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New Route to Amphiphilic Core-Shell Polymer Nanospheres: Graft Copolymerization of Methyl Methacrylate from Water-Soluble Polymer Chains Containing Amino Groups Pei Li,*,† Junmin Zhu,† Panya Sunintaboon,‡ and Frank W. Harris‡ Department of Applied Biology and Chemical Technology and Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis,§ The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China, and Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Received June 27, 2002 A novel method has been developed to prepare amphiphilic core-shell polymer nanospheres via graft copolymerizations of methyl methacrylate (MMA) from water-soluble polymer chains containing amino groups. Thus, amine-substituted biopolymers and synthetic polymers are treated with a small amount of tert-butyl hydroperoxide (TBHP, 0.08 mM) in water at 80 °C to generate free radicals on the amine nitrogens, which subsequently initiate the graft copolymerization of MMA. tert-Butoxy radicals are also generated that either initiate the homopolymerization of MMA or abstract hydrogen from the polymer backbones. The amphiphilic macroradicals generated in situ self-assemble to form polymeric micelle-like microdomains, which promote the emulsion polymerization of the monomer. Thus, well-defined, amphiphilic core-shell nanospheres, which range from 60 to 160 nm in diameter, are produced in the absence of surfactant. The conversion and grafting efficiency of the monomer strongly depend on the TBHP concentration and the structure of the amino-containing water-soluble polymer. Polymers containing primary amine groups are considerably more effective than those containing secondary or tertiary groups, while ammonium cations do not induce the polymerization. The particle size and stability strongly depend on the structure and molecular weight of the hydrophilic polymer, as well as the pH of the mixture. Transmission electron microscopic (TEM) images of the particles clearly show well-defined core-shell morphologies where PMMA cores are coated with hydrophilic polymer shells. The amphiphilic core-shell nanospheres can be produced in high concentrations (up to 22% solids content). This new method is scientifically and technologically significant because it provides a commercially viable route to a wide variety of novel amphiphilic coreshell nanospheres.
Introduction Interest in the design and controlled fabrication of composite nanoparticles that consist of hydrophobic polymer cores coated with hydrophilic polymer shells continues to increase. Applications for these particles are very diverse, including diagnostic testing, bioseparations, controlled release of drugs and other biological agents, gene therapy, catalysis, and water-borne coatings.1,2 Amphiphilic core-shell particles are also of interest from a fundamental and academic point of view, especially in the area of colloid and interface science. Such particles have been prepared according to four general approaches: (1) stepwise deposition of polyelectrolytes onto charged particle surfaces;3 (2) self-assembly of amphiphilic block copolymers followed by covalent cross-linking of the * Corresponding author. E-mail:
[email protected]. † The Hong Kong Polytechnic University. ‡ The University of Akron. § The University Grants Committee Area of Excellence Scheme (Hong Kong). (1) (a) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111-1114 and references therein. (b) Caruso, F. Adv. Mater. 2001, 13, 11-22. (2) Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1397. (3) (a) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf., A: Physicochem. Eng. Aspects 1998, 137, 253-266. (b) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; Mo¨hwald, H. Polym. Adv. Technol. 1998, 9, 759-767. (c) Caruso, F.; Mo¨hwald, H. J. Am. Chem. Soc. 1999, 121, 6039-6046.
