Preparation of Functionalized Polysilsesquioxane and

Communication

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DOI: 10.1002/marc.200600245

Summary: Network polysilsesquioxane spheres made solely of poly(vinylsilsesquioxane) (PVSQ) and poly(3-mercaptopropylsilsesquioxane) (PMPSQ) were prepared from heterogeneous mixtures of triethylamine (TEA), water, and either vinyltrimethoxysilane (VTMS), or 3-mercaptopropyltrimethoxysilane (MPTMS). The microscopic, macroscopic observations, and the relationship between the diameters of spheres and the amount of each ingredient in the reaction system, showed that spheres formed via a mechanism similar to emulsion polymerization and suspension polymerization, depending on the reaction conditions. Diameters of spheres could be controlled from tens of nanometers to a few micrometers by adjusting the amounts of TEA, water, and a surfactant. Heating aqueous solutions of metal ions with these spheres produced polysilsesquioxanes (PSQ)-metal nanoparticle composite spheres. The spheres prepared in this study were characterized by scanning electron microscopy, transmission electron microscopy, solid state NMR spectroscopy, IR spectroscopy, elemental analysis, and differential thermal analysis. These spheres would be useful in recovering metals from their ionic solutions and probes after chemical modifications. An image of the spheres of poly(vinylsilsesquioxane)-gold nanoparticle composite.

Preparation of Functionalized Polysilsesquioxane and Polysilsesquioxane-Metal Nanoparticle Composite Spheresa Young Baek Kim,*1 Young-A. Kim,1 Kyung-Sup Yoon2 1 2

Department of Nanotechnology, PaiChai University, Daejon 302-735, Korea R&D Center, Saimdang Cosmetics Co., Ltd., 805-5, Gyesan-ri, Yeongdong-eup, Yeongdong-gun, Chungbuk 370-802, Korea

Received: April 10, 2006; Revised: May 22, 2006; Accepted: May 24, 2006; DOI: 10.1002/marc.200600245 Keywords: emulsion polymerization; metal nanoparticles; polysilsesquioxane; poly(vinylsilsesquioxane); poly(3-mercaptopropylsilsesquioxane); spheres

Introduction Spheres containing functional groups are of great interest because they can find applications in different areas. Polysilsesquioxanes (PSQ) are among the most promising materials for preparing spheres of narrow size distributions that contain functional groups. PSQ are especially intera

: Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mrc-journal.de, or from the author.

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esting because many alkyltrialkoxysilane (ATAS) can be easily polymerized into spherical shapes[1–6] and different functionalized ATAS are easily available. PSQ also exhibit a variety of technologically important properties, with applications in coatings, optics, fabrication of fiber materials for photoconductors, and chromatography.[7–11] PSQ spheres containing vinyl, mercapto, and amines are the most interesting because these groups are stable under normal conditions but react readily with other reactive molecules. The PSQ containing mercapto groups were often used to remove heavy metal ions like Hgþ2.[12–15]

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Y. B. Kim, Y.-A. Kim, K.-S. Yoon

Amine containing PSQ spheres were prepared by cocondensing two different ATAS using either external[3] or internal[16] catalysts. The PSQ spheres prepared using internal catalysts were the first examples of PSQ spheres made solely of functionalized network PSQ. Spheres made solely of poly(vinylsilsesquioxane) (PVSQ) were reported only recently,[17] while spheres made solely of PSQ containing the mercapto group and amino group have never been reported. One of the reasons for our interest in these spheres was their potential application in preparation of templates of metal nanoparticles. For example, gold nanoparticles produced by chemical reduction were stabilized by thiols and glass of 3-aminopropylsilsesquioxane.[18] Poly(3mercaptopropylsilsesquioxane) (PMPSQ) spheres have thiol groups and their surfaces and inner voids, if any, would function as ligands and space where gold nanoparticles form. Our preliminary results showed PVSQ and PMPSQ could do more than these. In this report, we describe a very convenient method of preparing amine-free PVSQ and PMPSQ spheres from heterogeneous mixtures of vinyltrimethoxysilane (VTMS), 3-mercaptopropyltrimethoxysilane (MPTMS), water, and triethylamine (TEA) at room temperature, and their multiple roles in the production of metal nanoparticles as reductants, ligands, and templates.

