Surface Charged Polymeric Micelles – A Tunable Model System Studied by SANS

Micelles formed by n-alkyl – endcapped poly(ethylene oxides) polymers are excellent tunable model materials to study ultrasoft colloidal interactions. Parameters to tailor the “softness” start on a microscopic level by selecting the chemical structure of the hydrophobic block, by varying the solvophobic-to-solvophilic block ratio [1], the absolute block molecular weights [2], polydispersity of the hydrophilic block [3], and/or the interfacial tension [4]. The polymeric micelles are composed of light elements (hydrogen, carbon, and oxygen), resulting in a weak scattering signal with X-rays. However, by taking advantage of the strong scattering contrast between hydrogen and deuterium in neutron scattering, we achieved good contrast by using heavy water as the solvent for our hydrogenated polymer. Additionally, achieving zero average contrast for the core or shell through the polymerization of deuterated precursors and adjusting the solvent composition (by varying H2O/D2O ratios) enabled the development of a core-shell model. In the present work, we introduce additional surface charges on the micelles by oxidation of the terminal hydroxyl group of the PEO block into carboxylic group by Bobbitt’s salt [5]. Since the carboxylic group is a weak acid, the number of charges on the particles can be adjusted by modifying the pH or the ionic strength. Changes in electrostatic interactions influence the softness of our colloid, which is closely connected to its rheological behavior. The data were successfully fitted using the previously introduced core-shell model [6], which was adjusted with a Yukawa potential to account for long-range electrostatic interactions. This fitting demonstrated that the form factor was unaffected by the presence of charges or the concentration of polymer, allowing the structure factor to be recovered. In fig 1. The change in microscopic structure at different effective charge can be observed through 2D SANS images and scattering curves, where the appearance of Bragg peaks indicates the formation of an FCC crystalline structure at pH 6 (when the micelles are fully charged). In contrast, the 2D images of uncharged micelles (with OH-terminated polymer at pH 2) or those with screened charges (at pH 12) are isotropic, a characteristic typical of solutions. This significant shift in microscopic behavior is also reflected in the rheological properties. While the samples at pH 2, pH 12, and with OH-terminated polymer remain in a liquid state, the sample at pH 6 becomes non-flowing and demonstrates the characteristics of a repulsive glass highlighting the critical role of electrostatic charges in the micelle organization.