Synthesis of AB_n-type colloidal molecules by polymerization-induced particle-assembly (PIPA)

Abstract

Conventional synthesis of colloidal molecules (CMs) mainly depends on particle-based self-assembly of patchy building blocks. However, direct access to CMs by the self-assembly of isotropic colloidal subunits remains challenging. Here, we report the mass production of AB_n-type CMs by polymerization-induced particle-assembly (PIPA), using a linear ABC triblock terpolymer system. Starting from diblock copolymer spheres, the association of spheres takes place in situ during the polymerization of the third block. The third blocks aggregate into attractive domains, which connect spheres into CMs. The stability of CMs is ensured, as long as the conversions are limited to ca. 50%, and the pH is low. The valence of AB_n-type CMs (n = 2–6) is determined by the volume ratio of the polymer blocks. By tuning the volume ratio, 78.5% linear AB₂-type CMs are yielded. We demonstrate that polymerization-induced particle-assembly is successful for the scalable fabrication of AB_n-type CMs (50 g L⁻¹), and can be easily extended to vastly different triblock terpolymers, for a wide range of applications.

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Introduction

Colloidal molecules (CMs) are well-defined colloidal clusters with precise symmetry of molecular structures and have drawn extensive interest because of their unique properties^1,2 and potential as building blocks for low-coordination open structures.^3,4 With compartmental functional domains and directional interaction, CMs are promising in biological and medical applications,^5,6 self-assembly,^7,8 physical chemistry,^9–11 and materials science and technology.^12–14 Through particle-based self-assembly, CMs with molecule-like symmetries are enabled using monodisperse patchy building blocks with a suitable shape or interaction anisotropy. There are continuing efforts to develop methods toward patchy building blocks,^15,16 including colloidal crystal templating,¹⁷ seeded emulsion polymerization,¹⁸ assembly of oppositely charged particles,^19–21 DNA origami,^22–24 self-assembly of block copolymers,^25–28 and emulsion solvent-evaporation methods.^29–31 However, these methods struggle in fabricating controllable patches with desired size (10–100 nm). In this size regime, the synthesis of monodisperse building block is difficult, and irregularity causes defects which amplify throughout the hierarchical self-assembly. Therefore, efficient access to CMs remains challenging.

Over the past few years, self-assembly of block copolymers (BCPs) has emerged as an elegant bottom-up technique to produce soft nanoparticles with controllable structures, sizes, and functions.^32–34 The polymerization-induced self-assembly (PISA) technique has experienced great success in the mass production of self-assembled BCPs, where the synthesis and self-assembly of BCPs are achieved simultaneously.^35–41 Inspired by PISA, we envision that the synthesis of well-defined CMs might also be realized by chemical synthesis and self-assembly in one-step, without the fabrication and purification of patchy colloidal building blocks. Using isotropic diblock copolymer spheres as seeds, the polymerization of third blocks produces attractive patches on spheres, which combine colloids into anisotropic CMs in situ. Therefore, CMs are achieved directly via seeded polymerization, without the preparation and purification of patchy intermediates. We address this strategy as polymerization-induced particle-assembly (PIPA).

Herein, we demonstrate a direct approach for the mass production of BCP CMs based on dispersion RAFT polymerization. We perform the polymerization using spherical seeds of poly(N,N-dimethylaminoethyl methacrylate)-b-poly(benzyl methacrylate) (PDMA-b-PBzMA). The polymerization of 2-hydroxypropyl methacrylate (HPMA) produces a linear PDMA-b-PBzMA-b-PHPMA triblock terpolymer. Thus, PHPMA domains function as attractive patches and connect colloid atoms (CAs) into AB_n-type CMs in situ. The stability of CMs is ensured by limiting conversions to ca. 50%, and the pH is ∼ 2. Without post-purification, PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ yields 78.5% AB₂ CMs as prepared, in concentrated solution (50 g L⁻¹). The polymer-based AB₂-type CMs are intrinsically self-assembled, soft and programmable, offering the possibility to act as subunits for higher-level structures. We have realized the large-scale preparation of AB_n-type CMs via polymerization-induced self-assembly of spherical colloids, which is an efficient strategy achieving CMs and other hierarchical nanoparticles (HNPs) by synthesis and self-assembly in one step. This strategy can be readily applied to vastly different block copolymer systems, which would largely enrich the functional complexity of CMs and broaden their applications.

