Cubic to Hexagonal Phase Transition Induced by Electric Field

Cubic to Hexagonal Phase Transition Induced by Electric Field


Abstract :

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 of a relatively low dc electric field, ∼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 Et 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. Et 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.