Controlling micellar properties of styrene/isoprene copolymers by altering the monomer arrangement along the chain

By using appropriate anionic polymerization high-vacuum techniques, novel triblock copolymers of styrene and isoprene with normal tapered (TMB), inverse tapered (ITMB), and random (RMB) middle blocks were synthesized. In addition, diblock copolymers (SI) as reference materials and normal tapered (TBC) and random (RC) were prepared. All copolymers have practically the same composition (∼50 wt % PS). Their micellization behavior was investigated in dilute solutions in n-decane, a selective solvent for the isoprene part of the macromolecules, by static and dynamic light scattering as well as viscometry. Experiments were directed toward the determination of the micelles fundamental properties (average aggregation number, critical micelle concentration, critical micellization temperature) and compared with those obtained for pure diblock copolymers. No considerable differences, compared to the pure diblock copolymers, were observed for triblocks having a normal tapered middle block (TMB). A considerable decrease in the aggregation number and a corresponding decrease in the critical micellization temperature were found for the triblock copolymers having an inverse tapered middle block (ITMB) with length comparable to the length of the outer pure blocks. A similar decrease in the aggregation number was also observed in the case of a normal tapered copolymer. The differences in behavior are attributed to the different arrangement of the segments, which seems to have a considerable effect on the solubility characteristics of these copolymers.