shells to form shell-cross-linked “knedle” (SCK) micelles;4 (3) graft copolymerization of hydrophilic monomers onto reactive seeded particles;5 (4) free radical copolymerization of hydrophilic macromonomers or surfmers with hydrophobic monomers.6 Despite the success of these approaches, there are still some drawbacks, such as timeconsuming sequential polyelectrolyte deposition cycles and purification steps, tedious multiple-step syntheses, and the use of hydrophilic monomers, resulting in low surface incorporation and the formation of a large amount of watersoluble homopolymers. In addition, most of these methods (4) (a) Thurmond, K. B., II; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1996, 118, 7239-7240. (b) Huang, H.; Kowalewski, T.; Remsen, E. E.; Gertzmann, R.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 11653-11659. (c) Thurmond, K. B., II; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656-6665. (d) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805-3806. (e) Zhang, Q.; Remsen, E. E.; Wooley, K. L. J. Am. Chem. Soc. 2000, 122, 3642-3651. (f) Bu¨tu¨n, V.; Wang, X. S.; De Paz Ba´n˜ez, M. V.; Robinson, K. L.; Billingham, N. C.; Armes, S. P.; Tuzar, Z. Macromolecules 2000, 33, 1-2. (5) (a) Saito, R.; Ni, X.; Ichimura, A.; Ishizu, K. J. Appl. Polym. Sci. 1998, 69, 211-216. (b) Hritcu, D.; Muller, W.; Brooks, D. E. Macromolecules 1999, 32, 565-573. (c) Yun, Y.; Li, H.; Ruckenstein, E. J. Colloid Interface Sci. 2001, 238, 414-419. (d) Matsuoka, H.; Fujimoto, K.; Kawaguchi, H. Polym. J. 1999, 31, 1139-1144. (6) (a) Chen, M.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2213-2220. (b) Bu´csi, A.; Forcada, J.; Gibanel, S.; He´roguez, V.; Fontanille, M.; Gnanou, Y. Macromolecules 1998, 31, 2087-2097. (c) Soula, O.; Guyot, A.; Williams, N.; Grade, J.; Blease, T. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4205-4217. (d) Roy, S.; Favresse, P.; Laschewsky, A.; De La Cal, J. C.; Asua, J. M. Macromolecules 1999, 32, 5967-5969. (e) Serizawa, T.; Takehara, S.; Akashi, M. Macromolecules 2000, 33, 1759-1764.
10.1021/la0261343 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/25/2002
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Experimental Section
Figure 1. Schematic representation of the formation of amphiphilic core-shell nanospheres.
can only be carried out under very dilute conditions, which are not commercially viable. In this paper, we describe a novel and efficient method to prepare well-defined amphiphilic core-shell nanospheres in the absence of surfactant via the direct graft copolymerization of vinyl monomers from water-soluble polymers containing amino groups. Our approach is based on the reaction between alkyl hydroperoxides and the amino groups of the watersoluble polymer in water in the presence of dispersed methyl methacrylate. Amphiphilic graft copolymers and hydrophobic homopolymers are generated concurrently to form highly monodispersed particles with amphiphilic core-shell nanostructures. Alkyl hydroperoxides such as tert-butyl hydroperoxide (TBHP) are seldom used alone to initiate a polymerization reaction below 100 °C because of their high decomposition temperatures (e.g. t1/2 of TBHP ) 170 h at 100 °C). In addition, the hydrogen of the alkyl hydroperoxide (ROOH) is susceptible to abstraction by HO• and RO• radicals, forming a peroxy radical (ROO•), which is not sufficiently reactive to initiate polymerization.7 However, alkyl hydroperoxides can be activated by other reagents. For example, they have been extensively used with metal ions to form redox pairs and initiate polymerization at low temperatures.8 Low molecular weight polyamines also form redox pairs with alkyl hydroperoxides that decompose to alkoxy radicals (RO•) and amine radicals.9 These radicals can initiate polymerization under mild conditions. For instance, natural rubber/PMMA composite latexes have been prepared using a TBHP/tetraethylenepentamine initiation system.10 Although the amine/peroxide system has been reported and the decomposition mechanisms have been studied previously,9 the work presented here is the first to demonstrate that alkyl hydroperoxides will react with the amino groups of water-soluble polymers, thereby generating radicals on the nitrogen atom, which subsequently initiate the graft copolymerization of the vinyl monomers. Furthermore, the amphiphilic macroradicals generated in situ not only self-assemble to produce micellelike microdomains that promote the aqueous emulsion polymerization of vinyl monomers, but also act as electrosteric stabilizers to provide stability to the developing nanospheres. Therefore, the process can be carried out in the absence of surfactants (Figure 1). (7) Allcock, H. R.; Lampe, F. W. Contemporary Polymer Chemistry, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1990; p 53. (8) Sarac, A. S. Prog. Polym. Sci. 1999, 24, 1149-1204. (9) (a) Blackley, D. C. Emulsion Polymerization, Theory and Practice; Applied Science Publishers: London, 1975; p 242. (b) Feng, Xinde. Makromol. Chem., Macromol. Symp. 1992, 63, 1-18. (c) Feng, Xinde. Chin. J. Polym. Sci. 1985, 2, 109-118. (10) (a) Hourston, D. J.; Romaine, J. Eur. Polym. J. 1989, 25, 695700. (b) Hourston, D. J.; Romaine, J. J. Appl. Polym. Sci. 1990, 39, 1587-1594. (c) Schneider, M.; Pith, T.; Lambla, M. J. Appl. Polym. Sci. 1996, 62, 273-290. (11) (a) Rizzardo, E.; Solomon, D. H. J. Macromol. Sci., Chem. 1980, A14, 33-50. (b) Sharma, R. K.; Misra, B. N. J. Macromol. Sci., Chem. 1983, A20, 225-235.