Results and discussion

Scheme 1 shows the synthesis of AB₂-type CMs by aqueous seeded RAFT polymerization. Spherical PDMA₇₀-b-PBzMA₉₆ seeds are produced by dispersion RAFT polymerization of BzMA in ethanol, using a PDMA macro-chain transfer agent. The PDMA₇₀-b-PBzMA₉₆ seeds are then transferred into water by dialysis, and a desired amount of aqueous HCl is added to adjust pH to ∼ 2. The protonation of the PDMA stabilizer provided electrostatic repulsion for the stabilization of CMs.^42,43 The aqueous seeded polymerization of HPMA generated a PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ triblock terpolymer, which self-assembles hierarchically into AB₂-type CMs.

Scheme 1 The synthesis of AB₂-type CMs through polymerization-induced particle-assembly (PIPA).

Fig. 1a shows the PDMA₇₀-b-PBzMA₉₆ spheres with a core diameter of 31 nm. The polymerization of HPMA produces a linear PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ triblock terpolymer (Đ = 1.59), as studied by ¹H NMR and SEC (Fig. S3 and S4‡). A relatively high dispersity is due to the relatively low blocking efficiency caused by the embedding of CTA in PBzMA cores⁴⁴ and the crosslinking of PHPMA.⁴⁵ The growth of PHPMA blocks leads to the shift of particle size from 31 nm to 52 nm, indicating the formation of nanoparticle clusters (Fig. 1b). The cryo-TEM image (Fig. 1e) shows well-defined three-segment clusters (aka AB₂-CMs), resulting from seeded polymerization. We can find two dark grey CAs on each end as peripheral CAs, with the same size of PDMA₇₀-b-PBzMA₉₆ CAs (31 nm). The central CAs are much brighter in observation, suggesting a different chemical composition of middle CAs from cap CAs.

Fig. 1 (a) TEM image of PDMA₇₀-b-PBzMA₉₆ CAs after PTA-staining of the PDMA corona with the inset schematic structure. (b) DLS profiles of PDMA₇₀-b-PBzMA₉₆ CAs (black) and PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ CMs (red). (c and d) TEM image of PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ CMs after PTA-staining and RuO₄-staining, respectively; inset shows the schematic structure of AB₂-type CM. (e) Cryo-TEM image of CMs. (f) Distribution of AB_n-type CMs with varied valence numbers (N) obtained by statistically analysing 735 random particles by cryo-TEM. The valence number (N): 1, 2, 3, and 4+ correspond to AB₁, AB₂, AB₃, and AB_n (n ≥ 4), respectively. All the scale bars are 50 nm.

The composition of AB₂ CMs is further confirmed by TEM characterization of samples stained with RuO₄ and phosphotungstic acid (PTA), respectively. Fig. 1c shows that PDMA is located at the surface of peripheral CAs (PTA staining: PDMA black, PBzMA grey, and PHPMA not visible). And Fig. 1d reveals that satellite CAs are comprised of PBzMA (RuO₄ staining: PBzMA black, and PDMA and PHPMA not observable). Thus, we confirmed that central CAs were composed of newly formed PHPMA blocks. As described above, we have achieved PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ AB₂ CMs in situ during the polymerization, and the bonding angle of B-A-B is essentially 180° (Fig. S5‡), in analogy to the linear molecule of CO₂.

