Determining Lamellar Structure with Soft X-ray Reflectivity

Nanostructured polymer thin films have been studied for a variety of applications including electronic transistors, membranes for separations, and bit-patterned media. The formation of the nanostructure is dependent on three polymer properties: the composition of the block copolymer, f_A, the interaction parameter between the homopolymer components, χ, and overall chain length of the polymer, N. While composition and chain length can be easily tuned during polymer synthesis, χ is dependent on copolymer chemistry and, therefore, is difficult to tune. We have devised a model block copolymer system to systematically tune χ through use of modular A-b-(B-r-C) copolymers. The nanostructure can be tuned by changing the chemical identities of blocks B and C, as well as the volume fraction of component B to C, φ. Through resonant soft X-ray reflectivity (RSoXR), we have determined the effect of φ on the self-assembly of lamellar thin films through measurements of the interface width. The thin-film measurements are compared with predictions from bulk using strong segregation theory. Our results provide insights into the effect of molecular structure on block copolymer self-assembly to help enable predictive design of nanostructured materials.