Materials. Gelatin (granular), bovine serum albumine (BSA, protease free), polyethyleneimine, branched (b-PEI, 50% solution in water, Mw 50 000-60 000), poly(acrylamide) (nonionic, Mw 5 × 106), and tert-butyl hydroperoxide (70% solution in water) were all obtained from Acros. Chitosan (low molecular weight, 77% deacetylation determined by an elemental analysis), poly(allylamine) (PAA, 20% solution in water, Mw 17 000 and 65 000), polyvinylpyrolidone (PVP, Mw 55 000), poly(vinyl alcohol) (9899% hydrolyzed, Mw 31 000-50 000), poly(ethylene glycol) (Mw 2000), and poly(diallyldimethylammonium chloride) (PDADMAC, 20% in water, Mw 100 000-200 000) were all purchased from Aldrich. Poly(acrylic acid) (25% solution in water, Mw 23 000) was obtained from BDH Chemicals. Linear PEI (L-PEI, Mw 25 000) and poly(2-dimethylaminoethyl methacrylate) (PDMAEMA, 20% solution in tert-butyl alcohol) were obtained from Polysciences. All of these chemicals were used as received without further purification. Casein (Acros) was purified by mixing it with 1% EDTA solution at 50 °C for 48 h in order to remove metal ions in the casein. The phenolic inhibitor in methyl methacrylate (MMA, Aldrich) was removed by washing three times with 10% sodium hydroxide solution and then with deionized water until the pH of the water layer dropped to 7. It was further purified by vacuum distillation. Freshly deionized and distilled water was used as the dispersion medium. Synthesis of Amphiphilic Core-Shell Particles. The polymer containing amino groups (1.0 g) was dissolved in 95 mL of water and mixed with the purified MMA monomer (4.0 g) in a water-jacketed flask equipped with a thermometer, a condenser, a magnetic stirrer, and a nitrogen inlet. The stirred mixture was purged with nitrogen for 30 min. The appropriate amount of TBHP was added, and the mixture was heated at 80 °C for 2 h under nitrogen. The MMA conversion was determined gravimetrically. The grafting percentage and grafting efficiency were calculated as follows:
grafting percentage ) weight of the grafted PMMA × 100 weight of the soluble polymer grafting efficiency ) weight of the grafted PMMA × 100 weight of the total polymerized MMA The graft copolymer and PMMA homopolymer were isolated from the resultant polymers by Soxhlet extraction with chloroform for 48 h. Measurement and Characterization. Infrared spectra were recorded on a Nicolet 750 FT-IR spectrophotometer using KBr disks. Proton and carbon magnetic resonance spectral determinations were made on a Bruker Advance DPX 400 or a Varian Gemini 200. Gel permeation chromatography (GPC) was performed on a Waters Associate GPC system. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min. The GPC system was calibrated with polystyrene standard samples. Particle size and distribution were measured on a Coulter LS230 particle size analyzer. The ζ-potential was measured with a Brookheaven Zeta Plus Analyzer with 1 mM KCl aqueous solution as the suspension fluid. Transmission electron microscopy (TEM) photographs were obtained using a JEM 100 CX transmission microscope at an accelerating voltage of 100 kv. The sample was prepared by wetting either a Formvar-coated or a carbon-coated grid with a small drop of the dilute latex solution. Upon drying, it was stained with a small drop of 2% phosphotungstic acid (PTA) for 30 min and dried at room temperature before analysis.
