Synthesis and Characterization of Poly (N-isopropylacrylamide

Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering,. Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan...
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7998

Langmuir 1999, 15, 7998-8006

Synthesis and Characterization of Poly(N-isopropylacrylamide)-Coated Polystyrene Microspheres with Silver Nanoparticles on Their Surfaces† Chun-Wei Chen, Takeshi Serizawa, and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Received December 21, 1998. In Final Form: June 21, 1999 Dispersion copolymerization of styrene and a poly(N-isopropylacrylamide) macromonomer in ethanolwater media has been successfully carried out in the presence of AgNO3. Nearly monodisperse polystyrene microspheres with diameters ranging from 530 to 1250 nm were obtained. Nanoscopic silver particles were generated on their surfaces via in situ reduction of Ag+ by radicals generated from the initiator, 2,2′azobisisobutyronitrile (AIBN). The particle sizes of both polystyrene microspheres and silver nanoparticles were affected by the initial AIBN, AgNO3, and macromonomer concentrations. The diameters of the silvered microspheres and silver nanoparticles followed the relationships Dn ∝ [AIBN]0-0.107 [AgNO3]00.083[macromonomer]0-0.533 and dn ∝ [AIBN]00.027 [AgNO3]00.173 [macromonomer]0-0.137, respectively. Over 95.8% of the silver ions are converted into zerovalent metal and immobilized on the microspheres, according to atomic absorption spectroscopy measurements. The silvered microspheres were characterized by transmission electron microscopy, atomic force microscopy, and FTIR, UV-visible, and X-ray photoelectron spectroscopy. The surface-grafted PNIPAAm chains were found not only to serve as steric stabilizers to prevent the flocculation of the polystyrene particles but also to adsorb the Ag nanoparticles onto the surface of the microspheres. A mechanism for the formation of silvered polystyrene microspheres in dispersion copolymerization was presented.

Introduction The use of dispersion polymerization has proved to be of great scientific interest during recent years for the preparation of monodisperse polymer particles, which have, in turn, many attractive applications as supports of catalysts, carriers of biomolecules, and captors of human immunodeficiency virus-1 (HIV-1).1-7 In the dispersion polymerization process, polymeric microspheres are formed from an initially homogeneous reaction mixture, which contains monomer, initiator, solvent, and stabilizer. A graft or block copolymer can be used as the steric stabilizer to prevent the flocculation of growing particles. As an alternative strategy, the macromonomer technique gains increasing importance, since it not only generates a steric barrier of the particle covalently but introduces functional surface groups for future application.8-10 Bu´sci et al. * To whom correspondence should be addressed. † This paper is part XXIV in the series of the study on Graft Copolymers Having Hydrophobic Backbone and Hydrophilic Branches. Part XXIII is as follows: Chen, M.-Q.; Serizawa, T.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2155. (1) Paine, A. J.; Luymes, W.; A. McNulty, J. Macromolecules 1990, 23, 3104. (2) Kawaguchi, S.; Winnik, M. A.; Ito, K. Macromolecules 1995, 28, 1159. (3) Baines, F. L.; Dionisio, S.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3096. (4) Bu´sci, A.; Forcada, J.; Gibanel, S.; He´roguez, V.; Fontanille, M.; Gnanou, Y. Macromolecules 1998, 31, 2087. (5) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. J. Chem. Soc., Chem. Commun. 1998, 831. (6) Sakuma, S.; Suzuki, N.; Kikuchi, H.; Hiwatari, K.; Arikiawa, K.; Kishida, A.; Akashi, M. Int. J. Pharm. 1997, 149, 93. (7) Akashi, M.; Niikawa, T.; Serizawa, T.; Hayakawa, T.; Baba, M. Bioconjugate Chem. 1998, 9, 50. (8) Brindley, A.; Davis, S. S.; Davies, M. C.; Watts, J. F. J. Colloid Interface Sci. 1995, 171, 150 (9) Simmons, M. R.; Chaloner, P. A.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1998, 14, 611.

synthesized poly(ethylene oxide) macromonomers by anionic “living” polymerization, which constituted a hydrophobic head fitted with an unsaturation and of an acetal or aldehyde tail to enable the reaction of particles formed with the amino group of biomolecules.4 We recently prepared the thermosensitive polystyrene microspheres having poly(N-isopropylacrylamide) (PNIPAAm) branches on their surfaces by the macromonomer technique in both alcoholic and aqueous alcoholic media.10 The PNIPAAm macromonomer containing a styrene end group was prepared by the reaction of p-(chloromethyl)styrene with the hydroxyl group terminated oligoNIPAAm, which was obtained by the free radical polymerization of NIPAAm using 2-mercaptoethanol as a chain transfer agent in the presence of AIBN (see Figure 1). Using the PNIPAAm branches as the capping material, we succeeded in immobilizing platinum colloids on the microsphere surface in a well-dispersed way.5 The catalyst, separated from the reaction mixture by centrifugation, retains high activity on recycling in the aqueous hydrogenation of allyl alcohol. This class of polymer-metal nanocomposites synergize properties of both components, leading to many applications.11 Ionizing radiation has been applied to synthesize colloidal metal nanoparticles,12,13 making use of the reducing properties of the hydrated electrons and organic radicals which are formed in the radiolysis of the solvent and added organic compounds.14 To take the advantage of the γ-radiation method, Qian and co-workers synthesized polyacrylamide-silver nanocomposites via forma(10) Chen, M.-Q.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2213. (11) Krug, H.; Schmidt, H. New J. Chem. 1994, 18, 1125. (12) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129. (13) Marignier, J. L.; Belloni, J.; Delcourt, M. O.; Chevalier, J. P. Nature 1985, 317, 344. (14) Henglein, A. J. Phys. Chem. 1993, 97, 5457.

