Dielectric Properties of Micellar Aggregates Due to the Self-Assembly of Thermoresponsive Diblock Copolymers

Abstract

The radiowave dielectric properties of aqueous solutions of thermosensitive copolymers, consisting of poly(2-acrylamido-2-methylpropanesulfonate) [PAMPS] and poly(N-isopropylacrylamide) [PNIPAAM] with different block lengths, have been investigated over a broad temperature and frequency range. These copolymers PAMPS(n)-b-PNIPAAM(m) form temperature responsive aggregates (micelles) that represent a class of self-assembled structures in water of great interest because of their potential use as drug delivery formulations and in diverse biotechnological applications. Copolymers formed by hydrophilic segments covalently attached to a hydrophobic segments are capable of forming a micellar structure as soon as the temperature is raised above their lower critical solution temperature. We have investigated the dielectric properties of PAMPS(n)-b-PNIPAAM(m) diblock copolymers with different lengths of the hydrophilic and hydrophobic segments during the whole aggregation process driven by the progressive increase of temperature. The process has been followed by the changes resulting in the dielectric parameters (the dielectric increment Δε and the relaxation frequency ν(0)) of the whole aqueous solution. The dielectric response of the micelles has been described within the framework of the standard electrokinetic model for charged colloidal particles, and the main characteristic parameters have been evaluated. Subsequent cross-linking of these diblock copolymers by a cationic PEO(x)-b-PAMPTMA(y) polyelectrolyte yields hybrid core-shell-corona systems, with the PNIPAAM hydrophobic blocks collapsed in the core, an interpolyelectrolyte chain complex forming the shell, and the hydrophilic PEO chains as an external corona. In this case too, the dielectric spectra can be appropriately accounted for within the same theoretical framework.