Self-assembled complex micelle phase stability in ABA-type triblock copolymers and related core-shell bottlebrushes

We report the synthesis and temperature-dependent morphologies of a series of polylactide-block-poly (ϵ-decalactone)-block-polylactide (LDL) triblock copolymers with Mn=16.0-18.1kg/mol and volume fractions fL=0.27-0.31 and associated core-shell bottlebrush (csBB) polymers, which derive from enchaining LDL triblocks through a polymerizable midchain functionality. While the LDL triblocks form micellar Frank-Kasper A15 and σ phases due to the conformational asymmetry of this monomer pair, the csBB morphologies sensitively depend on the backbone degree of polymerization (Nbb). At low Nbb values, micellar Frank-Kasper phases with the brush backbone situated in the matrix domain are stable, albeit with a modest reduction in the mean interfacial curvature evidenced by a σ to A15 order-to-order transition. However, larger Nbb values drive csBBs to form hexagonally packed cylinders phases. This Nbb-dependent phase behavior is rationalized in terms of a star-to-bottlebrush transition. At low Nbb values, the csBBs are akin to star polymers with pointlike junctions that can support complex micelle packings. As Nbb increases, the csBBs adopt cylindrical molecular geometries with extended backbones situated in the matrix domain that prefer hexagonally packed cylinders morphologies.