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

Abstract

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.

Liquid‐to‐solid transitions in nanoparticle‐filled brush block copolymer composites

“…A dual power law expression (Equation 1) is used to model the full dynamic behavior from 0 < τ < τ max where H 0 , H 1 ,and H 2 are constants, τ 1 and τ 2 are relaxation times, and n 1 and n 2 are relaxation exponents. 34 H τ…”

“…44 The morphology of the nanocomposites was also investigated with temperature dependent SAXS to resolve such structure deep into the melt state. We previously reported the temperaturedependent phase behavior in lamellar forming polystyrene-block-poly(ethylene oxide) (PS-b-PEO) dbBBs where microphase separation persists up to T = 150 C. 34 The higher order peak 2q* disappears upon heating through T = 80 C (above T m of the PEO crystals) and the intensity of q* decreases due to electron density differences between crystalline and amorphous PEO domains, indicating a loss of long-range order in the lamellar packing. In the mobile melt state, the PS and PEO domains are microphase segregated, but the lamellar packing is macroscopically disordered.…”

“…Here, the primary mechanism for relaxation is the cooperative movement of internal slip layers (ISL) confined within each microphase separated domain, facilitated by a high density of free chain ends in the center of each layer. 34 The side chains within each block are unentangled and fully relaxed in the melt, which allows the backbone segments to rapidly slide along each other. However, the ISL becomes constrained upon the addition of NPs, which is indicated by the increase in n 1 at φ = 30 wt%.…”

“…[29][30][31][32][33] The relaxation processes in di-block bottle brushes (dbBB) are uniquely supported by a high density of free chain ends in the center of each domain that form internal slip layers (ISL), facilitating rapid relaxation. 34 Therefore, the phase behavior and rheological properties of dbBB/NP composites should be closely related and are of significant interest. The intermolecular interactions between the NPs and target domain places additional constraints on the relaxation of the polymeric chains, seriously impacting the effect of the reduced entanglements that are believed to be essential for the rapid self-assembly.…”

“…Specifically in BBCPs, the dense grafting of side chains suppresses chain entanglements in the melt state, resulting in highly mobile “liquid‐like” rheology 29–33 . The relaxation processes in di‐block bottle brushes (dbBB) are uniquely supported by a high density of free chain ends in the center of each domain that form internal slip layers (ISL), facilitating rapid relaxation 34 . Therefore, the phase behavior and rheological properties of dbBB/NP composites should be closely related and are of significant interest.…”