J. Phys. Chem. C 2009, 113, 3471–3477
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Controllable Formation of Nanorods through Electrostatic-Assisted Assembly of Star Poly(methacrylic acid) Induced by Surfactants Jianhua Ding, Li Wang,* Haojie Yu, Jia Huo, Qingquan Liu, and Anguo Xiao State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang UniVersity, Hangzhou, China, 310027 ReceiVed: August 23, 2008; ReVised Manuscript ReceiVed: December 29, 2008
Star poly(methacrylic acid) (Star-(PMAA)6) was prepared by hydrolysis of star poly(tert-butyl methacrylate) (Star-(PtBMA)6). The self-assembly behaviors of Star-(PMAA)6 induced by surfactant hexadecyl pyridinium chloride (HDPC) were investigated. It is of great interest that rodlike aggregates with different lengths and diameters could be obtained from the complexation of Star-(PMAA)6 and HDPC by adjusting the length of the polymer arm and the ratio of [HDPC]/[carboxyl group] in the system. The self-assembly mechanisms for the formation of rodlike aggregates were discussed. Introduction Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solution and make the polymers charged.1-4 Polyelectrolyte properties are thus similar to both electrolytes and polymers. Like polymers, polyelectrolytes can self-assemble to form a range of different morphologies such as vesicles and tubes through electrostatic or hydrogen bond interactions.5-8 Furthermore, many biological molecules are polyelectrolytes, for instance, polypeptides (thus all proteins) and DNA,9,10 and polyelectrolytes play a fundamental role in determining the structures, stabilities, and interactions of various molecular assemblies, while their unique properties are being exploited in a wide range of technological and industrial fields.11,12 Aggregates formed by the self-assembly of polyelectrolytes have many potential applications in biology and biomedicine.13,14 The morphologies and sizes of the aggregates formed by the selfassembly of polyelectrolytes have a great effect on their applications. Aggregates with required sizes and morphologies can be designed through modification of the structural parameter of the polyelectrolyte and the self-assembly conditions.15,16 It has been reported that the complexation of polyelectrolytes in water with either oppositely charged polyelectrolytes or surfactants can induce self-assembly.17-19 The complexation of polyelectrolytes with surfactants has been especially focused on, since there are a wide variety of available surfactants to choose from and the compositions of the resultant complexes and aggregates are readily adjustable. Star polyelectrolytes with a hexafunctional crystalline cyclotriphosphazene derivative core exhibit special three-dimensional shapes, which is explained in terms of a model consisting of six polymer chains arranged approximately perpendicular to the cyclotriphosphazene ring, and have the ability to self-assemble in bulk and in solution forming a range of different morphologies from their linear polymer counterparts. In the present study, six-arm star polyelectrolytes with a hexafunctional cyclotriphosphazene core were synthesized. The self-assembly behaviors of these star polyelectrolytes induced by hexadecyl pyridinium chloride (HDPC) were investigated. It was found that the * Corresponding author. Telephone: +86-571-8795-3200. Fax: +86-5718795-1612. E-mail:
[email protected].
carboxyl groups in polymer chains of Star-(PMAA)6 can complex with the pyridine groups of HDPC and the complexation between Star-(PMAA)6 and HDPC results in rodlike aggregates; the ratio of [HDPC]/[carboxyl group] in the system and the length of the polymer chain of Star-(PMAA)6 have great effects on the morphology of the aggregates formed. Experimental Section Materials. Hexachlorocyclotriphosphazene was recrystallized from dry hexane followed by sublimation twice before use. 4-Hydroxybenzaldehyde was purchased from Acros Organics. Sodium borohydride was purchased from Shanghai Chemical Reagents Corp. (Shanghai, China), and 2-bromoisobutyryl bromide was purchased from Sigma-Aldrich. Tetrahydrofuran (THF) was refluxed over potassium and distilled in argon atmosphere. Triethylamine (Et3N) was distilled from KOH. Anisole was treated with baked molecular sieves. tert-Butyl methacrylate was purchased from Acros Organics and washed with an aqueous solution of sodium hydroxide (NaOH) (5 wt %) three times and then with water until neutralization. After being dried with anhydrous magnesium sulfate, the tert-butyl methacrylate monomers were distilled at reduced pressure. Copper(I) (CuCl, AR grade) was purified by stirring in acetic acid, washed with methanol, and then dried under reduced pressure. 2,2′-Bipyridyl (bpy), AR grade, was used as obtained from Shanghai No. 1 Chemical Reagent Factory. 1,1,3,3,5,5Hexakis(4-(2-bromoisobutyryloxymethyl)phenoxy)cyclotriphosphazene (HBPC) was prepared by the literature method.20 Other reagents were of analytical grade and were used as received without further purification. Synthesis of Star Poly(tert-butyl methacrylate) (Star(PtBMA)6). 21,22 The polymerization was carried out at 90 °C with the feed ratio of [monomer]/[HBPC]/[CuCl]/[bpy] ) 600/ 1/6/12 (molar ratio). A typical procedure is described as follows. Into a 10 mL glass tube CuCl, bpy, HBPC, and tert-butyl methacrylate were successively added and then certain amount of anisole was added to the tube through syringe. The heterogeneous mixture was cycled between vacuum and nitrogen four times to remove air. The tube was sealed under vacuum and then immersed in an oil bath at the preset temperature. After the reaction was carried out for a prescribed time, the tube was rapidly cooled to room temperature with ice-water, and the
10.1021/jp8075468 CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
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Figure 1.
