Cationic Polyelectrolyte-Stabilized Nanoparticles via RAFT Aqueous

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Cationic Polyelectrolyte-Stabilized Nanoparticles via RAFT Aqueous Dispersion Polymerization M. Semsarilar, V. Ladmiral, A. Blanazs, and S. P. Armes* Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K. S Supporting Information *

ABSTRACT: We report the synthesis of cationic sterically stabilized diblock copolymer nanoparticles via polymerizationinduced self-assembly (PISA) using a RAFT aqueous dispersion polymerization formulation. The cationic steric stabilizer is a macromolecular chain-transfer agent (macroCTA) based on quaternized poly(2-(dimethylamino)ethyl methacrylate) (PQDMA), and the hydrophobic core-forming block is based on poly(2-hydroxypropyl methacrylate) (PHPMA). The effect of varying synthesis parameters such as the salt concentration, solids content, relative block composition, and cationic charge density has been studied. In the absence of salt, self-assembly is problematic because of the strong repulsion between the highly cationic PQDMA stabilizer chains. However, in the presence of salt this problem can be overcome by reducing the charge density within the coronal stabilizer layer by either (i) statistically copolymerizing QDMA monomer with a nonionic comonomer (e.g., glycerol monomethacrylate, GMA) or (ii) using a binary mixture of a PQDMA macro-CTA and a poly(glycerol monomethacrylate) (PGMA) macro-CTA. These cationic diblock copolymer nanoparticles were analyzed by 1H NMR spectroscopy, dynamic light scattering (DLS), transmission electron microscopy (TEM), and aqueous electrophoresis. NMR studies suggest that the HPMA polymerization is complete within 2 h at 70 °C. Depending on the specific reaction conditions, either spherical, wormlike or vesicular nanoparticles can be prepared with tunable cationic surface charge.

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INTRODUCTION Recently, there has been growing interest in the preparation of colloidal polymer particles using living radical polymerization.1−22 In particular, polymerization-induced self-assembly (PISA) of amphiphilic block copolymers to form spheres, worms/fibers or vesicles is a simple, direct method for obtaining these colloidally stable copolymer morphologies at relatively high concentrations in multigram quantities. For example, Charleux et al. have used aqueous emulsion polymerization to synthesize spherical diblock copolymer nanolatexes, copolymer fibers/worms, or copolymer vesicles using either reversible addition−fragmentation chain-transfer (RAFT) polymerization1−3,9−12 or nitroxide-mediated polymerization (NMP).7,8,23 The important parameters that influence the morphology of the nanostructures formed by polymerization-induced self-assembly are the chemical nature of the comonomers,1 the mean degrees of polymerization of the hydrophilic and hydrophobic blocks,8,11,12,15,18,20 their interactions with each other and with the solvent, and the monomer concentration.15,16,18,20 In the case of aqueous formulations, the solution pH and salt concentration can also be important parameters, particularly if the hydrophilic block has polyelectrolytic character.1−20,24 Recently, we have developed versatile and robust routes to obtain a range of block copolymer nano-objects based on RAFT aqueous dispersion polymerization.15−18,20,24 Such formulations require that the core-forming block comprises a © 2012 American Chemical Society

water-soluble vinyl monomer that forms a water-insoluble polymer. As far as we are aware, the only suitable commodity vinyl monomer is 2-hydroxypropyl methacrylate (HPMA). We have reported the RAFT polymerization of HPMA using a poly(glycerol monomethacrylate)-based macromolecular chaintransfer agent (PGMA macro-CTA) under RAFT aqueous dispersion polymerization conditions.15,16,18 This robust protocol enabled the efficient synthesis of block copolymer nanoparticles with tunable particle size and morphology by fixing the mean degree of polymerization of the stabilizer block and systematically varying the degree of polymerization of the core-forming PHPMA chains. Detailed phase diagrams were elucidated for both this system and also a closely related formulation based on a poly(2-(methacryloyloxy)ethyl phosphorylcholine)-based macro-CTA that enabled pure phases of spheres, worms, or vesicles to be reliably targeted.16,20 Careful sampling of the original PGMA-PHPMA formulation over time confirmed the in situ evolution from spheres to worms to vesicles, while transmission electron microscopy studies revealed remarkable intermediate morphologies that provide new mechanistic insights regarding the nature of the worm-tovesicle transition.15 Such RAFT polymerizations also proved to Special Issue: Interfacial Nanoarchitectonics Received: October 29, 2012 Revised: December 3, 2012 Published: December 3, 2012 7416

