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

• Published in: Macromolecules Vol. 52; no. 4; pp. 1557 - 1566
• Main Authors: Yavitt, Benjamin M, Fei, Hua-Feng, Kopanati, Gayathri N, Winter, H. Henning, Watkins, James J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.02.2019
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.8b01843

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 (N bb = 21–119). The difference in side chain length (PS M n = 2.9 kg/mol, PEO M n = 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 N bb results in an increase of domain spacing from d 0 = 29 to 90 nm. In the microphase separated melt state, dbBBs are thermorheologically simple and remain unentangled up to large molecular weight (M w > 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 (n 1) and long (n 2) time scales. Scaling on short time scales (n 1 ≈ 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 (n 2 ≈ 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.