Notably, PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ forms the major product of AB₂-type CMs (78.5%), with relative distribution shown in Fig. 1f. The sample was collected and analyzed as prepared, without multi-step purification. And the aqueous dispersion is more concentrated (50 g L⁻¹) than previously reported selective solvents (5 g L⁻¹).^25,46 The above experimental results demonstrate that this approach is robust and reliable for the mass preparation of AB₂-type CMs, which is especially beneficial for scalable applications.

The polymerization conditions for CM synthesis were optimized after kinetic study. Targeting PDMA₇₀-b-PBzMA₉₆-b-PHPMA₂₀₀ with a 10 wt% solid content, the polymerization reached 71.1% monomer conversion at 1.5 h (Fig. S7‡). And at 78.3% conversion (2 h), a mixture of AB₄ and large clusters was obtained. When the polymerization reached 90.7% conversion (4 h), large clusters were obtained possibly due to inter-cluster association and crosslinking between large PHPMA patches. To avoid clustering, we synthesize CM samples with the control of monomer conversion at ca. 50%.

A series of PDMA-b-PBzMA-b-PHPMA triblock terpolymer samples showed the morphology evolution of the AB_n-type CMs. PDMA₇₀-b-PBzMA₉₆-b-PHPMA₂₀ remains as symmetric spheres (Fig. 2a). This is understandable because PHPMA is too short to form visible domains by phase separation. For PDMA₇₀-b-PBzMA₉₆-b-PHPMA₄₆ with a longer PHPMA chain, 37.8% AB₂ CMs coexist with 58.3% spherical colloids (Fig. 2b). This is because, as the PHPMA domain exceed a critical size during continuing polymerization, the PDMA chains that stabilize the colloids are no longer able to cover them. As a result, the colloids become unstable due to largely increased surface energy (as will be discussed in detail later) and tend to bind together and form CMs to minimize the surface. Only a small number of Janus intermediates are captured in samples with a larger molecular weight (Fig. S6‡). Then AB₂ dominates PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀ with a purity of 78.5% (Fig. 2c). Further growth of PHPMA leads to AB_n-type CMs with n larger than two. PDMA₇₀-b-PBzMA₉₆-b-PHPMA₁₀₉ generates a mixture of 50.2% AB₂, 15.7% AB₃, 22.9% AB₄ (Fig. 2d). The hydrodynamic diameter of clusters grows significantly with the growth of PHPMA (Fig. 2e).

Fig. 2 (a–d) TEM images of PDMA₇₀-b-PBzMA₉₆-b-PHPMA_z with varied z (20, 46, 70, and 109) after PTA-staining (PDMA black, PBzMA grey, and PHPMA invisible). Scale bars are 50 nm. (e) DLS profiles of the colloid samples. PDMA₇₀-b-PBzMA₉₆-b-PHPMA_z is abbreviated to D₇₀B₉₆H_z. (f) Distribution of CMs with varied valence numbers (N) determined by statistically analyzing the TEM images with the inset V_H/V_B ratios. (g) Number average valence (N_n) of PDMA₇₀-b-PBzMA₉₆-b-PHPMA_z CMs against V_H/V_B. Error bars represent standard deviation.

The increasing DP of PHPMA leads to the increase of the PHPMA/PBzMA volume ratio (V_H/V_B), which plays a key role in determining the size and valence of CMs. Fig. 2f shows the distribution of valence shifts towards a higher value, as V_H/V_B increases. As V_H/V_B increases from 0.34 to 1.87, the number average of valence (N_n) gradually increases from 1 to 2.73 (Fig. 2g). For PDMA₇₀-b-PBzMA₉₆-b-PHPMA₇₀, the valence distribution is relatively narrow for a large-scale fabrication of CMs (N_n = 2.31, Đ = 1.11). Overall, we have generated a diversity of AB_n-type CMs with mean valence increasing with V_H/V_B.