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conducted using a Malvern Instruments Zetasizer Nano series instrument equipped with a 4 mW He−Ne laser operating at 633 nm, an avalanche photodiode detector with high quantum efficiency, and an ALV/LSE-5003 multiple-τ digital correlator electronics system. Aqueous electrophoresis measurements were performed on a 0.01 wt % aqueous copolymer solution using the same Malvern Instruments Zetasizer nanoseries instrument. The solution pH was adjusted by the addition of 0.01 M HCl or 0.01 M KOH using an autotitrator. Aqueous gel permeation chromatography (GPC) was used to characterize the cationic macro-CTA. The GPC protocol involved using a Pharmacia Biotech Superose 6 column connected to a Polymer Laboratories ERC-7517A refractive index detector. The eluent was an aqueous solution of 0.20 M NaNO3 containing 50 mM Trizma buffer at pH 7. Calibration was achieved using a series of near-monodisperse poly(ethylene oxide) standards ranging from 440 to 288 000 g mol−1. Synthesis of 4-Cyano-4-(2-phenylethanesulfanylthiocarbonyl)sulfanylpentanoic Acid (PETTC). 2-Phenylethanethiol (10.5 g, 76.0 mmol) was added over 10 min to a stirred suspension of sodium hydride (60% in oil; 3.15 g, 79.0 mmol) in diethyl ether (150 mL) at a temperature of between 5 and 10 °C. The vigorous evolution of hydrogen gas was observed, and the grayish suspension became a thick white slurry of sodium phenylethanethiolate over 30 min. The reaction mixture was cooled to 0 °C, and carbon disulfide (6.00 g, 79.0 mmol) was gradually added to provide a thick yellow precipitate of sodium 2-phenylethanetrithiocarbonate, which was collected by filtration after 30 min and used in the next step without purification. A suspension of sodium 2-phenylethanetrithiocarbonate (11.6 g, 4.09 mmol) in diethyl ether (100 mL) was treated by the portionwise addition of solid iodine (6.30 g, 25.0 mmol). The reaction mixture was then stirred at room temperature for 1 h, and the white sodium iodide was removed by filtration. The yellow−brown filtrate was washed with an aqueous solution of sodium thiosulfate to remove excess iodine, dried over sodium sulfate and evaporated to leave a residue of bis-(2phenylethane sulfanylthiocarbonyl) disulfide (∼ 100% yield). A solution of 4,4′-azobis(4-cyanopentanoic acid) (ACVA, 2.10 g, 75.0 mmol) and bis-(2-phenylethane sulfanylthiocarbonyl) disulfide (2.13 g, 5.0 mmol) in ethyl acetate (50 mL) was degassed by nitrogen bubbling and held at reflux under a N2 atmosphere for 18 h. After the removal of the volatiles under vacuum, the crude product was thoroughly washed with water (five 100 mL portions). The organic phase was concentrated and purified by a silica column (initially 7:3 petroleum ether/ethyl acetate, gradually increasing to 4:6) to afford 4cyano-4-(2-phenylethane sulfanylthiocarbonyl)sulfanylpentanoic acid (PETTC) as a yellow oil (78% yield). 1 H NMR (400.13 MHz, CD2Cl2, 298 K): δ 1.89 (3H, −CH3), 2.34−2.62 (m, 2H, −CH2), 2.7 (t, 2H, −CH2), 3.0 (t, 2H, −CH2), 3.6 (t, 2H, −CH2), 7.2−7.4 (m, 5H, aromatic). 13 C NMR (400.13 MHz, CD2Cl2, 298 K): δ 24.2 (CH3), 29.6 (CH2CH2COOH), 30.1(CH2Ph), 33.1 (CH2 CH2COOH), 39.9 (SCH2CH2Ph), 45.7 (SCCH2), 118.6 (CN), 127.4, 128.8, 129.2, 144.3 (Ph), 177.4 (CO), 222.2 (CS). Synthesis of [2-(Methacryloyloxy)ethyl] Trimethylammonium Iodide (QDMA). A round-bottomed flask was charged with 2-(dimethylamino)ethyl methacrylate (10.0 g, 0.06 mol) and THF (100 mL). This solution was stirred in an ice bath for 20 min prior to the addition of methyl iodide (9.93 g, 0.07 mol). A white precipitate was immediately formed, which was isolated via filtration, washed with THF, and dried under vacuum. The structure of quaternized 2(dimethylamino)ethyl methacrylate (QDMA) monomer was confirmed by 1H NMR analysis. Synthesis of Poly(2-(methacryloyloxy)ethyl trimethylammonium iodide) (PQDMA) Macro-CTA Agent. In a typical experiment, a round-bottomed flask was charged with QDMA (3.00 g, 10.0 mmol), PETTC (113 mg, 0.334 mmol, dissolved in 1.0 mL of dioxane), ACVA (18.7 mg, 0.067 mmol), and pH 5.5 buffer (9.00 g). The sealed reaction vessel was purged with nitrogen and placed in a preheated oil bath at 70 °C for 6 h. The resulting PQDMA macroCTA (QDMA conversion = 96%, Mn = 8 500 g mol−1, Mw = 9 800 g mol−1, Mw/Mn = 1.15) was purified by dialysis against deionized water

