Analysis of Structural Transformations and Water State in a Microfiltration Polyamide Membrane

Abstract

The paper presents the results of studies of structural transformations and the state of water in samples of MMK-045 nonworking, water-saturated, and working membranes using the methods of IR spectroscopy NTR and X-ray phase analysis. Three types of hydrogen bonds have been identified: (‒CO…H₂O…CO–) (III) type ~30 kJ/mol, H₂O…HN (II) type ~16 kJ/mol, and (I) type ~9 kJ/mol. Sorption water in a water-saturated sample, reducing the energy of intermolecular (NH…O=C) bonds, changes the configuration of PA6 macromolecules without changing the trans-conformers. At the same time, FTIR spectrum of the working membrane sample (absence of absorption band at 1478 cm^–1) shows breaking of H‑bonds, accompanying changes in the configurations of molecules with increasing gauche conformers. Manifestation of a complex absorption band at ~1723 and 1737 cm^–1, attributed to terminal carboxyl groups (COOH), indicates the destruction of PA6 macromolecules. Disappearance of the absorption band at ~3089 cm^–1 for the first time experimentally confirms the connection between the Fermi resonance phenomenon in polymer molecules and the destruction of PA6 macromolecules to elementary structural units. X-ray studies indicate structural changes mainly at the supramolecular level in MMK-045 membrane samples. In a water-saturated sample, the density of molecules in the amorphous phase increases (2θ = 26.4°), and in the working one it decreases (2θ = 20.9°). The total crystallinity in the working membrane sample relative to the nonworking one decreased from 36 to 33%, not as a result of recrystallization, but rather as a result of a reduction in crystallites of α-phases, and increased in crystalline formations of γ-phases with imperfect crystalline structure.