RAFT Polymerization-Induced Self-Assembly as a Strategy for

Letter Cite This: ACS Macro Lett. 2018, 7, 287−292

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RAFT Polymerization-Induced Self-Assembly as a Strategy for Versatile Synthesis of Semifluorinated Liquid-Crystalline Block Copolymer Nanoobjects Liangliang Shen, Huazhang Guo, Jinwen Zheng, Xiao Wang, Yongqi Yang, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: Polymerization-induced self-assembly is demonstrated as a powerful platform for the synthesis of block copolymers comprising a semifluorinated liquid-crystalline block. This strategy transforms the deficiency of polymer insolubility encountered in traditional homogeneous solution protocols to the strength for dispersion polymerization, thus, enabling direct access to polymorphic block copolymer nanoobjects at high concentrations and with quantitative conversions. The versatility of this strategy is highlighted by polymerizations in a wide selection of inexpensive solvents, from nonpolar to highly polar, to afford various block copolymers with distinct combinations of amorphous/crystalline or hydrophilic/hydrophobic/fluorinated segments. The utility of the nanoparticles is demonstrated as robust Pickering emulsifiers for commonly considered good solvents.

F

As such, block copolymers (BCPs) containing a semifluorinated liquid-crystalline (SFLC) block are intriguing materials in selfassembly studies.30−32 A widely used protocol in traditional BCP self-assembly is first dissolution of a BCP in a common solvent for both blocks, followed by either a temperature or solvent switch. However, such common solvents are notoriously difficult to find for BCPs containing a SFLC segment. Even with addition of fluorinated solvents, SFLC blocks can still associate to prevent complete dissolution of the BCPs.30,31 Thus, the insolubility problem greatly magnifies in selfassembly of SFLC BCPs due to the presence of disparate blocks and the rigid rod-like conformation adopted by the fluorinated group. As a result, a further limitation ensuesthe concentration of such SFLC BCP assemblies is extremely low, thus, it is difficult to scale up for mass production. Polymerization-induced self-assembly (PISA) has emerged as a powerful technique for rational synthesis of BCP nanoparticles with controllable morphologies at high concentrations and has been widely recognized as a viable method for scale-up production of BCP nanoparticles.33−47 We envision that dispersion PISA48,49 can be favorably employed as a versatile strategy for efficient synthesis of various BCPs containing a SFLC core-forming block (Scheme 1). A wide selection of inexpensive solvents, ranging from nonpolar (e.g., dodecane) to highly polar (e.g., N,N-dimethylforamide (DMF)), can be used for dispersion polymerization of heptadecafluorodecyl meth-

luorinated polymers have gained widespread use in optics, energy devices, surfaces and interfaces, separation, selfassembly, and biotechnology on account of their unique attributes, including amphiphobicity, low refractive indices, low surface energy, and high thermal and chemical stability.1−3 Thus, versatile synthetic strategies that enable facile access to a variety of fluorinated materials with controllable structures and tailorable properties are highly desirable. Controlled radical polymerization techniques 4−10 have been employed to synthesize fluorinated polymers of varying fluorine contents either in the side chain (e.g., perfluoroalkylethyl (meth)acrylate) or on the backbone (e.g., vinylidene fluoride).11−18 Despite great success, these synthetic protocols are often limited by poor solubility of fluoropolymers, lack of breadth in monomer family, or use of fluorinated ligands, initiators or solvents. Significant progress has recently been made via lightmediated atom transfer radical polymerization (ATRP) and reversible addition−fragmentation chain transfer (RAFT) for various semifluorinated (meth)acrylates.19−21 However, only oligomers were reported for monomers with a relatively large number of fluorocarbons, presumably due to insolubility of the corresponding polymers with high DPs. Therefore, development of a new paradigm that can circumvent these limitations encountered in traditional homogeneous solution polymerization with facile accessibility to various materials is of significant interest. Semifluorinated poly(meth)acrylates with side groups containing ≥7 fluorocarbons form ordered smectic liquidcrystalline (LC) phases,22,23 which provide a strong driving force for phase segregation between incompatible blocks.24−29 © XXXX American Chemical Society

Received: January 25, 2018 Accepted: February 12, 2018

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The size and morphology of BCP nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM), which revealed an interesting morphological transition sequence as DP of the core-forming SFLC block increased from 10 to 100 for dispersion polymerizations conducted at 20% solids (Figure 1).

