Structural Investigation on Water-Induced Phase Transitions of Poly(ethylene imine). 1. Time-Resolved Infrared Spectral Measurements in the Hydration Process

Abstract

Time-resolved infrared spectral measurements were made successfully for studying the microscopic mechanism of water-induced solid-state phase transitions of poly(ethylene imine) (PEI) in the hydration process at room temperature. The initial sample, obtained by cooling the melt under a dry atmosphere, showed the infrared spectra characteristic of the anhydrate phase consisting of doubly stranded helices. When this sample was exposed into an atmosphere of 100% relative humidity, the infrared spectra were found to change in a multistage mode from anhydrate to hemihydrate (molar ratio of ethylene imine unit/water = 1/0.5) to sesquihydrate (1/1.5) and to dihydrate (1/2), where the molecular chains in the latter three phases take the planar-zigzag all-trans conformation and form the complexes with water through the hydrogen bonds. The spectral measurements were made also for heavy water (D2O) as well as light water (H2O), and the quantitative analysis could be made more successfully for the former case, because the overlap of the polymer bands with water bands could be avoided due to the shift of the broad absorption bands of water molecules. A comparison was made for the strength of hydrogen bonds between PEI and PEI, PEI and water, and water and water. The infrared bands characteristic of the amorphous region could be detected in the frequency region of 1800−2500 cm-1, the intensities of which were found to decrease in parallel to the crystal phase transition, indicating that some parts of the amorphous region can also crystallize into the hydrates in the hydration process.