Letter pubs.acs.org/macroletters
Modular Monomers with Tunable Solubility: Synthesis of Highly Incompatible Block Copolymer Nano-Objects via RAFT Aqueous Dispersion Polymerization Baohua Zhang, Xiaoqing Lv, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *
ABSTRACT: The high incompatibility of block copolymers consisting of a neutral stabilizer block and a polyelectrolyte core-forming block is exploited to drive phase segregation during polymerization-induced self-assembly (PISA). A modular approach to systematically tune the solubility of ionic monomers/ polymers is developed to efficiently identify monomers suitable for aqueous dispersion polymerizations. The strong phase segregation ability of the neutral-polyelectrolyte block copolymers favors the formation of worms over a relatively broad composition range and even at very low solids. These findings suggest that the degree of incompatibility between the stabilizer block and the core-forming block should be considered as one of the key parameters when studying morphological transitions in PISA.
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more thermodynamically accessible for aqueous dispersion than emulsion polymerization.3,10 Despite significant amounts of work that have been reported for PISA via aqueous dispersion polymerization, there are only a few monomers that are suitable for formulations used in aqueous dispersion polymerization.18−35 As such, expanding the repertoire of monomers suitable for aqueous dispersion polymerization is highly desirable to push the PISA area even further. Herein, we focus on the use polyelectrolytes as the coreforming block in combination with neutral stabilizer blocks for PISA via aqueous dispersion polymerization. Such a neutral block−polyelectrolyte combination with a high chemical disparity is expected to provide a high χ, thus, promoting the PISA process. This design is inspired by the many outstanding literature precedents on the self-assembly of block copolymers. For example, Bates et al. demonstrated that selective addition of LiClO4 to the poly(ethylene oxide) block of poly(styrene-bisoprene-b-ethylene oxide) triblock copolymers enhanced the incompatibility and hence the segregation strength of the block copolymers.36 While this early report dealt with salt doping of block copolymers, Olsen and co-workers showed that protonation of the poly(2-vinylpyridine) block in poly(oligo ethylene glycol methyl ether methacrylate)-b-poly(2-vinylpyridine) to produce ion-containing block copolymers improved the effective segregation strength between the blocks and thus promoted phase-separated morphologies.37 Even more remarkably, Hawker and co-workers ingeniously showed
olymerization-induced self-assembly (PISA) is highly efficient and versatile for the synthesis of various block copolymer nano-objects with controlled morphologies at high solids.1−4 Two major polymer-related parameters, that is, block ratio and concentration, have been widely exploited to tune the nano-object morphologies.3 However, a survey of the immense literature on the self-assembly of block copolymers clearly suggests that many other parameters can be similarly harnessed in PISA to effectively influence the in situ self-assembly of block copolymers; one such prominent parameter is the Flory− Huggins interaction parameter χ, which provides the intrinsic thermodynamic driving force for the block copolymers to selfassemble.5 The product of χN, where N is the degree of polymerization, determines the segregation strength and must possess a minimum value for the self-assembly to occur. Thus, block copolymers with high χ have been actively studied in block copolymer lithography for the production of nanoscale patterns with increasingly shrinking feature sizes.6−9 Conceptually, block copolymers with high χ and, thus, high segregation ability can be exploited to promote the self-assembly process in PISA, which has yet to be demonstrated. In principle, PISA can be conducted in any solvent provided that amphiphilic block copolymers via chain-extension from soluble stabilizer blocks can be produced that in situ selfassemble after a critical length of the growing core-forming block is reached.10−15 However, synthesis of block copolymer nano-objects via aqueous PISA,16,17 particularly via aqueous dispersion polymerization, is extremely attractive for several reasons: (1) the use of water represents a green chemical process; (2) radical polymerization typically has a high polymerization rate; and (3) PISA morphological control is © XXXX American Chemical Society
Received: January 25, 2017 Accepted: February 16, 2017
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DOI: 10.1021/acsmacrolett.7b00056 ACS Macro Lett. 2017, 6, 224−228
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ACS Macro Letters
Scheme 1. (A) Modular Approach to Tune Solubility of Ionic Monomers/Polymers and (B) RAFT Aqueous Dispersion Polymerization of Ionic Monomer to Produce Neutral-Polyelectrolyte Block Copolymers