Scheme 1. RAFT Dispersion Polymerization-Induced SelfAssembly for the Synthesis of Block Copolymer Nanoparticles Containing a Semifluorinated LiquidCrystalline PHDFDMA as the Core-Forming Block

acrylate (HDFDMA) to produce an array of BCPs with adjustable structures and properties. This dispersion PISA strategy enables accelerated synthesis of high-concentration (≥15%), morphology-controlled SFLC BCP nanoparticles by directly transforming semifluorinated monomers into associating BCPs in the form of colloidal particles, thus, preempting the requirement for polymer solubility and temperature/solvent switching steps. Additional advantages include high polymerization rate and elimination of transesterification side reaction. Several macromolecular chain transfer agents (macro-CTAs) with low dispersities (Đ, 1.02−1.23) were synthesized by either esterification of poly(ethylene glycol) methyl ether (PEG) with 4-cyano-4-(ethylthiocarbonothioylthio)pentanoic acid or RAFT (reversible addition−fragmentation chain transfer) solution polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA), methyl methacrylate (MMA), stearyl methacrylate (SMA), and 2,2,2-trifluoroethyl methacrylate (TFEMA) using 4-cyano-4-(ethylthiocarbonothioylthio)pentanoic acid as a chain transfer agent. After standard purification procedures via either precipitation or dialysis, the macro-CTAs were fully characterized by 1H NMR spectroscopy (Figures S1−S5) and gel permeation chromatography (GPC; Figure S6), and both techniques provided molecular weights consistent with theoretical values. Initially, hydrophilic PEG113-CTA was chain-extended with HDFDMA in ethanol to produce a series of PEG113PHDFDMAx nanoparticles at 70 °C by systematically tuning the target DP at different solids content. The dispersion polymerization followed typical two-stage kinetics with an obvious rate acceleration upon nucleation at ∼2 h, and nearquantitative monomer conversion was achieved at the end of polymerization, which was typically run for 12−24 h (Figure S7). Use of a mixed solution CDCl3/CF2ClCFCl2 (3:2, v/v) enabled dissolution of the BCP nanoparticles and thus calculation of the BCP compositions via 1H NMR spectroscopy, which showed experimental results were in accord with targeted DPs (Figure S8). Although not very soluble in THF, GPC measurement (Figure S9) was conducted for PEG113PHDFDMA10 with a low SFLC block, which indicated a low dispersity (Đ = 1.18). These results together suggest that dispersion polymerization of HDFDMA using PEG113-CTA was under good RAFT control.

Figure 1. TEM micrographs of PEG113-PHDFDMAx nanoparticles synthesized in ethanol, 70 °C, 20% solids: (A) fusiform nanoparticles, (B) fusiform nanoparticles, (C) elongated vesicles, (D) fused vesicles, (E) porous nonspherical particles, (F) irregular solid particles.

An unusual anisotropic fusiform morphology was observed at DP = 10, and the average length of short and long axis was 28 and 49 nm, respectively. As DP increased to 30, the fusiform particles became larger along with a minor population of vesicles, while pure elongated vesicles were produced at DP = 35, which had an average diameter of 260 nm and an average membrane thickness of 51 nm. Fusion of vesicles occurred on further increasing DP, leading to the generation of nonspherical particles having 3−7 holes at DP = 80. This nonspherical particulate morphology with multiple holes (average diameter 42 nm) is a new morphology produced by PISA. Finally the holes were filled and solid particles with irregular rims were observed. Dispersion polymerizations mediated by PEG113CTA were also conducted at 15% and 30% solids. Vesicles were not observed for the series of polymerizations conducted at 15% solids (Figure S14). Higher solids facilitated morphological transitions, as expected, due to an enhanced probability of inelastic collisions between particles, and thus, all the morphologies observed for polymerizations conducted at 20% solids also appeared for those conducted at 30% solid, but morphological transitions occurred at comparably lower DPs in the latter case (Figure S15). The general appearing of nonspherical morphologies is caused by the LC nature of the core-forming block as evidenced by X-ray diffraction (XRD) and differential scanning calorim288

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ACS Macro Letters etry (DSC) measurements on dried nanoparticle powers (Figure 2). XRD spectrum shows four well-resolved peaks at

Figure 2. XRD spectrum (A) and DSC trace (B) of PEG113PHDFDMA100.

