Influence of Differential Distribution of Piperazine Conformers in Copolyamides on the Formation and Ion Separation Behaviors of Charged Nanofiltration Membranes

The effects of polymers’ architectures on the surface topography, morphology, and ion transport phenomena of thin film composite (TFC) charged nanofiltration (NF) membranes were investigated by probing the influence of distribution of piperazine (PIP) conformers within different copolyamide structures. TFC membranes having non-cross-linked and cross-linked copolyamides were prepared following in situ interfacial polymerization of a diamine mixture comprising PIP and 2,4-diamino benzenesulfonic acid and an acid chloride, either terephthaloyl chloride or trimesoyl chloride. Solid-state 13C CP/MAS NMR spectral analyses and density functional theory calculations revealed that tertiary amide segments of copolyamides consisted of cis and trans isomers of a PIP monomeric unit in unequal distributions within the spatially flexible non-cross-linked structure but in equal distributions within the spatially constrained cross-linked structure. Such variations in molecular structures between non-cross-linked and cross-linked copolyamides were found to dictate membranes’ surface topography - roughness and morphology - skin layer densification, upon curing treatment. The membrane with cross-linked copolyamide having a reduced degree of rotational freedom due to its constrained tertiary amide segment exhibited a higher rejection of SO4 2– during nanofiltration. The tertiary amide segment of a membrane with a cross-linked copolyamide was found to ensure a lower energy barrier during transmembrane transport of Cl– and a superior monovalent to bivalent anion selectivity for NO3 –/SO4 2– and Cl–/SO4 2– systems. This study enriches the understanding of membrane formation from molecular level and its bearing on membrane performance.