CRITICAL MICELLE LENGTHS FOR IONIC BLOCKS IN SOLUTIONS OF POLYSTYRENE-B-POLY(SODIUM ACRYLATE) IONOMERS

Polystyrene-b-poly(sodium acrylate) ionomers were found to form very stable reverse micelles in organic solvents such as tetrahydrofuran (THF), N,N-dimethylformamide (DMF), N-methylpyrrolidinone (NMP), and toluene. The micellar solutions in THF and NMP were investigated by size exclusion chromatography (SEC). Peaks were seen corresponding to both micelles and functionalized single chains; in some cases, homopolystyrene was observed as a separate peak. The single chains were thought to arise from an equilibrium solubilization of species with very short ionic chain lengths below a ''critical micelle length'' (CML). A computer model was developed in which the relative amounts of micelles and single chains were fitted to the experimentally obtained values using the molecular weight heterogeniety of the ionic block and the CML as the fitting parameters. The value of the CML was found to vary between 2 and 3 units of the ionic block and to be insensitive to the length of polystyrene block between 190 and 2300 units. The effects of counterions and solvents were found to be generally negligible. It is suggested that the CML is a more useful concept in these systems because of the very high free energy of micellization, which contrasts with the much lower value of that parameter for the hydrophobic core micelles. The materials were prepared by sequential anionic polymerization of styrene and tert-butyl acrylate in THF at -78-degrees-C using the alpha-methylstyryllithium-LiCl initiator system as reported by Teyssie et al., followed by hydrolysis and neutralization.