Polyisoprene-polystyrene diblock copolymer phase diagram near the order-disorder transition

The phase behavior often polyisoprene-polystyrene (PI-PS) diblock copolymers, spanning the composition range from 0.24 to 0.82 polyisoprene volume fraction (f(PL)), has been studied near the order-disorder transition (ODT). Dynamic mechanical spectroscopy, transmission electron microscopy, and neutron and X-ray scattering have been used to characterize phase transition temperatures and ordered state symmetries. Five distinct microstructures were observed for this chemical system: spheres, hexagonally packed cylinders (HEX), lamellae (LAM), hexagonally perforated layers (HPL), and a bicontinuous cubic phase having an Ia (3) over bar d space group symmetry. The bicontinuous Ia(<3>over bar>d phase only occurs in the vicinity of the ODT between the HEX and LAM states at compositions of 0.65 less than or similar to f(PI) less than or similar to 0.68 and 0.36 less than or similar to f(PI) less than or similar to 0.39 (prior report). Farther from the ODT, within these composition ranges, the HPL phase occurs. We did not find the ordered bicontinuous double diamond (OBDD) morphology at any composition or temperature studied, and the overall phase diagram is qualitatively different from those reported previously for PI-PS block copolymers.