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

Small-angleX-ray scattering and electrophoretic light scatteringmeasurements were made for dilute aqueous solutions of star-branchedpoly(N-isopropylacrylamide) (sPNIPAM) at differenttemperatures. In the constant heating process, relatively narrow-dispersedsphere particles with an average aggregation number between 10 and50 were observed in water at high temperatures (45-55 & DEG;C),regardless of the end groups: carboxyl, hydroxyl, and benzyl groups.Indeed, the test solutions for which the polymer mass concentrationwas 2-4 mg mL(-1) did not become turbid attemperatures higher than the phase separation temperature of the correspondinglinear polymer. Negative & zeta; potential was found for all solutionsat the high temperature, 45 & DEG;C, indicating that electrostaticrepulsion force stabilizes the dispersion state. Spinodal decompositionis the dominant mechanism of the microscopic phase separation becausenarrower distributed spherical particles were observed in the solutionupon rapid heating than that for the constant heating procedure.