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Langmuir 1999, 15, 5200-5203
Selective Incorporation of Palladium Nanoparticles into Microphase-Separated Domains of Poly(2-vinylpyridine)-block-polyisoprene Kiyoharu Tsutsumi,†,‡ Yoshinori Funaki,†,‡ Yoshitsugu Hirokawa,†,§ and Takeji Hashimoto*,†,| Hashimoto Polymer Phasing Project, ERATO, JST 15 Morimoto-cho, Shimogamo, Sakyo-ku, Kyoto 606-0805, Japan, and Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received March 2, 1999. In Final Form: June 1, 1999 Fine palladium (Pd) particles, which have a diameter of 4-5 nm and are coordinated with the poly(2-vinylpyridine) homopolymer (P2VP) or poly(2-vinylpyridine)-block-polyisoprene diblock copolymer (P2VPb-PI), were prepared by the reduction of palladium acetylacetonate (PdII(acac)2) with 1-propanol in the dilute benzene solution of P2VP or P2VP-b-PI. These Pd “nanoparticles” were selectively incorporated into a particular region of microdomains by casting a solution comprised of the Pd nanoparticles, P2VP-b-PI, and chloroform as a solvent into film specimens. The nanoparticles coordinated with P2VP were selectively incorporated into the P2VP domains, while those coordinated with P2VP-b-PI were around the interface between the P2VP and PI domains.
Introduction Fine metal particles having a diameter of the order of nanometers (designated hereafter as metal “nanoparticles”) are of interest as materials applicable to metal catalysts, optical devices, etc. Important factors in utilizing the nanoparticles are how to stabilize them to avoid their aggregation into large clusters and how to control their spatial positions in the materials. Lots of efforts have been devoted to stabilize the metal nanoparticles by various ways, for example, by surfactants1-3 or polymers.4-6 From a viewpoint of handling, the metal nanoparticles should be held on substrates. There are some reports on a chemical holding of the metal nanoparticles on the surface of the matrix polymer6 or on the homogeneous dispersion of the metal nanoparticles into the polymer matrix.7 If metal nanoparticles can be incorporated or arranged only into one of microphase-separated domains of block copolymers, we can obtain various structures having a unique spatial distribution of the nanoparticles in the matrix polymers. This “nanohybrid” may be used as highly functional materials in wide varieties of applications such as optical and electromagnetic devices and metal catalysts. There are mainly two methods to make the “nanohybrid”. One is the preparation of the nanoparticles in one of the microphase-separated domains, and another is * To whom all correspondence should be addressed at Kyoto University. † Hashimoto Polymer Phasing Project, ERATO. ‡ Present address: Research Center, Daicel Chemical Industries, Ltd., 1239, Shinzaike, Aboshi-ku, Himeji, Hyogo 671-1283, Japan. § Present address: Research & Development Center, Nippon Zeon Co., Ltd., 1-2-1 Yako, Kawasaki-ku, Kawasaki, Kanagawa 2108507, Japan. | Kyoto University. (1) Toshima, N.; Takahashi, T. Bull. Chem. Soc. Jpn. 1992, 65, 400. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (3) Pileni, M. P. Langmuir 1997, 13, 3266. (4) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (5) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000. (6) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem. Eur. J. 1996, 2, 1552. (7) Tamai, H.; Sakurai, H.; Hirota, Y.; Nishiyama, F.; Yasuda, H. J. Appl. Polym. Sci. 1996, 56, 441.
incorporation of the nanoparticles into one of the microphases. As for the former examples, there are reports on the in-situ preparation of the nanoparticles by a reduction of metal ions in the microdomain space which are selectively incorporated into one of the microphases of block copolymers.7-12 As for the latter example, there is a report on the incorporation of metal nanoparticles coordinated and stabilized by small molecules into the microphase-separated domains which are comprised of the block chains having side groups, the chemical structure of which is the same as that of the small molecule coordinating the metal particles.13 In this study, we focused on creation of a “nanohybrid” by using the latter method. We first prepared palladium (Pd) nanoparticles coordinated by either poly(2-vinylpyridine) homopolymer (P2VP) (denoted by Pd-P2VP hereafter) or poly(2-vinylpyridine)-block-polyisoprene block copolymer (P2VP-b-PI) (denoted by Pd-bcp hereafter) by the alcohol reduction of palladium acetylacetonate (Pd(acac)2) in benzene. The nanoparticles were then incorporated into the microphase-separated structure of P2VPb-PI by solvent casting of a mixture of P2VP-b-PI with Pd-P2VP or Pd-bcp into film specimens. We believe this is a new method to incorporate metal nanoparticles into the polymer matrix. In this Letter, we will report the preparation of the two types of Pd nanoparticles, Pd-P2VP and Pd-bcp, and selective incorporations of the Pd nanoparticles in the microphase-separated structure of P2VPb-PI. Experimental Section Synthesis of Polymers. P2VP and P2VP-b-PI were synthesized by the living anionic polymerization. Polymerizations were done in dried tetrahydrofuran (THF) at -78 °C under dried nitrogen atmosphere. sec-Butyllithium (8) Nakao, Y. J. Colloid Interface Sci. 1995, 171, 386. (9) Saito, R.; Okamura, S.; Ishizu, K. Polymer 1993, 34, 1183, 1189. (10) Chan, Y. N. C.; Craig, G. S. W.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 885. (11) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241. (12) Harada, M.; Hashimoto, T. In preparation. (13) Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L. Macromolecules 1997, 30, 417.
