Collapse Behavior of Polyion Complex (PIC) Micelles upon Salt Addition and Reforming Behavior by Dialysis and Its...

Temperature-responsive polyion complex (PIC) micelles were prepared by using two diblock copolymers composed of a sulfobetaine chain (poly(sulfopropyldimethylammonium propylacrylamide), PSPP) and ionic chains (poly(sodium styrenesulfonate), PSSNa, or poly(3-(methacrylamido)propyltrimethylammonium chloride), PMAPTAC). Because the core is PIC and the shell is sulfobetaine with UCST-type temperature response, the corona expands and contracts in response to temperature. To control the size and uniformity of the PIC micelles, the collapse of PIC micelles by salt addition and the reforming behavior by dialysis were investigated by transmittance, DLS, TEM, AFM, and ¹H NMR measurements. Investigation of the ionic species dependence of the added salt in the collapse behavior of PIC micelles revealed that it was dependent on the anionic species, although no dependence on the cationic species was observed. Its effectiveness was in the order of I^- > Br^- > Cl^- > F^-, which is in agreement with the order of ionic species with strong structural destruction in the Hofmeister series. Heterogeneous and large PIC micelles were formed by the simple mixing method. They collapsed by salt addition and were reformed by the dialysis method to form uniform and smaller PIC micelles. This is considered to be because a uniform and smaller micelle is formed to reform in equilibrium state by dialysis. The temperature response of PIC micelles formed by the simple mixing method and PIC micelles reformed by dialysis showed nearly the same temperature-transmittance curves. These results indicate that the temperature response of PIC micelles is affected by the concentration rather than the hydrodynamic radius. Furthermore, the stability of PIC micelles was found to be affected by the concentration temperature (the temperature at the time of concentration).