pubs.acs.org/Langmuir © 2009 American Chemical Society
A New Physical Route to Produce Monodispersed Microsphere Nanoparticle-Polymer Composites Ki Myoung Yun, Adi Bagus Suryamas, Chika Hirakawa, Ferry Iskandar, and Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan Received April 16, 2009. Revised Manuscript Received May 31, 2009 A new physical route for the production of monodispersed microsphere nanoparticle-polymer composites utilizing a beads milling method, followed by an electrospray method, has been developed. Poly(methyl methacrylate) (PMMA)TiO2 composites were used as a model to evaluate the performance of this route. SEM images showed that the products were monodispersed, spherical, and nonagglomerate. The mean diameter was in the range of 0.25-1.87 μm, with a standard deviation of 0.06-0.172. TEM images confirmed that nonagglomerated TiO2 nanoparticles were highly dispersed inside the polymer matrices. We found that the concentration ratio of TiO2 to polymer in the precursor led to changes in precursor properties, such as permittivity and electrical conductivity, and resulted in changes in the produced particle size.
1. Introduction Nanoparticles and polymer materials are of great scientific interest because of their unique properties. Nanoparticles show remarkably different electronic, optical, magnetic, and catalytic properties compared with their macroscale counterparts, due to the size-dependent quantum effects that arise in nanoparticles. More recently, there have been efforts to incorporate nanoparticles into polymer matrices to enhance the functionalities and properties of composites by utilizing the interaction between nanoparticles and polymer matrices. The interaction could result in unique and/ or new mechanical, electrical, optical, and thermal properties of nanoparticle-polymer composites.1-4 Advances in the preparation of nanoparticle-polymer composites will allow for the development of new types of data storage, as well as new optical and electro-rheological materials.4 In general, nanoparticle-polymer composites can be prepared by either in situ or ex situ methods. In the in situ method, nanoparticles are synthesized within a polymer matrix, but this method is limited by the fact that some kinds of nanoparticles cannot be easily synthesized within polymer matrices, and the precursor often results in byproducts. In the ex situ method, nanoparticles are prepared first and then incorporated into the polymer by a polymerization process.3 The ex situ method seems to be more feasible than the in situ method because many nanoparticles can be synthesized in large quantities by this inexpensive process.4 However, challenges still remain with this method. In nanoparticle preparation, agglomeration and bad dispersion of nanoparticles inside polymer matrices are the main problems. In the polymerization process, uncontrollable and polydispersed distributions of composite size are the main problems. It is therefore necessary to develop a general route to the production of nanoparticle-polymer composites that is effective for the nanoscale, yet applicable to macroscopic processing. In addition, *To whom correspondence should be addressed. Phone: +81-82-424-7716. Fax: +81-82-424-7850. E-mail:
[email protected]. (1) Mackay, M. E.; Tuteja, A.; Duxbury, P. M.; Hawker, C. J.; Horn, B. V.; Guan, Z.; Chen, G.; Krishnan, R. S. Science 2006, 311, 1740–1743. (2) Balazs, A. C.; Emrick, T.; Russel, T. P. Science 2006, 314, 1107–1110. (3) Hasell, T.; Yang, J.; Wang, W.; Li, J.; Brown, P. D.; Poliakoff, M.; Lester, E.; Howdle, S. M. J. Mater. Chem. 2007, 17, 4382–4386. (4) Inkyo, M.; Tokunaga, Y.; Tahara, T.; Iwaki, T.; Iskandar, F.; Hogan, C. J. Jr.; Okuyama, K. Ind. Eng. Chem. Res. 2008, 47, 2597–2604.