10.1021/la9817462 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/23/1999

Dispersion Copolymerization in AgNO3

Langmuir, Vol. 15, No. 23, 1999 7999 Table 1. Recipes for the Dispersion Polymerization at 60 °C

Figure 1. Structure of the poly(N-isopropylacrylamide) macromonomer.

tion of nanocrystalline metal particles and polymerization of monomer simultaneously.15 In a recent communication16 we reported the in situ formation of silver colloids on the surface of PNIPAAm-coated polystyrene microspheres (silvered microsphere). In this method, the metal salt and organic monomers are mixed homogeneously at the molecular level in the solution. During dispersion polymerization, silver ions are reduced to yield free atoms that subsequently coalesce to form larger particles by the radicals directly generated from AIBN or by the oligomeric radicals. The choice of silver metal as a model is based on it being the most frequently studied cheapest noble metal with a narrow intense plasmon absorption band in the visible region.17 At the same time, silver colloids have been widely used as the active substrates for surfaceenhanced Raman scattering,18 chemical, electronic and optical sensors,14,19 and photocatalysts for solar energy conversion.20 In the present paper we report a more detailed account of this work together with our latest results. The effects of initiator, silver nitrate, and PNIPAAm macromonomer concentrations on the particle sizes of polystyrene microspheres and silver colloids were examined. These parameters were identified by both transmission electron microscopy (TEM) and atomic force microscopy (AFM). The surface compositions of the silvered microspheres were investigated by X-ray photoelectron spectroscopy. The motivation for these investigations is not only from attractive applications of the totally innovative material but from a theoretical point of view. For example, the effect of inorganic electrolytes on emulsion polymerization has been intensively studied. Added electrolytes modify the properties of the final latex such as viscosity, freezing point, and pH, which depends on the chemical nature and concentration of electrolyte, surfactant, monomer, and initiator.21,22 Recently, Antonietti and co-workers reported the effect of ionic strength on the stabilizing properties of polyelectrolyte block copolymers in the surfactant-free emulsion polymerization.23 However, much less information is known about the effect of inorganic salts on the dispersion polymerization in the presence of a macromonomer. In particular, the transition metal ions are easily reduced by the radicals in the reaction mixture and surface-grafted polymer branches should serve as the (15) Zhu, Y.; Qian, Y.; Li, X.; Zhang, M. J. Chem. Soc., Chem. Commun. 1997, 1081. (16) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. Adv. Mater. 1998, 10, 1122. (17) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (18) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) Siiman, O.; Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983, 100, 163. (19) Armelao, L.; Bertoncello, R.; De Dominicis, M. Adv. Mater. 1997, 9, 736. (20) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (21) Blackley, D. C. In Emulsion Polymerization: Theory and practice; John Wiley and Sons: New York, 1975; p 382. (22) Blackley, D. C.; Sebastian, S. A. R. D. Br. Polym. J. 1989, 21, 313. (23) Mu¨ller, H.; Leube, W.; Tauer, K.; Fo¨rster, S.; Antonietti, M. Macromolecules 1997, 30, 2288.

styrene (mmol) PNIPAAm macromonomer, wt (g) AIBN, wt (mg) silver nitrate, wt (mg) ethanol/water (v/v, 7/3) (mL)

