Micellization of model macromolecular surfactants as studied by static light scattering

Micellization of ionically end-capped polystyrene-block-polyisoprene copoly mers (PS-b-PI) in N,N-dimethylacetamide (DMAc) is investigated by static li ght scattering. DMAc is a polar selective solvent for PS which ensures for sufficient dissociation of ion pairs in solution. The use of polyisoprene a s the nonsoluble block provides micelles in thermodynamic equilibrium since PI is a liquid at ambient temperature. Two different types of ionic end fu nctionality are considered: (i) a monofunctional diblock with a lithium sul fonate group at the PI chain end and (ii) an alpha,omega-macrozwitterionic species with an additional ammonium group at the PS chain end. They are bot h compared to a neutral diblock of same mass and block sizes (12 kg/mol per block). It is shown that micelles formed by block copolymers containing a sulfonate group at the PI end (core of the micelle) exhibit an aggregation number (N,,) which is only about 30% of N-agg of the neutral block copolyme r. The critical micelle concentration (cmc) in this case is increased by 1 order of magnitude. The macrozwitterionic samples behave qualitatively as t he monofunctionalized polymer, however, presenting a larger aggregation num ber and a lower cmc with respect to the monofunctionalized sample. This eff ect is attributed to a self screening mechanism in micelles of the alpha,om ega-macrozwitterions, where counterions are bound through chains of the sol uble block.