Effect of architecture on the micellar properties of amphiphilic block copolymers:: Comparison of AB linear diblock, A2B2, A3B3, and (AB)3 star block copolymers

The influence of architecture on aqueous micellar properties of amphiphilic block copolymers AB-1, A(2)B(2), A(3)B(3), and (AB)(3) (A = polyisobutylene (PIB), B = poly(methyl vinyl ether) (PMeVE)), with approximately constant A and B segment lengths, was investigated. A comparison of AB-1 with AB-2 and AB-3 diblock copolymers having similar composition but higher total molecular weight suggested spherical core-shell morphology for AB-3 and vesicles for AB-1 and AB-2. Similarly to AB-1 star block copolymers A(2)B(2), A(3)B(3), and (AB)3 also formed vesicles with unusually high aggregation number (N-agg) and hydrodynamic radius (R-h). The critical micelle concentration (CMC) measured by fluorescence spectroscopy at 20degreesC increased in the order (AB)(3) = AB-1 < A(2)B(2) similar to A(3)B(3). The temperature dependence of R-h and N-agg has been determined in the temperature range of 20-30degreesC. R-h and N-agg remained approximately constant in the whole temperature range for A(2)B(2), A(3)B(3), and (AB)(3). A substantial increase in both R-h and N-agg occurred at 25degreesC for AB-1. The particle size distribution for all block copolymers remained narrow in the whole temperature range. Below 25degreesC, R-h increased in the order (AB)(3) < AB-1 < A(2)B(2) < A(3)B(3) and N-agg increased in the (AB)(3) < A(3)B(3) < A(2)B(2) < AB order. The numbers of AB units in the micelles, however, were similar. Micelles of A(2)B(2) and A(3)B(3) consisted of 5600 and 5700 AB units respectively, while these numbers were slightly lower for AB-1 (4500) and (AB)(3) (4800).