Electrostatic control of block copolymer morphology

Journal name:
Nature Materials
Volume:
13,
Pages:
694–698
Year published:
DOI:
doi:10.1038/nmat4001
Received
Accepted
Published online

Energy storage is at present one of the foremost issues society faces. However, material challenges now serve as bottlenecks in technological progress1. Lithium-ion batteries are the current gold standard to meet energy storage needs; however, they are limited owing to the inherent instability of liquid electrolytes1, 2. Block copolymers can self-assemble into nanostructures that simultaneously facilitate ion transport and provide mechanical stability. The ions themselves have a profound, yet previously unpredictable, effect on how these nanostructures assemble and thus the efficiency of ion transport3. Here we demonstrate that varying the charge of a block copolymer is a powerful mechanism to predictably tune nanostructures. In particular, we demonstrate that highly asymmetric charge cohesion effects can induce the formation of nanostructures that are inaccessible to conventional uncharged block copolymers, including percolated phases desired for ion transport. This vastly expands the design space for block copolymer materials and is informative for the versatile design of battery electrolyte materials.

At a glance

Figures

  1. Schematic of block copolyelectrolytes and discussion of neutral block copolymers and charge solubility effects.
    Figure 1: Schematic of block copolyelectrolytes and discussion of neutral block copolymers and charge solubility effects.

    a, Schematic of a neutral diblock copolymer (top) of length N with blocks A and B that interact via an interaction parameter χ. A diblock copolyelectrolyte (bottom) includes charged monomers along the A-block of length fAN, with mobile and oppositely charged counterions nearby to maintain overall charge neutrality. b, Canonical phase diagram of neutral BCPs, which form a disordered phase (D) as well as hexagonal (H), lamellar (L), and inverse-hexagonal (H) nanostructures depending on the relative length of the A-block (if the immiscibility χ is sufficiently high). c,d, The influence of charge due to ion entropy and solubility effects but ignoring the (important) presence of electrostatic cohesion, when ΓA = 17.1 and ΓB = 27.8. (εr, A = 6.5 and εr, B = 4.0, respectively, for ion radii a = 0.25 nm). The effects can be captured by shifting χ in a manner proportional to the charge fraction, as shown in d, where the original phase boundaries (shown in c) collapse onto the uncharged phase boundaries for the shifted value χeff, using C = −160.0.

  2. Effect of charge cohesion on nanostructure phase behaviour.
    Figure 2: Effect of charge cohesion on nanostructure phase behaviour.

    a, Electrostatic cohesion between the charged A-blocks and the counterions. If the coupling parameter Γ > 1, the components are electrostatically correlated in a liquid-like ordered structure, having profound consequences for the phase diagram. We note that this is asymmetric; Coulombic cohesion manifests only in the charged A-block and not in the uncharged B-block. b, Nanostructure phase behaviour for a number of Coulombic interaction strengths Γ (black and red curves denote phase boundaries). At low values of Γ, nanostructure formation is suppressed by the entropy of the counterions that suppress demixing, whereas nanostructure formation is enhanced at higher values of Γ owing to Coulombic cohesion in the A-rich phases. We note that ordered states are observed even at χ = 0 for Γ ≥ 17.1. These results are contrasted with the uncharged phase behaviour (light red and grey) and the high-Γ limit of previous theories (orange, described in Supplementary Section V), which are both known to compare unfavourably with experiments3, 21.

  3. Charge control of percolating nanostructures.
    Figure 3: Charge control of percolating nanostructures.

    Comparison between two inverse-hexagonal nanostructures in the χ, fA-plane. Electrostatic cohesion opens the pathway to a different type of inverse-hexagonal structure, where the continuous phase is formed by the charged minority component—shown here for Γ = 17.1 and a charge fraction of the A-block of 15% (green cross). The chemical and dielectric properties of BCPs can only allow for those hexagonal phases where the continuous phase is formed by the majority component (black cross). These are schematically shown at the bottom, along with example paths for ion transport through the charged A phase (orange). Transport in charged systems is through a percolating minority phase that may possess transport properties associated with the A/B interface, whereas the uncharged system is primarily a bulk majority phase percolating around minority obstructions.

  4. Effect of charge fraction on nanostructure phase behaviour.
    Figure 4: Effect of charge fraction on nanostructure phase behaviour.

    ad, Effect of increasing the charge fraction of the A-block on the phase diagram of a two-dimensional block copolymer, with morphologies labelled (H, hexagonal; L, lamellae; D, disordered). Γ = 17.1 and N = 40. Even small values of the charge fraction significantly change the phase diagram, shifting all morphologies such that nanostructure formation is enhanced at low values of fA, but suppressed at higher values of fA. Neutral BCP phases (black) are plotted for comparison. The purple cross denotes a location immediately outside the ordered region that would be driven to nanostructure formation on including charge. At increasing values of the charge fraction, the entire set of morphologies is realized until the system once again becomes disordered at >20% charge on the A-block. e, Analogous behaviour in three dimensions, keeping χN = 18, N = 20 and fA = 0.25 constant but changing the charge fraction. Body-centred-cubic spheres, hexagonally packed cylinders, lamellae, and inverse hexagonally packed cylinders are all observed in between disordered states for charge fractions ranging between 2.5 and 17%.

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C.E.S., J.W.Z. and M.O.C. designed the research. C.E.S. and J.W.Z. developed the theoretical methods with input from M.O.C. C.E.S. performed the calculations. All authors contributed to the interpretation of the data and wrote the manuscript.

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