Microdynamics Mechanism of Thermal-Induced Hydrogel Network Destruction of Poly(vinyl alcohol) in D2O Studied by Two-Dimensional Infrared Correlation Spectroscopy

Microdynamics mechanism of thermal-induced hydrogel network destruction of poly(vinyl alcohol) (PVA) in D2O at heating (25-62 degrees 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 degrees C by dynamic rheological test at first, and then also monitored by MW2D FTIR spectra. The motion of v(s)(-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 degrees C, the number of D2O molecules in the swollen amorphous remains unchanged. At 32-37 degrees C, more D2O molecules enter into the swollen amorphous region, and the groups of -C-O-, together with -CH2-, are partially hydrated. At 37 degrees 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 degrees 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 degrees 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.