Fabrication and Performance of Novel Poly(piperazine-amide) Composite Nanofiltration Membranes Based on Various Poly(m‑phenylene isophthalamide) Substrates

The increasing pollution of freshwater resources and the severe shortage of clean water globally have raised concerns. Thus, to meet the global demand for clean water, developing highly effective and sustainable water production and purification techniques is imperative. Among the available water treatment technologies, advanced membrane-based filtration has attracted significant research attention owing to its numerous advantages, including environmental friendliness, low energy consumption, and high removal efficiency. In particular, nanofiltration is a kind of membrane separation method currently used in desalination and water softening processes.

Most nanofiltration membranes used in thin-film composite (TFC) structures are fabricated via interfacial polymerization. Because TFC membranes normally consist of a polyamide barrier layer and porous substrate with different materials and structures, their separation efficiency can be controlled by optimizing the substrate and the polyamide layer. Although many studies have focused on modifying the polyamide layer to enhance the performance of the composite membrane, research has shown that other parameters such as the material and substrate pore structure also have a considerable influence on the membrane’s performance. Unfortunately, these parameters are paid less attention to during membrane fabrication.

For analyzing the effects of substrate materials on composite membrane performance, polysulfone (PS) and polyethersulfone (PES) have been generally used as support materials due to their superior thermal ability and hydrophilicity. However, these two polymers have inadequate solvent resistance. Other substrates also have limitations like poor mechanical properties that limit their practical applications. As a distinctive polymer, poly(m-phenylene isophthalamide) (PMIA) exhibits outstanding, like high stability and better chemical resistance, than the above conventional polymers. Most importantly, it has been proved to be a superior substrate material for fabricating TFC nanofiltration membranes. However, the influence of its structure on the performance of the polyamide membrane is yet to be fully explored.

On this account, Beijing Institute of Technology researchers: Dr. Tao Wang, Mr. Xi Zheng, Miss Yajun Wang, Miss Luyao Zhang and Miss Zhenzhen Zhao in collaboration with Professor Jiding Li from Tsinghua University systematically investigated the influence of various PMIA substrates on the performance of novel poly(piperazine-amide) (PA) TFC membranes. The PA TFC nanofiltration membrane was fabricated via interfacial polymerization technique atop PMIA supports developed through phase inversion at different PMIA concentrations. The physicochemical properties, morphological structure as well as filtration performance of the resultant PA/PMIA TFC membranes were detailed. The work is currently published in the research journal, Industrial & Engineering Chemistry Research.

The authors’ findings showed that the observed changes on the PMIA substrates as well as the various parameters like porosity, topography and pore size significantly influenced the formation of the polyamide membranes. The resultant membrane exhibited a nodular structure as well as nodular distribution that declined with an increase in the polymer concentration. Owing to its excellent mechanical properties, the PMIA substrate provided the required structural stability during practical applications. Regarding the separation performance of the composite nanofiltration membrane, larger pore size and porosity resulted in a higher permeation. Additionally, TFC membranes developed on a substrate with 14% PMIA concentration exhibited excellent flux and salt rejection performance than membranes with other PMIA concentrations.

In summary, the research team successfully fabricated a series of polyamide nanofiltration membranes on different PMIA substrates with different concentrations to study the influence of PMIA structure on the property and performance of the resulting membranes. The characterization of the chemical properties revealed that different substrates could be used to achieve the formation of polyamide. Overall, PMIA substrates and different parameters have a considerable influence on the property and performance of the resulting membrane. In a statement to Advances in Engineering, Dr. Tao Wang explained their findings provided a new perspective regarding the influence of substrate structure on TFC membranes and would play a significant role in developing high-performance membranes for large-scale water treatment.