Effect of the soluble block size on spherical diblock copolymer micelles

While the effect of the insoluble block length on micelle properties is well understood, the effect of the soluble block is still controversial. We, therefore, have investigated the effect of the molecular weight of the soluble block on the critical micelle concentration (CMC), aggregation number, and hydrodynamic radius of spherical polymer micelles. Spherical micelles were formed from polystyrene-b-polyisoprene (PS-b-PI) in heptane, which was a good solvent for PI and a poor solvent for PS. Measurements were performed on two series of PS-b-PI with a constant PS block (19 and 39 kDa, respectively) and PI blocks varying from 10 to 100 kDa. For samples with large PI blocks, the experimental data were found to be in agreement with the commonly used star-like model. However, the experimental data for samples with short PI blocks deviated from the crew-cut micelle model. To correctly capture the crossover between the crew-cut and star-like regimes, it was found necessary to use recently developed scaling theory which explicitly considers all contributions to the free energy of the micelle. In agreement with theory, the aggregation number decreased while hydrodynamic radius and CMC increased with the molecular weight of the PI block. An interesting finding of these experiments is that the micelles of the 19 kDa series are in equilibrium at 25 degrees C, whereas the 39 kDa samples with the longer PS core block are "frozen" at room temperature. This was confirmed by SAXS measurements of core expansion upon heating which revealed a glass transition temperature of the 39 kDa samples at 28 +/- 1 degrees C. The temperature value is consistent with 10% swelling of the PS core with heptane as determined by SAXS and SLS.