Effect of Grafting Density on Self-Assembled Lamellar Phases of Core–Shell Bottlebrushes

This report examines the melt self-assembly behaviors of core–shell bottlebrush polymers (csBBs), in which a single AB diblock copolymer decorates each backbone repeat unit. To access these nonlinear polymer architectures, four norbornyl end-functionalized, symmetric composition poly(ε-decalactone)-block-poly(rac-lactide) (DL) diblock copolymers (Mn = 5.9–7.6 kg/mol, Đ = 1.20–1.29, with fL = 0.49–0.51) were first synthesized by sequential ring-opening polymerizations (ROPs). Living ring-opening metathesis polymerization (ROMP) of these DL macromonomers produces narrow dispersity csBBs (Đ = 1.02–1.18) with backbone degrees of polymerization Nbb = 6–37. For csBBs of a given DL macromonomer, small-angle X-ray scattering (SAXS) analyses reveal that the order-to-disorder transition temperatures (TODT’s) of their microphase-separated lamellar mesophases increase with increasing Nbb. From these data, the dependence of the critical macromonomer arm segregation strength for microphase separation (χNarm)ODT on Nbb is identified. Comparisons of these results with reports on related nonlinear block polymers suggest that the csBB architecture reduces the free energy penalty for chain arrangement into the self-assembled lamellar morphology, while brush grafting density directs the extent of side chain stretching, with implications for microphase-separated melt stability and the observed domain (d) spacings.