Polymer Composite

Department of Chemistry and Center for Advanced Materials Processing, Clarkson University,. Potsdam, New York 13699-5665. Received January 14, 2000...
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Reverse Micellar Synthesis of a Nanoparticle/Polymer Composite† Florentina M. Pavel and Raymond A. Mackay* Department of Chemistry and Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5665 Received January 14, 2000. In Final Form: May 2, 2000 Cadmium sulfide nanoparticle/polymer composites have been produced using a one-system reverse micellar synthesis. A monomer, methyl methacrylate (MMA), was used as the oil and polymerized following formation of 2-3 nm CdS particles in the fluid medium. When Aerosol OT (AOT) was employed as the surfactant, opaque solids containing 20-80 nm aggregates of the CdS nanoparticles were obtained. The aggregates were fairly uniform in size, their diameter depending on the AOT concentration. With a 1:1 weight ratio of MMA and a polyethylene diacrylate, aggregation was eliminated but the solid remained opaque. Replacing the AOT by the polymerizable surfactant didecyldimethylammonium methacrylate with MMA as the oil led to the formation of a transparent solid matrix containing nonaggregated CdS particles.

Introduction Producing nanosized, relatively monodisperse particles such as semiconductors, metals, or ultrahigh molecular weight latexes has received significant attention in the past decade because a change in a number of physical properties (optic, electric, and magnetic) compared with the bulk can be achieved by using such new materials.1-5 Inverse (W/O) microemulsions have been successfully used as reaction media to form monodisperse nanoparticles (5-50 nm) of inorganic materials such as metals (Pt, Pd, Au, Ag, Cu, Fe),6,7 semiconductors (CdS, ZnS, SiO2, ZnO, TiO2),8-10 cathode materials for lithium batteries (MnO2, LiMn2O4, LiNiO2, LiCoO2),11,12 and complex metal oxides (LaNiO3, PbZrO3, BaTiO3, Ba Fe12O19, YBa2Cu2O7-x).13-16 Composites of nanoparticles such as metals and semiconductors17,18 in a polymer matrix are under investigation for a variety of applications, including nonlinear optical † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. * To whom correspondence should be addressed. Telephone: (315) 268-7607. Fax: (315) 268-7615. E-mail: [email protected].

(1) Cahn, R. W. Nature 1992, 359, 591. (2) Hayashi, C. Phys. Today 1987, Dec, 44. (3) Ozin, G. A. Adv. Mater. 1992, 4, 612. (4) Awschalom, D.; DiVincenzo, D. Phys. Today 1995, 13, 43. (5) Tojo, C.; Blanco, M. C.; Lopez-Quintela, M. A. Langmuir 1997, 13, 4527. (6) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (7) Pileni, M. P.; Taleb, A.; Petit, C. J. Dispersion Sci. Technol. 1998, 19 (2&3), 185. (8) Xu, W.; Siow, K. S.; Gao, Z.; Lee, S. W. Chem. Mater. 1998, 10, 1951. (9) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (10) Ward, A. J. I.; O’Sullivan, E. C.; Rang, J. C.; Nedeljkovic, J.; Patel, R. C. J. Colloid Interface Sci. 1993, 8, 1049. (11) Huang, K. T.; Um, W. S.; Lee, H. S.; Song, J. K.; Chung, K. W. J. Power Sources 1998, 74, 169. (12) Rusling, J. F.; Zhou, D.; Gao, J. Proc. s Electrochem. Soc. 1997, 97, 137. (13) Ayyub, P.; Maitra, A. N.; Shah, D. O. Physica C 1990, 168, 571. (14) Kumar, P.; Pillai, V.; Bates, S. R.; Shah, D. O. Mater. Lett. 1993, 16, 68. (15) Kumar, P.; Pillai, V.; Shah, D. O. J. Magn. Magn. Mater. 1992, 116, L299. (16) Kumar, P.; Pillai, V.; Multani, M. S.; Shah, D. O. Colloids Surf., A: Physicochem. Eng. Aspects 1993, 80, 69. (17) LaPeruta, R.; Van Wagenen, E. A.; Roche, J. J.; Kitipichai, P.; Wnek, G. E.; Korenowski, G. E. SPIE Nonlinear Opt. Mater. 1991, 1497, 357.

