Small-Angle X-ray Scattering Studies of Thermoresponsive Poly( N -isopropylacrylamide) Star Polymers in Water

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The behavior of a range of different stimuli-responsive star poly( N -isopropylacrylamide) (PNIPAAM) polymers in water was investigated using small-angle X-ray scattering (SAXS). The samples of 1, 2, and 5 wt % PNIPAAM stars with 2, 3, 4, and 6 arms were measured at seven temperatures ranging from 15 to 36 °C, which covers the known lower critical solution temperature (LCST) for linear PNIPAAM. The data were fitted with a Gaussian star form factor, and interactions were accounted for by a random-phase approximation (RPA) expression. A clear LCST was observed at ∼32 °C for the four polymers in agreement with turbidity data on the same polymers. The molecular weight was calculated from the forward scattering, which showed reasonable agreement with the values expected from the synthesis and characterization. From the fits the root-mean-square radius of gyration ⟨ R g 2 ⟩ 1/2 and the second virial coefficient A 2 were obtained and compared with the literature.