Conceivably, monomers suitable for aqueous dispersion must possess a subtle balance between the hydrophilic and hydrophobic moieties, which could be more effectively adjusted with modular monomers having easily tunable structures. Thus, we have investigated the possibility of using readily available ionic monomers having separate cations and anions as modular monomers to identify suitable candidates for aqueous PISA to produce neutral−polyelectrolyte block copolymers with enhanced segregation strength. As shown in Scheme 1, using [2-(methacryloyloxy)ethyl]trimethylammonium as the model cationic monomer, its water solubility can be systematically tuned by varying the anions of different polarities. The monomer solubility decreases with anions in the order of chloride (Cl−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), and bis(trifluoromethanesulfonyl)imide ((CF3SO2)2N−). The corresponding polymers show a similar order in water solubility. While the polymer with Cl− is highly soluble in water as expected, the polymer with BF4− shows a temperaturedependent solubility with upper critical solution temperatures (UCSTs) being observed.42 In contrast, the polymers with either PF6− or (CF3SO2)2N− are completely insoluble in water even at high temperatures. Thus, this modular approach led to the identification of [2-(methacryloyloxy)ethyl]trimethylammonium hexafluorophosphate ([META+][PF6−]) being suitable for aqueous dispersion polymerization, which has a solubility of 0.6% and 8% w/v at 25 and 80 °C, respectively. Aqueous dispersion polymerization of [META+][PF6−] was conducted via reversible addition−fragmentation chain transfer (RAFT) polymerization as RAFT is a highly versatile controlled radical polymerization technique that has been shown to control a wide range of monomer families including cationic monomers.43,44 Two types of water-soluble macromolecular chain transfer agents (macro-CTAs) were synthesized and used
that the incorporation of a single ionic group in the junction of block copolymers dramatically increased the segregation strength of polydimethylsiloxane-based block copolymers with higher order−disorder transition temperatures being observed in comparison with the block copolymers having neutral junctions.38 These literature examples convincingly illustrate that high segregation strength can be effectively achieved when ionic groups are incorporated into block copolymers. Previously, polyelectrolytes have been used as stabilizer blocks in PISA by Armes et al., but morphological transition is hindered due to the high solvation and lateral electrostatic repulsion of the polyelectrolytes.39,40 Using an elegant polyion complexation (PIC) approach, Cai and co-workers studied aqueous dispersion polymerization of a water-soluble cationic monomer (2-aminoethylacrylamide hydrochloride) in the presence of an oppositely charged polyelectrolyte as the template, which only produced spheres or clusters of spheres as the morphologies.41 Interestingly, Detrembleur et al. reported aqueous dispersion polymerization of N-vinylimidazolium bromide to result in the formation of block copolymers consisting of double polyimidazoliums with different substitution groups to tune the water solubility of the two blocks.33,34 Because both the stabilizer and the core-forming blocks are polyelectrolytes in nature, their disparity may be diminished and thus very likely the segregation strength may not be enhanced in comparison with neutral-polyelectrolyte block copolymers. In addition, no higher-order morphologies were produced with only “rice-shaped” nanoparticles that slightly differ from the spherical shape being noted in one synthesis. To realize aqueous PISA that could be promoted by the enhanced segregation strength with ionic group incorporation, water-soluble ionic monomers to produce water-insoluble polyelectrolytes need to be identified in the first place. 225
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ACS Macro Letters for chain-extension with [META+][PF6−] to generate nanoobjects consisting of a neutral stabilizer block and a polyelectrolyte core-forming block. RAFT solution polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mn = 500 g/mol) in dimethylformamide targeting two different mean degrees of polymerization (DPs) using 4-cyano-4-(ethylthiocarbonothioylthio) pentanoic acid as the CTA afforded well-defined PPEGMA22 (Mn = 12.0 kg/mol, Đ = 1.35) and PPEGMA31 (Mn = 17.6 kg/mol, Đ = 1.20), as indicated by gel permeation chromatography (GPC, DMF and PMMA standards) analysis, which were in good agreement with 1H NMR data (Figures S1 and S2). Macro-CTAs derived from poly(ethylene glycol) monomethyl ether (mPEG−OH, Mn = 2 kg/mol and 5 kg/ mol), denoted as mPEG45-CTA and mPEG113-CTA, were prepared via esterification of mPEG−OH with excess 4-cyano4-(ethylthiocarbonothioylthio) pentanoic acid, providing mPEG-CTAs with essentially quantitative functionality (>99%), as revealed by 1H NMR analysis (Figures S3 and S4). RAFT aqueous dispersion polymerization of [META+][PF6−] was first conducted using PPEGMA31 at 80 °C and 5% w/v solids (Figure 1). The polymerization was very fast
5% w/v solids, as shown in Figure 2, spheres were obtained up to a DP of 33, after which point the spheres started to fuse into
Figure 2. TEM micrographs for RAFT aqueous dispersion polymerizations of [META+][PF6−] using PPEGMA31 at 80 °C and 5% w/v solids targeting different DPs.