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several applications in emerging technologies require highly monodispersed microspheres with very narrow size distributions.1 Many mechanical milling processes have been developed to break up the agglomeration of particles and disperse them in suspensions, including agitator discs, colloid mills, high-pressure homogenizers, triple roller mills, ball mills, and beads mills. Beads mills are often used in industrial processing to grind and disperse agglomerated particles with primary particles in the submicrometer size range.5 Our group has developed a new type of beads mill to disperse nanoparticles in an aqueous suspension without affecting particle crystallinity.4,5 In addition, electrospray is a robust method for particle production, in that it allows for not only controlled size but also controlled properties. With this method, liquid can be broken into ultrafine droplets, and then the droplets can be transformed into a collection of particles.6,7 We also have successfully produced particles with controlled sizes and properties using the electrospray method.6-8 Here, we report a novel physical route for the production of highly dispersed nanoparticles in polymer composite microspheres. The route consists of the utilization of a beads mill method followed by an electrospray method, as shown in Figure 1. Beads mill was used to avoid the agglomeration of nanoparticles inside polymer matrices, and electrospray was used to produce monodispersed microsphere particles. To the best of our knowledge, this is the first unification of beads mill and electrospray as a route to produce nanocomposites. The precursor of electrospray was first prepared by beads milling to obtain well-dispersed TiO2 nanoparticles, which was followed by electrospraying to obtain composites. As a comparison, we also produced nanoparticlepolymer composites from the precursor, which was not prepared using the beads mill process. The effects of the beads milling process and precursor composition were used to evaluate the electrospray method. The physical properties of the precursor also are reported. (5) Inkyo, M.; Tahara, T.; Iwaki, Y.; Iskandar, F.; Hogan, C. J. Jr.; Okuyama, K. J. Colloid Interface Sci. 2006, 304, 535–540. (6) Lenggoro, I. W.; Okuyama, K.; de la Mora, J. F.; Tohge, N. J. Aerosol Sci. 2000, 31, 121–136. (7) Lenggoro, I. W.; Xia, B.; Okuyama, K.; de la Mora, J. F. Langmuir 2002, 18, 4584–4591. (8) Hogan, C. J. Jr.; Yun, K.-M.; Chen, D.-R.; Lenggoro, I. W.; Biswas, P.; Okuyama, K. Colloids Surf., A 2007, 311, 67–76.
Published on Web 06/11/2009
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Figure 1. Schematic diagram of beads mill-electrospray route used in the experiment. Insets show spray modes usually occurring in electrospray: cone jet mode (i), silver bullet mode (ii), and pulsating mode (iii).
Poly(methyl methacrylate) (PMMA)-TiO2 composites were used as a model to determine the performance of this route for the production of nanoparticle-polymer composites. TiO2 is one of the most widely investigated metal oxides since it is an inexpensive and nontoxic material that has a high refractive index and possesses the ability to absorb ultraviolet light. PMMA is an important thermoplastic material that is transparent to visible light. TiO2 nanoparticle fillers in PMMA can be used to adjust the refractive index of PMMA as well as to enhance its UV light absorbing capabilities.4 PMMA-TiO2 composites also show ultrafast optical nonlinearity, which has great potential for optical switching and optical communication.9
2. Experimental Section Materials. Titania nanoparticles (TiO2) with a nominal particle size of 15 nm (MT150A, rutile phase; Tayca Co. Ltd., Japan) were used in the present experiment. The poly(methyl methacrylate) (PMMA, Mw = 90 000) was purchased from Asahi Kasei Co. Ltd., Japan. Oleic acid, C18H34O2 (Sigma Aldrich, tech. grade 90%), and silane coupling agent ((3-acryloxypropyl)trimethoxysilane; KBM5103, Shinetsu Co. Ltd., Japan) were used as surfactants in the dispersion process of nanoparticles. Diethylene glycol dimethyl ether (Kishida Chemicals Co. Ltd., Japan) was used as a solvent for the polymer solution prior to the electrospray process. Dispersion Process. TiO2 nanoparticles were dispersed first, before electrospray, to prevent agglomeration and to spread the distribution of nanoparticles as uniformly as possible. In the present study, two methods were used to disperse nanoparticles: stirrer mixing and beads milling. In the first method, nanoparticles were dispersed by mixing the TiO2 nanoparticles and oleic acid at a mass ratio of 1:1, followed by stirring of the solution for at least 8 h. In the second method, TiO2 nanoparticles were dispersed by beads (9) Elim, H. I.; Ji, W.; Yuwono, A. H.; Xue, J. M.; Wang, J. Appl. Phys. Lett. 2003, 82, 2691–2693.