Experimental Part Materials VTMS (97%), MPTMS (95%), TEA (99.5%), and Tween 80 were purchased from Aldrich Chemicals. Hydrogen tetrachloroaurate (III) trihydrate and sodium tetrachloropalladate (II) (ca. 36.4% Pd) were purchased from Acros, and hydrogen hexachloroplatinate (IV) hydrate was purchased from Kojima Chemicals. Silver nitrate solution (0.1 N) was purchased from DC Chemicals. All chemicals were used as received. Preparation of PSQ Spheres Poly(vinylsilsesquioxane) (PVSQ) and PMPSQ spheres were prepared by adding VTMS and MPTMS to aqueous solutions of TEA. The reaction mixtures were stirred using a magnetic stirring bar at room temperature overnight. In experiments using Tween 80, appropriate amounts of Tween 80 were dissolved in distilled water, and then TEA and VTMOS (or MPTMS) were added in sequence at room temperature. In a typical preparation of PVSQ, 1 g of TEA (10 mmol) was dissolved in 15 mL (833 mmol) of distilled water, and then 1.55 g (10 mmol) of VTMS was added to the solution. The reaction mixture was stirred at room temperature overnight. The spheres were recovered by centrifugation, resuspensed, agitated with 50 mL of water and then recovered by centrifugation. The spheres were washed with water three times, twice with 50 mL of ethanol and then finally once with 50 mL of methanol. The spheres were recovered by centrifugation and Macromol. Rapid Commun. 2006, 27, 1247–1253

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dried in a vacuum oven at 50 8C for 4 d. The yields of spheres with diameters larger than 1 mm were between 80 and 95%, assuming the final spheres had chemical formulae of [SiO1.5CH CH2]0.94[SiO2.5CH CH2]0.06 and [SiO1.5CH2CH2 CH2SH]0.87 [SiO2.5CH2CH2CH2SH]0.13, as determined by solidstate NMR spectroscopy. The yields of smaller spheres were lower due to difficulties in recovery by centrifugation. The practical yields of spheres with diameters of 70 nm were only 30% when the spheres were recovered by centrifugation. The effect of the amount of TEA on the sizes of spheres was examined using a fixed molar ratio between water and VTMS of 85:1. The effect of the amount of Tween 80 was examined using a fixed molar ratio among VTMS, TEA, and water of 9:2:4 444. The effect of the amount of aqueous medium was examined using a fixed molar ratio between VTMS and TEA of 1:1. Preparation of Polysilsesquioxane-Metal Nanoparticle Composite Spheres Approximately 5 mg of fully dried PSQ spheres were mixed with 50 mL of aqueous solutions of hydrogen tetrachloroaurate (III) trihydrate (0.25 103 M), sodium tetrachloropalladate (II) (0.25 103 M), hydrogen hexachloroplatinate (IV) hydrate (0.25 103 M), and silver nitrate (0.1 N). The solution of palladium ion was freshly prepared just before use and the reaction mixtures were wrapped with aluminum foil to avoid being exposed to light. The mixtures were allowed to stand or were stirred at 70 8C until the colors of spheres did not change any further as observed with naked eyes. The initially floating spheres usually settled down on the bottom of the reaction vessel as metal nanoparticles formed and were immobilized by the spheres. The spheres were recovered by centrifugation and washed with large amounts of water and ethanol. Visual Observation of the Formation of Spheres Visual observation of the formation of spheres was carried out both microscopically and macroscopically. In a microscopic observation, 0.1 mL of TEAwas dissolved in 2 mL of water. To the solution, 0.1 mL of VTMS was added, the mixture was shaken briefly by hand, and then a drop of the mixture was placed on a slide glass and covered with a cover glass. The change in the mixture was observed under an inverted microscope (x640) equipped with a digital camcorder. A macroscopic observation was carried out by mixing 100 mL of 5 wt.-% TEA and 2 mL of VTMS. For easier observation, a small amount of Oil Blue N was dissolved in VTMS. The change in the reaction mixture was recorded using a digital camcorder. The microscopically and macroscopically observations are available as Supporting Information. Miscellaneous The instruments used in this study were as follows: IR: Bomen 102; SEM: JEOL JSM-840 A; EDS: LINK system AN-10000/ 85S; Differential thermal analysis (DTA): TA Instruments SDT 2960; XRD: Rigaku D/MAX-2200V; EA: Fision EA-1108; ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Preparation of Functionalized Polysilsesquioxane and Polysilsesquioxane-Metal Nanoparticle Composite Spheres

