Microdynamics mechanism of thermal-induced hydrogel network destruction of poly(vinyl alcohol) in D2O studied by two-dimensional infrared correlation spectroscopy.

Abstract

Microdynamics mechanism of thermal-induced hydrogel network destruction of poly(vinyl alcohol) (PVA) in D2O at heating (25-62 °C) was studied by in situ Fourier transform infrared (FTIR) spectroscopy combining with moving-window two-dimensional (MW2D) technique and two-dimensional (2D) correlation analysis. The temperature range of hydrogel destruction was determined within 34-52 °C by dynamic rheological test at first, and then also monitored by MW2D FTIR spectra. The motion of vs(-C-O-, microcrystals) was important in the entire hydrogel destruction process. The microdynamics mechanism of PVA molecular chains can be elaborated as follows: At 32 °C, the number of D2O molecules in the swollen amorphous remains unchanged. At 32-37 °C, more D2O molecules enter into the swollen amorphous region, and the groups of -C-O-, together with -CH2-, are partially hydrated. At 37 °C, the intramolecular or intermolecular hydrogen bonds of PVA are dissociated. The physical cross-linking points of hydrogel are broken due to the melting of PVA microcrystals. At 42 °C, the dissociated hydroxyls from PVA microcrystals rapidly integrate solid hydrogen bonds with D2O molecules. The groups of -C-O- and -CH- are completely hydrated by D2O simultaneously. At 45-55 °C, PVA molecules are surrounded by more D2O molecules. The partially hydrated -CH2- is completely hydrated, and all of the PVA molecules are fully dissolved in D2O.