Synchrotron SAXS to probe cross-linked network of polyamide ‘reverse osmosis’ and ‘nanofiltration’ membranes

A typical ‘reverse osmosis’ (RO) or ‘nanofiltration’ (NF) polyamide cross-linked structure is formed by interfacial reaction between an aqueous solution of m-phenylene diamine (or piperazine) and a trimesoyl chloride solution in n-hexane. Using synchrotron SAXS, it was observed that a little change in the reactant concentrations resulted to different cross-linked networks of the polyamides. A clear difference between the cross-linking networks of RO and NF polyamides was observed even though there were some similarities in terms of the nature of polymer chain compaction and the cluster formation by aggregation. In both the polyamides, there was strong compaction of polymer chains forming globular like objects having smooth interface which was well deviated from the Gaussian polymer chain nature (I ∞ Q−2) wherein the NF polyamide had relatively more compaction with Porod scattering I ∞ Q−3.8 than I ∞ Q−3.4 of the RO polyamide. The size of the compacted globules containing about 1400 trimesamide monomeric units for the NF polyamide was slightly smaller than that of the RO polyamide. These globules were very strongly aggregated to form polymer nodules. The nodular cluster size for the NF polyamide was ∼2000–3000 Å which was ∼6–7 times larger than that of RO polyamide. These differences in the nature of polymer chain compaction and the cluster size agreed with the differences in effective cross-link density measured by equilibrium swelling in dimethyl acetamide and the polymer nodule sizes observed by the AFM.