10.1021/la990246l CCC: $18.00 © 1999 American Chemical Society Published on Web 07/08/1999
Letters
Langmuir, Vol. 15, No. 16, 1999 5201 Table 1. Characterizations of the Polymer Samples number average molecular weight (Mn) × 10-4
code
polymer
P2VP blocka
polymer 1 polymer 2 polymer 3
P2VP P2VP-b-PI P2VP-b-PI
5.1 3.8 5.7
PI blockb 3.0 3.0
microdomain morphologyc lamellae lamellae
remark used for coordinating Pd used for coordinating Pd used as a matrix polymer
a Calculated from the molecular weight of PI block precursor determined by GPC and the molar ratio of 2-vinylpyridine and isoprene monomer units determined by 1H NMR. b Determined by GPC with a standard polyisoprene calibration. c Cast from chloroform solution.
(Aldrich Co., cyclohexane solution) was used as an initiator. These polymers were purified by precipitating the THF solution into hexane. Gel permeation chromatography (GPC) and 1H nuclear magnetic resonance spectroscopy (1H NMR) were used to characterize these polymers. The characteristics of these polymers are summarized in Table 1. Preparation of Pd Nanoparticles. The Pd-P2VP and Pd-bcp were prepared in the homogeneous benzene solution. For this purpose, 0.2 g of P2VP (polymer 1) or P2VP-b-PI (polymer 2), 0.23 g of PdII(acac)2, and 15 mL of 1-propanol were first dissolved in 140 mL of benzene. This homogeneous and dilute solution was heated at 84 °C for 50 h at constant concentration for reduction of the metal ions into metals. After the alcohol reduction, benzene and 1-propanol were evaporated. The Pd-P2VP or Pd-bcp nanoparticles were then dissolved into chloroform solution and precipitated into acetone to remove unreacted salt. The precipitates were again dissolved by benzene and then purified by centrifugation three times to remove free P2VPb-PI chains. Finally, black powders were obtained by freeze-drying of benzene solution of the Pd-P2VP or Pdbcp nanoparticles. The resultant Pd nanoparticles were subjected to elemental analysis (YANAKO MT-5). The weight fractions of carbon, hydrogen, nitrogen, and Pd were calculated from the mole ratio of CO2, H2O, and N2 detected by a thermal conductivity detector14 and the weight of ash,15 respectively. The ash content gives an important conclusion with respect to numbers of coordinated polymer chains (P2VP homopolymers or P2VP-b-PI block copolymers) per single Pd nanoparticles. These numbers are equal to 5 for both cases, as will be discussed later. Preparation of Cast-Film of P2VP-b-PI with Pd Nanoparticles. The Pd nanoparticles thus obtained, either Pd-P2VP or Pd-bcp, were diluted in chloroform together with P2VP-b-PI (polymer 3) used as a matrix polymer for the particles after evaporation of chloroform. The homogeneous solutions thus obtained were cast into film specimens at room temperature in a Petri dish made by Teflon. The compositions of the Pd nanoparticles and the polymer 3 used in these works were ca. 1/25 (wt/wt) for both Pd-P2VP/polymer 3 and Pd-bcp/polymer 3. The cast-films were dried under vacuum at 60 °C for 24 h. The microdomain structures in the cast-films were observed under a transmission electron microscope (TEM, JEOL JEM-2000FXZ) operated at 160 kV on the ultrathin sections, obtained with a Reichert-Nissei Ultracut-S ultramicrotome and stained with OsO4 vapor for 30 min. (14) In elemental analysis, the organic samples including C, H, and N are thermally degraded and oxidized to CO2, H2O, and NO2, respectively, by CuO catalyst. The NO2 is then reduced to N2 by reduced copper. (15) Ash is generally an oxidized metal in the elemental analysis. However it is known that oxidized Pd (PdO2) becomes Pd metals by thermal degradation at 875 °C. Thus at the combustion temperature used in this study (950 °C), the ash may be Pd metal. The ash was subjected to analysis by electron probe microanalyzer (JEOL JSM5800, Oxford Instruments link ISIS), and only Pd metal was detected.