When V_H/V_B reached 5.48 (PDMA₇₀-b-PBzMA₉₆-b-PHPMA₃₁₉), we have observed CMs of different valences (n = 3–6) with molecular precision (Fig. 3a), i.e. linear AB₂, triangular AB₃, tetrahedral AB₄, trigonal bipyramidal AB₅, square pyramidal AB₅ and octahedral AB₆, which are colloidal analogues of molecules like beryllium chloride (BeCl₂), boron trifluoride (BF₃), carbon tetrachloride (CCl₄), antimonic chloride (SbCl₅), iodine pentafluoride (IF₅) and sulfur hexafluoride (SF₆), respectively. In contrast, using larger CAs targeting V_H/V_B ≤ 1, linear colloidal polymers (PDMA₇₀-b-PBzMA₃₂₁-b-PHPMA₁₆₉) are obtained, by the polymerization of “hamburger” intermediates (Fig. 3b).

Fig. 3 (a) Self-assembly of the PDMA₇₀-b-PBzMA₉₆-b-PHPMA₃₁₉ triblock terpolymer toward CMs with V_H/V_B > 1; inset shows the cryo-TEM image of the unstable Janus intermediate. TEM images of varied PDMA-b-PBzMA-b-PHPMA CMs after PTA-staining; inset shows schematic structures and molecules. (b) Illustrative self-assembly of PDMA-b-PBzMA-b-PHPMA at V_H/V_B ≤ 1. Cryo-TEM (bottom, left) and TEM images (bottom, right) after PTA-staining of PDMA₇₀-b-PBzMA₃₂₁-b-PHPMA₁₆₉ colloidal chains via the “Hamburger” (insert) structure. Scale bars are 50 nm.

We therefore propose the formation mechanism of AB_n-type CMs by polymerization-induced particle-assembly (PIPA): (1) PDMA-b-PBzMA forms spherical core–shell CAs with PBzMA blocks as rubbery cores (T_g = 56.8 °C),⁴⁷ and PDMA chains as soluble stabilizers locating on the surface. (2) The polymerization produces PHPMA blocks which phase separate with PBzMA blocks, and aggregate into PHPMA micro-domains. (3) When the attractive PHPMA domains reach a critical size, they combine into PHPMA central CAs and unify PDMA-b-PBzMA spheres into colloidal clusters. (4) The structure of colloidal clusters is determined by V_H/V_B. When V_H/V_B > 1, the assembly of unstable monovalent Janus intermediates produces AB_n-type CMs, with n regulated by V_H/V_B. When V_H/V_B ≤ 1, divalent hamburger-shape intermediates form colloidal chains.

This is in agreement with the principle of solvent-based fabrication of multicompartment micelles (MCMs), reported by Müller and coworkers.^25,26 They have systematically studied the self-assembly of an ABC triblock terpolymer system (A as the attractive corona, B as the core, and C as the repulsive corona). The strong phase separation between A and B generates a patchy structure based on V_A/V_B. V_A/V_B > 1 yields monovalent Janus intermediates, with one attractive A patch and one repulsive C patch, which generate cluster-like MCMs. Oppositely, V_A/V_B ≤ 1 leads to divalent “hamburger” subunits with two attractive A patches and one repulsive C patch, which then assemble into chain-like MCMs. This concept has been validated by vastly different linear triblock terpolymer systems for the fabrication of both spherical MCMs and linear MCMs.

