Assemblies of thermoresponsive diblock copolymers: micelle and vesicle formation investigated by means of dielectric relaxation spectroscopy.

The dielectric properties of aqueous solutions of two different thermoresponsive mixed copolymers, (3-acrylamidopropyl)trimethylammonium chloride and N-isopropylacrylamide, PAMPTMA-b-NIPAAM, and sodium 2-acrylamido-2-methylpropanesulfonate and N-isopropylacrylamide, PAMPS-b-PNIPAAM, have been investigated in the frequency range where marked interfacial polarization mechanisms occur, both below and above the lower critical solution temperature. In the presence of poly(ethylene oxide)-PNIPAAM block polymers, PEO-b-PNIPAAM, these classes of copolymers give rise to different types of aggregates with different compositions and different architectures. By the combined results from dielectric relaxation spectroscopy, dynamic light scattering, and ζ-potential measurements, we give evidence for assembling into two different composite structures, a core-shell-type micellar structure built up by a hydrophobic core surrounded by a hydrophilic charged layer, in the case of the PEO-b-PNIPAAM + PAMPS-b-PNIPAAM system, and a vesicular structure encompassing an aqueous core in the case of the PEO-b-PNIPAAM + PAMPTMA-b-PNIPAAM system.