Equilibrium Length and Shape of Rodlike Polyelectrolyte Micelles in Dilute Aqueous Solutions

Abstract

Supramolecular structure formation in dilute salt-free aqueous solutions of highly charged dodecyl-substituted poly(p-phenylene)sulfonates (PPPS) in the free acid form (PPPS-H) with molar masses between Mw,p = 21 × 103 and Mw,p = 58 × 103 g/mol is investigated by static, isotropic, and anisotropic dynamic light scattering as well as transmission electron microscopy. The PPPS-H moieties, which are models of rodlike polyelectrolytes, form cylindrical micelles in which the shape of the formed objects can be analyzed in terms of a weakly bending rod by Koyama's form factor. The cylindrical micelles exhibit constant diameters (d ∼ 5 nm) as well as radial and axial aggregation numbers (Nrad ∼ 15 and Nax ∼ 5) but variable weight-average length depending on the polymer molar mass. The lengthwise aggregation is not controlled by kinetics. The inferred structures are supported by images obtained by transmission electron microscopy. TEM using different preparation methods reveals worm- and looplike micelles. Depending on the preparation conditions, the examined structures exhibit slightly different average diameters ranging from d = 4.8 to 8.0 nm. The occurrence of the various morphologies and their relevance for a mechanism by which equilibrium sized micelles can be obtained could be illustrated by means of a model based on the nematic character of the individual micelles. Besides the expected polyelectrolyte micelles, ternary superstructures, formed by association of several individual micelles, were found.