Micellization in Block Polyelectrolyte Solutions. 2. Fluorescence Study of the Critical Micelle Concentration as a Function of Soluble Block Length and Salt Concentration

Abstract

Critical micelle concentrations (cmcs) of block polyelectrolyte micelles formed from poly(styrene-b-sodium acrylate) in aqueous and NaCl salt solutions were investigated by fluorescence measurements of solubilized pyrene. The measurements were made for a range of PS block lengths (6 to 110 units) and of PANa block lengths (15-2400 units). For the series consisting of 11 and 23 units of PS, which had been prepared with a wide range of PANa lengths, the cmc was found to pass through a maximum as a function of the soluble (PANa) block length. It was observed that as the length of the PS block increased, the dependence of the log cmc versus the PANa block length decreased. For all samples except the PS(6) series at low salt concentrations, the log cmc values were found to decrease linearly as a function of the square root of the NaCl concentration (√C s ) which was varied from 0.10-2.5 M. The shape of the gradient d(log cmc)/d(√C s ) versus the log of the number of PANa units was found to follow a curve of the same shape for the block polyelectrolyte micelles consisting of 6, 11, and 23 units of PS. The behavior of the curve was explained on the basis of polyelectrolyte conformations.