Results and Discussion Effect of the Water-Soluble Polymer. Various watersoluble polymers containing amino groups were dissolved in water containing TBHP and dispersed methyl methacrylate and stirred at 80 °C for 2 h under nitrogen. Table 1 shows the results of the TBHP-induced graft copolymerizations with respect to the MMA conversion, grafting percentage, grafting efficiency, and resulting particle size.
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Table 1. TBHP-Induced Graft Copolymerization of MMA from Water-Soluble Polymers Containing Amino Groupsa
polymer caseinc gelatin BSA chitosand branched PEI PAA linear PEIe PDMAEMAf PDADMAC
grafting grafting conversion percentage efficiency Dn (%) (%) (%) (nm) Dv/Dnb 82 80 82 87 96 88 22 42 12
131 144 206 52 187 343
40 45 63 15 49 98
82 160 63 126 132 75 139 112
1.17 1.16 1.18 1.09 1.12 1.21 1.11 1.13
a Refer to the procedure described in the Experimental Section. TBHP concentration ) 0.08 mM in all cases. Weight ratio of polymer to MMA is 1:4. b Dn and Dv are the number and volume average particle diameters, respectively. Dv/Dn is the polydispersity index of the particle size distribution. c Casein was dissolved in a 0.4 wt % sodium carbonate aqueous solution. d Chitosan was dissolved in a 1 wt % acetic acid solution. e The pH of the linear PEI solution was adjusted to 8 in order to obtain stable particles. f tert-Butyl alcohol was first removed, and then the poly(dimethylaminoethyl methacrylate) (PDMAEMA) was dissolved in distilled water, followed by adjusting the solution to pH 7.6.
The TBHP-induced graft copolymerizations of MMA from biopolymers, such as casein, gelatin, albumine bovine serum (BSA), and chitosan, generally gave high conversions, but the grafting efficiency and particle size were strongly dependent upon the nature of the biopolymer. For example, high grafting efficiency and stable nanospheres of BSA/PMMA were obtained, while the use of chitosan, a cationic polysaccharide derived from chitin, resulted in a much lower grafting efficiency. The low grafting efficiency indicates the formation of a large amount of PMMA homopolymer. In this case, the results are complicated by the presence of a small amount of acetic acid that was required to dissolve the chitosan in water. It is known that acid induces free-radical decomposition of the TBHP to tert-butoxy and hydroxy radicals, which could initiate the homopolymerization of MMA.11 The grafting of MMA from a variety of water-soluble synthetic polymers gave mixed results. The graft copolymerization of MMA from branched polyethyleneimine (b-PEI) containing 25% primary, 50% secondary, and 25% tertiary amino groups resulted in a high MMA conversion and a high grafting percentage. To gain more insight into the effect of the amino group structure, four water-soluble polymers, containing primary, secondary, and tertiary amino groups and ammonium cations, respectively, were subjected to the copolymerization conditions. The graft copolymerization of MMA from polyallylamine (PAA, Mw ) 65 000), which contains primary amino groups, proceeded to high conversion with 98% grafting efficiency. However, the use of both linear PEI and poly(dimethylaminoethyl methacrylate) (PDMAEMA), which contain secondary and tertiary amino groups, respectively, resulted in a much lower MMA conversion. The particles generated with these two polymers were also quite unstable. Moreover, a polymerization employing poly(diallyldimethylammonium chloride) (PDADMAC) gave only a 12% conversion. These results suggest that a primary amino group has much higher activating ability than a secondary or a tertiary group. The ammonium cation evidently cannot form a redox pair with TBHP. This finding also helps explain the variation in the results obtained with the biopolymers, since they contain different amounts of primary amino groups such as lysine and terminal amino acids. The use of other functional watersoluble polymers was also investigated. For example,
Figure 2. Effect of temperature on MMA conversion for various polymers: (9) chitosan; (b) branched PEI; (2) PAA; (1) casein; ([) BSA; (+) gelatin.