be very efficient: more than 99% HPMA conversion was achieved within 2 h at 70 °C even when targeting relatively high degrees of polymerization for the PHPMA chains.15,24 Moreover, DMF GPC analysis of the block copolymer chains indicated low polydispersities, as expected for well-controlled RAFT syntheses.15,24 To produce charged nano-objects, polyelectrolytes can be used as the stabilizing block. However, this approach often leads to only spherical morphologies being obtained when using anionic polyelectrolytes, as reported by both Charleux and co-workers and also ourselves.1,2,24 This morphological limitation is due to the strong electrostatic repulsion between the negatively charged chains forming the steric stabilizer layer. There are various technical solutions to this problem: (1) the addition of sufficient salt to screen the charge; (2) copolymerization with a nonionic comonomer; (3) the use of binary mixtures of polyelectrolytic stabilizer and nonionic stabilizer; and (4) the suppression of the ionization of acidic repeat units by lowering the solution pH in the case of poly(methacrylic acid)-based stabilizers.1,2,24 In our previous work, an anionic polyelectrolytic block based on poly(potassium 3-sulfopropyl methacrylate) (PKSPMA) was used to polymerize HPMA in order to form sterically stabilized colloidal particles.24 A systematic study of the effect of varying the diblock copolymer composition, the overall solids content, and the effect of added salt for this RAFT aqueous dispersion formulation was undertaken. In addition, the effect of diluting the coronal charge density was examined by either (i) statistical copolymerization of KSPMA with 2hydroxyethyl methacrylate (HEMA) or (ii) using binary mixtures of PKSPMA and PGMA macromolecular chaintransfer agents (macro-CTAs).24 The latter approach proved to be particularly useful in allowing access to worm and vesicle copolymer morphologies. This work examines the RAFT aqueous dispersion polymerization of HPMA using a macro-CTA based on quaternized poly(2-(dimethylamino)ethyl methacylate). In principle, the resulting cationic nano-objects could serve as colloidal templates for silicification,25 may find uses as antimicrobial coatings,26,27 or may act as model nanoparticles for electrostatic adsorption onto a range of anionic colloidal or planar surfaces. Several strategies are explored to vary the surface charge density, size, and morphology of such nano-objects systematically, including the nature of the macro-CTA, the total solids concentration at which the HPMA polymerization is conducted, and the concentration of added salt. To the best of our knowledge, there are no previous reports describing the direct preparation of cationic nanoparticles in water via polymerization-induced self-assembly.

