Core-Shell Gyroid in ABC Bottlebrush Block Terpolymers

Block polymer self-assembly provides a versatile platform for creating useful materials endowed with three-dimensional periodic network morphologies that support orthogonal physical properties such as high ionic conductivity and a high elastic modulus. However, coil configurations limit conventional linear block polymers to finite ordered network dimensions, which are further restricted by slow self-assembly kinetics at high molecular weights. A bottlebrush architecture can circumvent both shortcomings owing to extended backbone configurations due to side chain crowding and molecular dynamics substantially free of chain entanglements. However, until now, network morphologies have not been reported in AB bottlebrush block copolymers, notwithstanding favorable mean-field predictions. We explored the phase behavior by small-angle X-ray scattering of 133 poly(ethylene-alt-propylene)-b-polystyrene (PEP-PS) diblock and PEP-PS-PEO triblock bottlebrush copolymers prepared by ring-opening metathesis polymerization (ROMP) of norbornene-functionalized poly(ethylene-alt-propylene) (PEP), poly(styrene) (PS), and poly(ethylene oxide) (PEO) macromonomers with total backbone degrees of polymerization N_bb between 20 and 40. The PEP-PS diblocks exhibited only cylindrical and lamellar morphologies over the composition range of ca. 30-70%. However, addition of variable-length bottlebrush PEO blocks to diblocks containing 30-50% PS led to the formation of a substantial core-shell double gyroid (GYR) phase window containing 20 bottlebrush triblock specimens, which is the focus of this report. Encouragingly, the GYR unit cell dimensions increased as d ∼ N_bb^0.92, portending the ability to access larger network dimensions than previously obtained with linear AB or ABC block polymers. This work highlights extraordinary opportunities associated with applying facile ROMP chemistry to multiblock bottlebrush polymers.