Michelle Mok, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, Christopher J. Ellison, Dept. of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, Wesley Burghardt, Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Room E136, Evanston, IL 60208, and John M. Torkelson, Chemical and Biological Engineering and Materials Science and Engineering, Northwestern University, 2145 Sheridan Rd, TECH E136, Evanston, IL 60208.
Gradient copolymers are a class of polymers that exhibit a gradual change in composition along the entire chain from mostly A-monomer to mostly B-monomer. Theoretical work has predicted that gradient copolymers organize into sinusoidal composition profiles rather than the step-like profiles seen for block copolymers. Their unique combination of composition profiles and morphology in the segregated state raises questions regarding the extent to which gradient copolymers with composition gradients along the whole copolymer backbone will exhibit the signatures of block copolymer nanophase segregation such as long-range ordering and shear alignment. Here, small-angle x-ray scattering and low amplitude oscillatory shear studies were performed to investigate the impact of gradient design and comonomer choice on their unique ordering. Samples showed a variety of temperature-dependent, non-terminal behaviors consistent with their chain architecture relative to block copolymers, indicating highly tunable nanophase segregation. Scattering results were consistent with the degree of nanophase segregation observed via rheology. In addition, it was demonstrated for the first time that application of high amplitude oscillatory shear induced domain shear alignment in a manner similar to block copolymers, even though gradient copolymers do not possess distinct domain boundaries.