Ding et al.
1
H NMR spectra of Star-(PtBMA)6 (A) and Star-(PMAA)6 (B).
SCHEME 1: Synthesis Procedure of Star-(PMAA)6
polymer solution in THF was passed through a short column of neutral alumina to remove the copper salts. The polymer was precipitated from an excess of ethanol/water (v/v ) 70/30) mixture, filtered, and dried at 50 °C under vacuum to constant weight. 1H NMR (400 MHz, CDCl3): 7.20-6.91 (4H in -C6H4-), 4.98 (2H in -CH2O-), 2.10-1.70 (2H in -CH2-C-), 1.40 (12H in -COOC(CH3)3), 1.20-0.82 (3H in -CH2-C(CH3)-), 1.12 (6H in -C(CH3)2-).
Synthesis of Star Poly(methacrylic acid) (Star-(PMAA)6). The hydrolysis of tert-butyl side groups on the Star-(PtBMA)6 to acrylic acid was carried out following a literature procedure. Trifluoroacetic acid (5 times excess to the tert-butyl groups, 0.52 mL, 7.035 mmol) was added dropwise to a solution of Star-(PtBMA)6 (200 mg, 1.406 mmol tert-butyl groups) in 6.0 mL of dichloromethane. The reaction mixture was stirred at room temperature for 12 h, after which the solution became 23
Electrostatic-Assisted Assembly of Star-(PMAA)6
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3473 TABLE 1: Molecular Weight and Molecular Weight Distribution of Star-(PtBMA)6 polymera
Mna
Mw/Mna
Star-(PtBMA25)6 Star-(PtBMA36)6
2.26 × 104 3.07 × 104
1.27 1.24
a Mn(GPC) and Mw/Mn were determined by GPC analysis with polystyrene standards. THF was used as eluent.
Flash EA1112 element analysis system (Thermo Finnigan). TEM micrographs were obtained on a JEOL Model 1200EX instrument operated at an accelerating voltage at 160 kV. Results and Discussion Figure 2. GPC traces of Star-(PtBMA25)6 (A) and Star-(PtBMA36)6 (B).
