Zwitterionic and monofunctional block copolymers in a selective solvent: Model macromolecular surfactants

Abstract

Solutions of block copolymers containing ionic groups at the chain end in a selective solvent condition are investigated using small-angle neutron scattering (SANS). The self-assembling as a consequence of the competition between electrostatic and hydrophobic interactions is studied in those model macromolecular surfactants. The influence of the position of the ionic group on the block-copolymer chain on the structure and size of the aggregates is considered. Dilute solutions containing spherical micelles are investigated. The molecules considered are polystyrene-polyisoprene (PS-b-PI) block-copolymers in di-methyl acetamide which is a very bad solvent for the PI block. At low concentrations, block-copolymer micelles with a PI core are formed. At ambient temperature, the PI core is liquid and this ensures thermodynamic equilibrium. Three charged systems are studied: (1) sample S — polystyrene-polyisoprene (PS-PI) block copolymers containing a sulfonate ionic group at the PI end (non-soluble block); (2) sample Q — another monofunctional PS-b-PI containing, this time, a quaternized amine at the PS end (swollen polymer shell) and (3) sample Z — zwitterionic (PS-b-PI) with ionic groups of opposite charges at each chain end. It is found that the aggregation number of the charged micelles is smaller than that of the neutral micelles. Micelles formed by zwitterionic copolymers (sample Z) present an aggregation number which is similar to that observed for micelles formed by monofunctional block copolymers containing ionic groups at the core (sample S). The zwitterionic (Z) and the monofunctionalized (S) samples exhibits a soft correlation maximum at dilute concentrations, while sample Q, behaves much more like a colloidal particle with a pronounced structure factor.