Interests in application of organic solvent nanofiltration (OSN) technology based on synthetic
membranes to molecular separation processes have been growing rapidly in recent years. The
main classes of OSN flat sheet polymeric membranes are integrally skinned asymmetric (ISA)
and thin film composite (TFC) membranes. A general goal of OSN membrane research is to
improve membrane performance for specific non-aqueous applications, or to develop new
separation processes. Most of the time the research is performed via trial–and–error methods,
leading to extension of development time and increase of costs. This is partially because the
structure of OSN membranes, particularly the size of their permeation pathway is largely
unknown.
The filtration characteristics are mainly determined by the membrane structure, which is
dependent on various fabrication methods as well as polymer chemistry. However, a direct
correlation between these factors has not been understood in detail, because the current
characterisation techniques have limitations in studying polymer structures with dimensions at
the macromolecular level. The pore size in nanofiltration (NF) membranes is believed to be less
than 2 nm, which is a lengthscale at the edge of most available material characterisation
techniques. For this reason, advanced methods to study the membrane morphology need to be
explored or developed with the aim of elucidating the NF membrane structure, transport
mechanisms, and to understand the relationship between the membrane structure and the
separation characteristics.
These objectives guided the work to development of a nanoscale characterisation method based
on imaging the porous regions via probing the NF pores with nanoparticles (NP). Given that the
probes provide high electron contrast, it is possible to map the pores formed between the
polymer entanglements in the transmission electron microscope (TEM). This technique measures
the pore size in situ, thus, a membrane is characterised during its operational state. The pore size
was found to correlate well with the solute rejection and flux measured for a range of ISA and
TFC membranes. The pore size distributions were then used together with a pore–flow model to
simulate rejection curves.
A further insight into the membrane structure, particularly the surface structure, was provided by
atomic force microscopy (AFM), particularly phase imaging. This method was applied to
characterisation of polymer packing at the membrane surface, leading to analysis of the
correlations between the phase shift, filtration parameters and membrane preparation methods.