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2008, 112, 5265-5267 Published on Web 04/10/2008
Microemulsion Polymerizations via High-Frequency Ultrasound Irradiation Boon M. Teo, Muthupandian Ashokkumar,* and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: February 21, 2008; In Final Form: March 26, 2008
The synthesis of nanosized polymer latex particles using high-frequency ultrasound (213 kHz) has been successfully performed. The effects of surfactant type and concentration of surfactants on the rates of polymerization, latex size, and molecular weights of the polymers produced are presented.
Introduction Polymer nanoparticles find increasing use in a wide range of advanced applications such as electronic devices, coatings, and, in particular, controlled drug delivery systems. The recent development of biopolymer nanoparticles as an effective drug delivery vehicle has the potential to revolutionize medical drug delivery practices. By incorporating a drug into polymer nanoparticles, it is possible to facilitate efficient delivery and release of the drug molecules to specific sites in the body.1 The synthesis of polymer latex particles by the classical emulsion polymerization method using chemical initiators has been extensively studied.2,3 In recent years, the use of ultrasound to promote free-radical emulsion polymerizations (without the use of chemical initiators or hydrophobe) has emerged as a novel technique to synthesize ultraclean polymer nanoparticles.4-6 Ultrasound-assisted emulsion polymerization provides many advantages over conventional emulsion polymerization; apart from the absence of a chemical initiator and hydrophobe, fast polymerization rates, the formation of nanosized latex particles, and polymers of high molecular weight can be achieved with this method.4 The overall mechanical and reaction processes associated with sonochemical preparation of polymer colloids has been explained in detail elsewhere.5 Many sonochemical miniemulsion polymerization studies to date have reported on the use of 20 kHz.4-9 No study has been reported on the use of high frequency for polymerization reactions. One reason for this is that the physical effects generated during high-frequency cavitation are not strong enough to produce the shear forces that are necessary to disperse the oil phase in the aqueous medium. Microemulsions are optically transparent and thermodynamically stable mixtures of oil, water, and surfactants.10-12 The oil and water components are located in distinct domains separated by a surfactant-rich interfacial region. The oil domains in water are so small (100 nm) are present.13 This suggests that, at higher SDS concentration, the particle size distribution becomes narrower. In a previous study by Bradley et al.,7 it was found that the surfactant is adsorbed at the latex surface rather than chemically bonded to the latex particles. This indicates that the surfactant only acts as a stabilizer for the monomer droplets and is not intercepted by the radicals. However, as the concentration of surfactant used in this study is much higher than that used in Bradley et al.,7 the surfactant may well be intercepting the radicals formed. Further investigation is required to clarify this issue.
Letters
Figure 2. (a) Conversion vs sonication time for the polymerization of BMA using different surfactants. (b) Evolution of particle size during polymerization in 6.5 wt % SDS, DDDAB, and Triton-X 100. (c) Photograph of the final polymer latexes (after 15-min sonication) stabilized with (i) DDDAB, (ii) SDS, and (iii) Triton-X 100.
TABLE 1: Average Number of Polymer Chains Per Particle (Nc), Weight-Average Molar Mass (Mw), Number-Average Molar Mass (Mn) and Polydispersity Index (PDI) at Final Conversions SDS (wt %)
Mw (g/mol)
Mn (g/mol)
PDI
Nc (chains/ particle)
0.5 2.5 4.5 6.5 10.5
7.0 × 106 6.3 × 106 5.1 × 106 5.4 × 106 4.4 × 106
6.8 × 106 5.7 × 106 4.7 × 106 4.9 × 106 3.9 × 106
1.0 1.1 1.1 1.1 1.2
5 3 3 2 2
Table 1 shows the molecular weights of PBMA latex particles and the average number of polymer chains per particle. It can be noted that, within experimental error, the molecular weights decrease with increasing amount of SDS. This phenomenon has been observed by other authors.14,15 One likely explanation for the decrease in molecular weight is chain transfer to the surfactant (the ratio of surfactant to monomer for 10.5 wt % was 4.2:1). At higher surfactant concentrations, the droplet size