standard recipe

experimental variations

3.0 0.5 10.0 5.1 5.0

3.0 0.12-0.5 5.0-40.0 2.55-10.2 5.0

capping agent for the formed metal colloids to prevent their aggregations. A mechanism for the formation of silvered polystyrene microspheres in dispersion polymerization is presented. Experimental Section Materials. The PNIPAAm macromonomer was prepared by the method reported in the previous paper.10 In short, hydroxyl group terminated NIPAAm oligomers were synthesized via free radical oligomerization of NIPAAm (Kohjin Co. Ltd.) using AIBN and 2-mercaptothanol (Wako Pure Chemical Ind., Ltd.) as an initiator and chain transfer agent, respectively, in ethanol at 60 °C. After evaporation, the product was poured into distilled water to redissolve the PNIPAAm oligomer. The PNIPAAm oligomer was isolated by heating to 50 °C and then separated by centrifugation at 4000 rpm. The NIPAAm macromonomer was prepared in N,N-dimethylformamide (DMF) by reacting the PNIPAAm oligomer with a 5-fold excess of p-chloromethylstyrene (CMSt, supplied by Nippon Oil and Fats Co. Ltd.) in the presence of a 5-fold excess of potassium hydroxide at room temperature. The KCl salt formed was removed by filtration. The product was purified by dialyzing in distilled water for several days. In the present study, molecular weight and molecular weight distribution of the PNIPAAm macromonomer were 5300 and 2.2, respectively, determined using gel permeation chromatography (GPC). A general structure of the PNIPAAm macromonomer is shown in Figure 1. The structure and composition are verified by 1H NMR spectroscopy in methyl-d6 sulfoxide with a JEOL GSX-400 spectrometer operating at 400 MHz. Styrene (Wako Pure Chemical Ind., Ltd.) was distilled under partial vacuum to remove inhibitor before use. 2, 2′-Azobisisobutyronitrile (AIBN) from Wako was recrystallized from methanol. Silver nitrate (Nacalai Tesque, Inc.) was used as received. The mixture of distilled water and ethanol (Nacalai Tesque, Inc., purified by distillation) was used as a polymerization medium. Dispersion Polymerization. All dispersion copolymerizations were carried out batchwise in a glass tube, using the recipes given in Table 1. In the case of the standard recipe, the initiator concentration was 2.0 mol % and the molar ratio of styrene/ macromonomer was 33.3/1. The general procedure used to prepare all the silvered polystyrene microspheres was as follows, which was similar to the preparation of usual polystyrene microspheres.10 Each batch of copolymerization in a glass tube was repeatedly degassed by freeze-thaw cycles on a vacuum apparatus, sealed off, and then placed in an incubator at 60 °C for 24 h. The resulting microspheres were first dialyzed in distilled deionized water using a cellulose dialyzer tube to remove unreacted monomer, and then the latex particles were centrifuged and redispersed in water. In an attempt to ascertain the effects of radicals directly generated from AIBN, the silvered microsphere was also prepared by a two-step process. In the first step, the unsilvered polystyrene microsphere was obtained by dispersion copolymerization in the absence of AgNO3, as described above. The obtained unsilvered microsphere, AIBN, and AgNO3 were dissolved in the ethanol/ water mixture and then introduced into a glass tube. The reduction of Ag ions by radicals directly generated from AIBN was also carried out at 60 °C for 24 h. The amount of residual metal in supernatant was analyzed on a SAS-7500A (Seiko Instruments) atomic absorption spectrophotometer. Microsphere Characterization. Transmission electron microscopy, dynamic light scattering, and atomic force microscopy were used to determine the particle size. TEM images were obtained with a Hitachi H-700H microscope operation at an acceleration voltage of 150 kV at a magnification of 36 000 or

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Chen et al.

Table 2. Effect of Varying Synthesis Parameters on the Particle Sizes and Distributions for the Polystyrene Microspheres and Silver Colloids on Their Surfacesa microspheresb

Ag colloidsb

expt AIBN AgNO3 PNIPAAm DnTEM DnDLS dn SD no (mol %) (M) (mol %) (nm) PDIc (nm) (nm) (nm) 1 2 3 4 5 6 7 8 9

1 2 4 8 2 2 2 2 2

0.006 0.006 0.006 0.006 0.0 0.003 0.012 0.006 0.006

3.0 3.0 3.0 3.0 3.0 3.0 3.0 1.5 0.75

660 600 570 530 480 580 650 1000 1250

1.02 1.01 1.02 1.01 1.01 1.01 1.01 1.01 1.01

1123 950 863 825 680 935 1146 1632 1785

13.7 14.0 14.3 14.5

5.4 4.7 4.0 4.9

12.5 15.9 16.3 16.9

5.3 6.6 4.7 4.9

a The number-molecular weight of PNIPAAm was 5300 (confirmed by GPC). b The sizes of microspheres were determined by TEM and DLS, respectively. The size and dispersion of silver colloids were measured by TEM. c The distribution of microspheres is given by the polydispersity index (PDI) from TEM images.

100 000. Specimens of the various silvered microspheres were prepared by slow evaporation of a drop of the appropriately diluted solution deposited onto a collodium-coated copper mesh grid, followed by carbon sputtering. The particle size and polydispersity index (PDI) of the polystyrene microsphere were measured from TEM images. The dynamic light scattering measurements (Coulter model N4SD) were carried out on dilute dispersions at 25 °C using a fixed 90° scattering angle. Samples for atomic force microscopy (AFM) imaging were prepared by placing a drop of dispersion of polystyrene microspheres on freshly cleaved mica and allowing it to dry in the air. The samples thus prepared were imaged immediately after deposition as well as after several days without noticeable changes in appearance and average dimensions. AFM imaging was performed using a Nanoscope III-SPM system (Digital Instruments) with a J-type vertical engage piezoelectric scanner and operated in tapping mode in air. The internal morphology of the silvered microspheres was studied by TEM on the thin unstained section. The dried samples were embedded in epoxy resin and cured at 35 °C for days. The embedded samples were sectioned with an LKB 8800 Ultratome III with a glass knife, and the thickness of ultrathin cross sections is less than ∼100 nm. The sections were transferred to a copper grid, dried under reduced pressure, and examined in the Hitachi H-700H transmission electron microscope. UV-visible measurements are performed with a JASCO model V-550 recording spectrophotometer, working in a spectral range between 200 and 800 nm. FTIR spectra are recorded in KBr pellets with a Shimadzu Fourier transform Nicolet spectroscope. Pressure was applied to the sample powder until the pellet was transparent. X-ray photoelectron spectra were obtained with a Shimadzu ESCA 1000 apparatus employing Mg KR radiation (1253.6 eV) and a pass energy of 31.5 eV. Peaks were referenced to carbon at 285.0 eV to account for sample charging. Good quality survey spectra were obtained with a single scan; core-line high-resolution spectra were integrated over 5-10 scans depending on the intensity of the spectral region of interest. Percentages for each carbon environment quoted within the text are derived from peak areas in the high-resolution spectra.

Results and Discussion Silvered Microsphere Syntheses. The PNIPAAm macromonomer with a mean molecular weight of 5300 was applied for all the following experiments. Under these experimental conditions, the initial reaction mixture is completely clear. As polymerization and reduction of silver ions proceed, the solution becomes turbid and yellowish particles are formed within 2-3 h. The characteristics of polystyrene microspheres prepared with varying AIBN, AgNO3, and macromonomer concentrations are summarized in Table 2.

