Static light scattering characterisation of microporous membrane filters

Some of the more commonly employed microporous membrane filters have been studied in quasi index matching solvents by means of small-angle static light scattering. We report data obtained from membranes made of esters of cellulose with rated pore size p(s) ranging from 0.1 to μm. The high q behaviour of the scattered intensity follows a power law I(q) ∞ q(-α) where α is either 1.9 or 3.8 depending on the membrane chemical composition. Such values reveal that on a short length scale the filters have a fractal nature. Furthermore, the scattered intensity distribution exhibits a peak at a finite wave vector q(m), like in spinodal decomposition processes (which indicates mass anticorrelation effects), a behaviour similar to that observed in Vycor porous glasses via neutron and X-ray small angle scattering. The peak position moves towards smaller q values when increasing p(s), although the characteristic length A=2π/q(m) is always larger than the pore size and it is not trivially related to it. In particular, for membranes made of mixed- esters of cellulose, A is found to be roughly proportional to p(s) for small pore sizes, while for p(s) > 1.2 μm it increases only slightly with the pore size and eventually tends to saturate. In conclusion, these results indicates that small-angle static light scattering promises to be a very useful technique for characterising microporous membrane filter morphology, and also for quality control.