Tuning of structure and kinetics of chain exchange in star-like PEP-PEO block copolymer micelles

A study of structural and dynamical properties of star-like polymeric micelles is presented. The micelles are formed by highly asymmetric poly(ethylene-alt-propylene)-poly(ethylene oxide)(PEP1-PEO20)(the numbers indicate the molecular weight in kg/mole) block copolymers. Using small angle neutron scattering (SANS) we have studied the global aggregation behaviour of these micelles in different water/dimethyl formamide (DMF) solvent mixtures. Using Pendant drop tensiometry as a complementary technique we measured the interfacial tension, γ, between the insoluble PEP block and the corresponding solvent mixtures. The results reveal a good agreement between the proposed power law valid for star-like micelles: P∼γ6/5, where P is the aggregation number. Furthermore, we demonstrate using a special time-resolved SANS technique that the kinetics of chain exchange can be studied. The kinetics was found to be very slow; however, it can be tuned by γ into an appropriate time scale so that it can be followed by SANS. The obtained relaxation curves show a multiexponential decay that suggest that the exchange kinetics is quite complex