short worms (DP = 45), and relatively long worms (average 246 nm) with a uniform diameter (38 nm) were observed at DP 50. Further increasing the DP to 59 resulted in entanglement and fusion of the worms and high-density network structures were finally observed at DP 92. These worm dispersions were turbid and fluidic without forming freestanding gels possibly due to insufficient worm contact at such low solids. Attempts to further increase the DP to induce precipitation did not significantly alter the network morphology. Polymerizations conducted at 8% w/v solids yielded colloidally stable nano-objects up to DP 56, which had a morphology composed of highly fused worms with the tendency toward formation of bilayers (Figure S8). Worm phases typically occupy very narrow composition space.23,24 However, the PPEGMA-b-P([META+][PF6−]) worm phase appeared to cover a relatively broad composition range (45 ≤ DP < 100) for the nano-objects synthesized at 5% w/v. While it is a decisive advantage to synthesize nano-objects with morphology control at high solids for PISA vs traditional block copolymer self-assembly, the observation of the wide worm space for these PPEGMA-b-P([META+][PF6−]) formulations prompted us to explore the morphology further at even lower solids to see whether higher-order morphologies could be possibly obtained. To this end, we conducted polymerizations at 3% and 2% w/v. Again, worms were clearly produced at DP 36 at 3% w/v solids (Figure S9). It is wellknown that the probability of inelastic collision between nanoparticles leading to morphological transition is favored at high solids (typically >10%).3 The fact that the usually elusive worm phase can be reliably accessed at such a low solids serves to explicitly prove the extraordinary phase segregation strength of the neutral-polyelectrolyte block copolymers. Although the polymerizations conducted at 2% w/v became increasingly inefficient, network structures were still obtained at high DPs (Figure S10), which was quite similar to the final morphology observed for polymerizations conducted at 5−8% w/v solids. Worms could also be produced using PPEGMA having a different molecular weight (PPEGMA22), and the final morphology for this PPEGMA22-b-P([META+][PF6−]) was also a network structure albeit with somewhat lower pore density (Figure S11). Surprisingly, these polyelectrolyte-based nano-objects are very resistant to dissolution in salt solution with NaCl concentrations up to 0.6 M (data not shown).
Figure 1. Polymerization kinetic results (A) and TEM micrographs (B−D) during RAFT aqueous dispersion polymerization of [META+][PF6−] using PPEGMA31 at 80 °C, 5% w/v solids and a molar ratio of [PPEGMA31]/[monomer]/[V-50] = 1:100:0.1.
