Structural investigation on water-induced phase transitions of poly(ethylene imine), Part IV: Changes of intra- and intermolecular hydrogen bonds in the hydration processes as revealed by time-resolved Raman spectral measurements

Poly(ethylene imine) (PEI) exhibits water-induced phase transitions among four kinds of crystalline hydrates; anhydrate (EI monomer unit/water molecule = 1/0), hemihydrate (1/0.5), sesquihydrate (1/1.5) and dihydrate (1/2). The chain conformation changes from a double helix in the anhydrate to a planar-zigzag form in the three types of hydrates. Time-resolved Raman spectral measurements have been successfully performed for the first time in the hydration processes of PEI using light and heavy waters. Raman spectral profiles characteristic of each crystal form were obtained in the frequency region of 30–3500 cm⁻¹. Exchange of water from H₂O to D₂O was helpful for shifting the many overlapped bands to identify the characteristic bands. Details of the change in intra- and intermolecular hydrogen bonds have been clarified by quantitative interpretation of the observed Raman spectral data. The conformational disordering occurring in the transition process from the double-stranded helices to the planar-zigzag chains was also discussed.