Non-centrosymmetric superlattices in block copolymer blends

Materials with a macroscopic electric polarization display a variety of use ful properties, such as piezo- and pyroelectricity and second-order nonline ar optical activity(1). Macroscopic polarization results when dipolar molec ules are orientated in the same direction, or when ions are organized in a non-centrosymmetric crystal structure(2). Centrosymmetric molecules have no dipole moment and so cannot generate a macroscopic polarization. Non-centr osymmetry in amorphous materials can be engineered by depositing particular sequences of layers on top of each other, or by applying external fields ( generally electric) to orientate the molecules(3). Here we report the forma tion of a non-centrosymmetric structure in an amorphous material through sp ontaneous self-assembly. Block copolymers are known to form ordered structu res at the microscale owing to segregation of the different blocks(4,5). We show that a mixture of a ternary triblock copolymer and a binary diblock c opolymer will organize itself into a noncentrosymmetric layered structure i n which the layers are occupied by different blocks. The structure is perio dic with a length scale of around 60 nm.