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EXPERIMENTAL SECTION

Materials. All reagents were purchased from Sigma-Aldrich (U.K.) and were used as received unless otherwise stated. 4,4′-Azobis-4cyanopentanoic acid (ACVA, >98%) was used as an initiator. Glycerol monomethacrylate (GMA, 99.8%) was kindly donated by Cognis (Hythe, U.K.). HPMA comprises approximately 75% 2-hydroxypropyl methacrylate and 25 mol % 2-hydroxyisopropyl methacrylate.28 Analytical Techniques. NMR spectra were acquired in either D2O or CD3OD using a Bruker 250 or 400 MHz spectrometer. All chemical shifts are reported in ppm (δ). TEM studies were conducted using a Philips CM 100 instrument operating at 100 kV. To prepare TEM samples, 5 μL of a dilute aqueous copolymer dispersion was placed on a carbon-coated copper grid, stained with ammonium molybdate, and dried under ambient conditions. DLS studies were 7417

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Scheme 1. RAFT Aqueous Dispersion Polymerization of 2-Hydroxypropyl Methacrylate at 70 °C to Produce Sterically Stabilized Cationic Diblock Copolymer Nanoparticlesa

a

Either (a) poly(2-(methacryloyloxy)ethyl trimethylammonium iodide) macro-CTA or (b) cationic poly(2-(methacryloyloxy)ethyl trimethylammonium iodide)-stat-glycerol monomethacrylate) macro-CTA was used.

Figure 1. Representative transmission electron micrographs obtained for spherical PQDMA32-PHPMA500 block copolymer nanoparticles synthesized by RAFT aqueous dispersion polymerization of HPMA at 70 °C for 10% w/w solids in the presence of (a) no salt, (b) 0.10 M NaCl, or (c) 0.30 M NaCl (entries 1, 5, and 9, respectively, in Table S1).

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and isolated by lyophilization. A mean DP of 32 was calculated for this macro-CTA using 1H NMR spectroscopy by comparing the integrated signal intensity due to the aromatic protons at 7.2−7.4 ppm with that due to the methacrylic polymer backbone at 0.4−2.5 ppm (Figure S1a). Synthesis of Poly(2-(methacryloyloxy)ethyl trimethylammonium iodine-stat-glycerol monomethacrylate) (P(QDMAstat-GMA)) Macro-CTA Agent. In a typical experiment, a roundbottomed flask was charged with GMA (6.00 g, 37.5 mmol), QDMA (1.25 g, 4.16 mmol), PETTC (353 mg, 1.04 mmol, dissolved in 1 mL of dioxane), ACVA (29.16 mg, 0.104 mmol), and pH 5.5 buffer (6.00 g). The sealed reaction vessel was purged with nitrogen and placed in a preheated oil bath at 70 °C for 2.5 h. The resulting P(GMA-statQDMA) macro-CTA (conversion = 96%, Mn = 15 500 g mol−1, Mw = 18 300 g mol−1, Mw/Mn = 1.19) was purified by dialysis against deionized water and isolated by lyophilization. A mean DP of 46 was calculated for this macro-CTA using 1H NMR spectroscopy (Figure S1b), which corresponds to a mean copolymer composition of P(GMA41-stat-QDMA5). Block Copolymer Nanoparticle Synthesis via RAFT Aqueous Dispersion Polymerization. In a typical aqueous dispersion polymerization of PQDMA32-PHPMA100 at 10% w/w solids, HPMA (1.00 g, 6.94 mmol), ACVA (3.89 mg, 0.014 mmol), and PQDMA32 macro-CTA (0.664 g, 0.069 mmol) were dissolved in water (15.0 g). The reaction mixture was sealed in a round-bottomed flask, purged with nitrogen for 15 min, and then placed in a preheated oil bath at 70 °C for 24 h. The more concentrated formulations (15, 20, and 25% w/ w) were obtained similarly by reducing the amount of solvent, as required.

