We developed a synthetic protocol for tapered bottlebrush copolymers using ring-opening metathesis polymerization (ROMP). This new bottlebrush copolymer architecture is prepared by copolymerizing exo- and endo-norbornene functionalized macromonomers. The vastly different ROMP reactivities of the two norbornene configurations lead to a unique compositional profile, where homopolymer domains at both ends of the molecule are separated by a gradient compositional transition. A model system comprised of polystyrene (PS) and polylactide (PLA) side chains was designed to gain insight into the effect of gradient interface on melt state polymer self-assembly. A series of near symmetrical PS-taper-PLA bottlebrush copolymers was synthesized. Self-assembled samples were analyzed by small-angle X-ray scattering (SAXS) and revealed a morphology with a hexagonal symmetry. Inspection by transmission electron microscopy evidenced the formation of a rare perforated lamellae morphology. Shear aligned samples, on the other hand, exhibited a well-ordered cylindrical morphology, confirmed by 2D SAXS and scanning electron microscopy after selective etching of the PLA domain. Such molecular packing is drastically different from the self-assembly of bottlebrush block and random copolymer analogs with similar composition and dimensions. Domain spacings of PS-taper-PLA bottlebrush copolymers increased linearly with the backbone length and were consistently smaller than pitch sizes obtained for their PS-block-PLA bottlebrush copolymers analogs, which suggested a non-perpendicular molecular orientation at the domain interface. SAXS studies also revealed an unusual and large d-spacing changes with temperature, which was observed only for tapered bottlebrush systems. Thus, gradient interface in bottlebrush copolymers has a strong influence on molecular packing and orientation unlike their linear copolymer counterparts. In summary, the newly developed tapered bottlebrush copolymer system exhibits morphological behavior that is different from previously established copolymer architectures and provides a new avenue to direct molecular assembly.

Synthesis and melt self-assembly of tapered bottlebrush copolymers

255th National Meeting (2018)

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