2θ 2.8°, 5.6°, 8.4° and 17.9°, corresponding to an ordered smectic B phase in which the rod-like perfluoroalkyl side chains (1.58 nm) are arranged hexagonally (0.5 nm) with a lamellar periodicity of 3.16 nm.22 Diffraction peaks at 19° and 23.5° due to PEG crystalline domains are also clearly assigned.50 The phase transition temperatures, i.e., LC to isotropic liquid transition (85 °C) for PHDFDMA and melting point (61 °C) of PEG, are also consistent with literature values and further confirm the crystalline nature of PEG-PHDFDMA BCPs synthesized by PISA.50 The PEG113-PHDFDMAx BCP nanoparticles synthesized in ethanol can be dispersed into a wide range of solvents commonly used in the laboratory, including DMF, DMSO, acetone, acetonitrile, ethyl acetate, 1,4-dioxane, CH2Cl2, CHCl3, THF, toluene, and trifluoroethanol. In principle, the ability of the nanoparticles to resist solvent destruction is dependent on DP of PHDFDMA. Nevertheless, the nanoparticles, as represented by PEG113-PHDFDMA35, were extraordinarily stable and retained their particle size and morphology in these solvents even after being heated at 70 °C (Table S4 and Figure S16). This remarkable stability exhibited by the SFLC BCP nanoparticles in various solvents promoted further exploration of PISA in different solvents using macro-CTAs with a variety of structures and solubilities. For instance, syntheses of PEG113PHDFDMAx nanoparticles were also conducted in dioxane at 30% solids (Figure 3A). Besides PEG-CTA, PDMAEMA30CTA, a pH- and temperature-responsive polymer, was similarly used as a suitable stabilizer for ethanolic dispersion polymerization of HDFDMA. A representative TEM image of PDMAEMA30-PHDFDMA100 microtubes synthesized at 30% solids is shown in Figure 3B. These microtubes, with a wall thickness of 35 nm, extend several micrometers and appear to have both hollow and solid segments (Figure S17). It is worth noting that PDMAEMA is a weak polybase, which may be able to catalyze transesterification of HDFDMA with ethanol. To check this point, an ethanolic solution of PDMAEMA and HDFDMA was heated at 70 °C for 12 h. Analysis by 1H NMR spectroscopy indicated no discernible new species were produced (Figure S18). Thus, dispersion polymerization of HDFDMA in ethanol using PDMAEMA as a macro-CTA proceeded without transesterification side reaction. Figure 3C shows an example of nanoparticles synthesized in DMF using PMMA43-CTA, which is an amorphous and hydrophobic polymer. In contrast, PSMA is a semicrystalline and hydrophobic polymer, and the use of PSMA15-CTA for dispersion polymerization of HDFDMA conducted in n-dodecane produced PSMA15-PHDFDMA50 short rods (Figure 3D). The

Figure 3. Representative TEM micrographs of block copolymer nanoparticles synthesized using various macro-CTAs in different solvents at indicated solids: (A) fusiform nanoparticles, (B) microtubes, (C) fusiform nanoparticles, (D) rodlike nanoparticles, (E) fusiform nanoparticles, (F) vesicles.