Letters is smaller. This may indicate that the growing polymeric radical in the droplet is closer to the surfactant layer and therefore the probability of chain-transfer reaction with surfactant is higher.15 In order to further explore the effect of surfactant properties on microemulsion polymerization kinetics and on the properties of latex particles, experiments were carried out in the presence of different surfactants. The effects of ionic surfactants SDS and DDDAB and a nonionic surfactant Triton-X 100, as stabilizers, were investigated. The conversion percentage versus time plots for different surfactants at 6.5 wt % are presented in Figure 2a. It can be observed that when ionic surfactants were used as stabilizers, full conversion to polymer was obtained within 15 min. However, for the case when a nonionic surfactant was used, the rate of polymerization was significantly slower and the conversion percentage only reached ∼50% after ∼15 min of sonication (Polymerization reached to almost full conversion after 1 h of sonication.). These results are consistent with previous studies on conventional microemulsion polymerization systems,16,17 where slower polymerization rates was observed when nonionic surfactants were used. It is likely the reason for this rests with the droplet size produced in this system. A larger droplet size corresponds to a smaller number of nucleation sites; i.e., the number density of particles is low and, as a consequence, results in a slower polymerization rate. It is also postulated that a bigger droplet size signifies a smaller total surface area and this means that the probability of any monomeric radicals entering a droplet is lower, and therefore, the polymerization rate is retarded. Figure 2b shows the evolution of polymer particle sizes as a function of polymerization time. It can be inferred from Figure 2b that the charged surfactants, SDS and DDDAB, are better stabilizers for the colloidal systems, resulting in smaller and more stable oil droplets and latex particles; alternatively, the steric stabilization effect of Triton-X 100 alone was not sufficiently strong to completely prevent coagulation of the primary emulsion droplets, thereby resulting in larger particles. For the latex particles stabilized by Triton-X 100, the size of the particles increases during the initial stage of polymerization whereas the particles stabilized by SDS and DDDAB have approximately the same droplet/particle size throughout the polymerization reaction. The photograph of the final latex particle samples shown in Figure 2c provides further support to the particle size data (Figure 2b). When the latex particles were stabilized with the nonionic surfactant, the solution appears
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5267 more turbid, indicating that particles with a diameter greater than 100 nm are present. However, when the latex particles were stabilized by the ionic surfactants, the solutions appear bluish translucent, an indication that the size distribution is narrower. In summary, we have shown that ultrasound-initiated microemulsion polymerization at high frequency can be readily achieved. The use of ionic surfactants as stabilizers leads to the formation of transparent latex particles. Acknowledgment. The authors thank James Wiltshire and Tor Kit Goh for their assistance in collecting the molecular weights data. B.M.T. gratefully acknowledges the receipt of a Melbourne International Research Scholarship. The financial support from the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council is also acknowledged. References and Notes (1) Dai, L. M. Intelligent Macromolecules for Smart DeVices: From Materials Synthesis to DeVice Applications; Springer-Verlag: London, 2004. (2) Xu, X. L.; Ge, X. W.; Zhang, Z. C.; Zhang, M. W.; Zuo, J.; Niu, A. Z. Polym. Int. 1998, 47, 393. (3) Landfester, K.; Bechthold, N.; Tiorks, F.; Antonietti, M. Macromolecules 1999, 32, 679. (4) Bradley, M. A.; Prescott, S. W.; Schroonbrood, H. A. S.; Landfester, K.; Grieser, F. Macromolecules 2005, 38, 6346. (5) Teo, B. M.; Prescott, S. W.; Ashokkumar, M.; Grieser, F. Ultrason. Sonochem. 2008, 15, 89. (6) Ooi, S. K.; Biggs, S. Ultrason. Sonochem. 2000, 7, 125. (7) Bradley, M. A.; Grieser, F. J. Colloid Interface Sci. 2002, 251, 78. (8) Zhang, C. H.; Wang, Q.; Xia, H. S.; Qiu, G. H. Eur. Polym. J. 2002, 38, 1769. (9) Xia, H. S.; Wang, Q.; Liao, Y. Q.; Xu, X.; Baxter, S. M.; Slone, R. V.; Wu, S.; Swift, G.; Westmoreland, D. G. Ultrason. Sonochem. 2002, 9, 151. (10) Tauer, K.; Ramirez, A. G.; Lo´pez, R. G. C. R. Chim. 2003, 6, 1245. (11) Gan, L. M.; Lee, K. C.; Chew, C. H.; Ng, S. C. Langmuir 1995, 11, 449. (12) Go´mez-Cisneros, M.; Trevin˜o, M. E.; Peralta, R. D.; Rabelero, M.; Mendiza´bal, E.; Puig, J. E.; Cesteros, C.; Lo´pez, R. G. Polymer 2005, 46, 2900. (13) Franses, E. I.; Scriven, L. E.; Miller, W. G.; Davis, H. T. J. Am. Oil Chem. Soc. 1983, 60, 1029. (14) Vanderhoff, J. W.; Distefano, F. V.; El-Aasser, M. S.; O’Leary, R.; Shaffer, O. M.; Visioli, D. L. J. Dispersion Sci. Technol. 1984, 5, 323. (15) Candau, F.; Leong, Y. S.; Fitch, R. M. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 193. (16) O ¨ zdegˇer, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3827. (17) Napper, D. H.; Netschey, A.; Alexander, A. E. J. Polym. Sci., Part A-1 1971, 9, 81.