with ∼90% conversion was achieved in 1 h using 0.1 equiv 2,2′azobis(2-methylpropionamidine) dihydrochloride (V-50) as the initiator. A linear kinetic plot of Ln([M]0/[M]) vs time was obtained, suggesting well-controlled RAFT polymerization was achieved. Surprisingly, the solution turned turbid within just 2.5 min, corresponding to a conversion of only 3% and a block copolymer composition of PPEGMA31-b-P([META+][PF6−])3, which is a highly asymmetric block copolymer with a very minor weight fraction (5.7%) of the hydrophobic block. This result suggests that this neutral-polyelectrolyte block copolymer has a remarkable ability to in situ self-assemble. The samples withdrawn during the polymerization were analyzed via transmission electron microscopy (TEM), which revealed morphological transitions from spheres (30% conv.) to short worms (46% conv.) to fused worms (52% conv.) and finally to percolated networks (Figure S7). The morphological transitions observed over monomer conversion during a single polymerization encouraged us to conduct a systematic investigation of PPEGMA-b-P([META+][PF6−]) morphologies by targeting various DPs and solids. At 226
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ACS Macro Letters
P([AETA+][PF6−])29. The detailed study of RAFT dispersion polymerization of [AETA+][PF6−] will be disclosed in a full article. In summary, a modular approach has been developed with ionic monomers to allow easy tuning of the solubility of monomers/polymers, which is expected to facilitate the identification of monomers suitable for aqueous dispersion polymerization formations. Using this approach, RAFT aqueous dispersion polymerization formulations based on [META+][PF6−] have been developed using PPEGMAs and mPEG-CTAs of different molecular weights. Spheres-toworms-to-networks morphological transitions were observed when using PPEGMAs as the stabilizer blocks while spheres-tolamellae-to-networks morphological transitions were observed when using mPEG-CTAs as the stabilizer blocks. Significantly, worms could be reliably produced at very low solids (3% w/v). The weight fractions of the polyelectrolyte block on the formation of higher-order morphologies were found to be much lower than those reported in the literature using other block copolymer compositions. Potentially, these polyelectrolyte-based nano-objects may be used for ion-responsive demulsification when used as Pickering emulsifiers or the encapsulation of DNA/RNA for gene delivery. The modular approach to identify suitable monomers for aqueous dispersion polymerization and the use of highly incompatible block copolymer compositions to enhance phase segregation in PISA is generally applicable and should provide new methods to the PISA fields.
The ability to phase-segregate of block copolymers is determined by the incompatibility and the block ratio. To gain a more direct comparison of the incompatibility and thus the intrinsic thermodynamic driving force of these neutralpolyelectrolyte block copolymers with those used in other PISA formulations, we calculated the weight fractions of the block copolymers produced in PISA when higher-order morphologies started to form, which was tabulated in Table S3. Ideally, volume fractions should provide a more direct correlation with the phase behavior but the densities of some of the polymers are not known from the literature. One of the lowest weight fractions for the core-forming block when worms were produced was 51% for the poly(N,N-dimethylacrylamide)-bpoly(N,N-dimethylacrylamide-co-acrylic acid)-b-poly(N-isopropylacrylamide) at 15% solids.19 In comparison, the worms started to form at a P([META+][PF6−]) weight fraction of 47.5% at only 5% solids for the PPEGMA31-P([META+][PF6−])45 in this study, which is substantially lower than many other formulations reported in the literature. To further probe the strong propensity of the neutralpolyelectrolyte block copolymers for the formation of higherorder morphologies, mPEG-CTAs of two different molecular weights were also used as the stabilizer blocks for RAFT aqueous dispersion polymerization of [META+][PF6−] at 5% w/v solids. Figures 3, S12, and S13 show some of the
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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.7b00056. Experimental details, NMR spectra, GPC traces, TEM micrographs, and tables (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 3. TEM micrographs for RAFT aqueous dispersion polymerizations of [META+][PF6−] using mPEG45-CTA (A-C) and mPEG113CTA (D-F) at 80 °C and 5% w/v solids targeting different DPs.
ORCID
Zesheng An: 0000-0002-2064-4132 Notes
representative TEM micrographs. When mPEG45-CTA was used to produce mPEG45-P([META+][PF6−]) block copolymers, lamellae were formed over a wide range of DPs (19−81). When mPEG113-CTA with a higher molecular weight was used to produce mPEG113-P([META+][PF6−]) block copolymers, the spheres started to fuse at DP 28, lamellae were observed at DP 55, and fusion of lamellae resulted in the formation of lamellar networks at DP 91. Finally, to demonstrate the generic nature of the modular monomer approach for the identification of suitable ionic monomers for aqueous PISA, we also applied this approach to the acrylate analogue of [META+][PF6−] and developed aqueous PISA formulations based on 2-[(acryloyloxy)ethyl]trimethylammonium hexafluorophosphate ([AETA+][PF6−]), which has a higher solubility than [META+][PF6−]. RAFT aqueous polymerization of [AETA+][PF6−] conducted at 70 °C and 15% w/v solids using poly(N,N-dimethylacrylamide) (PDMA37, Mn = 5.1 kg/mol, Đ = 1.10) could also produce a worm phase (Figure S19) at the composition of PDMA37-
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank National Natural Science Foundation of China (21674059) for funding support. REFERENCES
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