Figure 3. Effect of TBHP concentration on the graft copolymerization of MMA from branched PEI: (9) conversion; (b) grafting efficiency; (2) mean particle diameter (Dn, nm).
polymers containing amide groups such as polyacrylamide and polyvinylpyrolidone were examined under similar reaction conditions. However, less than an 8% conversion of MMA was obtained after 2 h. The use of poly(vinyl alcohol), poly(ethylene glycol), and poly(acrylic acid) also resulted in minimal MMA polymerization. Effect of the Reaction Temperature. Figure 2 shows the effect of reaction temperature on MMA conversions for various water-soluble polymers containing amino groups. Except for the case with chitosan, where the process was complicated by the independent acid catalyzed breakdown of TBHP as explained earlier, the reaction temperature had to be higher than 70 °C in order to achieve a reasonable conversion in 2 h. Although such a reaction temperature is acceptable for synthetic polymers, it is not very suitable for some biopolymers such as proteins because they may be denatured at this temperature. Effect of TBHP Concentration. During the reaction, the water-soluble polymer is transformed into a macroinitiator. This could lead to gelation if multisite macroradicals were generated. To avoid such gelation, a TBHP concentration below 0.2 mM was used in an attempt to create approximately one to two graft sites per polymer. Figure 3 shows that, in the absence of TBHP, less than 5% MMA was converted to polymer after 2 h at 80 °C. When 0.08-0.10 mM TBHP were used, high conversion and grafting efficiency were achieved. Surprisingly, a further increase of the TBHP concentration had little influence on the conversion but greatly reduced the grafting efficiency. This means that more PMMA homopolymer was formed at higher TBHP concentrations. This effect may be due to the fact that higher TBHP concentrations result in higher tert-butoxy and hydroxy radical concentrations through thermal decomposition. These reactive radicals could subsequently initiate the homopolymerization of MMA. Figure 3 also shows that the TBHP concentration and grafting percentage have little influence on particle size and size distribution. Monomer
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Figure 4. Effect of molecular weight of poly(allylamine) and pH of the reaction medium on nanosphere size: (a) pH ) 7; (b) PAA Mw 65 000.
conversion seems to be the key factor controlling the particle size. Compared to the concentration of radical initiators (e.g. potassium persulfate, >6 mM)12 or metal ion oxidants (e.g. Ce4+, >3 mM)13 commonly used in grafting reactions, the amount of TBHP used in our system is extremely low. This efficient initiation method is particularly useful for biopolymer systems, because it greatly reduces the potential for oxidative degradation, which is a major problem associated with other initiation systems. Furthermore, the copolymers obtained are metal ion-free and surfactant-free, which is highly desirable for many biological applications. Effect of Polymer Molecular Weight and pH of the Solution. When a lower molecular weight sample of poly(allylamine) (PAA) (Mw ) 17 000) was used in the process (pH of the PAA solution was 11.5), the latexes formed were unstable, and a low MMA conversion was obtained. However, when the PAA solution was neutralized to pH 7 prior to the reaction, high conversion and very stable particles were obtained. To further understand the effect of the pH, the graft copolymerization of MMA from a higher molecular weight sample of PAA (Mw ) 65 000) was carried out at pH 11.5 and 7.0. Comparable results were obtained, indicating that pH had little effect on the conversion and particle stability. It is speculated that the strong influence of pH on the results obtained with the low molecular weight PAA is due to electrostatic stabilization of the particles. At lower pH, some amino groups are converted to ammonium cations, which enhance the particle stability through electrostatic repulsion. With the high molecular samples, steric stabilization is sufficient to stabilize the particles formed. Thus, two important criteria must be satisfied in order to obtain stable amphiphilic core-shell nanospheres: the water-soluble polymer must contain primary amino groups, and it must be able to stabilize the particle in water via either electrostatic or steric interactions or a combination of both effects. The molecular weights of the polymers and the pH of the solution also affect particle sizes as illustrated in Figure 4. Under the same pH conditions, a higher molecular weight PAA forms larger particles than a low molecular weight sample. For PAA samples with the same molecular weight, a lower pH results in a larger particle and a broader particle size distribution. The increase of particle size in pH 7 solutions can be attributed to the protonation of amine groups, leading to an expansion of the PAA shell layer due to intramolecular charge repulsion. Effect of Solids Content. One of the major disadvantages of previous methods to prepare amphiphilic (12) (a) Mohan, D.; Radhakrishnan, G.; Rajadurai, S. J. Appl. Polym. Sci. 1990, 39, 1507-1518. (b) Somanathan, N.; Sanjeevi, R. Eur. Polym. J. 1994, 30, 1425-1430. (c) Gao, J.; Li, Z.; Wang, W.; Huang, M. J. Appl. Polym. Sci. 1998, 68, 1485-1492. (13) Kla´sek, A.; Bae`a´kova´, M.; Sˇ imonı´kova´, J.; Pavelka, F.; Tka´•, J. J. Appl. Polym. Sci. 1983, 28, 2715-2728.