RESULTS AND DISCUSSION As an extension of our recently published study based on the use of anionic polyelectrolyte macro-CTAs,24 herein we examine the synthesis of sterically stabilized diblock copolymer nanoparticles via RAFT aqueous dispersion polymerization of HPMA using a cationic polyelectrolyte macro-CTA as a steric stabilizer (Scheme 1). The RAFT synthesis of the PQDMAPHPMA diblock copolymer nanoparticles was performed in two steps. First, QDMA was polymerized using PETTC as the CTA and 4,4′-azobis(4-cyanopentanoic acid) as the free radical initiator (CTA/initiator molar ratio = 5.0) in an aqueous buffer solution at pH 5.5 so as to minimize the hydrolysis of the RAFT chain ends29 and hence ensure good blocking efficiencies. A small amount of 1,4-dioxane cosolvent was used in this macro-CTA synthesis to ensure complete dissolution of the PETTC. The solution polymerization was terminated after 6 h, and aqueous GPC studies (vs poly(ethylene oxide) calibration standards) indicated a polydispersity of 1.15 for the resulting PQDMA macro-CTA. The second step was performed under RAFT aqueous dispersion polymerization conditions using this near-monodisperse macro-CTA dissolved in distilled water along with HPMA and 4,4′azobis(4-cyanopentanoic acid) initiator. The macro-CTA/ initiator molar ratio was fixed at 5.0, and the HPMA polymerization was conducted at 70 °C. Unlike our earlier studies,14−20 it was not feasible to use GPC to assess the living character of these RAFT syntheses because there is no suitable 7418

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Figure 2. Representative transmission electron micrographs obtained for spherical nanoparticles of P(QDMA11-stat-GMA116)-PHPMAx synthesized by RAFT aqueous dispersion polymerization at 70 °C for 10% w/w solids in the presence of 0.30 M NaCl with mean degrees of polymerization for the core-forming PHPMA block of (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, and (f) 900 (entries 3−8, respectively, in Table S2).

Figure 3. Representative transmission electron micrographs obtained for P(QDMA5-stat-GMA41)-PHPMAx particles prepared by RAFT aqueous dispersion polymerization at 70 °C for 10% w/w solids in the presence of 0.30 M NaCl with target PHPMA DPs of (a) 150, (b) 200, (c) 220, (d) 250, and (e) 300 (entries 1−5, respectively, in Table S3).

common organic solvent that dissolves both the highly cationic PQDMA block and the weakly hydrophobic PHPMA block. Nevertheless, on the basis of our previous experience of using various macro-CTAs to polymerize HPMA via RAFT aqueous dispersion polymerization, we would expect reasonably good living character to be achieved under such conditions.14−20 A series of PQDMA32-PHPMAx diblock copolymers were synthesized in aqueous media targeting mean degrees of polymerization (x) of 100, 200, 300, and 500 for the coreforming PHPMA block in the presence or absence of NaCl (Table S1). In the absence of added salt, spherical particles could be obtained, but these proved to be rather polydisperse (Figure 1a). This is because strong repulsive electrostatic forces between neighboring stabilizer chains in the coronal layer hinder efficient self-assembly, as reported previously.24

However, in the presence of either 0.10 or 0.30 M NaCl this systematic approach led to a series of spherical nanoparticles with increasing mean diameters (Figures 1 and S1). To promote more efficient chain packing, the charge density of the polyelectrolyte stabilizer chains was reduced by statistically copolymerizing QDMA with a neutral comonomer, glycerol monomethacrylate (GMA). The following three different macro-CTAs containing approximately 10% QDMA were synthesized: P(QDMA11-stat-GMA116), P(QDMA5-statGMA41), and P(QDMA2-stat-GMA20). The longest macroCTA, P(QDMA11-stat-GMA116), produces only spherical nanoparticles. Nevertheless, the mean diameter of these spherical nanoparticles could be accurately tuned, which is in accordance with our previous reports.15,16 As shown in Figure 2 and Table S2, uniform spherical particles can be obtained at 7419

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Figure 4. Representative transmission electron micrographs obtained for particles of P(QDMA2-stat-GMA20)-PHPMAx prepared at 10% w/w solids by RAFT aqueous dispersion polymerization at 70 °C in the presence of 0.10 M NaCl with target PHPMA DPs of (a) 50, (b) 80, (c) 100, and (d) 150 and in the presence of 0.30 M NaCl with target PHPMA DPs of (e) 50, (f) 80, (g) 100, and (h) 150 (entries 2−9, respectively, in Table S4).

Figure 5. (a, b) Conversion vs time data calculated from 1H NMR spectra (D2O) for the RAFT aqueous dispersion polymerization of HPMA using a P(QDMA5-stat-GMA41) macro-CTA at 70 °C, conducted at either 10, 15, or 20% w/w solids. The targeted diblock composition was P(QDMA5stat-GMA41)-PHPMA500 in each case. Transmission electron micrographs obtained for (c) P(QDMA5-stat-GMA41)-PHPMA80, (d) P(QDMA5-statGMA41)-PHPMA250, and (e) P(QDMA5-stat-GMA41)-PHPMA500 prepared at 15% w/w solids.

