Disordered Bicontinuous Morphology from Frustrated Dumbbell-Shaped ABC Bottlebrush Block Terpolymers

We report the phase behavior of twenty-three poly(ethylene-alt-propylene)-b-poly(d,l-lactide)-b-polystyrene (PEP-b-PLA-b-PS) bottlebrush block terpolymers based on small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) measurements. Polymers were synthesized by sequential ring-opening metathesis polymerization (ROMP) of norbornene end-capped macromonomers with PEP and PS number-average molecular weights M_n = 4.7 and 3.9 kg/mol, respectively, and M_n = 1.5 kg/mol PLA. These architecturally asymmetric block terpolymers produce morphological packing frustration due to the sequence of segment-segment interactions χ_PEP-PLA > χ_PLA-PS ≈ χ_PEP-PS. SAXS and TEM reveal the expected ordered hexagonal and lamellar (both "two-color" and "three-color") morphologies, but additionally, there is an unanticipated bicontinuous disordered morphology across a significant range of the compositional space. Larger PEP and PS side chain lengths relative to PLA, i.e., a "dumbbell" shape, combined with packing frustration due to the higher interfacial energies of the mandatory PEP/PLA interfaces compared to PEP/PS and PLA/PS, apparently stabilize an equilibrium disordered bicontinuous structure even at relatively strong segregation. This study expands the diverse phase behavior available with ABC bottlebrush block terpolymers, while revealing a new approach for producing cocontinuous materials attractive for various applications.