The compositions of both silvered and unsilvered polystyrene microspheres were characterized by FTIR and 1H NMR (in CDCl ). Solid samples of silvered and 3 unsilvered microspheres were obtained by lyophilizing dialyzed aqueous samples. The FTIR spectra of microspheres are essentially identical to that of polystyrene, but the amide I and II bands of surface-grafted PNIPAAm chains are clearly observed for both silvered and unsilvered polystyrene microspheres.16 The assignments for bands in the spectrum of silvered microsphere are overall very similar to those for unsilvered one. However, some differences are observed. For example, the band observed near 1539 cm-1 due to N-H bending vibration in the spectrum of unsilvered microsphere is shifted to about 1545 cm-1. The band assigned to N-H stretching vibration shows considerably decreased intensity in the spectrum of silvered microsphere compared with the unsilvered microsphere. It is expected that the band shift and intensity change reflect an interaction between the silver colloid and surface-grafted PNIPAAm chain. A similar intensity change of N-H stretch and band shift of amide II band have been observed for the PNIPAAm chains grafted on the polystyrene microspheres on immobilizing of platinum colloids on their surfaces.24 In the 1H NMR spectrum of silvered microsphere, the methine proton of the isopropyl group in a PNIPAAm unit has a resonance centered at 4.01 ppm and its intensity is used to check the content of PNIPAAm branches. We found that the PNIPAAm contents in silvered and unsilvered microspheres are almost the same. Considering the in situ reduction of silver ions by radicals in the polymerization, the amount of AIBN was doubled in the standard recipe compared with the synthesis of unsilvered microspheres. After the dispersion polymerization was finished in 24 h, the sample was centrifuged and redispersed in water. UV-visible spectra of both the silvered microsphere dispersion and the supernatant were recorded. The extinction spectrum of a silvered microsphere solution showed an obvious absorption band at around 400 nm, which is attributed to the surface plasmon excitation of the silver particles.25 However, the supernatant was colorless and showed no absorption band at ca. 400 nm.16 These results indicate that all the formed Ag nanoparticles were adsorbed on the surface of the polystyrene microspheres. Further evidence that silver colloids were immobilized on the microspheres in a high yield is available from atomic absorption measurements. The amount of residual metal in supernatant is lower than 10.0 ppm for the standard preparation, indicating that over 95.8% of silver ions are converted into metal and immobilized on the microspheres. As the AIBN concentration was increased from 2 to 8 mol %, a slight increase in the conversion of silver ions up to 98.5% was observed. For the synthesis of an unsilvered microsphere,10 it was found that over 80% of styrene and macromonomers were converted to microspheres in 24 h at 60 °C in the presence of 1 mol % of AIBN. These results suggested that enough radicals are generated for both polymerization and metal ion reduction at the AIBN concentration of 2 mol % in the standard preparation. The initial color of the microsphere is white, and then changes to yellow, which reflects the induction period of silver reduction. The rate of Ag+ reduction by radicals in the reaction solution may be low. Upon formation of a small number of silver particles, silver ions are reduced (24) Chen C.-W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (25) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773.

Dispersion Copolymerization in AgNO3

Langmuir, Vol. 15, No. 23, 1999 8001

Figure 2. TEM images of silvered polystyrene microspheres prepared according to recipes of run 2 (a), run 6 (b), and run 9 (c) given in Table 2. The TEM image of silvered microspheres (c) in thin section was shown in (d).

autocatalytically at the surface of the primary colloidal particles. A silver particle may collect electrons from many radicals. The electron accumulation on the Ag particle alters the position of the Fermi level in the particle and establishes a cathodically polarized metal nanoelectrode.26,27 The silver ions are reduced readily by the stored electrons at the particle surface and, in turn, the particle grows smoothly. Upon addition of AgNO3, the initial polymerization rate of the dispersion copolymerization of styrene and PNIPAAm macromonomer may decrease. In fact, the time for solution changing from clear to turbid is longer for silvered microsphere synthesis than that for unsilvered microspheres. However, the monomer conversion and the composites of microspheres formed are almost not affected by the addition of AgNO3, which is confirmed by the results of FTIR and NMR measurements. Chew et (26) Henglein, A. Chem. Mater. 1998, 10, 444. (27) Rogach, A. L.; Shevchenko, G. P.; Afanas’eva, Z. M.; Sviridov, V. J. Phys. Chem. B 1997, 101, 8129.

al. recently reported that an increase in initiator concentration increased the initial polymerization rate of styrene and a poly(ethylene oxide) (PEO) macromonomer but showed little effect on the final styrene conversion.28 Particle Size and Morphology. Figure 2 shows TEM images of three different silvered polystyrene microspheres prepared according to recipes given in Table 2. The silvered microspheres produced with varying initiator, silver nitrate, and PNIPAAm macromonomer concentrations have a nearly monodisperse spherical morphology with number-average particle diameters from 530 to 1250 nm. As expected, particle diameters of the polystyrene microspheres determined by dynamic light scattering (DLS) are much larger than those of dried particles derived from TEM measurements. The difference may be attributed to either polystyrene core shrinkage upon drying or the presence of a PNIPAAm corona layer around a (28) Liu, J.; Chew, C. H.; Gan, L. M.; Teo, W. K.; Gan, L. H. Langmuir 1997, 13, 4988.

