Small-Angle X-ray Scattering Study of Charged Triblock Copolymers as a Function of Polymer Concentration, Temperature, and Charge Screening

In the current study, the influence of electrostatic interaction on micelle formation has been investigated by characterizing a series of charged thermo-responsive triblock copolymers by densitometry and small-angle X-ray scattering in a wide temperature range from 20 to 90 C and by varying the salt concentration of the solvent. The copolymers, MPEG(45)-b-P(NIPAAM)(n) -b-P(SSS)(22), were composed of methoxypoly(ethylene glycol) (MPEG, hydrophilic), poly(N-isopropylacrylamide) (PNIPAAM, temperature sensitive), and poly(4-styrene; sulfonic acid sodium) (PSSS, charged). The PNIPAAM block constituted between 20 and 50% of the polymer repeat units. The single polymers were described with a Gaussian chain model, where the repulsive interchain interactions were taken into account in aqueous solution. Above the critical micelle temperature (CMT) the polymer with the medium PNIPAAM block formed spherical micelles, whereas the copolymer with the largest PNIPAAM block formed cylindrical micelles. Addition of salt to the solution affected the formed micelles as well as the CMT of the system.