Study of the Surface Acetate-Cellulose Layer in the Compositional Membranes by the Vibrational Spectroscopy Method

Abstract

The paper presents the structural organization of the surface (selective) layer. The amorphous phase of the cellulose acetate composite membrane is formed at a certain stage of the technological cycle of producing the cellulose acetate membrane. It presents a mass of broken, entangled, and twisted macromolecular chains connected by a network of hydrogen bonds of various strengths. Analysis of the IR spectra, where the shapes of the absorption bands of the valence vibrations of hydroxyl groups at ν = 3339.14–3366.2 cm^–1 with the asymmetry coefficient varying from ~1 for a dry sample to 0.79 for a wet sample, suggests that the bulk supramolecular structure of the cellulose acetate layer of the air-dry sample is formed by two types of hydrogen bonds and the dipole–dipole interactions of the carbonyl groups. The interactions of macromolecules in the equatorial plane are formed by a network of hydrogen bonds of the OH…O type with the participation of a single hydroxyl group of the pyranose ring in cellulose acetate. In the axial direction, the supramolecular structure is organized through the hydrogen bonds of the CH…O=C type between the methane and carbonyl groups, as well as possibly the dipole–dipole interactions between the coplanar ordered dipoles of the C=O groups. The coefficient of asymmetry of the absorption bands of hydroxyl groups is reduced to 0.81–0.79 and the intensity of the absorption band of methyl groups is reduced by factors of 2.56, 3.3, and 3.8 in water-saturated samples due to the destruction of the supramolecular structure and the reorganization of the hydrogen bonds between the active groups of cellulose acetate and water molecules. The absence of an absorption band of ν = 873.53–851.9 cm^–1 in the water-saturated samples indicates a conformational rearrangement of the macromolecules into a linear form with the formation of narrow mesopores of cellulose acetate between the macromolecules.