Thermotropic transition from a state of liquid order to a macrolattice in asymmetric diblock copolymers

The disorder-to-order transition in asymmetric diblock copolymers of polystyrene/polyisoprene is studied using small angle x-ray scattering. In between the homogeneously disordered state at high temperature and the bcc ordered array of spheres we find a stable state of liquidlike order between the spherical domains. Quantitative analysis of this order is possible in the framework of the Percus-Yevick theory for hard sphere liquids. The transition to the ordered state occurs at a sphere volume fraction of 0.47. The size of the spheres may be determined in the disordered as well as the ordered state and is found to increase continuously with decreasing temperature.