Amide A band is a fingerprint for water dynamics in reverse osmosis polyamide membranes

D Surblys and T Yamada and B Thomsen and T Kawakami and I Shigemoto and J Okabe and T Ogawa and M Kimura and Y Sugita and K Yagi, JOURNAL OF MEMBRANE SCIENCE, 596, 117705 (2020).

DOI: 10.1016/j.memsci.2019.117705

Reverse osmosis membranes based on aromatic polyamide (ar-PA) are widely used in desalination of seawater, yet the microscopic mechanism of water diffusion through a polyamide layer remains elusive. Here, we study the structure and dynamics of polymer chains and water molecules in ar-PA in comparison to nylon 6 (one of aliphatic polyamides) under various water contents (0.0-15.9 wt%). The infrared (IR) difference spectrum between dry and moist ar-PA shows little change in amide A bands, in contrast to that of nylon 6 which yields a prominent dip. Theoretical analyses using molecular dynamics simulations and quantum electronic and vibrational calculations reveal that the dip in nylon 6 is caused by breaking of hydrogen bonds (HBs) among amide groups. The incoming water molecules that break amide-amide HBs are bound to polyamide chains nearby and diffuse slowly. On the other hand, the amide-amide HBs of ar-PA are kept upon hydration. Such polymer structure facilitates growth of large water clusters with more than 100 water molecules and rapid diffusion of water molecules. The amide A band serves as a fingerprint to characterize the water permeability of polyamide materials.

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