Precise hierarchical self-assembly of multicompartment micelles

Abstract

Hierarchical self-assembly offers elegant and energy-efficient bottom-up strategies for the structuring of complex materials. For block copolymers, the last decade witnessed great progress in diversifying the structural complexity of solution-based assemblies into multicompartment micelles. However, a general understanding of what governs multicompartment micelle morphologies and polydispersity, and how to manipulate their hierarchical superstructures using straightforward concepts and readily accessible polymers remains unreached. Here we demonstrate how to create homogeneous multicompartment micelles with unprecedented structural control via the intermediate pre-assembly of subunits. This directed self-assembly leads to a step-wise reduction of the degree of conformational freedom and dynamics and avoids undesirable kinetic obstacles during the structure build-up. It yields a general concept for homogeneous populations of well-defined multicompartment micelles with precisely tunable patchiness, while using simple linear ABC triblock terpolymers. We further demonstrate control over the hierarchical step-growth polymerization of multicompartment micelles into micron-scale segmented supracolloidal polymers as an example of programmable mesoscale colloidal hierarchies via well-defined patchy nanoobjects.

Figure 1. Funnel concept for the directed hierarchical self-assembly of ABC triblock terpolymers in solution.

Minimization of kinetic traps is realized by step-wise reduction of conformational freedom via pre-assembled subunits. In the first step, triblock terpolymers self-assemble into subunits, that is, pre-assembled intermediates, in a non-solvent for the middle block B (black), leaving different corona conformations of A (red) and C (grey). In the second step, the collapse of block A is triggered by exposure to a non-solvent for A and B, and higher-level assembly of the subunits occurs into the final superstructure. The process is accompanied by a refinement of the initial corona structure of A and C, and the conformational space narrows down into MCM superstructures of low polydispersity (Scale bar is 50 nm).

Figure 2. Structures of the linear ABC triblock terpolymers.

S, polystyrene; B, polybutadiene; M, poly(methyl methacrylate); T, poly(tert-butyl methacrylate; D, poly(2-(dimethylamino)ethyl methacrylate); V, poly(2-vinylpyridine); C, poly(2-(cinnamoyloxy)ethyl methacrylate). In each case, the red block builds up the corona.

Figure 3. Spherical and linear MCMs formed by SBMs with various core volume ratios (V_PS/V_PB).

MCMs were prepared by dialysis of subunits with mixed- or compartmentalized PS/PMMA corona and PB core in DMAc into acetone/isopropanol (60/40 v/v). Staining was achieved with OsO₄ (PB black, PS grey, PMMA corona invisible). Subunits: (a) Weakly phase-segregated corona of PS and PMMA of SBM9 subunits with a PB core in DMAc. (b,c) DLS CONTIN plots of pre-assembled subunits and final MCMs of SBM3 and SBM9. MCMs: (d) SB₂ (BSB) 'hamburgers' of SBM6; (e) SB₃ 'clovers' of SBM3; (f) SB₄ 'Maltese crosses' of SBM4; (g) SB₇ 'footballs' of SBM5; (h,i) SB_x 'footballs' of SBM1 and SBM2. The inset (lower right, i) depicts the Fourier transform of the TEM micrograph. (j) SBSBS=(SBS)₂ 'double burgers' of SBM7 (see also Supplementary Fig. S3); (k) (SBS)_x linear MCM colloidal polymers of SBM9. The kinks in the colloidal polymers will be discussed below (Scale bars are 200 nm and 50 nm in the insets).

Figure 4. Detailed mechanism for the preparation and directed hierarchical self-assembly of well-defined MCMs.

First, the ABC triblock terpolymers are forced into (a) corona MCMs via dissolution in a non-solvent for B (termed subunits). Upon dialysis into a non-solvent for A and B, these subunits self-assemble further via a refinement of the corona structure (b,c) into various MCMs with well-defined number of patches (d,e). (f) Under suitable conditions, the colloidal polymerization into segmented worms can be triggered (TEM images, OsO₄-stained; scale bars are 50 nm PS grey, PB black and PMMA not visible due to e-beam degradation).

Figure 5. Mesoscale polymerization of linear MCMs and subunit exchange of spherical MCMs promoted by changing the corona volume.

The red corona chains emerge from the black compartments, but are mostly omitted for clarity (TEM images, OsO₄-staining except (b) RuO₄, acetone/isopropanol fractions are given in v/v). (a) Scheme for reversible mesoscale colloidal polymerization (SBM9). Note that the branching points are composed of (SBS)₃ centres. (b) SBS-refined subunits in acetone/isopropanol (90/10), (c) (SBS)₂ 'double burgers' formed by two SBS in acetone/isopropanol (80/20) and their polymerization (d) in acetone/isopropanol (50/50). (e) Scheme for reversible exchange of refined subunits in spherical MCMs (SBM3). (f–h) In situ switching of spherical MCMs. (f) 'Hamburgers' and refined subunits successively merge into (g) 'clovers' and (h) 'footballs' triggered by reduction of the solvent quality for the corona (addition of isopropanol from 90/10 to 60/40 to 50/50). Refined subunits clearly reappear upon the expansion of the corona (addition of acetone from 50/50 to 90/10). Scale bars are 200 nm and 50 nm in insets.

Figure 6. MCMs of different triblock terpolymers.

(TEM images, OsO₄-stained if not mentioned otherwise; scale bars are 200 nm and 50 nm in insets). (a) SBV in acetone/isopropanol 20/80 (PB black, PS bright grey, P2VP corona grey). (b) SBT2 in ethanol (PB black, PS grey, PtBMA invisible due to e-beam degradation). (c) TVB1 in dodecane (I₂ staining; P2VP black, PtBMA white). (d–f) pH-programmable colloidal polymerization of TCD in water: (d) pH=3 completely protonated, (e) pH=6 partially protonated and (f) pH=10 deprotonated PDMAEMA corona (RuO₄ staining; PCEMA black, PtBMA bright grey, PDMAEMA corona not visible).