Solid state NMR: Varian Unity Infinity Plus; UV/vis. spectrophotometer: Shimatzu 2450; BET specific surface analysis: Micrometritics ASAP 2020; micropore volume: Micrometrics Tristar 3000. Samples for BET specific surface analysis were outgassed for 24 h at 100 8C under vacuum. Calcination was carried out using a furnace heated from room temperature to 1 000 8C at a ramp of 2 8C min1. The DTAwas scanned from room temperature to 1 450 8C at a ramp of 10 8C min1 in atmospheres of nitrogen and air. The average diameters of spheres were calculated by measuring the diameters of at least 50 spheres from SEM images. Further characterization details are available in Supporting Information.

Results and Discussion Figure 1(a) and 1(b) show SEM images of typical PVSQ and PMPSQ spheres prepared in this study. The molar ratio of water:TEA:VTMS and MPTMS was 450:1:1. VTMS

and MPTMS were added to a homogeneous solution of TEA in water. As discussed below, PMPSQ spheres had larger diameters than PVSQ spheres when the molar ratio between each ingredient was identical. The reaction mechanism, and, subsequently, the sizes and size distribution of spheres, were affected by many factors. Above all, the sequence in which the reaction ingredients were added, and whether or not the reaction mixtures were agitated, were important. When TEA was added to inhomogeneous mixtures of water and VTMS (or MPTMS), instead of adding VTMS (or MPTMS) to the homogeneous solutions of TEA in water, the spheres had larger size distributions than when VTMS (or MPTMS) was added to a homogeneous solution of TEA in water. These results were ascribed to the inhomogeneity of the reaction mixture that led to the formation of nucleation at different moments. The size distribution of PVSQ spheres obtained from nonagitated reaction mixtures [Figure 1(c)] was drastically different from those obtained from normal reaction mixtures

Figure 1. SEM images of (a) monodisperse PVSQ spheres, (b) monodisperse PMPSQ spheres, (c) PVSQ spheres with large size distribution obtained from a non-agitated reaction system, and (d) a film of PVSQ obtained from a non-agitated reaction system. Macromol. Rapid Commun. 2006, 27, 1247–1253

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[Figure 1(a)]. When the amount of VTMS was high, films shown in Figure 1(d) formed along with these spheres. Microscopic and macroscopic observations of nonagitated reaction mixtures gave us clues to understanding the reaction mechanisms. In a macroscopic observation, 0.5 mL of VTMS was added to 5 mL of a 3.5 wt.-% aqueous solution of TEA in a 10 mL vial. In this case, the amount of VTMS was not enough to cover the water layer fully and formed a floating oil layer. Shortly, the VTMS layer started to move, indicating that the reaction of VTMS had started. Soon white clouds formed in the water phase near the interface, and those particles slowly moved down to the bottom of the vial with long tails. The VTMS layer continued to decrease in size and disappeared completely. When the amount of VTMS was excessive, the final thin oil layer hardened to form a film shown in Figure 1(d). Microscopic observation of the white particles formed in the aqueous phase proved that these particles were viscous oil drops [Figure 2(a)]. The overall morphology and distribution of sizes of the oil drops in Figure 2(a) corresponded to those of network PVSQ spheres obtained from these mixtures [Figure 1(c)]. On the other hand, microscopic observation of thoroughly agitated reaction mixtures prepared in the same composition did not show any visible object in the mixtures for approximately 2 min, and then the view window of the microscope was filled with a large number of small particles that grew in size with time. These spheres were not liquid, as they did not combine with each other to form larger droplets. The spheres prepared from agitated reaction mixtures had narrow size distributions as shown in Figure 2(b). These observations led us to the conclusion that the large spheres in Figure 1(c) and homogeneous spheres shown in Figure 1(a) and 2(b) formed by mechanisms such a suspension polymerization and emulsion polymerization, respectively. In a non-agitated system, the interfacial area between water and the VTMS layers was very small, and the initial number of VTMS molecules in the water phase was low. Consequently, the number of growing particles formed from nuclei was small and the monomeric species produced at the water–VTMS interlayer were not consumed fast enough to keep their concentration low near the VTMS layer. The monomeric species condensed to form oligomers that were soluble in VTMS. These oligomers were more hydrophilic than VTMS and had surface tension that was high enough to form oil drops as shown in Figure 2(a). During this process, the VTMS phase floating on the aqueous layer self-disintegrated and dispersed into the water phase, which looked just like oil layers on water would do when they meet surfactants. In agitated systems, the formation of spheres with narrow size distribution was ascribed to the formation of larger numbers of growing particles due to the higher interfacial area between the VTMS and water layers. The monomeric species formed at the VTMS and water interface were Macromol. Rapid Commun. 2006, 27, 1247–1253

