Phase formation in architecturally asymmetric coil-brush diblock copolymers

Diblock copolymers are known to self-assemble into a variety of structures, and the effects of molecular weight, volume fraction, compatibility between the blocks, and block chemistry have been extensively studied. The effect of architectural differences between the blocks on phase behavior are less well understood. In this work, five sets of norbornene-based coil-block-bottlebrush copolymers with high conformational asymmetry were synthesized using grafting-through living ring-opening metathesis polymerization with poly(norbornene-exo,exo-dimethyl ester) as the coil block and bottlebrush blocks containing oligomeric poly(ethylene-alt-propylene) side chains. The consequences of changing the volume fraction of the coil block, and the side chain length in the brush block, were studied as a function of temperature using small-angle X-ray scattering (SAXS). Increasing the side chain length of the brush block shifts the phase diagram towards higher compositions of the coil block, while retaining the classical lamellar, gyroid, and cylindrical morphologies. Small-amplitude oscillatory shear rheology was used to estimate the statistical segment lengths of each block, where the conformational asymmetry increases with increasing side chain length.