Cubic to Hexagonal Phase Transition Induced by Electric Field

The possibility of electric field induced phase transitions in soft matter systems was studied by means of small-angle X-ray (SAXS) and neutron (SANS) scattering measurements. By dissolving a diblock copolymer PS-b-PEP (polystyrene-block-poly(ethylene-co-propylene)) in a mixture of cyclohexane (CH) and dimethylformamide (DMF), it was possible to create a liquid 3D cubic structure in which spherical microdomains of DMF were embedded into a liquid CH (major component) matrix with the liquid liquid interfaces covered by PS-b-PEP diblock copolymer chains. When sited under an external electric field, the experimental SAXS and SANS results revealed that the initial self-organized 3D cubic structure is converted into an hexagonal arrangement. The order-to-order transition was reached by the application or a relatively low de electric field, similar to 1.25 kV/mm. The electric field generates dipole moments in DMF-rich spherical microdomains that are deformed and further interconnected, leading to the formation of the hexagonal packed cylinders. The electric field strength E-1 needed to induce such transition depends on the magnitude of the generated dipole moment in the DMF-rich spherical microdomains and hence depends on their size and dielectric contrast irrespective of the surrounding liquid. The latter must have the lower dielectric constant for the transition to occur. E-1 also increases with increasing block copolymer concentration. The chain statistics does not change at the transition and always corresponds to that of a polymer in good solvent. The HEX-cylinders phase developed under external electric field is unstable, and as soon as the field is switched off, the cylinders undergo an order-to-order transition back to the cubic phase. Finally, another HEX-cylinders phase thermodynamically stable without electric field was created by dissolving a PS-b-PI diblock copolymer (polystyrene-block-polyisoprene) in a CH-DMF mixture. In this last case, the structure essentially does not feel the presence of an electric field of the same magnitude.