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nonshrinking polystyrene hard core. In the previous study,10 we showed that the hydrodynamic particle size of an unsilvered polystyrene microsphere decreased from 420 nm at 20 °C to 370 nm at 30 °C and then 320 nm at 35 °C. This behavior was attributed to the “coil-to-globule” transition of PNIPAAm chains on the particle surface.29 A recent paper by Elaı¨ssari et al. proposed a similar explanation for the particle size change.30 They investigated the hydrodynamic diameter of a cationic aminocontaining N-isopropylacrylamide-styrene copolymer latex by DLS. They found that the diameter of the latex is 430 nm by DLS at 20 °C, while the size is 306 nm by TEM. The particle size and size distribution of Ag colloids on the polystyrene microsphere were measured from TEM images. It is clearly seen from Figure 2a-c that the Ag colloids with diameters ranging from 12.5 to 16.9 nm have adsorbed in a controlled, well-separated manner on the polystyrene microsphere surface. The silvered microsphere of run 9 in Table 2 was embedded in epoxy resin and sectioned with a microtome. In the TEM micrograph of an unstained thin section, the particle appears as elliptical (see Figure 2d). This is an artifact introduced by the microtome, which compresses the section. The cutting direction is perpendicular to the long axis of the apparently elliptical particles. An obvious core-corona structure was observed with all the Ag nanoparticles located in the PNIPAAm corona layer on the exterior. The polystyrene core appeared as a dark domain, and the boundary between the core and corona phase seems sharp. The morphologies of individual silvered and unsilvered microsphere were also observed by atomic force microscopy (AFM). Evaluation of the microsphere diameters from the particle heights is in good agreement with that obtained from TEM measurements. The AFM image of a silvered microsphere (see Figure 3b) confirms that polystyrene particles are spherical and their surfaces are covered with isolated Ag colloids. The colloid heights are found to be in the range of 15-20 nm, which agree with the diameter values of Ag colloids by TEM. However, surfaces of the unsilvered polystyrene microspheres are relatively smooth and featureless (see Figure 3a). Additional AFM images (2 µm × 2 µm and 5 µm × 5 µm) of the silvered and unsilvered microsphere also show the narrow size distribution of spherical particles (Supporting Information). Effect of AIBN Concentration. The data in Table 2 showed that the particle size of silvered microspheres decreases with an increase in initiator concentration. This trend of decreasing particle size with increasing initiator was observed by other workers using macromonomer technology.2,31 By plotting the particle size against AIBN concentration as shown in Figure 4, the slope of the line is -0.107. In the absence of AgNO3, unsilvered polystyrene microspheres were obtained, and the value of such a slope was -0.13.32 In the case of silvered microsphere synthesis, part of radicals will serve as the reducing agent of silver ions and the real AIBN concentration to initiate the polymerization is lower than that for the unsilvered microsphere synthesis. Thus, one would expect that silvered microspheres will have a larger particle size than unsilvered ones at the same AIBN concentration. The dependence of Ag colloid size on the AIBN concentration is also shown in Figure 4. The Ag colloid size increases (29) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352. (30) Nabzar, L.; Duracher, D.; Elaı¨ssari, A.; Chauveteau, G.; Pichot, C. Langmuir 1998, 14, 5026. (31) Liu, J.; Gan, L. M.; Chew, C. H.; Quek, C. H.; Gong, H.; Gan, L. H. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3575. (32) Chen, M.-Q.; Serizawa, T.; Akashi, M. Polym. Adv. Technol. 1999, 10, 120.

Chen et al.

Figure 3. Tapping-mode AFM images (1 µm × 1 µm) of an unsilvered polystyrene microsphere (a) and a silvered microsphere (b), deposited on freshly cleaved mica.

Figure 4. Double-logarithmic plots of particle sizes of silvered microsphere (9) and Ag colloid (b) vs AIBN concentration.

slightly with increasing AIBN concentration, which may be attributed to the increase in the electron accumulation on Ag particles at higher AIBN concentrations.

Dispersion Copolymerization in AgNO3

Langmuir, Vol. 15, No. 23, 1999 8003 Table 3. XPS Data of the Silvered Microspheres and Unsilvered Microspheresa sample PS-PNIPAAm/Ag

PS-PNIPAAm

elemental environments

BE (eV)

fwhm (eV)

area (%)

C 1s (C-C/C-H) C 1s (N-C) C 1s (NCdO) N1s (free) N1s (coordinated) O 1s Ag 3d5/2 Ag 3d3/2 C 1s (C-C/C-H) C 1s (N-C) C 1s (NCdO) N 1s (free) O 1s

285 286.3 288 399.9 400.8 532 368.9 374.8 285 286.5 287.9 400.0 532.1

2.0 1.6 1.6 1.7 1.0 2.5 1.3 1.2 1.9 1.4 1.4 1.9 2.8

84.6 7.7 7.7 94.4 5.6 100 59.7 40.3 84.2 7.9 7.9 100 100

a Peak positions were corrected for sample charging, consistent with published data.35

Figure 5. Double-logarithmic plots of particle sizes of silvered microsphere (9) and Ag colloid (b) vs AgNO3 concentration.

Figure 6. Double-logarithmic plots of particle sizes of silvered microsphere (9) and Ag colloid (b) vs macromonomer concentration.