materials. As separate classes of materials, both the organic matrix and metal nanoparticles show promise for such applications. When combined to form composites, interactions between the polymer host and entrapped metal nanoparticles can potentially be used to produce synergistic effects such as larger optical nonlinearities. Tailoring the size, shape, and composition of the particles to increase local fields internal to the composite and having charge-transfer interactions between the polymer and metal particles are among the mechanisms by which an increase in the nonlinear polarizabilities can be achieved. For many of these applications a transparent matrix is required. Since 1980, many attempts have been made to prepare porous polymeric materials by polymerization of methyl methacrylate or styrene in water-in-oil (W/O) microemulsions,19,20 bicontinuous (middle-phase) microemulsions,21-24 or both.25 Nonpolymerizable sodium dodecyl sulfate (SDS) was most often used for stabilizing those polymerized microemulsions. Polymeric materials thus obtained were usually opaque and often underwent phase separation during polymerization.26 A detailed study of the stability regions of styrene/polystyrene water-in-oil microemulsions stabilized by sodium dodecyl sulfate with n-pentanol or butylcellosolve as cosurfactant was performed by Gan et al.27 They showed that the stability of a microemulsion toward phase separation during polymerization is directly (18) Han, M. Y.; Huang, W.; Chew, C. H.; Gan, L. M.; Zhang, X. J.; Ji, W. J. Phys. Chem. B 1998, 102, 1884. (19) Stoffer, J. O.; Bone, T. J. Dispersion Sci. Technol. 1980, 1, 393. (20) Menger, F. M.; Tsuno, T.; Hammond, G. S. J. Am. Chem. Soc. 1990, 112, 1263. (21) Sathav, M.; Cheung, H. M. Langmuir 1991, 7, 1378. (22) Palani Raj, W. R.; Sathav, M.; Cheung, H. M. Langmuir 1991, 7, 2586. (23) Palani Raj, W. R.; Sathav, M.; Cheung, H. M. Langmuir 1992, 8, 1931. (24) Qutubuddin, S.; Haque, E.; Benton, W. J.; Fendler, E. J. In Polymer Association Structures: Microemulsions and Liquid Crystals; El-Nokaly, M., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989; Vol. 384. (25) Chieng, T.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 36, 1941. (26) Chew, C. H.; Li, T. D.; Gan, L. H.; Quek, C. H.; Gan, L. M. Langmuir 1998, 14, 6068. (27) Gan, L. M.; Chew, C. H.; Friberg, S. E. J. Macromol. Sci. 1983, A19, 739.

10.1021/la000045d CCC: $19.00 © 2000 American Chemical Society Published on Web 07/01/2000

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Figure 1. Absorption spectra of CdS nanoparticles in the 0.2 M AOT/water/MMA (w ) 5) system as a function of time. The overall CdS concentration is 4.5 × 10-4 M.

Figure 2. Kinetics of CdS particle growth in the system of Figure 1: (a) diameter (d) versus time; (b) log-log plot of (d d0) versus time (vide text). The line represents a linear leastsquares fit.

related to the limited solubility of the polymer in the microemulsion. Menger et al.20 reported the polymerization of a W/O microemulsion system consisting of styrene, the crosslinker divinylbenzene, water, and the anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or simply AOT). The polymerization was initiated by benzoyl peroxide/UV light, and a spongelike polymer was obtained. Although the reacting systems were optically clear mixtures, after polymerization opaque polymers were obtained. The existence of porosity reflects the tendency for water pools to assemble within the polymer composite. The synthesis of transparent polymeric solids in W/O microemulsions has been attempted by using polymerizable surfactants. These surfactants have the advantage over nonpolymerizable surfactants that the templating effect of the amphiphilic interface may be better preserved during polymerization.28 It is desirable to use a polymerizable surfactant that can be copolymerized with the monomer being employed, for example methyl methacrylate (MMA) or styrene, to form an integrated polymeric material. Systems previously investigated include MMA/ acrylic acid/water and a copolymerizable anionic surfactant of sodium (acrylamido)undecanoate or sodium (acrylamido)stearate.29-31 Only opaque polymeric materials were obtained from these microemulsion systems with water contents > 15 wt %. Although transparent polymeric (28) Dreja, M.; Pyckhout-Hintzen, W.; Tieke, B. Macromolecules 1998, 31, 272.

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Figure 3. Variation of initial CdS particle size (d0) as a function of AOT concentration in the w ) 5 water/MMA system: overall CdS concentration ) (a) 1.5 × 10-4 M or (b) 4.5 × 10-4 M.

