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J. Phys. Chem. B 2004, 108, 17378-17383
Photoinduced, Controlled Generation of Palladium Crystallite Structures in Polyimide Films G. A. Gaddy,*,†,‡ Edward P. Locke,*,†,§ M. E. Miller,| R. Broughton,⊥ T. E. Albrecht-Schmitt,# and G. Mills*,# National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia 23681, and Departments of Biological Sciences, of Textile Engineering, and of Chemistry, Auburn UniVersity, Auburn, Alabama 36849 ReceiVed: May 13, 2004; In Final Form: August 26, 2004
Structures of Pd crystallites with nanometer dimensions are formed on the surface of and inside polyimide films by a combination of photochemical and thermal processes. Photoinduced transformations of polyamic acid films containing Pd2+ complexes prior to thermal curing affect the subsequent thermal imidization of the polymer and reduction of the remaining palladium ions. The extent of photolysis controls the formation and characteristics of surface and subsurface metallic layers as well as the sizes of Pd particles generated within and below the interlayer region. Interference between waves reflected from the continuous embedded Pd layer and waves reflected from the continuous surface metal layer is the origin of the colors observed in photolyzed and cured films. Multicolored areas are a result of nanometer range variations of the interlayer distance. Optical properties typical of Fabry-Perot filters are observed when the thickness of the surface layer and internal metal layer are similar. The etalon-like films experience significant reversible dimensional changes upon exposure to white light.
Introduction Metal-containing polymers have received much attention in the last twenty years because of their technological significance.1 For instance, numerous applications for metallized polymers are found in the microelectronics and automotive industries.2,3 Continuous interest in these materials is supported by potential applications to space technologies currently under development.4 Polyimides (PI) are established aerospace materials, and metallization of their surfaces allows these polymers to fulfill demanding tasks in the severe environment of space. Electrochemical methods can produce layers of metal particles either on the surface of PI films or internally in the case of swollen polymers, but high-temperature sintering of the particles is needed to form compact metal films.5,6 A simpler method is based on the in situ reduction of metal ions present inside films of polyamic acids, which occurs during the high-temperature cyclodehydration of these polymers to form polyimides.7,8 This method offers a single step process for achieving conductive and/or reflective metallic layers on the surface of PI films, which exhibit excellent metal to polymer adhesion. The high-temperature procedure can also be employed to generate nanometersized particles dispersed in the bulk of the film. Formation of metal particles with nanometer dimensions inside polymers is also of interest because these metallic crystallites exhibit size-dependent properties.9-13 Polymeric * Corresponding authors. E-mail: G.A.G.,
[email protected]; E.P.L.,
[email protected]; G.M.,
[email protected].. † National Research Council Postdoctoral Fellow. NASA-Langley Research Center. ‡ Current address: United States Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783. § Current address: K & M Environmental, Inc., Virginia Beach, VA 23454. | Department of Biological Sciences, Auburn University. ⊥ Department of Textile Engineering, Auburn University. # Department of Chemistry, Auburn University.
matrixes can inhibit the undesirable growth of metal crystallites due to spontaneous agglomeration of clusters and particles.14 In fact, these matrixes play an important role in established technologies that exploit the size-dependent properties of Ag particles, such as imaging with photographic emulsions or thermally developed photographic materials.15 Photogeneration of metal crystallites within dry or swollen polymers is an attractive technique because the swelling agent or the macromolecules can aid the reduction of metal ions under illumination.16-18 Additionally, long-term stabilization of metal clusters containing fewer than 10 metal atoms has been demonstrated in polymer films.17 This simple procedure, using illumination to induce the formation of metal crystallites, seemed a logical option in attempts to gain further control of the high-temperature metal crystallite formation in PI films.7,8 Results from the present study indicate that the formation of Pd crystallite arrays can be manipulated by a combination of photochemical and thermal methods yielding polymer films with unusual optical and photomechanical properties. Experimental Section 3,3′, 4,4′-Benzophenonetetracarboxylic dianhydride (BTDA) and 4,4′-oxydianiline (ODA) were obtained from Allco Chemical and Wakayama Seika Kogyo, respectively. PdCl2, N,Ndimethylacetamide (DMAc) and dimethyl sulfide were obtained from Aldrich. All chemicals were used as received without further purification. Polyamic acid resins (12-15 wt %) were prepared by reacting equimolar amounts of BTDA and ODA in anhydrous DMAc, resulting in resins with ηinh ≈ 1.6. The resins were stirred under N2 for 15-20 h and then stored at 10 °C under N2. Pd[S(CH3)2]2Cl2 was synthesized from PdCl2 as previously described for the Pt analogue of this compound.19 Resins were doped at room temperature with 5.0 wt % Pd in the form of Pd[S(CH3)2]2Cl2. The resins were sealed under N2 and placed on an automatic shaker for 4 h, then stored at 10
10.1021/jp0479421 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
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Figure 1. Digital images of a 15 h irradiated and thermally cured polyimide film containing 5 wt % Pd from Pd[S(CH3)2]2Cl2: (a) when completely illuminated with white light; (b) when only partially illuminated with white light. The arrow in (a) indicates the blue-green region corresponding to the location in the photoreactor where the smaller films were placed. The arrows in (b) identify the gray and black line features that are the origin of the scattered light.
