Complex Phase Behavior of Polyisoprene-Polystyrene Diblock Copolymers Near the Order-Disorder Transition

The phase behavior of polyisoprene-polystyrene (PI-PS) diblock copolymers near the order-disorder transition was investigated using dynamic mechanical measurements, transmission electron microscopy, and small-angle X-ray and neutron scattering at polyisoprene volume fractions 0.33 < f_PI < 0.42. Two new ordered morphologies have been documented between hexagonally packed cylinders (HEX) and lamellae (LAM). At low temperatures a hexagonally perforated layered (HPL) microstructure occurs, that transforms into a bicontinuous cubic phase characterized by Ia3d space group symmetry near the order-disorder transition (ODT). The ordered-bicontinuous double diamind (OBDD) phase was not found at any temperature or composition studied. We demonstrate that the bicontinuous Ia3d phase exists at equilibrium over a narrow range of compositions, 0.35 ≲ f_PI ≲ 0.40 bounded by the HEX, LAM, HPL, and disordered states, and is restricted to within about 60 °C of the ODT, suggesting the importance of finite size effects. This cubic phase also exhibits pronounced metastability, remaining intact for extended periods of time after cooling below the Ia3d-HEX or Ia3d-HPL phase boundary. Cooling from above the ODT to below the PS glass transition temperature leads to a vitrified form of the Ia3d state.