Figure 4. Conductivity versus volume fraction of water plus surfactant for the w ) 5 water/MMA (2) and isooctane (9) systems at 22 °C. A magnified conductivity scale for the water/ MMA system is shown in the inset.

materials could also be produced from these systems at water contents < 15 wt %, they did not reveal any obvious microstructures, as indicated by scanning electron microscopy.31 The less readily polymerizable anionic surfactant of potassium undecenoate (PUD) can also be incorporated in a bicontinuous microemulsion23 consisting of MMA, PUD, water, and the cross-linking agent ethylene glycol dimethacrylate (EGDMA). However, PUD is very prone to allylic chain-transfer reactions,32 leading to poor copolymerization for the microemulsion system. Polymerization in microemulsions with polymerizable surfactants has been studied in bicontinuous systems23,33,34, and in W/O35 and O/W microemulsions.36 Recently, several types of transparent nanoporous polymeric materials have been successfully prepared from the polymerization of microemulsions containing very reactive polymerizable surfactants, such as 11-(N-ethylacrylamido)undecanoate,37 (acryloxy)undecyltrimethylammonium (29) Gan, L. M.; Chew, C. H. J. Dispersion Sci. Technol. 1983, 4, 291. (30) Gan, L. M.; Chew, C. H. J. Dispersion Sci. Technol. 1984, 5, 291. (31) Chew, C. H.; Gan, L. M. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2225. (32) Paleos, C. M.; Stassinopoulou, C. I.; Mallaris, A. J. Phys. Chem. 1983, 87, 251. (33) Li, T. D.; Chew, C. H.; Ng, S. C.; Gan, L. M.; Teo, W. K. J. Macromol. Sci., Pure Appl. Chem. 1995, A32, 969. (34) Li, T. D.; Chew, C. H.; Ng, S. C.; Gan, L. M.; Teo, W. K. J. Macromol. Sci., Pure Appl. Chem. 1995, A32, 221. (35) Hammouda, A.; Pileni, M. P. Langmuir 1995, 11, 3656. (36) Dreja, M.; Tieke, B. Macromol. Rapid Commun. 1996, 17, 825. (37) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Langmuir 1994, 10, 4022.

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Figure 5. TEM of CdS (0.45 mM) nanoparticles in a polymerized 0.2 M AOT/water/MMA (w ) 5) system (vide text): (a) lower magnification (bar is 200 nm); (b) higher magnification (bar is 10 nm).

bromide,33,38 and (acryloxy)undecyldimethylammonioacetate,38,39 and ω-methoxypoly(ethyleneoxy)undecyl-R-methacrylate.40 Different approaches have been employed to prepare nanoparticle/polymer composites. A variety of such composites have been prepared, including silver in a polyurethane bearing a tricyanovinyl moiety,17 amorphous iron and cobalt embedded in poly(methacrylate) or poly(methyl methacrylate),41 CdS in a cross-linked polyphosphazenebased polymer network,42 and CdS attached to the sulfhydryl groups of a thiol-containing polyphenol.43 The objectives of this study were first to produce randomly dispersed (nonaggregated) particles and second to do so in a transparent polymer matrix. We report here the results of an initial study to produce inorganic nanoparticles in a reverse micellar system using a monomer as the oil, followed by polymerization to produce a solid inorganic/organic composite. The specific materials employed were CdS in methyl methacrylate using either AOT or didecyldimethylammonium methacrylate (DDAMA) as the surfactant. While it is anticipated that this methodology will have general applicability, CdS in the (38) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K. Langmuir 1996, 12 (2), 5863. (39) Li, T. D.; Gan, L. M.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (40) Xu, W.; Siow, K. S.; Gao, Z.; Lee, S. Y.; Chow, P. Y.; Gan, L. M. Langmuir 1999, 15, 4812. (41) Wizel, S.; Margel, S.; Gedanken, A.; Rojas, T. C.; Fernandez, A.; Prozorov, R. J. Mater. Res. 1999, 14, 3913. (42) Olshavsky, M. A.; Allcock, H. R. Chem. Mater. 1997, 9, 1367. (43) Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1997, 9, 1342.