Table 2. Effect of Solids Content on the Formation of PMMA/PEI Core-Shell Nanospheresa solid content (wt %)
conversion (%)
5 9 17 22 29
96 93 85 85 95
particle size (nm) Dn Dv 132 131 135 135 144
148 142 148 152 277
polydispersity, Dv/Dn 1.12 1.08 1.10 1.13 1.92
a MMA to PEI ratios were kept to 4:1; [TBHP] ) 0.08-0.1 mM, at 80 °C for 2 h.
Figure 5. Comparison of FTIR spectra of various polymers: (1) casein; (2) casein-g-PMMA; (3) gelatin; (4) gelatin-g-PMMA; (5) BSA; (6) BSA-g-PMMA; (7) chitosan; (8) chitosan-g-PMMA; (9) PEI-g-PMMA; (10) PAA-g-PMMA.
core-shell particles is that stable particles can only be obtained at very low concentrations. As shown in Table 2, in our system, stable particles with comparable particle sizes and size distributions could still be obtained up to a 22 wt % solids content. A further increase to 29 wt % resulted in a larger particle size and a broader size distribution. This indicates that our new process is commercially viable. Characterization of the Graft Copolymers. The amphiphilic graft copolymers were isolated by Soxhlet extraction with chloroform. The FTIR spectra of caseing-PMMA, gelatin-g-PMMA, BSA-g-PMMA, chitosan-gPMMA, PEI-g-PMMA, and PAA-g-PMMA show strong carbonyl peaks at 1730 cm-1, which clearly indicate the presence of PMMA (Figure 5). The casein-g-PMMA polymer was also treated with 6 N HCl for 24 h under reflux to hydrolyze the casein backbone. The PMMA grafts were isolated and characterized. The number average molecular weights of the PMMA grafts and the homopolymer of PMMA, which were determined with GPC, were 281 800 and 315 690, respectively. The small dif-
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Figure 6. TEM micrographs of nanospheres stained with 2% phosphotungstic acid on either Formvar or carbon-coated grids: (a) casein/PMMA; (b) gelatin/PMMA; (c) chitosan/PMMA; (d) b-PEI/PMMA; (e) PAA/PMMA; (f) b-PEI/PMMA under high magnification.
ference in their molecular weights indicates that the grafting occurred from the backbones rather than onto the backbones.14 Furthermore, it suggests that the graft copolymerization and homopolymerization of MMA start approximately at the same time and proceed concurrently. Determination of Nanosphere Size and Morphology. The average particle sizes of the PMMA-based amphiphilic core-shell particles were found to be strongly dependent upon the nature of the water-soluble polymer (Table 1). Grafting reactions with BSA, casein, and PAA generated nanospheres less than 100 nm in diameter. In addition, highly monodispersed particles were obtained with all these systems. Transmission electron microscopy (TEM) micrographs of casein/PMMA, gelatin/PMMA, chitosan/PMMA, b-PEI/PMMA, and PAA/PMMA particles clearly show the core-shell particle morphologies, where PMMA cores are coated with various hydrophilic polymer shells (Figure 6a-e). Interestingly, dense and hairy (14) Stejskal, J.; Strakova´, D.; Kratochvil, P. J. Appl. Polym. Sci. 1988, 36, 215-227.
Figure 7. pH dependence of the ζ-potential of PEI/PMMA nanospheres in a 1 mM KCl aqueous solution.