The formation of AB_n-like CMs requires PHPMA domains protruding from PBzMA cores, not growing inside PBzMA cores. The interfacial energy is the critical parameter determining the CM formation during polymerization. Compared to the entropy cost of stretching chains, and the repulsive interactions among stabilizer chains, interfacial energies are dominant in the formation of AB_n-type CMs. After neglecting free energies contributed by core-stretching and corona repulsion, the whole free energy change is written as

γ_i and A_i: interfacial tension and area of the i^th interface. For an ABC linear triblock terpolymer with A as the stabilizer, B phase separates with C. when the interfacial tension between B cores and solution (γ_B–S) is higher than that between C cores and solution (γ_C–S) (γ_B–S > γ_C–S), the C block tends to aggregate at the B/solvent interface. As a result, the C block forms microdomains serving as attractive patches. Otherwise, C microdomains might aggregate inside the B cores, and raspberry-like MCMs are possibly obtained as Armes and coworkers have reported.^48–50 As shown in Table S5,‡ the interfacial tensions were measured by the Owens–Wendt–Rabel–Kaelble method.^51,52 The interfacial tensions of PHPMA–water (γ_H–W) and PBzMA–water (γ_B–W) interfaces are 9.0 ± 1.6 mN m⁻¹ and 27.1 ± 2.5 mN m⁻¹, respectively. Since γ_H–W < γ_B–W, the PHPMA blocks aggregate at the PBzMA/water interfaces. Therefore, CMs are favored compared to raspberry-like MCMs (Scheme 2).

Scheme 2 Illustrations of PIPA targeting colloidal molecules, colloidal polymers, and raspberry-like MCMs, respectively, by tuning the interfacial tensions (γ_B–S and γ_C–S) and the volume ratio (V_C/V_B).

The PIPA strategy is an extension of PISA for the rational design and mass production of BCP nanoparticles with structural hierarchies. The design of the ABC triblock terpolymer requires (1) strong phase separation between B and C blocks and (2) monomer C suitable for PISA. And the morphology of HNPs is tuned by the interfacial tension and the V_C/V_B of the ABC triblock terpolymer. Based on these requirements, PIPA could be applied to vastly different triblock systems. For example, we also studied the aqueous seeded polymerization of diacetone acrylamide (DAAm), using PDMA₇₀-b-PBzMA₉₆ spheres as seeds. PDMA₇₀-b-PBzMA₉₆-b-PDAAm₉₅ (Đ = 1.38) assembled into a mixture of AB₂ and AB₄ CMs (Fig. S21‡), with PDAAm as an attractive domain. As the DP of PDAAm increased, PDMA₇₀-b-PBzMA₉₆-b-PDAAm₁₁₄ (Đ = 1.52) produced majorly AB₄ CMs (Fig. S22‡). Our strategy could be readily applied to different triblock terpolymer systems for the fabrication of HNPs with different hierarchies and functionalities. Moreover, the discussed considerations of directing the process may well serve as an inspiration for other hierarchical self-assembly systems.

Conclusions

In summary, we have developed a straightforward way to fabricate BCP AB_n-type CMs by seeded polymerization. A variety of well-defined CMs have been fabricated, including linear AB₂, triangular AB₃, tetrahedral AB₄, trigonal bipyramidal AB₅, square pyramidal AB₅ and octahedral AB₆. At a low volume ratio, colloidal chains were achieved by assembly of “hamburger” intermediates. Polymerization-induced particle-assembly allows the large scale production of the CMs, e.g. AB₂-CMs (78.5% purity, more than 10¹⁷ particles, 50 g L⁻¹). The CMs can serve as new building blocks for more complex structures such as colloidal chains and colloidal networks. The PIPA approach could be readily applied to vastly different triblock terpolymer systems for the fabrication of functional HNPs and has unique advantages for further applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Danyang Li and Xiaofeng Hu at the National Center for Protein Science (Beijing) Tsinghua Cryo-EM Facility for the technical support, and Prof. Xiaosong Wang at the University of Waterloo for helpful discussion. This work was supported by the National Natural Science Foundation of China (21871162 and 51573086).

Notes and references

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Footnotes

† This work is dedicated to Professor Caiyuan Pan, University of Science and Technology of China, for his 80th birthday in May, 2020.

‡ Electronic supplementary information (ESI) available: Experimental details and figures. See DOI: 10.1039/d0sc00219d

§ These authors contributed equally.

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