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10% w/w solids in the presence of 0.30 M NaCl. The mean TEM particle diameter increases from 40 to 82 nm as the target DP of the core-forming PHPMA block increases from 300 to 900. To access so-called higher order morphologies such as worms or vesicles, a somewhat shorter P(QDMA5-stat-GMA41) macro-CTA was used. The target PHPMA DPs varied were between 100 and 300, and polymerization was again conducted at 10% w/w in the presence of 0.30 M NaCl. TEM studies (Figure 3) confirm that a full range of copolymer morphologies were obtained, ranging from spheres to worms to vesicles. The lowest targeted PHPMA DP of 150 results in relatively uniform spherical particles of 50 nm diameter (Table S3). As the PHPMA block length is gradually increased, distinctly anisotropic wormlike particles are formed, followed by a mixed worm/vesicle phase and ultimately pure vesicles with a relatively low DLS polydispersity of 0.07. The shortest macro-CTA is P(QDMA2-stat-GMA20), which contains on average only two cationic units per chain. This steric stabilizer shows a similar trend when the target DP of the core-forming PHPMA block is varied between 50 and 150 (Table S4). In the presence of 0.10 M NaCl, spherical nanoparticles are observed for PHPMA DPs of 50 or 80 (Figure 4a,b). Increasing the target DP to either 100 or 150 produces a purely vesicular phase (Figure 4c,d). When the salt content is increased to 0.30 M NaCl, spherical particles are formed when targeting a PHPMA DP of 50 (Figure 4e). At a target DP of 80 (Figure 4f), there is some evidence for a possible worm phase, but an alternative interpretation could be simply random aggregates of nanoparticles. Higher DPs (100 or 150) result in the formation of vesicles (Figure 4g,h). Kinetic studies (Figure 5) were conducted using the P(QDMA5-stat-GMA41) macro-CTA at either 10, 15, or 20% w/w solids and monitored by 1H NMR spectroscopy. Monomer conversions were calculated on the basis of the disappearance of the vinyl signals relative to the growing signals resulting from the methacrylic backbone. The reaction mixture was initially transparent at all three concentrations, but in each case the turbidity increased as the polymerization proceeded, indicating in situ phase separation. This is because the HPMA monomer is water-soluble but the growing PHPMA chains gradually become insoluble, which leads to polymerizationinduced self-assembly to form diblock copolymer nano-objects. Micellar nucleation occurs after around 70 min at 10% w/w, 30 min at 15% w/w, and 20 min at 20% w/w solids. Thereafter, an enhanced rate of polymerization is observed as a result of the relatively high local concentration of HPMA monomer within the micelle cores (first-order kinetic plot in Figure 5). At 10% w/w, polymerization is essentially complete after 150 min. As expected, this characteristic time scale is reduced to either 70 or 40 min as the total solids concentration is increased to 15 or 20% w/w, respectively. Unfortunately, none of the diblock copolymers could be analyzed using GPC because of their insolubility in common GPC eluents (e.g. THF, water, chloroform, or DMF). Figure 6 shows the aqueous electrophoretic curves obtained for a series of block copolymer nanoparticles prepared using differing cationic charge densities in their stabilizer chains at a fixed target DP of 200 for the core-forming PHPMA block. As expected, the particles produced using the PQDMA32 macroCTA exhibit the most positive zeta potentials (at least +45 mV) over the entire pH range studied. The zeta potential decreases as the charge density in the stabilizer layer is reduced but

Figure 6. Aqueous electrophoresis curves obtained for (a) PQDMA32PHPMA200 (■), (b) P(QDMA11-stat-GMA116)-PHPMA200 (▲), (c) P(QDMA5-stat-GMA41)-PHPMA200 (●), and (d) P(QDMA2-statGMA20)-PHPMA200 (▼). In each case, these block copolymer nanoparticles were prepared via RAFT aqueous dispersion polymerization at 10% w/w solids in the presence of 0.30 M NaCl. Electrophoretic measurements were conducted using 0.01 wt % dispersions in the presence of 10−3 M background NaCl.

