Non-centrosymmetric superlattices in block copolymer blends

Materials with a macroscopic electric polarization display a variety of useful properties, such as piezo- and pyroelectricity and second-order nonlinear optical activity(1). Macroscopic polarization results when dipolar molecules 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-centrosymmetry 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 formation of a non-centrosymmetric structure in an amorphous material through spontaneous self-assembly. Block copolymers are known to form ordered structures 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 copolymer will organize itself into a noncentrosymmetric layered structure in which the layers are occupied by different blocks. The structure is periodic with a length scale of around 60 nm.