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milling. Suspensions containing TiO2 particles, silane coupling agent, and diethylene glycol dimethyl ether were used in this process. Nanoparticle suspensions were pumped through the beads mill with a recirculation mass flow rate of 15.6 kg/h (29 s residence time in the milling region, 25 cycles per hour, 144 s total recirculation time). Centrifugal forces for bead separation in the upper part of the vessel were generated by rotating the outer cylindrical wall at a speed of 10 m/s. Zirconia beads with a primary size of 15 μm occupied 65% of the vessel volume. A schematic of the beads milling method is depicted in Figure 1a. Details of the beads milling method could be found in our previous reports.4,5 Electrospray. Nanoparticle-polymer composites were produced from a mixture of PMMA and TiO2 suspensions by electrospray. Figure 1b shows the schematic of the electrospray system used in the experiment. Dry nitrogen (N2) was flowed to keep the relative humidity of the environment below 30% for all experiments. These conditions allowed for complete evaporation of the solvent prior to particle deposition on the ground plate. In order to get the optimum experimental parameters, three different precursors were prepared for electrospray. After that, the concentration of TiO2 nanoparticles was varied to study the effect of TiO2 concentration on the physical properties of the prepared composite. For the first precursor (Precursor-1), as-prepared oleic-acid-coated TiO2 nanoparticles were mixed together with PMMA and diethylene glycol dimethyl ether. For the second precursor (Precursor-2), PMMA was directly dissolved in a suspension prepared by beads milling that contained 1 at. wt % of TiO2 and 1 at. wt % surfactant. For the third precursor (Precursor-3), the polymer solution of PMMA and diethylene glycol dimethyl ether solvent was mixed with a suspension prepared by beads milling that contained 5 at. wt % TiO2 and 5 at. wt % surfactant. For these three kinds of precursors, the mass ratio of TiO2, surfactant, PMMA, and diethylene glycol dimethyl ether was kept at 1:1:10:100, respectively. For the precursors used to vary the concentration of TiO2 nanoparticles, the various concentrations of polymer solutions for the PMMA, surfactant, and diethylene glycol dimethyl ether solvent were mixed with a beads milled suspension used for Precursor-3. The mass ratio of surfactant, PMMA, and diethylene glycol dimethyl ether was kept at 1:10:100, respectively, but the concentration of TiO2 nanoparticles was varied to 0.09, 0.45, and 0.9 at. wt %. All compounds were dissolved at room temperature, followed by stirring for at least 8 h. The precursor solution was placed in a hypodermic syringe, which was operated using a syringe pump (Harvard Apparatus PHD 2000) at a constant rate of 2 μL/min. The Taylor cone jet was generated by applying a high voltage, around 5 kV (MATSUSADA Precision, HER10R3), to the stainless steel capillary needle. The highly charged polymer particles were deposited on the ground plate, which was 6 cm away from the tip of the capillary needle for the experiments. Characterization. The morphology of prepared nanoparticlepolymer composites was observed using a field emission scanning electron microscope (FE-SEM; S-5000, Hitachi, Tokyo, Japan). TiO2 nanoparticles inside polymer matrices were observed using transmission electron microscopy (JEOL-JEM-2010, 200 kV). To obtain TEM images, the composites were cut into thin layers using a microtome, and a single thin layer was placed on a TEM grid. Particle size distribution of the solutions was measured using a dynamic light scattering (DLS) method using an HPPS-5001 Malvern Instrument. Solution electrical conductivities were measured using a TCX-90i conductivity meter (Toko Chemical Laboratories Co., Ltd., Japan), and dielectric-constant measurements were made using a Solartron 1296 dielectric interface (Ametex Inc., Hampshire, England).
