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Notes Unusual Size Formation of Polymeric Nanospheres Synthesized by Free Radical Polymerization in Ethanol-Water Mixed Solvents Takeshi Serizawa, Ming-Qing Chen, and Mitsuru Akashi* Faculty of Engineering, Kagoshima University, Kagoshima 890, Japan Received September 3, 1997. In Final Form: November 17, 1997
Introduction Polymeric nanospheres are useful materials in regards to current technology. Many kinds of nanospheres have been synthesized and utilized for technological or biological applications.1 In our research group, we also synthesized polymeric nanospheres by the free radical dispersion polymerization of hydrophilic macromonomers and hydrophobic comonomers in a polar solvent without any emulsifier and then characterized them.2 To develop their functionality, their surfaces were modified by functional polymers such as poly(vinylpyrrolidone), poly(1-butylvinylpyridine), poly(N-isopropylacrylamide), poly(tert-butyl methacrylate), poly(ethylene glycol), poly(N-vinylacetoamide), and their derivatives. In regards to strict control of their size, the effect of the monomer ratios, the monomer concentrations, and the molecular weight of macromonomers as well as other factors, have been investigated. For example, an increase in hydrophobic comonomers such as styrene relative to the macromonomers resulted in a larger diameter of the nanosphere. Other research groups also used the same procedure.3 On the other hand, we also studied their assembly in an aqueous phase or onto a substrate.4 The assemblies will be the accumulations of their functionality. † This paper is Part 17 in the series “Graft Copolymers Having Hydrophobic Backbone and Hydrophilic Branches.” Part 16: see ref 11.
(1) Polymer Latexes: Preparation, Characterization and Applications; Daniels, E. S., Sudol, E. D., El-Aasser, M. S., Eds.; American Chemical Society: Washington, DC, 1992. Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. Emulsion Polymerization and Emulsion Polymers; Lowell, P. A., El-Aasser, M. S., Eds.; John Wiley & Sons: Chichester, England, 1997. Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 385, 57. Burmeister, F.; Schafle, C.; Matthes, T.; Bohmisch, M.; Boneberg, J.; Leiderer, P. Langmuir 1997, 13, 2983. Microbeads, macrocapsules and liposomes; Arshady, R., Ed.; STC Books: London and Los Angeles, CA, 1997. (2) Akashi, M.; Kirikihara, I; Miyauchi, N. Angew. Makromol. Chem. 1985, 132, 81. Akashi, M.; Yanagi, T.; Yashima, E.; Miyauchi, N. J. Polym. Sci. Polym. Chem. Ed. 1989, 27, 3521. Akashi, M.; Chao, D.; Yashima, E.; Miyauchi, N. J. Appl. Polym. Sci. 1990, 39, 2027. Capek, I.; Akashi, M. J. Macromol. Sci.sRev. 1993, 33, 369. Riza, M.; Tokura, S.; Iwasaki, M.; Yashima, E.; Kishida, A.; Akash, M. J. Polym. Sci., Part A: Polym. Chem. Ed. 1995, 33, 1219. Chen, M.-Q.; Kishida, A.; Akashi M. J. Polym. Sci., Part. A: Polym. Chem. Ed. 1996, 34, 2213. (3) Takeuchi, S.; Okie, M.; Kowitz, C.; Shimasaki, C.; Hasegawa, K.; Kitano, H. Makromol. Chem. 1993, 194, 551. Kawaguchi, H.; Winnik, M. A.; Ito, K. Macromolecules 1995, 28, 1159. Ishizu, K.; Tahara, N. Polymer 1996, 37, 1729. (4) Serizawa, T.; Akashi M. Chem. Lett. 1997, 809.
