According to thermodynamics of polymer solutions, polymers show conformational changes depending on the solvent quality. A transition of single polymer chain induced by the solvent quality change, known as a coilglobule transition, has been extensively studied because of their importance in not only polymer physics but also application aspects for the collapse of a gel network, the folding of a protein, and so on. [1][2][3][4][5][6] Poly(N-isopropylacrylamide) (PNiPAM) chain in water has a Θ temperature at 30.6°C, where the second virial coefficient is zero, and shows the coil-globule transition by heating. Indeed, there are many reports studying PNiPAM solutions by scattering methods, 7-10 fluorescence, 11 electronic paramagnetic resonance, 12 IR spectroscopy, 13,14 and calorimetry. 15,16 A considerable amount of work on PNiPAM solutions is collected in a review. 17 However, thermal diffusion phenomena, also called the Ludwig-Soret effect, have not been studied with taking into account the coil-globule transition in the vicinity of the Θ temperature.The Ludwig-Soret effect accounts for a mass flux, J 1 , in a mixture which is induced by a temperature and a concentration gradient. [18][19][20][21] Under a steady temperature gradient a steady concentration gradient is built up, where the flux vanishes (J 1 ) 0). The Soret coefficient, S T , is expressed as where D is the translational mass diffusion coefficient, D T the thermal diffusion coefficient, c 1 the mass fraction of component 1, and T the temperature.The sign of S T indicates whether component 1 migrates to the warm side or the cold side of the fluid. 22 If the system is dominated by short-range interactions, the thermal diffusion behavior can be attributed to single particle and collective contributions. 23 There are several detailed and systematic studies of thermal diffusion phenomena of organic polymer systems covering scaling behavior, concentration effects, and preferential solvation effects in mixed solvents. [24][25][26][27][28][29] In contrast to those organic systems, there are only few studies for aqueous polymer solutions.Observations of the Ludwig-Soret effect in the vicinity of coil-globule transition of PNiPAM/water will illuminate the importance of segment-segment and segment-solvent interactions. These key features can be varied experimentally according to the consequence of coil-globule transition of PNiPAM. This approach will lead to deeper understanding of thermal diffusion behavior of aqueous polymer systems as well as provide new insight into the solution properties of PNiPAM in a thermodynamically nonequilibrium state.PNiPAM was purified by a fractionation method. 8 One fraction was used in this study which had the weightaveraged molecular weight M w ) 3.0 × 10 6 g/mol with a polydispersity M w /M n of 1.20. Details of sample preparation and characterizations are described elsewhere. 30 Water was deionized by a Milli-Q system. In this study 1.0 g/L PNiPAM in water was prepared with a tiny amount of the dye, Basantol Yellow 215. 31 ...