flexibility in solvent selection also enables synthesis of BCP nanoparticles consisting of blocks with unique properties. For example, PTFEMA39-PHDFDMA40 fusiform nanoparticles (Figure 3E) and PTFEMA39-PHDFDMA60 vesicles (Figure 3F) comprising two different semifluorinated blocks were prepared by employing PTFEMA39-CTA for dispersion polymerizations conducted in DMF at 30% solids. These BCPs having different compositions were fully characterized by 1 H NMR spectroscopy using mixed CDCl3/CF2ClCFCl2, THF-GPC (for low DPs), XRD, and DSC analyses (Figures S22−S29). BCP nanoparticles represent an important class of stabilizers for the formation of Pickering emulsions, which have attracted interest from across several fields.51,52 We hypothesize that these extremely stable SFLC BCP nanoparticles with diverse surface wetting properties can be directly employed as Pickering stabilizers for solvents with wide-ranging polarities. In contrast, commonly used BCP nanoparticles have to be cross-linked to prevent disintegration for emulsification of “good” solvents (e.g., toluene) and under high shearing forces,53,54 which increases the complexity of nanoparticle synthesis, often with inherent limitations in morphology control or achievable cross-linking density.34,55 Thus, both oil-in-water and oil-in-oil Pickering emulsions were prepared, including toluene-in-water, DMF-in-dodecane, and cyclohexane-in-DMSO, using PEG 113 -PHDFDMA 30 , PSMA 15 PHDFDMA50, and PMMA43-PHDFDMA50 nanoparticle dispersions, respectively (Figure 4A−C). Given that toluene, DMF, and DMSO are commonly used low-polarity and highpolarity good solvents in the laboratory, formation of stable 289

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00070. Detailed experimental procedures and supplementary data, including Tables S1−S4 and Figures S1−S34 (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zesheng An: 0000-0002-2064-4132 Notes

The authors declare no competing financial interest.

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Figure 4. Optical micrographs and digital photographs (insets) of Pickering emulsions: (A) toluene-in-water, 1 mL toluene + 1 mL 0.5 wt % PEG113-PHDFDMA30 aqueous dispersion; (B) DMF-indedocane, 1 mL DMF + 1 mL 5.0 wt % PSMA15-PHDFDMA50 ndodecane dispersion; (C) Cyclohexane-in-DMSO, 1 mL cyclohexane + 1 mL 3.0 wt % PMMA43-PHDFDMA50 DMSO dispersion; (D) Cyclohexane-in-DMSO, 0.8 mL cyclohexane + 0.2 mL 5.0 wt % PMMA43-PHDFDMA50 DMSO dispersion. Scale bar is 40 μm.

ACKNOWLEDGMENTS We thank financial support by National Natural Science Foundation of China (21674059, 21674050) and assistance of Instrumental Analysis and Research Center (Shanghai University).

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REFERENCES

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Pickering emulsions with these solvents is particularly noteworthy, which not only serves to illustrate the robustness of these nanoparticles, but also significantly broadens the scope of emulsions for applications in biphasic catalysis. More intriguing is the ability to form high internal phase emulsions (HIPEs, with internal phase volume ≥ 74.05%)56 using some of these nanoparticles, as represented by the formation of highly viscous cyclohexane-in-DMSO (internal phase 80%) HIPEs using PMMA43-PHDFDMA50 nanoparticles (Figure 4D). Formation of stable HIPEs is challenging, which typically requires a large amount of surfactants/nanoparticles with a limited solvent scope. To our knowledge, formation of such oil/oil Pickering HIPEs is rare. TEM confirmed that the nanoparticles essentially remained intact after removal of solvents from the emulsions (Figure S34). In conclusion, dispersion polymerization has been demonstrated to be a viable and effective strategy for the synthesis of BCPs containing a SFLC block. The insolubility of PHDFDMA, a significant problem in traditional solution polymerization protocols, can be favorably utilized in dispersion polymerizations to directly afford block copolymer nanoparticles via a PISA mechanism. PISA syntheses have been conducted in various common solvents using several hydrophilic/hydrophobic/fluorinated or amorphous/crystalline macro-CTAs, leading to the production of BCP nanoparticles with rich morphologies. The LC nature of the core-forming PHDFDMA is responsible for the general formation of interesting nonspherical morphologies. The PHDFDMAcontaining BCP nanoparticles can be used as robust Pickering emulsifiers without resort to additional cross-linking chemistries. Several types of stable oil/water or oil/oil emulsions have been fabricated even with some generally considered good solvents such as DMF. We believe this PISA strategy can be extended to the synthesis other fluorinated BCPs via judicious choice of solvents. This will not only lead to facile access to a broad range of BCPs, but also enable intriguing new particle morphologies to be discovered. 290

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