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Thermosensitive LCST phase separation behaviors of poly( N-isopropylacrylamide) (PNIPAM) aqueous solutions have been investigated by a power compensation differential scanning calorimetry which can be subjected to relatively high cooling and heating scanning rates. Significant difference in the thermal properties is observed in four different concentration regions. The typical demixing enthalpy recovery behavior is observed in region III and IV, but cannot be observed in region II. In concentration region I, only one concentration-dependent glass transition behavior can be observed. The distinct thermal features in different concentration regions were interpreted by the complexity of chemical structure, and there are more than one polar groups in the repeating unit of PNIPAM, hydrogen bonded with different numbers of water molecules in different concentration regions. The endothermic phase separation process involved both a chemical process of the breakage of hydrogen bonding between polymer and water molecules and a physical process of the coil-to-globule transition of swelling polymer chains. [ABSTRACT FROM AUTHOR]