Self-Assembly of Shaped ABC Coil-Bottlebrush Block Terpolymers

Abstract

A series of 100 “shaped” ABC coil–brush triblock terpolymers were prepared by sequential living ring-opening metathesis polymerization (ROMP) of 5-norbornene-2,3-exo,exo-dicarboxylic acid dimethyl ester (M) and norbornenyl diester macromonomers P_a and P_b, each bearing two identical side chains that are precise oligomers of poly(ethylene-alt-propylene) containing a,b = 1, 2, 4, or 9 repeat units. These MP_aP_b block terpolymers were designed such that the more polar coil M block melt microphase separates from the mutually miscible P_a and P_b segments (a ≠ b), which form a digitally shaped bottlebrush segment, in order to assess how bottlebrush shape affects block copolymer network phase stability. Thus, samples with total M_n = 7–30 kg/mol, dispersities 1.02 ≤ Đ ≤ 1.04, and volumetric compositions 0.26 ≤ f_M ≤ 0.58 were chosen to target the region where network phases such as the double gyroid (GYR) might be expected. Small-angle X-ray scattering (SAXS) analyses revealed that all of these polymers were well-segregated based on SAXS measurements at 100 °C, with no evidence of proximity to an order–disorder transition. Most samples formed hexagonal (HEX) or lamellar (LAM) phases, with some persistent HEX/LAM coexistence, with a critical f_M-value at the HEX/LAM order-to-order transition that depends on the length and sequence of P blocks. However, the formation of GYR was systematically suppressed by the shaped brush architecture, especially when the longer P block was P₉; this is surprising, given that previously reported MP_a diblocks exhibited significant GYR phase windows. Furthermore, despite the low glass transition temperatures of the blocks (T_g,M = 69 °C, T_g,Pa ≤ –42 °C), there was no evidence of structural refinement over a time scale of hours, indicating very slow ordering kinetics.