| Abstract: | Thermal response of N-isopropylacrylamide (NiPAM) gel particles of submicron size was investigated by means of ultrasonic velocity and density focusing on the hydration and dehydration behavior. Hydration number, defined as the number of water molecules bound to one NiPAM monomer unit, was quantitatively evaluated in the course of the volume phase transition. Hydration numbers at low and high temperatures were about 7.5 (20 °C) and 3 (40 °C), respectively. Hydration number decreases markedly near the volume phase transition temperature, and the decrease is responsible to the formation of hydrophobic bonding and dehydration. The hydration number is independent both of the size and the composition of gel particles in the shrunken state, although there exists a composition dependence in the swollen state. It was also found that the self-assemblies formed by hydrophobic interaction are more bulky compared to those in the hydrated state. |
