Atomic force microscopy of dense and asymmetric cellulose-based membranes

The surface structures of dense and integrally skinned cellulose acetate (CA) and cellulose acetate butyrate (CAB) membranes, prepared by phase inversion under different casting conditions, are investigated by tapping mode atomic force microscopy (TM AFM). The results obtained show that: (i) The top and bottom surfaces of the dense CA membrane were quite uniform in comparison with the corresponding faces of asymmetric CA and CAB membranes. Despite the casting conditions the active and support layers of the asymmetric membranes display large differences on the roughness parameters. (ii) The asymmetric membranes prepared with an organic system as a non-solvent pore-former (method IV) display smaller nodule aggregates and lower values of the roughness parameters than the ones prepared using an inorganic system as swelling agent (method I). This is more pronounced for the CA membranes than for the CAB membranes. (iii) In the active layer of asymmetric CA membranes casted at longer evaporation times, the measured values of surface roughness parameters tend to decrease. Also, for these CA membranes, as the evaporation time increases the average size of the depression areas observed on the surface decreases.

The laboratory-made CA and CAB membranes display a wide range of nanofiltration and reverse osmosis permeation characteristics. These characteristics are correlated to surface roughness parameters of the active layers.