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Figure 2. Optical microscope images of (a) oil drops produced at the interface of water and the VTMS layers, and (b) powers produced in an agitated reaction mixture of VTMS, water, and TEA.

consumed efficiently for the growth of growing particles, and the formation of oligomers, if any, did not affect the formation of spheres with homogeneous diameters. The increase in the number of growing particles was expected to decrease the size of spheres. The number of growing particles can be increased by increasing the amount of TEA and the amount of water medium, and using surfactants in the reaction mixture. Figure 3 shows the relationship between the diameters of spheres and the amount of each ingredient in the reaction system. The results in Figure 3 showed that increasing the number of growing particles decreased the diameters of spheres, and these results corresponded to what was expected in the emulsion polymerization process. Microscopic and macroscopic observation of the reaction of MPTMS showed that MPTMS formed spheres ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


Figure 3. The diameters of spheres against (a) the amount of TEA at a fixed ratio of water and VTMS, (b) the amount of Tween 80 at a fixed ratio of VTMS, water, and TEA, and (c) the amount of water at a fixed ratio of TEA and water.

more slowly than VTMS. The slower reaction of MPTMS was ascribed to the bulkiness and hydrophobicity of the 3-mercaptopropyl group, like the phenyl group in phenyltrimethoxysilane that formed spheres with significant amounts of soluble molecules[5,19,20] due to poor reactivity; however, MPTMS gave virtually identical results to those derived from VTMS. The formation of films at the interface of MPTMS and water layers took place more markedly because MPTMS had a higher density than water and formed a lower layer. The oil-drop-like products that correspond to those shown in Figure 2(a) were gathered at the surface of the MPTMS layer due to the operation of gravity in non-agitated systems. Nitrogen adsorption measurements showed that BETspecific surface area of PVSQ and PMPSQ spheres with average diameters of approximately 1.5 mm were 10.589 and 6.0806 cm3 g1, respectively. These measurements also indicated that both spheres were microporous. The Macromol. Rapid Commun. 2006, 27, 1247–1253

