From hard spheres to soft spheres: The effect of copolymer composition on the structure of micellar cubic phases formed by diblock copolymers in aqueous solution

Small-angle X-ray scattering (SAXS) was used to determine the packing of mi celles in cubic phases formed by poly(oxyethylene)-poly(oxybutylene) (EB) d iblock copolymers in water. SAXS with large amplitude oscillatory shear was used to identify structures formed by the highly asymmetric molecules E96B 18,E184B18, E315B17, and E398B19 (where the subscripts denote the number of repeats). At a constant temperature (20 degrees C) and concentration (10 w t %), We find that the two copolymers with shorter hydrophilic blocks form face-centered cubic (fcc) gels, whereas the two copolymers with longer coro na-forming blocks form body-centered cubic (bcc) gels. SAXS also confirmed that the sols formed at lower copolymer concentrations. are micellar liquid s. Our results for the gel structures are in accord with the observation th at micelles with relatively short coronas behave as hard spheres, and pack in a fee structure, whereas micelles with large coronas act like soft spher es 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 diblock s studied previously by us.