Mesoscopic Morphologies in Frustrated ABC Bottlebrush Block Terpolymers

Bottlebrush block polymers, characterized by densely grafted side chains extending from a backbone, have recently garnered significant attention. A particularly attractive feature is the accessibility of ordered morphologies with domain spacings exceeding several hundred nanometers, a capability that is challenging to achieve with linear polymers. These large morphologies make bottlebrush block polymers promising for various applications, such as photonic crystals. However, the structures observed in AB diblock bottlebrushes are generally limited to simple lamellae and cylindrical phases, which restricts their use in many applications. In this study, we synthesized a library of 50 ABC bottlebrush triblock terpolymers, poly(DL-lactide)-b-poly(ethylene-alt-propylene)-b-polystyrene (PLA-PEP-PS), spanning a wide range of compositions using ring-opening metathesis polymerization (ROMP) of norbornene-functionalized macromonomers. This constitutes a frustrated system, in that the mandatory internal interfaces (PLA/PEP and PEP/PS) have larger interfacial energies than PLA/PS. We systematically explored phase behavior using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Morphological characterization revealed a series of intriguing mesoscopic structures, including layered microstructures, core-shell hexagonally packed cylinders (CSHEX, plane group p6mm), alternating tetragonally packed cylinders (ATET, plane group p4mm), and rectangular centered cylinders-in-undulating-lamellae (RCCUL, plane group c2mm). Adjustments in molecular weight resulted in a wide range of unit cell dimensions (exemplified by RCCUL), from 40 nm to over 130 nm. This work demonstrates that multiblock bottlebrushes offer promising opportunities for developing materials with diverse structures and a broad range of domain dimensions.