Electric Field Alignment of Diblock Copolymer Thin Films

Block copolymers self-assemble into arrays of microdomains, e.g. lamellae, cylinders, or spheres, depending on the volume fraction of the components and _N, where _ is the Flory-Huggins segmental interaction parameter and N is the degree of polymerization [1, 2]. The sizes of the microdomains are dictated by the total molecular weight of the copolymer and, hence, are tens of nanometers in size. This size scale cannot be easily achieved using conventional photolithographic techniques and makes block copolymer thin films ideal can- didates for template and scaffolds for the fabrication of nanostructured ma- terials [3-8]. For applications, controlling the microdomain orientation is es- sential. In a copolymer thin film, the grains of the copolymer microdomains are found where the ordering is high locally, but globally there is no preferred orientation of the grains; on average, they are randomly arranged. Various external fields, such as shear, electric and surface fields have been used to manipulate the microdomain alignment in block copolymer thin films. Here, we focus primarily on the electric field alignment of diblock copolymers in thin films a few hundred nanometers in thickness. The preferential interac- tion and segregation of one block to the substrate orients the microdomains parallel to the substrate surface. Electric fields normal to the surface are used to overcome these interfacial interactions and orient the microdomains in the direction of the applied field [9-14]. We will first discuss the influence of the interfacial interaction on the microdomain orientation in a copolymer thin film; then the effect of interfacial interaction on the electric field alignment. We will subsequently discuss the electric field alignment of the lamellar and cylindrical microdomains in thin films and the electric field induced sphere-to- cylinder transition. Finally, we will discuss the effect of the ionic impurities, such as lithium ions, on the electric field alignment of copolymer thin films. Discussion of the electric field alignment of block copolymers in the bulk or solution can be found elsewhere [9-12, 15, 16].