The formation in a polar solvent of the polymeric nanospheres, which were synthesized by the macromonomer technique, will contain the following steps. The oligomers and the grafted oligomers that derived from hydrophobic comonomers and/or the macromonomers are produced in the first step. Subsequently, they selfassemble as an origin for nanosphere formation in a polar solvent because of their lower solubility, though both of monomers are completely soluble in the reaction media. Then, the hydrophilic polymers that derived from the macromonomer stabilize sterically the origin. After that, the nanosphere formed becomes gradually larger by uptaking other monomers. The type of self-assembly will be influenced by the reaction media. Kimizuka et al.5 found that certain synthetic amphiphiles formed bilayer membranes in an ethanol-water mixed solvent. They had specific ratios in the bilayer formation. This phenomenon indicates that the self-assembling process for the nanosphere formation may be influenced by fine solvent structure. In addition, the ethanol-water mixed solvents show a clustered structure of ethanol at suitable mixture ratios.6 This type of solvent structure on a molecular level may influence the polymerization of the macromonomers and the comonomers. In reality, the solvent effects of the polymerization on nanosphere formation or size control have been already studied to some degree by another group of researchers.7 They selected an ethanol-water mixed solvent for the medium in order to study influences of the ethanol content. Their sizes increased with an increase in the content in the medium. However, other conditions or combinations may induce a significant dependence on the content. In this paper, we polymerized methacryloyl-terminated poly(ethylene glycol) (MMA-PEG) macromonomers and methyl methacrylate (MMA) in ethanol-water mixed solvents of various mixture ratios and analyzed the effects of the solvent on their sizes. The unusual size formation that we found, which was influenced by the solvent component, is a significant polymerization system for nanosphere synthesis by using macromonomers. Experimental Section 2,2′-Azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride (VA044) (Wako Pure Chemical Ind., Ltd.) was used without further purification. MMA (Wako Pure Chemical Ind., Ltd.) was distilled under reduced pressure in a nitrogen atmosphere. Ethanol and water were purified in the usual way prior to use. A MMA-PEG macromonomer (Mn ) 4000) was kindly donated by Nippon Oil and Fats Co. (Tokyo, Japan). This macromonomer has a narrow molecular weight distribution (Mw/Mn is less than 1.10) according to gel permeation chromatography (GPC) analysis. A dialysis tube was purchased (the cutoff molar mass: 12 000-14 000) from Wako Pure Chemical Ind., Ltd. (Osaka, Japan) and used after rinsing with pure water. The free radical dispersion polymerization is as follows: MMA-PEG macromonomers (0.05 mmol) and MMA (2.0 or 4.0 mmol) were added into a glass tube together (5) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Tokuhiro, M.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 5808. (6) Nishi, N.; Koga, K.; Ohshima, C.; Yamamoto, K.; Nagashima, U.; Nagami, K. J. Am. Chem. Soc. 1988, 110, 5246. (7) Ishizu, K.; Tahara, N. Polymer 1996, 37, 2853.
S0743-7463(97)00992-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/06/1998
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Figure 1. TEM image of the polymeric nanosphere, which was synthesized by MMA-PEG (0.05 mmol) and MMA (4.0 mmol) with VA-044 (1 mol % to the total monomer) in a ethanolwater (35/65, v/v) mixed solvent at 60 °C for 24 h. with VA-044 (1 mol % to the total monomers) as an initiator and ethanol-water mixed solvents (5 mL). All of the reactants were mutually soluble before the polymerization. After being degassed by freeze-thaw cycles on a vacuum apparatus, the solution was sealed and shaked in an incubator at 60 °C for 24 h without stirring. The reaction media were dialyzed in distilled water for 5 days by using a cellulose dialyze tube in order to remove unreacted monomers. Morphologies of the products were observed by means of the transmission electron microscopy (TEM) (Hitachi H-7010A). The samples for TEM observation were prepared as follows: the aqueous dispersions of the nanosphere were cast to a copper mesh on a glass seat and dried at room temperature under a reduced pressure. Particle sizes of the nanosphere were determined by means of a dynamic light scattering apparatus (Coulter model N4SD).
Results and Discussion The MMA-PEG macromonomers and MMA were copolymerized with VA-044 as a radical initiator in an ethanol-water mixed solvent in order to obtain polymeric nanospheres. The hydrophilic PEG that derived from the macromonomer will sterically stabilize polyMMA that is insoluble in an ethanol-water mixed solvent after the reaction, as a core of the nanosphere. Figure 1 shows a TEM image of the reaction product after it had been dried on copper mesh. The polymeric nanospheres that were spherical in form were observed and the coefficients of variation were 24 ( 6%. Those values were the almost same as those for other systems in the nanospheres.2 In another paper on the same topic, we found, by using dynamic laser light scattering, that the hydrophilic poly(ethylene glycol) branches on nanosphere surfaces stabilize sterically and cover the relatively hydrophobic polyMMA core.8 Further researches about the synthesis of the nanospheres in detail such as the conditions involved in nanosphere formation will be reported elsewhere.9 Figure 2 shows the particle size of the nanospheres that were synthesized in ethanol-water mixed solvents with various mixture ratios. The nanosphere sizes that were formed under conditions of MMA ) 4.0 mmol changed from 150 to 220 nm with a change in the ethanol volume fraction, as is shown in Figure 2a. Their sizes tended to decrease slightly with an increase in the ethanol volume in the reaction solvents. They did not, however, decrease linearly with it. Surprisingly, the size reached its minimum at an ethanol content of around 30 vol %. We did not predict that from the size dependence on the ethanol volume. In particular, their sizes were 138 and (8) Wu, C.; Akashi, M.; Chen, M.-Q. Macromolecules 1997, 30, 2187. (9) Chen, M.-Q.; Serizawa, T.; Kishida, A.; Akashi M. J. Polym. Sci., Part. A: Polym. Chem. Ed., in preparation.
