Evaluation of the cross section of elongated micelles by static and dynamic light scattering

We describe simultaneous static (SLS) and dynamic light scattering (DLS) measurements on dilute solutions of a series of poly(ferrocenyldimethylsilane-b-isoprene) (PFS(50)-PI(1000)) block copolymer micelles of uniform length in tert-butyl acetate (tBA) and in decane. The subscripts in the term PFS(50)-PI(1000) refer to the mean degree of polymerization of each block. The SLS experiments show that in both solvents the micelles formed are elongated and rigid. We also observed that the large length of the PI block (1000 units) contributes to the SLS signal. From the SLS data, we calculated the mass per unit length (linear aggregation number), as well as the cross section of the micelles in both solvents. Interestingly, the linear aggregation number and the micelle cross sections, as deduced by SLS, were the same in decane and in tBA. However, the fitting of DLS data indicates that the hydrodynamic cross section of the micelles in tBA is much larger than that in decane, and both values are larger than the values determined by SLS. We hypothesize that the difference between cross sections deduced from SLS and DLS data fitting is related to the shape of the segment density profile of the corona. In tBA, the PI chains are more stretched than in decane, increasing the hydrodynamic radius of the micelle cross section.