Temperature-Induced Nanostructure Formation Behavior of Core Cross-Linked Star-Shaped Poly(N-isopropylacrylamide) in Water

Small-angle X-ray scattering and electrophoretic light scattering measurements were made for dilute aqueous solutions of star-branched poly( N -isopropylacrylamide) (sPNIPAM) at different temperatures. In the constant heating process, relatively narrow-dispersed sphere particles with an average aggregation number between 10 and 50 were observed in water at high temperatures (45–55 °C), regardless of the end groups: carboxyl, hydroxyl, and benzyl groups. Indeed, the test solutions for which the polymer mass concentration was 2–4 mg mL –1 did not become turbid at temperatures higher than the phase separation temperature of the corresponding linear polymer. Negative ζ potential was found for all solutions at the high temperature, 45 °C, indicating that electrostatic repulsion force stabilizes the dispersion state. Spinodal decomposition is the dominant mechanism of the microscopic phase separation because narrower distributed spherical particles were observed in the solution upon rapid heating than that for the constant heating procedure.