3. Results and Discussion It has been understood for most of this century that high electric fields can deform and attract fluid interfaces.10 The (10) Zeleny, J. Phys. Rev. 1917, 10, 1–6.
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Figure 2. SEM images of TiO2 nanoparticles (a) and PMMATiO2 composites: Sample-1 (b), Sample-2 (c), and Sample-3 (d). Inset in (a) shows a TEM image of TiO2 nanoparticles.
electrospray method utilized the electric field to pull out the precursor from the needle and then produce particles and collect them on the ground plate. The stability of the meniscus shape plays a crucial role in controlling and dispersing particle size uniformly in an electrospray method. Three spray modes of fluid shape, which can be controlled by adjusting the electric field, usually occur in electrospray: the cone jet, silver bullet, and pulsating modes.6,8,11,12 Images of these modes are shown in the insets of Figure 1b. The cone jet mode was utilized in all experiments in the present study. The cone jet mode is stable enough to produce not only monodispersed but also spherical shape particles in an electrospray method.8 From electrodynamics point of view, a droplet contains many charged molecules. Molecules interact with one another until the electrodynamics force is balanced. The balancing of electrodynamic forces leads to a sphere geometrical shape. The details of the mathematical and physical description of droplet formation can be found elsewhere.13 In addition, it can be understood that a stable cone jet and electrodynamics forces will result in droplets and particles of a uniform size. SEM and TEM images of as-purchased TiO2 nanoparticles, without further treatment, are shown in Figure 2a. It is apparent that primary TiO2 particles are rod-shaped (15 nm for minor axis, 50 nm for major axis), and that the nanoparticles are hardly agglomerated. Figure 2b-d shows SEM images of prepared composite samples from Precursor-1, Precursor-2, and Precursor-3, which are labeled as Sample-1, Sample-2, and Sample-3, respectively. All precursors resulted in nonagglomerated composite particles with spherical morphology. Figure 3a-c shows the distributions of composites from all precursors obtained from SEM images. A monodispersed distribution was shown by Sample-1 and Sample-3. Their average diameters, D, were 1.87 and 1.18 μm, with relative standard deviations, σ, of 0.17 and 0.07, respectively. Since relative standard deviation σ < 0.1 is a requirement for good uniformity,14 only Sample-3 was found to have good uniformity. On the other hand, a polydispersed distribution was shown by Sample-2. The diameters of the composites were arranged into two groups, the small group (S)
and the large group (L), with average diameters of 0.25 and 1.10 μm and standard deviations of 0.17 and 0.07, respectively. Polydispersed particles could be obtained when either the molecular weight of the polymer or the concentration of the polymer solution was too low when it was used as the precursor in the electrospray process.8 Cone jets of polymer solutions with sufficient resistance to shear stress will not break up to form droplets, rather, a polymer fiber forms from them after solvent evaporation by a process called electrospinning.15 Particle agglomeration in the suspension could also prevent spherical and controlled-size particle formation in the electrospray process.16 In these cases, however, a broader range of particle diameter would result. The arrangement into two groups can be explained by the distortion of the cone jet during the electrospray process. This phenomenon is affected by fluid dynamics that depend on the properties of the solution and operating conditions in the electrospray. The cone jet distortion could have resulted from a tiny branching cone jet mode that could not be observed by the camera. Branching in the cone jet mode can result in polydispersed particles, depending on the number of cone jet branches, branch size, and, of course, the stability of the cone jet branches.17 A coulombic explosion of electrosprayed droplets also could cause multipeak distribution. The coulombic explosion could have occurred when the highly charged droplets produced by electrospray were reduced in size to a point at which the repulsion between charges on the surface of the droplet was greater than the force of the surface tension, before solvent evaporation.18 The coulombic explosions would increase the polydispersity of the produced particle size distribution. However, a number of our repeat experiments show that only Sample-2 resulted in polydispersed distribution, even though the measured solution properties and chemical compositions of Sample-2 were almost the same as Sample-3. Presumably, the method for the preparation of nanoparticle-dispersed polymer solution has influenced the compatibility between PMMA and silane coupling agents to enclose
(11) Yun, K.-M.; Hogan, C. J. Jr.; Matsubayashi, Y.; Kawabe, M.; Iskandar, F.; Okuyama, K. Chem. Eng. Sci. 2007, 62, 4751–4759. (12) Cloupeau, M.; Prunet-Foch, B. J. Electrostat. 1989, 22, 135–159. (13) Collins, R. T.; Jones, J. J.; Harris, M. T.; Basaran, O. A. Nat. Phys. 2008, 4, 149–154. (14) Duby, M.-H.; Deng, W.; Kim, K.; Gomez, T.; Gomez, A. J. Aerosol Sci. 2006, 37, 306–322.