cloudy with observed precipitation, which indicated hydrolysis of the tert-butyl side groups. The resultant star polymer was filtered and washed with THF three times, and then dried at 50 °C under vacuum to constant weight. 1H NMR (400 MHz, DMSO): 10.50-12.00 (1H in -COOH), 7.25-6.92 (4H in -C6H4-), 4.98 (2H in -CH2O-), 2.10-1.25 (2H in -CH2-C-), 1.15-0.80 (3H in -CH2-C(CH3)-), 1.12 (6H in -C(CH3)2-). Self-Assembly of Star-(PMAA)6 Induced by Surfactants. Extremely dilute solutions of complexes between Star-(PMAA)6 and hexadecylpyridinium chloride monohydrate (HDPC) containing 0.20 wt % Star-(PMAA)6 were prepared by dissolving a certain amount of Star-(PMAA)6 and different amounts of HDPC in water followed by sonication for 1 h. The selfassembly structures with different sizes and morphologies formed spontaneously. Samples for transmission electron microscopy (TEM) measurement were prepared by aerosol spraying a dilute micellar solution (ca. 10 µL) onto 200-mesh gilder copper TEM grids. The samples were air-dried before introduction into the TEM. The morphology and structure of the prepared samples were examined with TEM. Characterization. 1H NMR was obtained from a 400 MHz AVANCE NMR spectrometer (Model DMX400). For protons, the chemical shifts were relative to tetramethylsilane at δ ) 0 ppm. The molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) with a laser scattering detector. The eluent was THF at a flow rate of 1.0 mL min-1, and calibrations were narrow-distribution polystyrene standards. Element analysis was obtained from a
Synthesis and Characterization of Star-(PtBMA)6 and Star-(PMAA)6. We prepared six-armed Star-(PtBMA)6 with cyclotriphosphazene core by atom transfer radical polymerization (ATRP) of tert-butyl methacrylate with HBPC as initiator. Star-(PMAA)6 was prepared through hydrolysis of the tert-butyl moieties in the polymer chain of Star-(PtBMA)6. The synthesis route is depicted in Scheme 1. The chemical structures of the prepared star polymers were characterized by 1H NMR analysis, as shown in Figure 1. The 1 H NMR spectrum of Star-(PtBMA)6 (Figure 1A) showed peaks with following shifts: peaks at 2.10-1.70 ppm are assigned to the protons of the -C-CH2- group, the peak at 1.40 ppm is attributed to the protons of the tert-butyl group, and peaks at 1.20-0.82 ppm are attributed to the protons of the methyl group. The signals corresponding to the protons of HBPC can also be clearly obtained from the 1H NMR spectra of Star-PtBMA. The peak at 1.12 ppm can be attributed to the methyl protons of HBPC, peaks at 7.20-6.91 ppm correspondto the aromatic protons of HBPC, and the peak at 4.98 ppm is assigned to the methylene protons of HBPC. The peak at 3.74 ppm corresponds to the proton of methine at the end of the each polymer chain. This indicates that Star-(PtBMA)6 was prepared. Figure 1B is the 1H NMR spectrum of Star-(PMAA)6, compared with the 1H NMR spectrum of Star-(PtBMA)6; peaks at 10.50-12.00 ppm corresponding to carboxyl proton of methacrylic acid groups appeared and the peak at 1.40 ppm attributed to the protons of the tert-butyl group disappeared. The comparison of 1H NMR spectra indicates the entire hydrolysis of Star-(PtBMA)6 and Star-(PMAA)6 was obtained. The molecular weight of Star-(PtBMA)6 is shown in Table 1. The GPC curves (Figure 2) of star Star-(PtBMA)6 are symmetrical and monomodal, suggesting that no mixture of star and linear polymers was formed and these Star-(PtBMA)6 have well-controlled molecular weights and relatively narrow molecular weight distributions.
Figure 3. TEM images of self-assembly aggregates from Star-(PMAA25)6 at various polymer concentrations in H2O. Polymer concentrations: 0.1 (A) and 0.3 wt % (B).
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Figure 4. TEM images of self-assembly aggregates from Star-(PMAA25)6-HDPC at various HDPC/Star-(PMAA25)6 ratios at Star-(PMAA25)6 concentration of 0.2 wt % and H2O as solvent. HDPC/complex units: 0.8/1 (A1, A2), 1/1 (B1, B2), and 1.2/1 (C1, C2). Histogram of length distribution of rods with HDPC/complex units ratio of 1/1 (D1) and 1.2/1 (D2).