hydrophilic shells of b-PEI on the PMMA particles are clearly visible under high TEM magnification (Figure 6f). Characterization of Particle Surfaces. The presence of the hydrophilic polymers on the particle shells was confirmed with ζ-potential analyses. For example, Figure 7 shows the ζ-potential of the PEI/PMMA nanospheres as
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Scheme 1. Proposed Mechanism for the Graft Copolymerization of MMA from Water-Soluble Polymers Containing Amino Groups
a function of pH in a 1 mM KCl solution at 25 °C. The positive value of the nanospheres decreases as the pH increases, indicating that labile cationic PEI is coated on the particle surfaces. The PEI shell layer was further characterized with 1H and 13C NMR. The 1H NMR spectrum of the PMMA/PEI core-shell nanospheres contains peaks between 2.6 and 2.8 ppm due to methylene groups containing different amine substitutents. The chemical shifts are similar to those obtained with pure branched PEI (2.4-2.5 ppm), but the peaks are broader. The broadening of the proton peaks indicates a decrease in the molecular mobility of the PEI segments. This is consistent with the attachment of the PEI molecules to the PMMA particle surface. The 13C NMR spectrum of PEI contains characteristic well-separated carbon peaks in the area between 38 and 57 ppm. The 13C NMR spectrum of the PEI/PMMA nanospheres also exhibits peaks in the same chemical shift region, indicating the presence of the PEI on the particle surface. In addition, the two singlet peaks at 38 and 40 ppm, which are attributable to the methylene carbons containing primary amine substituents,15 appear as multiple peaks with decreased intensities. These results suggest that the grafting reaction preferably occurred at the primary amine, which is consistent with our other results. Reaction Mechanism. Plausible grafting and particle formation mechanisms are proposed in Scheme 1. The hydroperoxide (ROOH) initially interacts with amino groups on the polymer backbone, forming redox pairs. One electron is then transferred from nitrogen to ROOH, resulting in the formation of a nitrogen cation radical and an alkoxyl radical (RO•). Subsequent loss of a proton from the nitrogen generates an amino radical, which is capable of initiating the graft copolymerization of MMA.9 The RO• produced can either initiate homopolymerization of MMA in the polymeric micelles or abstract a hydrogen atom from the polymer backbone, thus generating a backbone radical that can initiate MMA graft copolymerization. For example, PAA contains tertiary hydrogens, which are highly susceptible to radical abstraction. This may explain why such a high grafting percentage was obtained with this polymer. Webber et al studied micelles made from poly(acrylic acid)-graft-polystyrene in water.16 Their TEM (15) Harpe, A. V.; Peterson, H.; Li, Y.; Kissel, T. J. Controlled Release 2000, 69, 309. (16) Ma, Y.; Cao, T.; Webber, S. E. Macromolecules 1998, 31, 17731778.
images revealed a unique micelle morphology in which small spherical polystyrene cores were joined together in a “multicore” structure surrounded by poly(acrylic acid) loops. Hiborn et al. have also reported the micelle formation of amphiphilic graft copolymers and demonstrated that poly(acrylic acid)-graft-polystyrene is an efficient stabilizer in aqueous emulsion polymerization of latex particles.17 On the basis of these studies, we propose that once the amphiphilic graft copolymers are generated in situ in our reaction system, they act like polymeric surfactants, self-assembling to form micelle-like microdomains. Thus, the emulsion polymerization of MMA monomer can take place within the hydrophobic microdomains to form core-shell particles with PMMA as the core and the water-soluble polymer as the shell. Conclusions Hydrophilic biopolymers and synthetic polymers containing primary amino groups undergo reaction with a small amount of tert-butyl hydroperoxide in water at 80 °C in the presence of MMA to generate amphiphilic graft copolymers. PMMA homopolymer is also formed in the process. During the reaction, polymeric micelle-like microdomains are formed, and well-defined core-shell nanospheres are produced in the absence of surfactant. Since nanospheres containing up to 22 wt % solids can be produced, this process appears to be amenable to the commercial production of a wide variety of novel amphiphilic core-shell nanomaterials with different sizes, compositions, structures, and functions. Important features include nanosphere diameters ranging from 60 to 160 nm, narrow particle size distributions, covalent bonding between the core and shell, the ability to tailor the core diameter and shell thickness, the use of aqueousbased chemistry, and a simple synthetic approach. These new materials should be extremely useful in a wide range of applications. Further studies in this direction are currently in progress. Acknowledgment. We gratefully acknowledge The Hong Kong Polytechnic University, Research Grants Council of the Hong Kong Special Administrative Region (Project No. 5301/01P), and The University Grants Committee Area of Excellence Scheme (Hong Kong) for their financial support of this research. LA0261343 (17) Carrot, G.; Hiborn, J. Polymer 1997, 26, 6401-6407.