Effect of AgNO3 Concentration. Figure 5 shows the effect of AgNO3 concentration on particle sizes of both polystyrene microsphere and silver colloid. The scaling relationships obtained are Dn ∝ [AgNO3]0.083 and dn ∝ [AgNO3]0.173, respectively. A high AgNO3 concentration leads to the formation of silver colloids with a larger mean diameter, which will consume more radicals for the silver ion reduction. Thus, the amount of radical for the polymerization is relatively low. As is discussed in the previous subsection, the polystyrene microsphere size will increase with decreasing AIBN concentration. However, the effect of AgNO3 concentration on the microsphere size is not as significant as that of AIBN concentration. Effect of Macromonomer Concentration. Figure 6 shows the response of particle sizes of the polystyrene microsphere and silvered colloid to PNIPAAm macromonomer concentration. Marked increase in microsphere size is observed with decreasing macromonomer concentration, and the slope of this line is -0.533, which is much larger than that for AIBN or AgNO3 concentrations. In the absence of AgNO3, a similar trend was observed in our previous work, where the slope was -0.76.32 In the literature,31 the effect of PEO-based macromonomer concentration on particle size has been studied and the value of such a slope was -0.60. It is expected that the hydrophilic PNIPAAm macromonomer

is covalently attached on the particle surface by its copolymerization with styrene in an extended configuration away from the surface. The stability of the latex particle strongly depends on the level of surface PNIPAAm. In the case of silvered microspheres, the surface-grafted PNIPAAm chains not only serve as steric stabilizers to prevent the flocculation of the polystyrene particles but adsorb the Ag nanoparticles onto the surface of the microspheres. Therefore, the Ag colloid size decreases with increasing PNIPAAm macromonomer concentration. This is consistent with the dependence of colloidal particle size on the concentration of the stabilizing polymer in the preparation of polymer-protected metal sol.33,34 XPS Characterization. The surface compositions derived from XPS analysis of unsilvered and silvered polystyrene microspheres are given in Table 3. Confirmation of the presence of surface PNIPAAm on silvered microsphere can be obtained by closer inspection of the C 1s core level spectrum shown in Figure 7a. The C 1s peak has a main component centered at 285.0 eV due to the C-C/C-H bonds and two components with the same intensity at 286.3 and 288.0 eV due to C-N and N-CdO groups, respectively. The latter two features are due to the PNIPAAm stabilizer, in good agreement with the literature.35 The theoretical proportion of N-CdO or C-N environment is 4.3%, which was obtained from the mole fractions in the initial monomer feed and the structural formulas of the monomers. According to XPS analysis, however, the proportion of the two environments for the silvered microspheres is 7.7%, clearly indicating the presence of a PNIPAAm-enriched surface layer. The same conclusion can be drawn for unsilvered microspheres (see Figure 8a). The N 1s signal for an unsilvered microsphere has the only component centered at 400.0 eV due to the amide group of the PNIPAAm chains (see Figure 8b). However, for the N 1s signal of the silvered microsphere, a new peak at 400.8 eV is required to obtain the most satisfactory peak fit (see Figure 7b). About 5.6% of the amide groups of PNIPAAm coordinated with the silver colloids. Similar results were obtained for a poly(N-vinyl-2-pyrrolidone)protected rhodium colloid36 and a platinum-supported (33) Chen, C.-W.; Akashi, M. Langmuir 1997, 13, 6465. (34) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (35) (a) Bearinger, J. P.; Castner, D. G.; Golledge, S. L.; Rezania, A.; Hubchak, S.; Healy, K. E. Langmuir 1997, 13, 5175. (b) Baumgarten, E.; Fiebes, A.; Stumpe, A.; Ronkel, F.; Schultze, J. W. J. Mol. Catal. A: Chem. 1996, 113, 469. (36) Wang, Y.; Liu, H.; Jiang, Y. J. Chem. Soc., Chem. Commun. 1989, 1878.

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Figure 7. High-resolution XPS spectra for a silvered microsphere (run 2 in Table 2): (a) C 1s region; (b) N 1s region; (c) Ag 3d region.

Figure 8. High-resolution XPS spectra for the unsilvered microsphere: (a) C 1s region; (b) N 1s region.

catalyst based on a novel polymeric carrier with amide groups in the side chains.35b Therefore, it is confirmed that grafted PNIPAAm chains on the surface of silvered

Chen et al.