remains positive from pH 2 to pH 8, even when each copolymer stabilizer chain contains only two cationic quaternary amine methacrylate units. In Figure 7, aqueous

Figure 7. Aqueous electrophoresis curves obtained for the following block copolymer nano-objects: (a) P(QDMA 5 -stat-GMA 41 )PHPMA100 (▲), (b) P(QDMA5-stat-GMA41)-PHPMA150 (■), and (c) P(QDMA5-stat-GMA41)-PHPMA250 (●). In each case, these aqueous dispersions were prepared at 10% w/w solids in the presence of 0.30 M NaCl. Electrophoretic measurements were conducted on 0.01 wt % dispersions in the presence of 10−3 M background NaCl.

electrophoretic curves are shown for spherical, wormlike, and vesicular particles prepared using the same P(QDMA5-statGMA41) macro-CTA when targeting DPs of 100, 150, and 250 for the core-forming PHPMA chains. The spherical particles exhibit the highest zeta potential (+26 mV) at pH 2, with values of +22 and +19 mV being observed for the block copolymer worms and vesicles, respectively. All three copolymer morphologies retain positive zeta potentials over the entire pH range from 2 to 10, although the zeta potential steadily decreases at higher pH. As reported previously,24 the polymerization of HPMA in the presence of a binary mixture of two macro-CTAs is a convenient strategy for modulating the coronal charge density of anionic block copolymer nanoparticles (Scheme 2). From an 7421

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example, nonionic PGMA60 stabilizer chains dilute the strong lateral electrostatic repulsion between highly cationic PQDMA32 stabilizer chains within the coronal layer, which promotes polymerization-induced self-assembly. The proportion of PQDMA macro-CTA chains was systematically varied from 5 to 50 mol %, and the mean DP of the core-forming PHPMA chains was fixed at 500, with all RAFT syntheses being conducted at 10% w/w solids in the presence of either 0.15 M or 0.30 M NaCl. To confirm that true entropic mixing of the two types of stabilizer chains had occurred within the coronal layer, we examined the raw mobility data obtained at pH 4.0 prior to calculating the zeta potentials. As expected, nonionic PGMA60PHPMA500 block copolymer nanoparticles (which were prepared as a reference) exhibited essentially zero mobility. In contrast, PQDMA32-PHPMA500 nanoparticles exhibited a relatively high mobility of around +3.5 μm cm/V s whereas the mixed corona (3 PQDMA 32 + 7 PGMA 60 )-PHPMA 500

Scheme 2. Schematic Representation of the Synthesis of Cationic Diblock Copolymer Nanoparticles with Mixed Coronas via RAFT Aqueous Dispersion Polymerization of HPMA at 70 °C Using a Binary Mixture of an Anionic PQDMA Macro-CTA and a Nonionic PGMA Macro-CTAa

a

The latter stabilizer reduces the overall coronal charge density and hence enables polymerization-induced self-assembly to occur in the presence of sufficient added salt.

entropic viewpoint, this approach is expected to produce block copolymer nanoparticles with mixed coronas. Hence, for

Figure 8. Electrophoretic mobilities obtained for 0.10% aqueous dispersions of the following diblock copolymer nanoparticles at pH 4.0: (a) PGMA60-PHPMA500, (b) PQDMA32-PHPMA500, and (c) (3 PQDMA32 + 7 PGMA60)-PHPMA500. Originally, PGMA60-PHPMA500 was prepared at 10% w/w solids in the absence of any salt, and PQDMA32-PHPMA500 and (3 PQDMA32 + 7 PGMA60)-PHPMA500 were prepared at 10% w/w solids in the presence of 0.30 M NaCl. 7422

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Figure 9. Transmission electron micrographs obtained for the RAFT aqueous dispersion polymerization of HPMA at 70 °C using various xPQDMA32 + yPGMA60 binary mixtures in the absence of added salt: (a) x/y molar ratio = 1:1, (b) x/y molar ratio = 1:4, and (c) x/y molar ratio = 1:9. The total solids concentration was 10% w/w and the target DP of the core-forming PHPMA block was 500 in each case.

