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Small-angle X-ray scattering (SAXS) was used to determine the packing of micelles in cubic phases formed by poly(oxyethylene)−poly(oxybutylene) (EB) diblock copolymers in water. SAXS with large amplitude oscillatory shear was used to identify structures formed by the highly asymmetric molecules E96B18, E184B18, E315B17, and E398B19 (where the subscripts denote the number of repeats). At a constant temperature (20 °C) and concentration (10 wt %), we find that the two copolymers with shorter hydrophilic blocks form face-centered cubic (fcc) gels, whereas the two copolymers with longer corona-forming blocks form body-centered cubic (bcc) gels. SAXS also confirmed that the sols formed at lower copolymer concentrations are micellar liquids. Our results for the gel structures are in accord with the observation that micelles with relatively short coronas behave as hard spheres, and pack in a fcc structure, whereas micelles with large coronas act like soft spheres and pack in a bcc array. This is confirmed by assembling a phase diagram as a function of copolymer asymmetry and concentration using results from the four copolymers discussed here and a series of gels of other EB diblocks studied previously by us.

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