(15) Li, D.; Xia, Y. N. Adv. Mater. 2004, 16, 1151–1170. (16) Widiyandari, H.; Hogan, C. J. Jr.; Yun, K.-M.; Iskandar, F.; Biswas, P.; Okuyama, K. Macromol. Mater. Eng. 2007, 292, 495–502. (17) Yarin, A. L.; Kataphinan, W.; Reneker, D. H. J. Appl. Phys. 2005, 98, 064501-1–064501-12. (18) de la Mora, J. F. J. Colloid Interface Sci. 1996, 178, 209–218.
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Figure 3. Particle size of PMMA-TiO2 composites: Sample-1 (a), Sample-2 (b), and Sample-3 (c).
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Figure 4. TEM Images of PMMA-TiO2 composites: Sample-1 (a), Sample-2 (b), and Sample-3 (c).
Figure 5. Size distribution of TiO2 particles dispersed in organic solvent before and after beads milling. Inset shows a TEM image of TiO2 particles after beads milling.
TiO2 nanoparticles, which is also significant for the production of polymer particles by the electrospray process. TEM analysis of the PMMA-TiO2 composites was also performed to investigate the distribution of TiO2 nanoparticles inside the polymer matrix. Figure 4 shows the TEM images of prepared composites Sample-1, Sample-2, and Sample-3. Sample-1 TiO2 particles are shown to be agglomerated inside the polymer matrix. Figure 5 shows the size distributions of TiO2 particles measured by DLS, before and after beads milling. It is apparent that the TiO2 nanoparticles that were dispersed in the organic solvent without beads milling were hardly agglomerated. The agglomerations of TiO2 occurred because in the dispersion process, which was done only by a stirrer mixing method, the external dispersion force was not strong enough to break up particle agglomeration, so the particles naturally agglomerated. On the other hand, different results were found with Sample-2 and Sample-3. In both samples, the beads milling process was done prior to the electrospray to break up the agglomeration. DLS measurement and TEM measurement ensure that the beads mill process will break up the TiO2 agglomerates to an average diameter of about 20 nm. Furthermore, the primary particles appeared to have undergone a morphology change from rod-shape to more spherical by beads milling.5 In Sample2, TiO2 particles tended to disperse inside the polymer matrices. However, a small amount of agglomeration of TiO2 particles was still observed inside the polymer matrices. Well-dispersed TiO2 nanoparticles were shown in Sample-3. TiO2 particles were dispersed inside the polymer matrices without agglomeration. On the basis of these findings, we can conclude that, of the three samples, Sample-3 yielded the most suitable results. The beads milling method was found to be very useful, not only for the avoidance of particle agglomeration inside the polymer matrices but also for the dispersion of nanoparticles. To further investigate the prepared composites, energy-dispersive X-ray spectroscopy (EDS) spectra of composites produced by all methods were obtained, as shown in Figure 6. The elements detected by EDS largely comprised C, O, and Ti. This Langmuir 2009, 25(18), 11038–11042
Figure 6. EDS spectrum of PMMA-TiO2 composites: Sample-1 (a), Sample-2 (b), and Sample-3 (c).
