Effect of Temperature and Ionic Strength on Micellar Aggregates of Oppositely Charged Thermoresponsive Block Copolymer Polyelectrolytes

• Published in: Langmuir 2019-10, Vol.35 (42), p.13614-13623
• Main Authors: Fehér, Bence, Zhu, Kaizheng, Nyström, Bo, Varga, Imre, Pedersen, Jan Skov
• Format: Article
• Language: English
• Subjects: composite polymers; electrolytes; electrophoresis; electrostatic interactions; hydrophobic bonding; hydrophobicity; ionic strength; light scattering; micelles; mixing ratio; nanomaterials; small-angle X-ray scattering; temperature; thermosensitivity; turbidimetry
• ISSN: 0743-7463; 1520-5827; 1520-5827
• DOI: 10.1021/acs.langmuir.9b01896

The self-assembly of two oppositely charged diblock copolymers that have a common thermosensitive nonionic block of poly­(N-isopropylacrylamide) (pNIPAAM) has been investigated. The effect of the mixing ratio and total polymer concentrations on the self-assembly of the components and on the phase stability of the mixtures was studied by dynamic light scattering, electrophoretic mobility, and turbidimetry measurements in water at 20 °C. The effect of the competing electrostatic and hydrophobic interactions on the nanostructure of negatively charged electrostatically self-assembled micelles bearing a pNIPAAM corona was investigated by small-angle X-ray scattering (SAXS). The electrostatic and hydrophobic interactions were controlled independently by tuning the ionic strength (from pure water to 50 mM NaCl) and the temperature (20–50 °C) of the investigated mixtures. The SAXS data could be fitted by a spherical micelle model, which has a smoothly decaying radial profile and a Gaussian star term that describes the internal structure of the micellar structures and possible attractive interactions between the polymer chains. At high temperature, a cluster structure factor was included for describing the formation of bulky clusters of the formed micelles. At low temperature and ionic strength, the formation of micelles with a coacervate core and hydrated pNIPAAM shell was observed. The structural evolution of the self-assembled micelles with increasing ionic strength and temperature could be followed, and finally at high ionic strength and temperature, the formation of inverted micelles with a hydrophobic core and polyelectrolyte shell could be identified.