Self-Assembly Behaviors of Star-(PMAA)6 Induced by Surfactants. Generally speaking, with the interaction of polymer chains and the oriented arrangement of cyclotriphosphazene derivative cores, Star-(PMAA25)6 would self-assembly to form spherical giant vesicles of micrometer scale in dimension at a low polymer concentration without the effect of HDPC as shown in Figure 3. While induced by the surfactant of HDPC, Star(PMAA25)6 formed rodlike aggregates with an average diameter of 30 nm in water. Star-(PMAA25)6 was mixed with HDPC in water at a fixed star polyelectrolyte concentration of 0.2 wt % but with varying [HDPC]/[carboxyl group] ratio (denoted as
HCR) of 0.8/1, 1/1, and 1.2/1; rodlike aggregates with different lengths and shapes were obtained. The morphologies of the resultant aggregates were observed by transmission electron microscopy (TEM). The TEM images for the aggregates formed by the Star-(PMAA25)6-HDPC system with different HCRs are shown in Figure 3. When the HCR was 0.8/1, rodlike aggregates with a uniform diameter of 30 nm and broad length distribution of 60-300 nm appeared. Also, a large number of irregularly shaped aggregates with an average dimension of less than 30 nm appeared with the rod (Figure 3A1,A2). At the HCR of 1/1, rodlike aggregates with
Electrostatic-Assisted Assembly of Star-(PMAA)6
Figure 5. Possible schematic model for formation of a rod. Green, cyclotriphosphazene core; pink, PMAA; blue, HDPC.
uniform dimension were obtained and the rods obtained had an average diameter of 30 nm and an average length of 200 nm (Figure 3B1,B2). The self-assembly of Star-(PMAA25)6 induced by HDPC at the HCR of 1.2/1 results in rodlike aggregates with a uniform diameter of 30 nm but the length of the rods exhibits a bimodal distribution. The first type of rod possesses a relatively uniform length of 200 nm, and this type of rod is abundant in aggregates obtained, while the second type of rod has a relatively uniform length of 400 nm and the number of rods with longer length is less than those with shorter length. It is interesting to find that these two types of rods have a relation of 2-fold in length (Figure 4C1,C2). After carefully observing and analyzing the TEM images, we made statistics about the length distribution of the aggregates formed in the Star-(PMAA25)6-HDPC system with HDPC/complex unit ratios of 1/1 and 1.2/1, respectively. The statistical results of the length distribution of the rods were translated to histograms and are shown in Figure 4D1,D2. These histograms indicate the length distribution of rods which were transformed from unimodal distribution to bimodal distribution after the HDPC/complex unit ratio increased from 1/1 to 1.2/1. The carboxyl groups in polymer chains of Star-(PMAA25)6 can complex with the pyridine groups of HDPC. One star polyelectrolyte molecular can complex with many HDPC molecules, and the complexation of Star-(PMAA25)6 with many HDPC molecules in water results in the formation of a multimolecular associate. Star-(PMAA25)6 exhibits special threedimensional shapes, which are explained in terms of a model consisting of six polymer chains arranged approximately perpendicular to the cyclotriphosphazene ring and can be regarded as disk shapes. Thus, the multimolecular associate formed by Star-(PMAA25)6 and HDPC can be treated as a disk with its two faces covered with hydrophobic cetyl chains. These hydrophobic cetyl chains tend to aggregate in water, and with the inducement of cetyl chains multimolecular associates were forced to adopt a head-to-end association to form rodlike aggregates. Based on the self-assembly morphologies of the Star-
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3475 (PMAA25)6-HDPC system and the analysis above, we propose a model for the self-assembly behaviors of Star-(PMAA25)6 induced by HDPC with an electrostatic effect as shown in Figure 5. HDPC possesses a critical micelle concentration (cmc) value of about 0.34 mg/mL;24 therefore, the HDPC concentration in the self-assembly solution system should be above the cmc value. Even at a ratio of HDPC/complex units as low as 0.8/1, the HDPC concentration is also above the cmc value. In principle, when the HCR in the system is 0.8/1, there should be a remarkable number of carboxyl groups free to complex with HDPC. The multimolecular associates formed have many defects and lead to an irregular arrangement of the associates, and the head-to-end association of multimolecular associates becomes more difficult. Thus, the rodlike aggregates formed with broad length distribution and there also exist a large number of irregularly shaped aggregates. When the HCR is 1/1 in the system, theoretically speaking, all the carboxyl groups are complexed with HDPC and the multimolecular associates formed possess a perfect