On the selection of FCC and BCC lattices in poly(styrene-b-isoprene) copolymer micelles

Spherical micelles of poly(styrene-b-isoprene) (SI) diblock copolymers in selective solvents have been reported to pack onto either face-centered cubic (fee) or body-centered cubic (bcc) lattices. The selection rule for fee and bcc lattices has been understood in terms of the intermicellar potentials, and they have been quantified using the ratio of the corona layer thickness to the core radius, L/R_c, as suggested by McConnell and Gast. In order to test the validity of the McConnell-Gast criterion, this study compared the L/R _c values from various solutions i.e. nine SI copolymers in several different selective solvents. The McConnell-Gast criterion was not found to be a determining factor, even though it could explain the fcc/bcc selection qualitatively. From the phase diagrams, the transition between fee and bcc phases was also considered as a function of concentration and temperature, and their physical mechanisms are discussed based on the recent mean-field calculation reported by Grason.