almost constant over the whole pH range. As the cationic stabilizer character is gradually diluted by incorporating a higher proportion of nonionic PGMA chains, the zeta potential gradually decreases at any given pH. For the 1:1 mixture of PQDMA32 and PGMA60, the zeta potential remains positive (+40 to +2 mV) from around pH 2 to 10. A systematic shift to lower zeta potentials occurs as the binary mixture composition is adjusted to 30, 20, or 10 mol % PQDMA32. The latter compositions exhibited isoelectric points at pH 6.2, 6.2, and 4.7, respectively, with the latter lying close to the IEP of PGMA60PHPMA500 particles at around pH 4.

nanoparticles prepared using 30 mol % PQDMA32 macro-CTA had an intermediate mobility of around +2.5 μm cm/V s (Figure 8). This binary mixture of macro-CTAs strategy provides fine control over the electrophoretic footprint of the nanoparticles as well as convenient access to either wormlike or vesicular morphologies (Figure 9). Thus when HPMA polymerizations were performed at 10% w/w solids using 5 to 20 mol % PQDMA macro-CTA, block copolymer vesicles were formed even in the absence of salt (Figure S3d,e). TEM studies confirmed the presence of worms when using 20 mol % PQDMA stabilizer (Figures 9b and S3c), whereas only spheres were observed when 30 or 50 mol % PQDMA stabilizer was employed (Figures 9a and S3a,b). Block copolymer vesicles were observed only when using 20, 10, and 5 mol % PQDMA stabilizer in the presence of 0.15 M NaCl (Figure S3h−j). Finally, regardless of the precise binary mixture of cationic/ nonionic macro-CTAs employed, HPMA polymerizations conducted at higher salt concentration (e.g., 0.30 M NaCl) always resulted in the formation of block copolymer vesicles (Figure S3k−o). This approach also provided good control over the electrophoretic footprint exhibited by the block copolymer nanoparticles (Figure 10). As expected, the highest zeta potential (+50 mV) was observed for a stabilizer shell comprising pure PQDMA32 chains, and this value remained

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CONCLUSIONS

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ASSOCIATED CONTENT

A range of cationic polyelectrolyte-stabilized diblock copolymer nanoparticles were prepared via RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a near-monodisperse PQDMA-based macro-CTA. The strongly cationic character of the PQDMA stabilizer hinders polymerization-induced self-assembly because strong lateral repulsive forces between adjacent copolymer chains prevent their efficient packing. This leads to the formation of rather illdefined pseudo-spherical diblock copolymer nanoparticles. However, this problem can be overcome by the addition of sufficient salt, which screens these repulsive forces and leads to the formation of significantly denser, more well-defined nanoparticles. Alternatively, the anionic charge density of the stabilizer chains can be systematically reduced by statistically copolymerizing QDMA with a nonionic comonomer such as glycerol monomethacrylate. The latter approach allows the electrophoretic behavior of these cationic nanoparticles to be precisely tuned. A third strategy for promoting polymerizationinduced self-assembly involves using a binary mixture of the cationic PQDMA macro-CTA and a nonionic poly(glycerol monomethacrylate) macro-CTA in order to modulate the repulsive electrostatic forces. This latter approach enables both wormlike and vesicular morphologies to be obtained, particularly for syntheses conducted in the presence of salt in relatively concentrated solution.

Figure 10. Aqueous electrophoresis data obtained for the following cationic block copolymer nanoparticles: (a) PQDMA32-PHPMA500 (●), (b) (1PQDMA32 + 1PGMA60)-PHPMA500 (□), (c) (3PQDMA32 + 7PGMA60)-PHPMA500 (○), (d) (1PQDMA32 + 4PGMA60)PHPMA500 (▲), (e) (1PQDMA32 + 9PGMA60)-PHPMA500 (■), and (f) PGMA60-PHPMA500 (▽). All particles were prepared at 10% w/w solids in the presence of 0.30 M NaCl.

S Supporting Information *

Additional TEM images and DLS data obtained for these cationic diblock copolymer nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. 7423

dx.doi.org/10.1021/la304279y | Langmuir 2013, 29, 7416−7424

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AUTHOR INFORMATION

Corresponding Author

*E-mail: s.p.armes@sheffield.ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC for the postdoctoral support of M.S. (EP/G007950/1). REFERENCES

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