few nanometer size range was chosen as a model particle because it has been well-studied and characterized. To the best of our knowledge, this is the first report of a “onesystem” synthesis of nanoparticles dispersed in a polymer matrix using a reverse micellar system. Experimental Section Materials. Methyl methacrylate 99% (MMA) was obtained from Aldrich. Dioctylsulfosuccinate sodium salt (AOT), cadmium nitrate tetrahydrate, and sodium sulfide nonahydrate were purchased from Aldrich and were of 98% purity. Benzoyl peroxide 97% was obtained from Aldrich, and 2,2′-azobis(2-methylpropionitrile) 98% (AIBN) from Alfa Aesar. Poly(ethyleneglycol)ndiacrylate (n ) 200) was purchased from Polysciences, and deionized water was used throughout. The polymerizable surfactant didecyldimethylammonium methacrylate (DDAMA) was synthesized as previously reported.44 Apparatus. The transmission electron microscope used is a JEOL-1200 EX operating at 120 kV. The UV spectra were recorded using a Milton Roy Spectronic3000 Array instrument. The conductivity measurements were carried out using a YSI model 32 conductance meter operating at 1 kHz, with a YSI electrode model 3403 (K ) 1.0/cm). The quasi-elastic lightscattering measurements were performed on a Leeds and Northrup Ultratrac UPA 150 instrument. Polymerizations. All polymerizations were carried out in an oven at 55-60 °C. In all of the reactions 0.1-0.3 M AOT or DDAMA was dissolved in MMA, and water was added to yield a water-to-surfactant mole ratio (w) of 5. The amount of benzoyl (44) Moumen, N.; Pileni, M. P.; Mackay, R. A. Colloids Surf., A: Physicochem. Eng. Aspects 1999, 151, 409.

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Figure 6. TEM of CdS (0.45 mM) nanoparticles in a polymerized 0.3 M AOT/water/monomer system, where the monomer is (a) MMA or (b) MMA/poly(ethylene glycol) (200) diacrylate, 1:1 w/w. peroxide used was 3%, and that of AIBN 0.5-3 wt %, on the basis of the total weight of monomer. Transmission Electron Microscopy. One drop of unpolymerized microemulsion was allowed to dry on a 200-mesh carboncoated copper grid. For polymerized microemulsions, the TEM copper grid with a drop of microemulsion was placed in an oven at 55-60 °C and the microemulsion was allowed to polymerize prior to microscopy. Synthesis of CdS in Reverse Micelles. CdS nanoparticles were prepared using a procedure described previously.45 The synthesis was carried out by mixing two reverse micellar solutions with the same ratio of water (w ) 5) containing Cd2+ and S2-, respectively. The concentrations of Cd2+ and S2- in the microemulsions before mixing were the same, 3 × 10-4 or 9 × 10-4 mol‚L-1. The mixing was produced by rapid injection of 3 mL of each of the solutions (X ) [Cd2+]/[S2-] ) 1) into a 20 mL vial, and the fluid was then transferred into a quartz cuvette for Uvvisible spectroscopy or polymerized as described above. All of these experiments, both CdS formation and MMA polymerization, were carried out in normal atmosphere, and no attempt was made to exclude oxygen.

acterized by means of UV-visible spectroscopy.55 Nonetheless, some examination of CdS nanoparticle formation in this W/MMA system was undertaken to develop an understanding of the compositional boundaries within which composites might be formed. It should be emphasized that, as indicated in the Experimental Section, the particles were produced by mixing equal volumes of two identical micellar solutions, one containing Cd2+ and the other S2-. The mixing time was about 5 s before starting the clock. Therefore, time “zero” was on the order of 5 s after mixing. Previous studies in similar media have shown that the primary events leading to the nucleation and initial particle formation normally take place in 10) have attractive interactions, and their conductivity curves exhibit percolative behavior in the vicinity of Φ g 0.2.59,60 These conductivity curves resemble those re(59) Van Dijk, M. A. Phys. Rev. Lett. 1985, 55, 1003.

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Figure 8. TEM of a polymerized 0.2 M DDAMA W/MMA (w ) 5) system (vide text). (a) lower magnification (bar is 100 nm); (b) higher magnification (bar is 10 nm).

ported61 for a w ) 55 AOT/W/isooctane system with and without added poly(ethylene glycol) (PEG). Both isooctane curves are again similar except that at w ) 55 the microemulsion exhibits a percolation threshold in the normal range while the sharp rise for the w ) 5 system occurs at Φ ≈ 0.6. Since this value is near that for random close packing of spheres (0.64), this result may indicate a transition to a bicontinuous, O/W, or sponge phase. The shapes of the MMA and PEG curves are also very similar except that the absolute values of the conductivity are somewhat higher in the case of MMA. This behavior with added PEG was attributed to a change in droplet-droplet interaction from attractive to repulsive. It should be noted that, below a value of w ≈ 10, there is no water that is not strongly hydrating the surfactant and counterions.62 The exact value of w could vary slightly depending upon the surfactant and the organic fluid, but above this value, the system is a W/O microemulsion, and below it, it is more properly referred to as a reverse micellar system, although both terms are frequently used somewhat interchangeably, since there is only one continuous single-phase region. It is clear that these results require further study. However, since the principal objective of this study was to form a nanocomposite, the AOT concentration employed was always