Assessment of atomic force microscopy for characterisation of nanofiltration membranes

Shortly after atomic force microscopy (AFM) became a common technique for characterisation of surfaces; it was applied to image surface morphologies of polymeric membranes. The main interest is in analysis of the nodule size, the pore size and the roughness. Because nanofiltration (NF) membranes comprise nanostructures with dimensions lying within the lateral resolution of the microscope, the interpretation of NF membrane features is challenging. In this paper, we describe and critically discuss the outcomes from experiments on NF membranes using AFM combined with carbon nanotube probes. This combination increases the microscope resolution. The surface roughness, the nodule size, and the pore size were analysed for a range of P84 co-polyimide integrally skinned asymmetric NF membranes formed by phase inversion, and compared with the experimentally measured separation performance. Furthermore, we demonstrate a new concept for membrane characterisation through the quantitative analysis of phase images to elucidate the macromolecular packing at the membrane surface, and correlate this with the membrane functional performance.