The effect of different functional groups of methylated urea on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) aqueous solutions has been studied by a high-sensitivity differential scanning calorimetry. The results reveal that with the addition of osmolytes with N-H groups, the enthalpy change increases with the number of DSC cycles, presumably due to the gradual formation of hydrogen bonds with dehydrated C=O groups of PNIPAM at high temperature. Moreover, with the addition of tetramethylurea (TMU) without hydrogen bond donor groups, the enthalpy change of PNIPAM solution remains unchanged with the number of DSC cycles and decreases with the TMU concentration, suggesting that the van der Waals interactions between TMU and isopropyl groups of PNIPAM and the weakening of hydrophobic interactions between isopropyl groups play a dominant role in the effect of TMU on the phase transition of PNIPAM. (C) 2016 Wiley Periodicals, Inc.