Power Law Relaxations in Lamellae Forming Brush Block Copolymers with Asymmetric Molecular Shape

We report the linear viscoelasticity of densely grafted poly­(styrene)-block-poly­(ethylene oxide) (PS-b-PEO) diblock bottlebrush block copolymers (dbBB) of equal mass fraction over a wide range of backbone degree of polymerization (Nbb = 21–119). The difference in side chain length (PS Mn = 2.9 kg/mol, PEO Mn = 5 kg/mol) produces an asymmetry between the molecular shape of the two blocks despite their equal mass fractions. The dbBBs rapidly self-assemble into lamellar morphologies upon thermal annealing. Increasing Nbb results in an increase of domain spacing from d0 = 29 to 90 nm. In the microphase separated melt state, dbBBs are thermorheologically simple and remain unentangled up to large molecular weight (Mw > 500 kg/mol). Oscillatory shear rheology data shows distinct power law relationships analogous to critical gels across a wide range of time scales. The viscoelasticity is expressed by a dual power law relaxation time spectrum H(τ), consisting of relaxation processes at short (n1) and long (n2) time scales. Scaling on short time scales (n1 ≈ 0.83) is attributed to the cooperative mobility of internal slip layers (ISLs) confined within the microphase separated domains. Slipping is facilitated by a high concentration of free chain ends in the middle of each domain. Longer time scales (n2 ≈ 0.67) are dominated by the microphase separation, which is globally disordered. The results suggest a weakly percolating structure with rapid dynamic rearrangements of bottlebrushes within the PS/PEO interface