Morphological Behavior of Compositionally Gradient Polystyrene-Polylactide Bottlebrush Copolymers

Morphological behavior of PS-PLA bottlebrush copolymers with a compositional gradient along the backbone was investigated by small-angle X-ray scattering (SAXS) analysis and compared to that of their block copolymer analogs. Side chain-symmetric gradient copolymers with varying volume fractions were prepared by one-step ring-opening metathesis polymerization of the corresponding exo- A nd endo-norbornene-functionalized macromonomers of similar lengths. The morphological map constructed using the SAXS data revealed a wide cylindrical morphology window, including for symmetric compositions, well-ordered lamella morphologies at very low PS volume fractions, and the formation of a rare bicontinuous gyroid morphology. In addition to the highly asymmetric nature of the morphology diagram, the domain spacings obtained for gradient bottlebrush copolymers were significantly smaller (by 30-40%) than the corresponding bottlebrush block copolymer analogs, which was attributed to a nonperpendicular orientation of the gradient bottlebrush backbone at the domain interface. Side chain-asymmetric gradient bottlebrush copolymers were synthesized from PS and PLA macromonomers of different lengths and were demonstrated to assemble into well-ordered cylindrical and lamella morphologies. The results of these studies demonstrate that the gradient interface plays an important role in determining molecular packing during bottlebrush copolymer self-assembly. A rich morphological behavior of the gradient bottlebrush copolymers combined with their "user-friendly" one-step synthesis provides a robust and versatile platform for nanostructured material-design and fabrication.