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micropore volumes of PVSQ and PMPSQ spheres were 0.004030 and 0.001411 cm3 g1. The interactions between metal ions and PVSQ and PMPSQ spheres were examined. Alcohols and amines are known to reduce metal ions; therefore, it was important to prepare spheres free of alcohol, alkoxysilane, and TEA to elucidate the interaction among metal ions, vinyl and mercapto groups in PVSQ and PMPSQ spheres. 29Si NMR spectra of PVSQ and PMPSQ spheres showed peaks only for T3 and T2 silicones in ratios of 94:6 and 87:13, respectively. The higher content of T2 silicon in the PMPSQ spheres also indicated that the condensation between MPTMS molecules took place to a lesser extent than that between VTMS molecules. Elemental analysis (EA) of these spheres showed that PVSQ spheres contained no nitrogen, while PMPSQ spheres contained 0.15 wt.-% nitrogen. Repeated washing did not reduce the amount of nitrogen in PMPSQ, indicating that the nitrogen was trapped deep inside the ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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spheres. Considering that the amount of nitrogen was low and that it was trapped deep inside, it was expected that the nitrogen in the PMPSQ spheres would not be reactive, and thus this was disregarded in interpreting results in this study. The EA results showed that the ratios of C, H, and S were very close to theoretical values; [SiO1.5CH CH2]0.94[SiO(OH)CH CH2]0.06 (C:H, calculated 29.97: 3.85, actual 30.15:3.87), and [SiO1.5CH2CH2CH2SH]0.87[SiO(OH)CH2CH2CH2SH]0.13 (C:H:S, calculated 27.84:5.50: 24.77, actual 28.06:5.60:24.97). IR spectra of these spheres did not change at all after heating at 100 8C for 24 h. These results showed that the PVSQ and PMPSQ spheres prepared in this study were virtually free of amine and any source of alcohol. DTA of PVSQ was very unusual because significant weight gains were observed respectively at 250 and 850 8C when DTA analysis was carried out in air and nitrogen. We do not have detailed explanations for these behaviors other than that there might be reaction with oxygen and nitrogen. These behaviors might indicate that the vinyl groups in VTMS had very unusual reactivities. The interactions between metal ions and PVSQ and PMPSQ spheres were examined by heating mixtures of aqueous solutions of metal ions and the spheres at approximately at 60 8C. Remarkably, the TEM images of spheres recovered from such mixtures showed nanoparticles on the surface and inside of the spheres as shown in Figure 4(a) and (b). Figure 4(a) and (b), respectively, show PVSQ spheres with gold nanoparticles both inside and outside and a PMPSQ sphere coated with palladium nanoparticles with insets of magnified images. TEM, XRD, UV/vis. spectroscopy, and EDX analyses showed that PVSQ spheres were active to silver, gold, platinum, and palladium ions while PMPSQ spheres were active to gold, platinum, and palladium ions. None of the metal nanoparticle-PSQ composite spheres changed color during 6 months of observation, indicating that the particles were stable. As a control, we prepared spheres of PMSQ by the same method. PMSQ, indeed, did not show any activity regarding either reduction of metal ions or fixation of metal nanoparticles. It was thus clear that vinyl and thiol groups were responsible for the formation and fixation of metal nanoparticles in this study. However, IR spectrum of PVSQ spheres and PVSQ-gold metal nanoparticle composites did not show any noticeable changes in the position of peaks and relative intensity. A few IR peaks near 3 000 cm1 showed shoulders that might be caused by the association of palladium nanoparticles with double bonds. The PVSQ spheres gave the most interesting products. Figure 4(a) shows that PVSQ spheres had metal nanoparticles with diameters of approximately 10 nm inside and diameters of 1–2 nm on the surface. The fact that new 10 nm nanoparticles were visible when focusing on different points indicated that the nanoparticles were inside the Macromol. Rapid Commun. 2006, 27, 1247–1253

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Figure 4. A TEM image of the surface and the whole image (inset) of (a) a PVSQ-gold nanoparticle composite sphere, and (b) the surface (inset) and the whole image of a PMPSQ-palladium nanoparticle composite sphere.

spheres. It was also remarkable that PVSQ spheres prepared using Tween 80 gave gold nanoparticles with diameters of 1–2 nm only on the surface. These observations indicated that PVSQ prepared in the absence of Tween 80 had voids of approximately 10 nm diameter inside, and Tween 80 prevented formation of such voids. Extended reaction between metal ions and PVSQ spheres did not cause formation of larger metal particles. Neither the color of the PVSQ spheres nor the supernatant aqueous metal ion solutions used in excess changed any further usually after 48 h. The interaction between gold nanoparticles and PVSQ spheres was seen in DTA thermograms of PVSQ and PVSQ-gold nanoparticle composite spheres. The DTA ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


thermogram of PVSQ-gold nanoparticle composite spheres showed no weight gain at 250 8C in air. The chemical driving force behind the formation of metal nanoparticles is yet to be investigated; however, the results of this study proved that vinyl groups and mercapto groups in PVSQ and PMPSQ were very useful for the preparation of metal nanoparticles without the aid of any other reductant or stabilizer.

Conclusion The results of this study showed that spheres with narrow size distributions could be prepared by emulsion polymerization from heterogeneous mixtures of water-insoluble ATAs, TEA, water, and surfactants. The diameters of spheres were controlled from tens of nanometers to a few micrometers, and spheres made of fully-functionalized network polysilsesquioxanes were obtained. The vinyl and mercapto groups in the spheres were able to produce metal nanoparticles and immobilized metal nanoparticles within themselves. The vinyl and mercapto groups can be further modified chemically to obtain spheres with new functions. These spheres can also be applied to recovering metal from aqueous solutions of metal ions. Since polysilsesquioxanes can be fabricated in different shapes[21] and coated on different templates, materials coated with metal nanoparticles might be obtained by treating materials made of and coated with PVSQ and PMPSQ with metal ion. Acknowledgements: This work was supported by Korea Research Foundation Grant 160 (KRF-2003-015-C00438).

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