Hydrodynamics of polystyrene-polyisoprene miktoarm star copolymers in a selective and a non-selective solvent

The hydrodynamics of PSnPIn miktoarm (mixed arm) star copolymers made from polyisoprene (PI) and polystyrene (PS) arms are studied in a selective and a non-selective solvent, n-hexane and THF, respectively. It is found that in n-hexane, the number of arms affects the organization of the miktoarm copolymers: stars with 2 arms (a linear diblock for reference purposes) or 4 arms show aggregation in this selective solvent, whereas no aggregation is observed for stars with 8 and 16 arms in the concentration region studied. This behavior is due to shielding posed by the soluble blocks, which prevents the insoluble blocks from coming together. Interestingly, the contribution from aggregates observed for the two arm star (PS1PI1 diblock) at the highest concentration studied is rather small because the chains predominantly exist as single diblocks in n-hexane. This result may be due to the fact that low molecular weight PS is slightly soluble in linear hydrocarbon solvents. The hydrodynamic sizes found in THF are similar to those in n-hexane for the 2 and 4 arm stars but smaller for the 8 and 16 arms stars. We propose that this is a result of both the limited free space needed for motion of the chains and also because of an increased probability of heterocontacts between the collapsed PS blocks and the swollen PI arms near the star's core.