Figure 1. TEM micrographs of Pd nanoparticles coordinated by (a) P2VP (polymer 1) and (b) P2VP-b-PI (polymer 2).
Results and Discussions Pd Nanoparticles. Benzene solutions of the Pd nanoparticles were dropped onto electron microscope grids coated with a thin film of poly(vinyl formal). After drying, these grids were subjected to TEM observation. Figure 1 shows the TEM images of the Pd nanoparticles. The diameter of Pd-P2VP was about 4 nm (Figure 1a), while that of Pd-bcp was about 5 nm (Figure 1b). These images show that the diameter of nanoparticles is monodisperse and stabilized by either P2VP or P2VP-b-PI. Position of the Pd Nanoparticles in the Microphase-Separated P2VP-b-PI Matrix. Figure 2 shows TEM images of the microphase-separated structures in the as-cast films for Pd-P2VP/polymer 3 (part a) and Pd-bcp/polymer 3 (part b). Both micrographs in parts a and b show an alternating lamellar structure where dark and bright phases correspond to the stained PI lamellae and the unstained P2VP lamellae, respectively. These microphase-separated structures were the same as those of pure polymer 3, i.e., P2VP-b-PI matrix polymer.
5202 Langmuir, Vol. 15, No. 16, 1999
Letters Scheme 1. Schematic Models for Possible Configurations of P2VP-b-PI Coordinating the Pd Nanoparticle: (a) and (b) Isotropic Configuration and (c) “Polarlizable” Configurationa
Figure 2. TEM micrographs of microphase-separated structures as observed in thin films cast from chloroform solution of P2VP-b-PI (polymer 3) mixed with Pd nanoparticles coordinated by P2VP (polymer 1) (a) and P2VP-b-PI (polymer 2) (b). Ultramicrotomed thin specimens were stained with OsO4.
The micrographs revealed the following intriguing features. The Pd-P2VPs which appear as small dark dots in Figure 2 were localized in the center of the P2VP lamellae (Figure 2a), while the Pd-bcps were localized in the specific place of the P2VP lamellae which is close to the interface between the P2VP and the PI phases (Figure 2b). In both cases, the Pd nanoparticles exist in the P2VP phase, indicating that they have a preferential miscibility with P2VP. It seems to be quite natural that the Pd-P2VPs are incorporated into P2VP lamellae, as the Pd particles are coordinated by P2VP. The localization of Pd-P2VP around the center of P2VP lamellae may be reasonable because a cost of the conformational entropy loss for the P2VP block chains involved by the incorporation of the nanoparticles may be minimized under this condition. Moreover, the result that the Pd-bcps are localized at the interface between the P2VP and the PI phases revealed that these Pd nanoparticles also have a part which has a selective miscibility with PI as well. Consequently the results in Figure 2b infer unique configurations of the polymer 2 (P2VP-b-PI) which are coordinated on Pd nanoparticles, as will be suggested below. Configurations of P2VP-b-PI around the Pd Nanoparticle. Many nanoparticles prepared in the micelles of block copolymers4-6 must be coordinated by the block chains forming the micelles (e.g., P2VP), and the other block chains (e.g., PI) must be emanating from the P2VP micelle as coronae, as shown in Scheme 1a. If such Pd
a The solid lines, dotted lines, and black meshed circles represent P2VP chains, PI chains, and Pd particles, respectively.
