Chain conformation and hydration structure of an aqueous solution of linear poly(ethylene imine) (LPEI) have been studied by the use of mid-infrared (mid-IR) and near-infrared (NIR) spectrometries to explore the molecular mechanism of an upper critical solution temperature (UCST)-type solubility change. When LPEI was dissolved in water, the solution became a gel at room temperature. Mid-IR and NIR absorption bands of the LPEI gel appeared at the same positions as the bands of the dihydrate crystal of LPEI in a film, which suggested that LPEI formed dihydrate crystallite in water at room temperature. This means that LPEI is in a planar-zigzag structure even in water at room temperature. When the gel was heated, it was changed to a transparent aqueous solution. Both mid-IR and NIR bands of an LPEI aqueous solution above 64 °C exhibited broader band width than those at room temperature. Judging from the change of the band width, LPEI chain in water was disordered by heating to take a random coil form. In addition, band locations of the CH stretching first-overtone vibration bands in a NIR spectrum of the solution above 64 °C were higher than those of melted dry LPEI, which was reasonably understandable that the disordered LPEI chains in the aqueous solution were in a highly hydrated state. These spectral results have revealed that the change from the planer-zigzag form to the random-coil one in water is a trigger to begin the UCST-like solubility change on heating followed by hydration.
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