Effect of the casting solvent on the morphology of poly(styrene-b-isoprene) diblock copolymer/magnetic nanoparticle mixtures

We investigate the effect of the casting solvent on the phase behavior of a cylinder-forming poly(styrene-b-isoprene) (PS-b-PI) diblock copolymer mixed with marginally selective gamma-Fe2O3 nanoparticles. Notably, for the mixtures of PS-b-PI/gamma-Fe2O3 nanoparticles cast with toluene, which is a neutral solvent for both PS and PI blocks, the latticelike aggregates of gamma-Fe2O3 nanoparticles, originating from magnetic dipole interactions, induce an intriguing transition from hexagonal cylinders to body-centered cubic spheres via undulated cylinders whereas the neat block copolymer does not show such an order-order transition. In contrast, when PI-selective hexane is used, the nanoparticles are selectively incorporated into the PI domains, and the microphase-separated hexagonal cylinder morphology gradually loses its long-range order upon increasing the nanoparticle concentration. The structural information obtained from small-angle X-ray scattering is also in good agreement with transmission electron microscopy images and differential scanning calorimetry results.