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 zeta-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. These different structures, governed by a delicate interplay between electrostatic and hydrophobic interactions, are characterized by dielectric parameters (dielectric increments and relaxation frequencies) that can be properly deduced from suitable dielectric models, in the framework of heterogeneous system theory. These structures and in particular hallow particles with a large internal aqueous core, where large hydrophilic compounds can be encapsulated, offer novel and interesting properties in biomedical technologies and in particular in drug delivery as drug mesoscopic carriers.