nanoparticles as shown in Scheme 1a are incorporated into the microphase-separated lamellae of P2VP-b-PI, the nanoparticles must be localized around the center of the PI lamellae, because the nanoparticles are coordinated by the P2VP block chains which are then shielded by the PI coronae. The same conclusion will be obtained in the case when the Pd particles prepared by the reduction of Pd2+ ions in a dilute homogeneous solution of P2VP-b-PI are coordinated by many block chains in such a way that the particles are isotropically coordinated by the P2VP block chains which are then shielded by the PI block chains (see Scheme 1b). However, in this study, the Pd-bcp nanoparticles coordinated by P2VP-b-PI were localized in the P2VP phases close to the interface between P2VP and PI phases. In this case, the Pd nanoparticles must have a part having miscibility with the P2VP phase and a part having miscibility with the PI phase, as described in the last part of the preceding sections, and these two parts must be “polarized” or “polarizable”. Thus, a Pd nanoparticle is expected to have the P2VP block coordinated on its surface and the uncoordinated, more or less, free polarizable PI block as shown in Scheme 1c. In Scheme 1c, the nanoparticle must need to be coordinated by a few P2VP block chains in order to have the “polarizable” configuration for the PI block chains. Thus, we tried to evaluate the number of polymer chains
Letters
Langmuir, Vol. 15, No. 16, 1999 5203 Table 2. Results of Elemental Analysis of the Pd Nanoparticles nanoparticles
C (wt %)
H (wt %)
N (wt %)
ash (Pd metals) (wt %)
remark
Pd coordinated by P2VP Pd coordinated by P2VP-b-PI Pd coordinated by P2VP-b-PI
37.8 32.3 62.9
3.2 3.1 6.1
6.0 3.1 5.7
44.2 54.1 18.6
after the removal of free polymer after the removal of free polymer before the removal of free polymer
which coordinate one Pd nanoparticle by elemental analysis. From the weight ratio of nitrogen atoms and Pd metals (from ash)15 determined by the elemental analysis (Table 2) and the average diameter of the Pd nanoparticles determined by a TEM image (4 nm for Pd-P2VP and 5 nm for Pd-bcp), the number of polymer chains coordinating with one nanoparticle was estimated to be five for both Pd-P2VP and Pd-bcp. This value seems to be consistent with the model shown in Scheme 1c. Microphase-Separated Structure of the Pd Nanoparticles. If the model of Scheme 1c is true, the Pd-bcp nanoparticles will form a microphase-separated structure by themselves even without the matrix block copolymer due to its polarizable configuration. Thus, we investigated microphase separation of the Pd-bcp nanoparticles by casting from solution without adding the P2VP-b-PI matrix polymer. Because the purified Pd-bcp nanoparticles did not form any obvious microphase-separated structure,16 we explored the microphase separation by using the Pd nanoparticles before the removal of free polymer 2 (polymer 2 uncoordinating the metals) by centrifugation in order to reduce the number density of the Pd-bcps in the system. A composition of Pd-bcp and free polymer 2 in the system studies was about 11/4 (wt/wt), or about one Pd particle (coordinated by five P2VP-b-PI chains)/6.5 free P2VP-bPI chains, which was calculated from the result of elemental analysis shown in Table 2. To obtain a strong contrast between the P2VP and PI phases, the P2VP phases in the as-cast film were selectively cross-linked and stained by 1,4-diiodobutane, and the microphaseseparated structure of the film was observed by TEM without further staining by OsO4. Figure 3a shows the microphase-separated structure where the dark phases, bright phases, and black dots are the cross-linked and stained P2VP phases, the unstained PI phases, and the Pd particles, respectively. In this figure, we can observe network-type P2VP microdomains which contain the Pd particles, while the original microphase-separated structure of the polymer 2 is the alternating lamellae of P2VP blocks and PI blocks as shown in Figure 3b. This result indicates that polymer 2 cannot keep the lamellar structure when concentration of the Pd nanoparticles becomes high. Thus the Pd-bcp nanoparticles strongly affect the microphase separation and microphase-separated structures of polymer 2 (P2VP-b-PI). Conclusion In conclusion, the Pd nanoparticles coordinated by P2VP or P2VP-b-PI were prepared by the reduction of Pd(II)(acac)2 with 1-propanol in a dilute homogeneous benzene solution of P2VP or P2VP-b-PI. These Pd nanoparticles were selectively incorporated into the P2VP phase of the microphase-separated structure of P2VP-b-PI (polymer 3). However, the positions of the nanoparticles in the P2VP phase are different; the Pd nanoparticles coordinated by P2VP are localized around the center of the P2VP phase, (16) Because the bulk of the purified Pd nanoparticles has high Pd density, the attractive interactions between Pd metals may be stronger than the repulsive interactions between P2VP and PI, and thus microphase-separated structures could not be formed.
Figure 3. TEM micrographs of microphase-separated structure of (a) Pd nanoparticles coordinated by P2VP-b-PI (polymer 2) and of (b) P2VP-b-PI (polymer 2) without the Pd particles. In both cases, the P2VP phase was selectively cross-linked and stained by 1,4-diiodobutane. The dark phase, bright phase, and black dots in part a are the P2VP phase, PI phase, and Pd particles, respectively. The dark and bright phases in part b are the P2VP and PI lamellae, respectively.
while the nanoparticles coordinated by P2VP-b-PI are localized near the interface between the P2VP and PI lamellae, revealing that P2VP-b-PI block chains coordinating the Pd nanoparticle have the “polarizable” configuration. This provides a new method for a controlled incorporation of metal nanoparticles into the polymer matrix. The influence of the molecular weight and the composition of the coordinating block copolymer and the matrix block copolymer on the spatial distribution of Pd nanoparticles in the microphase separation are now under investigation in our laboratory.17 Acknowledgment. We express our sincere thanks to Dr. J. Yamanaka, Mr. A. Okumura, and Miss Y. Kanazawa for their technical assistance and useful discussions. LA990246L (17) Okumura, A.; Tsutsumi, K.; Hashimoto, T. In preparation.