microspheres not only serve as steric stabilizers to prevent the flocculation of the polystyrene particles but adsorb the Ag colloids onto their surfaces. The Ag 3d core level spectrum for silvered microsphere is shown in Figure 7c. The Ag 3d doublet is identified at 368.9 and 374.8 eV for Ag 3d5/2 and Ag 3d3/2, respectively. The binding energy (BD) maximum of Ag 3d5/2 is 0.6 eV higher than that of zerovalent silver in the literature (368.3 eV).37 However, the intensity ratio of the Ag 3d doublet is near 1.5 and the difference in the BD maximum, i.e., spin-orbit splitting, is about 6.0, which fairly well coincides with the values in the literature. The shift of Ag 3d5/2 peak to higher binding energies may be attributed to the oxidation of surface silver atoms. As mentioned in the previous section, the Fermi level in Ag particles on the polystyrene microsphere lies at a rather negative potential since they store many electrons. Once the silvered microsphere solution is exposed to air, electron transfer from the silver colloids to O2 will take place. As Ag+ ions are formed at the surface, the Fermi level in the silver particles moves toward a more positive potential until the oxidation ceases. Therefore, it is suggested that the majority of the silver atoms in the colloids must be in the Ag0 state and some Ag+ ions are chemisorbed on their surfaces. Kim et al. investigated the formation of dodecanethiol-derivatized silver nanoparticles using NaBH4 as a reducing agent.38 A similar Ag 3d5/2 peak shift to higher BE by 0.7 eV was observed, as is attributed to the outer silver atom oxidation upon thiolate formation. Scheme of Silvered Microsphere Formation. Figure 9 gives a schematic representation of the mechanism for the formation of silvered microspheres in the dispersion copolymerization. At the start of the process, monomer, macromonomer, initiator, and AgNO3 dissolve completely in a homogeneous solution. PNIPAAm macromonomers can coordinate to silver ions before reduction though the nitrogen atom in the amide group (see Figure 9a). The polymer-metal ion complex plays an important role in the synthesis of polymer-stabilized colloidal metal sols by the alcohol-reduction method with respect to the nucleation and the controlled growth of the formed nuclei.39 Toshima et al. suggested that metal ions can be reduced under mild conditions to give colloidal metal cluster of smaller size and narrower size distribution owing to the formation of polymer-metal ion complex.40 Once AIBN decomposes, linear oligomers, polymers and graft copolymers are all produced by the polymerization in the continuous phase. Radicals in the polymerization system will serve as the reducing agent for the conversion of silver ion to the zerovalent state (see Figure 9b), just as the hydrated electrons and organic radicals generated by radiation do in radiation chemical methods.14 As a control, the experiment was performed in the absence of AIBN. The resulting solution showed no obvious adsorption at ca. 400 nm, indicating that silver ions are hard to be reduced by ethanol under these conditions. We believe that silver ions are reduced both by the radicals directly generated from AIBN and by the oligomeric radicals to yield free atoms. To demonstrate the (37) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation: Eden Prairie, MN, 1992; p 120. (38) Kang, S. Y.; Kim, K. Langmuir 1998, 14, 226. (39) Hirai, H. Makromol. Chem., Suppl. 1985, 14, 55. (40) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91 (22), 4111.

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Figure 9. A schematic representation of silvered microsphere formation in dispersion copolymerization of styrene and a PNIPAAm macromonomer.

effect of radicals directly from AIBN, we studied the formation of Ag colloids on the surface of the unsilvered polystyrene microsphere in the presence of AIBN and AgNO3 using the two-step process (see Experimental Section). Although no oligomer radical is generated, wellseparated Ag colloids are observed on the microsphere just as in the one-pot procedure. The conversion of Ag+ to Ag0 is about 87.3% according to the atomic absorption spectroscopy (AAS) analysis and is slightly lower than that in the one-pot procedure. This may imply that the oligomeric radicals are involved in the reduction of Ag ions. Very recently, Yanagihara et al. reported the kinetic observation on the polymerization of methyl methacrylate (MMA) in the presence of a silver(I) complex using AIBN and benzoyl peroxide (BPO).41 After a postheating process, the silver solid sol in PMMA was obtained. They suggested that the formation and growth of silver particles in PMMA are markedly influenced by the kind and concentration of initiator. The solubility of these polymers generated in the reaction solution is a function of their molecular weight and the composition of the graft copolymer. Polymers with a molecular weight larger than a certain critical value will precipitate and coagulate to form particles (see Figure 9c). The reaction solution becomes turbid upon formation of the primary polystyrene particles. The hydrophilic PNIPAAm macromonomer is covalently attached on the particle surface by its copolymerization with styrene in (41) (a) Yanagihara, N. Chem. Lett. 1998, 305. (b) Yanagihara, N.; Uchida, K.; Wakabayashi, M.; Uetake, Y.; Hara, T. Langmuir 1999, 15, 3038.

an extended configuration away from the surface. The reduction rate of Ag ions by radicals is relatively low in solution, as discussed above. Once the critical concentration of Ag0 atoms necessary for nucleation to occur is built up around polystyrene particles, the primary Ag particle will be generated on their surfaces. At this time, the color of the solution changes from white to yellow. The induction period for the nucleation of Ag particles is about 2 h, while precipitation and coagulation of the polymers are observed in 30 min. Duff et al. studied the induction period for the formation of a polymer-stabilized Pt sol by methanol reduction of ionic platinum.42 They proposed that colloidal metal sol preparations usually have induction periods when mild reductants are used. The grafted PNIPAAm chains on the polystyrene particle build up high-local densities of silver ions which assist the nucleation of Ag particles. The stability of the latex particle strongly depends on the level of surface hydrophilic polymer.43 The increase in particle size will continue until there are sufficient PNIPAAm chains grafted onto the surface of the latex particle to provide the steric stabilization (see Figure 9d). As is discussed in the previous section, the primary Ag particle is able to react with many radicals, leading to the accumulation of electrons. Thus, the Ag+ reduction occurs at the surface of the cathodically polarized Ag particles, and the particles grow smoothly: (42) Duff, D. G.; Edwards, P. P.; Johnson, F. G. J. Phys. Chem. 1995, 99, 15934. (43) Akashi, M.; Yanagi, T.; Yashima, E.; Miyauchi, N. J. Polym. Sci., Polym. Chem. Ed. 1989, 27, 3521.