structure. Therefore, the association of multimolecular associates become more regular and the rodlike aggregates formed possess uniform dimension and longer length. When the HCR in the system is 1.2/1, there should be a remarkable number of HDPC molecules free to complex with Star-(PMAA25)6. Nevertheless, it is found that these uncomplexed HDPC molecules have a great effect on the self-assembly behavior of the Star-(PMAA25)6-HDPC system, compared with that at HCR 1/1. First, all of Star-(PMAA25)6 complexes with a stoichiometric quantity of HDPC molecules to form multimolecular associates, and then these associates adopt a head-toend association to form rodlike aggregates. There still exists a certain quantity of HDPC molecules free to complex with carboxyl groups, and these molecules will absorb to the end of the short rod. Short rods with the end absorbed with HDPC molecules are forced to adopt an end-to-end association and two short rods joined together form a rod with a length the sum of two short rods. Based on the self-assembly morphologies and the analysis above, we propose a model for the selfassembly behaviors of Star-(PMAA25)6 induced by an excessive quantity of HDPC molecules as shown in Figure 6. When the HDPC/complex unit ratio in the system is 0.4/1, which would also be more than the cmc value of HDPC, the number of HDPC molecular in the solution is too insufficient to have a great effect on the self-assembly of Star-(PMAA25)6, and the self-assembly behavior of the Star-(PMAA25)6-HDPC system is just similar to the self-assembly behavior of Star(PMAA25)6 (Figure 7). Star-(PMAA36)6 possesses longer polymer chains than Star(PMAA25)6 and, mixed with HDPC in water at a fixed star polyelectrolyte concentration of 0.2 wt % and a fixed HCR of 1/1, can form rodlike aggregates with an average diameter of 45 nm in water induced by HDPC. The TEM images for the
Figure 6. Possible schematic model for the formation of rod with increased length as an effect of excessive HDPC. Green, cyclotriphosphazene core; pink, PMAA; blue, HDPC.
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Figure 7. TEM images of self-assembly aggregates from Star-(PMAA25)6-HDPC at Star-(PMAA25)6 concentration of 0.2 wt % in H2O with HDPC/complex units ratio of 0.4/1.
Figure 8. TEM images of self-assembly aggregates from Star-(PMAA)6-HDPC at various HDPC/Star-(PMAA)6 ratios at Star-(PMAA)6 concentration of 0.2 wt % and H2O as solvent. HDPC/complex units in Star-(PMAA36)6 ) 1/1 (A, B, C), HDPC/complex units in Star-(PMAA25)6 ) 1/1 (D).
aggregates formed by the Star-(PMAA36)6 and HDPC system with an HCR of 1/1 are shown in Figure 8. The resultant aggregates are mainly of a short rodlike morphology with an uniform diameter and a broad length distribution of 100-300 nm (Figure 8A-C). Compared with the rodlike aggregates formed by the Star-(PMAA25)6-HDPC system at an HCR of 1/1 (Figure 8D), the rodlike aggregates formed by the Star(PMAA36)6-HDPC system at the same condition possess a larger diameter but a shorter length. Star-(PMAA36)6 molecules with more carboxyl groups can complex with more HDPC molecules than Star-(PMAA25)6. Multimolecular associates formed by the Star-(PMAA36)6HDPC system at an HCR of 1/1 possess a larger dimension and this characteristic will make the migration and arrangement of the multimolecular associates difficult. Thus, the aggregates formed possess a larger diameter but a shorter length, and the length distribution of rodlike aggregates is broad.
polyelectrolyte induced by HDPC in the effect of electrostatic bonds were investigated carefully. It was found that the complexation between Star-(PMAA)6 and HDPC results in rodlike aggregates; the ratio of [HDPC]/[carboxyl group] in the system and the length of the polymer chain of Star-(PMAA)6 have a great effect on the morphology of the aggregates formed. At the Star-(PMAA25)6 concentration of 0.2 wt %, when HCR is 0.8/1, short rodlike aggregates and irregularly shaped aggregates with a with a broad dimension distribution were obtained. When HCR is 1/1, rodlike aggregates with uniform dimensions were formed, and when HCR is 1.2/1, two types of rodlike aggregates with a uniform diameter of 30 nm but 2-fold in length are formed. The self-assembly of Star-(PMAA36)6 with HCR of 1/1 and polymer concentration of 0.2 wt % leads to the formation of rodlike aggregates with larger diameter and shorter length. The resultant aggregates possess a special rodlike structure, and the morphology of the aggregates can be adjusted by the HCR and the length of the polymer chains in the system.
Conclusions In conclusion, star-shaped polyelectrolyte Star-(PMAA)6 with a cyclotriphosphazene derivative core was successfully synthesized and the self-assembly behaviors of the star-shaped
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