Figure 2. Effect of ethanol volume fraction in ethanol-water mixed solvents on the nanosphere size. The polymerizations were carried out by MMA-PEG (0.05 mmol) and MMA (a, 4.0 mmol; b, 2.0 mmol) with VA-044 (1 mol % to the total monomers) at 60 °C for 24 h.
220 nm at ethanol contents of 30 and 40 vol %, respectively. The other research group found that the size of the nanospheres, which were synthesized by poly(methacrylic acid) macromonomers and MMA, increased continuously with an increase in the ethanol volume.7 However, they did not observe the minimum value against the ethanol volume by using the same solvents. This must have been caused by the difference in the macromonomers. The concentration in the feed of the MMA changed the dependence of the particle sizes on the ethanol volume fraction. As shown in Figure 2b, all of the sizes were relatively smaller than those in Figure 2a. The opposite type of dependence on the ethanol volume fraction was observed at MMA ) 2.0 mmol. In that case, we observed a maximum value at around 30 vol % of ethanol in the ethanol-water mixed solvent. We are as yet unsure of the reason for this difference. Tentatively, we can say that the size determination of the nanosphere is very sensitive to not only the volume fraction of the solvent but also the concentrations of hydrophobic comonomer in the feed. In general, free radical polymerization does not exhibit the solvent effect except when there is a strong interaction such as a hydrogen bond between monomers or between a solvent and a monomer.10 In reality, all of the conversions were about 80%, so we will ignore those effects on size determination. Ethanol exhibits a clustered structure in water at suitable mixture ratios. There is no research that shows that the clustered structure drastically influences this radical polymerization. However, our polymerization system may be influenced by this phenomenon. On the other hand, self-assembly of the synthetic amphiphiles occurred in the ethanol-water mixed solvent; subsequently, a bilayer vesicle was prepared.5 A specific ethanol volume against the water was necessary for the formation to occur. In the case of the nanosphere, the process involved in the nanosphere formation included the self-assembling process of the hydrophobic core polyMMA and/or the amphiphilic grafted copolymers, which were made up of a hydrophobic MMA backbone and hydrophilic PEG grafted polymers. This process may also be influenced by the clustered structure. The amphiphiles described above dissolve in pure ethanol (10) Ito, K.; Uchida, K.; Kitano, T.; Yamada, E.; Matsumoto, T. Polym. J. 1985, 17, 761. Kodaira, T.; Yang, J.; Aida, H. Polym. J. 1988, 20, 1021. Takemoto, K.; Akashi, M.; Inaki, Y. J. Polym. Sci., Part. A: Polym. Chem. Ed. 1974, 12, 1861. Akashi, M.; Kita, Y.; Inaki, Y.; Takemoto, K. J. Polym. Sci., Part. A: Polym. Chem. Ed. 1979, 17, 301. (11) Chen, M.-Q.; Serizawa, T.; Akashi M. Polym. Adv. Tech. in press.
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easily, but not in pure water. The specific volume fraction of the solvent induced their self-assembled structure. In fact, polyMMA as a core of the nanosphere can dissolve in pure ethanol, indicating that our system might be also affected by the fine solvent structure. This unusual size formation against the ethanol content in the solvent is the first system in nanosphere synthesis.
Notes
particle sizes in the nanospheres were very dependent on the ethanol volume fraction in the ethanol-water mixed solvents. The sizes reached their minima or maxima, which were dependent on the mole fraction of MMA used, at an ethanol content of around 30 vol %. The solvents may affect a self-assembling process for the nanosphere formation.
Conclusion We presented a novel polymerization system for the nanosphere synthesis by the free radical dispersion polymerization. We copolymerized MMA-PEG macromonomers and MMA with a radical initiator in ethanolwater mixed solvents of various mixture ratios and analyzed the effects of the solvent on their sizes. The
Acknowledgment. This work was financially supported in part by a Grant-in-Aid for Scientific Research in the Priority Areas of “New Polymers and Their NanoOrganized Systems” (No. 277/09232249) from the Ministry of Education, Science, Sports, and Culture, Japan. LA970992V
