The whole process of phase transition and relaxation of poly(N-isopropylacrylamide) aqueous solution.

We observed the whole dynamics from the phase transition to the molecular relaxation of a poly(N-isopropylacrylamide) (PNIPAM) aqueous solution using the heterodyne transient grating (HD-TG) method combined with the laser temperature-jump technique. The initial phase transition corresponding to coil to globule conformational transition occurred with a time constant of 96 ± 7 μs. Then, the globule molecules aggregated due to hydrophobic interaction, and the size of the aggregates increased sufficiently to scatter light (11 ± 1 ms) when the temperature achieved by heating was high enough to exceed the phase transition temperature. The aggregates loosened due to thermal diffusion with a time constant of 44 ± 5 ms. Even after that, the released globule molecules returned back to the coil state with a lifetime of 1.5-5 s, as we reported in a previous paper (H. Inoue et al., Phys. Chem. Chem. Phys., 2012, 14, 5620-5627).