation between Λ and the pore size ps depends on the type of membrane. In general Λ grows with ps, but while for AC membranes Λ is roughly proportional to the pore size, for MEC filters a more complex behavior is observed. On the basis of these results and on mass density measurements we propose two different models for the AC and MEC membrane structure. The proposed picture is further supported by the observation of scanning microscope images.