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Agnm- + Ag+ f Agn+1(m-1)-

Conclusions and Outlook

The hydrophobic nature of polystyrene oligomers or polymers would be responsible for the adsorption of Ag nanoparticles on their surfaces, where the Ag nanoparticles are sterically stabilized by surface-grafted PNIPAAm chains. The formation of noble metal nanoparticles within the micelles of amphiphilic block copolymers, such as poly(styrene-b-vinylpyridine) (PS-b-PVP) and poly(styreneb-ethylene oxide) (PS-b-PEO), in toluene has been studied by some research teams.44 Inorganic precursors are loaded into the hydrophilic micellar cores by binding either directly to the polymeric ligand44a or indirectly as counterions.44b Even if the precursor salt is soluble in the solvent, e.g., for the PS-b-PVP/THF system, over 98% of metal ions are fixed in the micellar core because of the stronger coordination to the PVP units.45 Considering the complexation of Ag+ to PNIPAAm macromonomers and the weak adhesion to polystyrene, one would expect that it is hard for a silver colloid to be generated in the polystyrene core. Tsai et al. investigated the conformation of 2-vinylpyridine/styrene (2VP/S) block copolymers adsorbed onto silver surfaces using surface-enhanced Raman scattering.46 They confirmed that the 2VP block preferentially adsorbed to the surfaces while the styrene block was positioned away from the surfaces. The 2VP block was adsorbed though the nitrogen atom of the pyridine ring, with the plane of the pyridine ring oriented perpendicular to the silver surface. It can be expected that the PNIPAAm chains would adsorb onto Ag particles though the amide nitrogen atoms, where the polymer chains will show some local order in the vicinity of the silver. The polystyrene chains show less interaction with Ag particles, but they can be linked to the Ag particles through “surface entanglements” with PNIPAAm loops at the Ag surface. These entanglements restrict the lateral motions of the polymer chains and define an effective network for the stabilization of Ag particles. This mechanism, proposed by Shull et al., explained the pinning of gold particles to an interface between PS and P2VP.47 We also found that the Ag particles cannot be effectively adsorbed on the polystyrene microspheres when a PEO macromonomer was used in the preparation. Most of them aggregate together and precipitate from the microsphere solution. This is attributed to the quite weak interaction of PEO/ Ag compared with that of PNIPAAm/Ag. It has been reported that a single gold particle had been formed in each PS-b-PEO micellar compartment owing to the weak interaction between gold particles and PEO block.44c

In this report we investigated the dispersion copolymerization of styrene and a poly(N-isopropylacrylamide) macromonomer in ethanol-water media in the presence of silver nitrate. Well-dispersed Ag nanoparticles were formed in situ on the surface of poly(N-isopropylacrylamide)-coated polystyrene microspheres, as confirmed by our TEM and AFM results. According to AAS measurements, over 95.8% of the silver ions are converted into zerovalent metal and immobilized on the microspheres. The surface-grafted PNIPAAm chains not only serve as steric stabilizers to prevent the flocculation of the polystyrene particles but adsorb the Ag nanoparticles onto the surface of the microspheres. By variation of the initiator, silver nitrate, and macromonomer concentrations, the particle sizes and distributions for both polystyrene microspheres and Ag colloids can be changed. The nucleation of silver particles has an induction period for the low rate of Ag+ reduction by radicals in the solution. The electron accumulation on the initial Ag particles is responsible for the Ag+ reduction at the particle surfaces and their smooth growth. It is noteworthy that silver nanoparticles have been widely used as the active substrates for surface-enhanced Raman scattering. Therefore, the silvered microsphere may serve as a practical analytical probe with which to study the conformation of both polystyrene and poly(Nisopropylacrylamide) chains on the surface. In addition, we have previously reported that the Pt colloids on the microspheres show higher activity than the commercial Pt/C catalyst and retain high activity on recycling in the same reaction.5,24 The silvered microsphere is expected to facilitate higher reaction rates in electrochemistry or catalysis applications. Ongoing research in our group suggests that other metal colloids such as Pt, Au, and bimetallic colloids can be generated in situ on the surface of polystyrene microspheres.

(44) (a) Klingelho¨fer, S.; Heitz, W.; Greiner, A.; Oestreich, S.; Fo¨rster, S.; Antonietti, M. J. Am. Chem. Soc. 1997, 119, 10116. (b) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (c) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8, 337. (d) Chan, Y. N. C.; Schrock, R. R. Chem. Mater. 1993, 5, 566. (45) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (46) Tsai, W. H.; Boerio, F. J.; Clarson, S. J.; Parsonage, E. E.; Tirrell, M. Macromolecules 1991, 24, 2538. (47) Kunz, M. S.; Shull, K. R.; Kellock, A. J. J. Colloid Interface Sci. 1993, 156, 240.

Acknowledgment. C.-W. Chen thanks the Ministry of Education, Science, Sports, and Culture, Japan for the scholarship. This work was financially supported in part by a Grant-in-Aid for Scientific Research in Priority Areas of New Polymers and Their Nano-Organized Systems (277/ 101266248) and a Grant-in-Aid for Scientific Research (10555326) from the Ministry of Education, Science, Sports, and Culture, Japan. Dr. M.-Q. Chen is thanked for his assistance with the microsphere synthesis and helpful discussion. We also want to thank Mr. T. Kakoi, Mr. W. Sakamoto, and Mr. Y. Ozono for the help with TEM, AFM, and XPS measurements. The reviewers are thanked for helpful comments. Supporting Information Available: AFM images (2 µm × 2 µm and 5 µm × 5 µm) of silvered and unsilvered polystyrene microspheres. This material is available free of charge via the Internet at http://pubs.acs.org. LA9817462