Figure 7. SEM images of composites with variation in TiO2 concentration: 0 wt % (a), 0.09 wt % (b), 0.45 wt % (c), and 0.9 wt % (d).
result is consistent with the composition of the precursor used for electrospray. The presence of other peaks was also detected in the EDS spectra of Sample-2 and Sample-3. These peaks represent silicon elements, which were caused when the silanecoupling agent was used as the surfactant in the beads milling process. In order to investigate the effect of TiO2 concentration on the morphology, size, and physical properties of composites, experiments were performed using the same procedures as with Sample3, with variations in TiO2 concentration. The concentrations of TiO2 particles were 0, 0.09, 0.45, and 0.9 at. wt %. The SEM images of the prepared composites with variations in TiO2 concentration are shown in Figure 7. It is apparent that all prepared composites had spherical morphologies and were monodispersed. The particle size decreased as the TiO2 concentration increased, as shown in Figure 8. A semiempirical model that relates process parameters and solution properties to the size of the droplets produced by the electrospray method has been developed by Rosell-Llompart and de la Mora19 as well as by (19) Rosell-Llompart, J.; de la Mora, J. F. J. Aerosol Sci. 1994, 25, 1093–1119.
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Figure 8. Particle size and standard deviation of composites as a function of TiO2 concentration.
Chen and Pui.20 This scaling law to predict the initial droplet size and particle size yielded by the electrospray method is given as Q 1=3 Dcomp ¼ f ðKÞ φKε0 K
ð1Þ
where Dcomp is the diameter of the composite particle, κ is the dielectric constant of the liquid, ε0 is the permittivity of a vacuum, Q is the precursor flow rate, K is the electrical conductivity of liquid, φ is the volume fraction of all solutes in the liquid, and a dimensionless function of f(κ) is given by f ðKÞ ¼ -10:9K -6=5 þ 4:08K -1=3
ð2Þ
Figure 8 shows the SEM-measured particle sizes, calculated particle sizes, and standard deviations. The calculated particle sizes were slightly different with measured particle sizes, but overall, they tended to agree. The deviation from experimental results at a TiO2 concentration of 0%, presumably due to the electrical conductivity, was not measured by precision because the value was too low (around 1/30 times lower than the second lowest value). On the basis of the scaling laws,19,20 the size of produced polymer composite would be a function of the electrical conductivity and dielectric constant of the precursor, so that experimental conditions were identical. Thus, the TiO2 concentration led to changes in electrical properties of the precursor. The measured dielectric constant and electrical conductivity of precursors as a function of TiO2 concentration are shown in the Figure 9. Dielectric constant and electrical conductivity of the solutions increased as TiO2 concentration increased overall. (20) Chen, D.-R.; Pui, D. Y. H. Aerosol Sci. Technol. 1997, 27, 367–380.
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Figure 9. Dielectric constant and electrical conductivity of precursor as a function of TiO2 concentration.
The liquid, which has higher electrical properties, could carry a larger amount of charges, which produced a thinner liquid column and smaller droplets at the tip of the capillary in the electrospray process due to coulomb repulsion.10 As a result, the precursor with the higher TiO2 concentration produced smaller composite particles.
4. Conclusions A new physical route to produce monodispersed microsphere nanoparticle-polymer composites was successfully developed, using a beads milling method followed by an electrospray method. TiO2-PMMA composites were used as a model to observe the performance of this route. Nonagglomerated, monodispersed, and microspherical composites were observed via SEM characterization. Furthermore, TEM images confirmed that nanoparticles existed inside the polymer matrices. Also, the beads milling method was found to be necessary, not only to avoid particle agglomeration inside the polymer matrices but also to disperse the nanoparticles. The EDS spectra showed that the prepared composites were largely composed of C, O, and Ti, which is consistent with the composition of the precursor used in the experiment. Variations in TiO2 concentration were used in the experiment to study the effect of TiO2 concentration on the morphology and physical properties of as-prepared composites. It can be concluded that increased TiO2 concentration led to decreased particle size and made the distribution uniform. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (A) No. 18206079 by the Japan Society for the Promotion of Science (JSPS). K.-M.Y. and A.B.S. thank the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan for doctoral scholarship provision.
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