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

Abstract

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 (M_n = 5.9–7.6 kg/mol, Đ = 1.20–1.29, with f_L = 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 N_bb = 6–37. For csBBs of a given DL macromonomer, small-angle X-ray scattering (SAXS) analyses reveal that the order-to-disorder transition temperatures (T_ODT’s) of their microphase-separated lamellar mesophases increase with increasing N_bb. From these data, the dependence of the critical macromonomer arm segregation strength for microphase separation (χN_arm)_ODT on N_bb 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.