IR-Spectroscopic Investigations of Adsorbed Water and Structural Changes in Hydrophobic and Hydrophilic Microfiltration Membranes

Abstract

In this work, the effects of adsorbed water on the structural evolution in the PVDF layer of composite hydrophobic MFFC-3 and hydrophilic MFFC-3G membranes were studied using ATR IR spectroscopy. It has been found that adsorbed water molecules initiate the conformational rearrangement of PVDF polymer molecules in the hydrophilic membrane layer, while the conformation of PVDF polymer molecules in the hydrophobic membrane layer does not change. The deconvolution of the absorption bands of the stretching vibrations of OH groups was carried out using four Gaussians. Based on the frequencies of the Gaussian peaks, the energy was calculated and the hydrogen bonds of adsorbed water were classified. It is shown that the structure of adsorbed water with hydrogen bond energies E_OH = 42.8, 35.7, 17.9, and 14 kJ/mol are “clusters” with three or four hydrogen bonds. Energy values E_OH = 17.9, 10.6, 16.8, and 14 kJ/mol correspond to OH bonds of dimeric and monomeric water molecules. It is concluded that the structure of adsorbed water in the PVDF layer can be characterized by a mixed model of hydrogen bonds, consisting of monomeric, dimeric molecules, and associated clusters with hydrogen bond energies from 10.6 to 42.8 kJ/mol. A comparative analysis of the ATR IR spectra of water-saturated samples indicates that the adsorbed water molecules in the PVDF “polymer–water” interfacial space of the hydrophobic membrane do not form a layer of bound water, while, in the hydrophilic membrane, water enters the structure of molecules, forming a layer of bound water.