Improvement in semipermeable membrane performance of wholly aromatic polyamide through an additive processing strategy

Abstract

A new concept for the method to provide semipermeability in ultrathin and single-component wholly aromatic polyamide membranes has been developed for the first time. It was found that water molecules could permeate through the membrane prepared not from polyamides containing flexible ether, bulky binaphthyl, or fluorene rigid units, but one with carboxylic acid groups under a reverse osmosis mode. However, the enhancement of water transport properties by introducing the hydrophilic group of polyamide was not substantial. Therefore, polyamide membranes were prepared from the solution containing aqueous additives in order to weaken hydrogen bonds between polymer chains and thereby to suppress the aggregation of the polymer chains. As a result, water flux was dramatically improved with slightly improved NaCl rejection. Our analyses based on attenuated total reflectance Fourier transform infrared spectroscopy and solid-state carbon polarization and magic angle spinning nuclear magnetic resonance ( 13 C CPMAS NMR) spectroscopy confirmed that the aggregation of polymer chains due to the hydrogen bonds among the amide linkages was suppressed by the coordination of the aqueous additives to the amide linkage. The state of water in the membranes analyzed by differential scanning calorimetry also supported the formation of pores.

Effect ofN-methyl amide linkage on hydrogen bonding behavior and water transport properties of partiallyN-methylated random aromatic copolyamides

“…Despite the introduction of the N ‐methyl amide linkage, the water permeation cannot be observed for PA‐0‐20 . These results agree with our previous report, which revealed that the ultrathin semipermeable membrane based on PAs without a carboxylic acid group could not show any water permeation even if a flexible or rigid molecular unit was incorporated into the polymer backbone . On the other hand, PA‐20‐0 , PA‐20‐20 , PA‐20‐40 , and PA‐20‐60 with a carboxylic acid group show a water permeation property.…”

Structure–Property Relationships of Random Aromatic Copolyamide Membranes by the Partial N‐Methylation of Amide Linkages

“…In our previous study, we reported that the introduction of the N ‐methyl amide linkage suppressed the hydrogen bonding, conformational changes, and inter‐/intramolecular interactions among the polymers, implying that there would be differences in the internal structure of these membranes . In fact, the decrease in the dry density of the polymer was confirmed after the N ‐methylation (Figure S1, Supporting Information).…”

“…N ‐Methylpyrrolidone (NMP) was purchased from Wako Pure Chemical Industries. Partially N ‐methylated random aromatic polyamides were prepared according to a previous report …”

“…Previously, we reported the permselectivity of novel partially N ‐methylated random aromatic coPAs (PA‐ X ‐ Y , where X and Y indicate the molar percent of 3,5‐diaminobenzoic acid (DABA) and N , N′ ‐dimethyl‐4,4′‐diaminodiphenyl ether (MDAE) among the diamine comonomers in the feed, respectively) composed of three repeating units: the hydrophilic secondary amide unit (which contains a carboxylic acid group), the N ‐methylated tertiary amide unit, and the secondary amide unit (see Figure ) . The suppression of the hydrogen bonding behavior and the conformational preferences of the amide linkages affected the aggregation behavior in the obtained membranes.…”

Effect of primary structure on permselectivity of ultrathin semipermeable polybenzimidazole membrane

“…27 However, in the case of PBI ultrathin membrane, it is obvious that water permeation phenomenon was confirmed without introducing the hydrophilic groups into the polymer backbones (Table II). This result is probably due to the nature of the imidazole ring, which makes PBI molecules self-charged in aqueous environment, resulting in the assistance for the formation of water transporting channels.…”