°C. All films were cast on 18 × 31 cm glass substrates and cut at 330 µm with a 2 in. Gardco Microm Film Applicator. The freshly cast films were placed directly into a horizontally positioned cylindrical Rayonet 100 photochemical reactor equipped with 16 RPR-3500A lamps and exposed to 350 nm photons. An average light intensity (I0) of 1017 hν s-1 was determined in the central region of the reactor. However, a gradual decrease in I0 occurred between the central region and the zones at the ends of the Rayonet. For instance, the light intensity was 2.5 times lower at the ends of the reactor as compared with the I0 value measured in the central region. After photolysis, the films were cured using a programmable forcedair oven (Blue M model DC-256 C) to remove the DMAc solvent, induce imidization of the polyamic acid, and further reduce the remaining Pd2+ ions. The oven was programmed to heat the samples in successive steps of 100, 200, and 300 °C for 1 h each, returning to ambient temperature over 1 h. Cured films had a thickness of 15-60 µm, as determined with a TMI 49-60 micrometer. Film properties reported here do vary significantly with film thickness. Diffuse reflectance spectra of film strips were recorded with a Shimadzu UV-2501-PC spectrophotometer equipped with a Shimadzu ISR 2200 integrating sphere using BaSO4 as a reference. Transmission electron microscopy (TEM) analysis of microtomed film samples was performed with a Zeiss EM 10CR microscope operating at 60 kV. X-ray diffraction (XRD) data were collected using a Rigaku Miniflex diffractometer using Cu KR radiation. The response of preloaded films to illumination was determined using an Instron 1122-4400R; white light was
provided by a microscope stage illuminator with an intensity of 0.17 W, as determined with a Coherent 210 power meter. Qualitative tests on the response of films to different wavelengths were performed with light from a PTI A-1010-S system equipped with a 150 W Xe arc lamp. A Cr-coated glass filter was employed to eliminate the IR component and wavelengths shorter than 290 nm. Different ranges of wavelengths were selected by means of broadband glass filters. Thermomechanical measurements were carried out on a TMA-2940 apparatus from TA Instruments. Films in Figure 1 were illuminated with an I2R Glow Box SD12-18. Results and Discussion Irradiated polyamic acid films doped with Pd2+ appear darker than nonirradiated doped films; they also appear darker than irradiated, nondoped polymer films. This result suggests that some transformation of the films occurs during photolysis. However, imidization of nonirradiated samples, or of polymers photolyzed for only a few hours yields mirror-type metal films similar to those reported in the literature for Ag+ containing polyimides7a,8 but differing from previous reports of red-brown Pd-containing polymers.7d,7e The origin for this difference in behavior is presumably the higher Pd2+ concentration used in the present study. Longer light exposures significantly altered the appearance of thermally cured PI films. Digital images of a sample irradiated for 15 h and subsequently thermally cured are depicted in Figure 1. The brightly colored regions of the film are only evident after the high temperature curing process,
17380 J. Phys. Chem. B, Vol. 108, No. 45, 2004 and form exclusively on the side of the film exposed to the atmosphere. Hence, polymer coloration results from the combined effects of irradiation and thermal processes. This implies that thermal curing acts as a complementary process to the photochemical transformation, resembling somewhat the behavior of photothermographic systems.15 It should be noted that Pd particles exhibit a brown color typical of metal crystallites devoid of surface plasmons in the visible region.20 All doped and thermally cured films exhibit the brown coloration on the substrate side of the film. Hence, color formation in the polymer is not due to sized-induced variations in the optical properties of the formed Pd nanoparticles. Figure 1a was obtained by placing the PI film such that the entire surface was directly illuminated by the light source, whereas Figure 1b was obtained by angling the sample such that only the lower portion was directly illuminated. Only the gray and black line features indicated by the arrows on both sides of the film appear to extend into the top nonilluminated region of Figure 1b, indicating that efficient light scattering takes place in these regions. Another interesting observation drawn from Figure 1a is that reflective mirrors form at both the left and right ends of the irradiated/thermally cured films. The mirror regions are believed to be a result of the low light intensity near the ends of the photochemical reactor because nonirradiated films doped with Pd2+ yield analogous mirrored films after thermal curing. A logical conclusion is that the color changes shown in Figure 1a originated from the gradient in I0 that was found between the central region and the ends of the photoreactor. Additional experiments were performed using smaller film samples of Pd2+ doped polyamic acid, which were irradiated in a position of the photoreactor that yielded the blue-green region of the large film (indicated by the arrow in Figure 1a). This position is close to the central region of the reactor where the maximum I0 was measured. Illuminations for 5, 15, 20, and 25 h followed by thermal curing produced mirrored, blue-green, red-brown, and gray-brown films, respectively. Inspection of Figure 1a reveals similar color changes upon moving from the center of the film toward the mirrored regions. The combined results of experiments with large films and with the smaller samples indicated that a short irradiation using a high photon flux produced the same result as a long irradiation with a low I0. Hence, the outcome of the photolysis depended mainly on the total number of photons absorbed by a certain region of the films. This means that, within the range of light intensities available in the photoreactor, I0 affected the extent of the photoreaction but not the mechanism of this process. According to this interpretation, regions exhibiting the same color were subjected to similar photon fluxes. The same holds true for the jagged colored lines shown in Figure 1a, which exhibit increasingly larger peaks as they progress from the lateral borders to the central horizontal axis of the film. Analogous features have been found in intensity distribution plots of interference patterns produced by the combined effects of reflection and scattering of light.21 Hence, the doped films are able to produce a color image that represents the variations in photon flux experienced by a light absorber present inside the reactor. X-ray diffraction measurements taken from all irradiated/ thermally cured films showed a broad signal at a 2θ angle of ∼19° corresponding to the polymer, and reflections from the {111}, {200}, {220}, and {331} lattice planes of Pd. Representative diffraction patterns of a nondoped, irradiated/thermally cured PI sample and a Pd2+ doped PI film irradiated for 15 h and cured are shown in Figure 2a,b, respectively. TEM images
Gaddy et al.
Figure 2. Diffraction patterns of (a) nondoped, 25 h irradiated/ thermally cured PI film (control) and (b) 25 h irradiated/thermally cured PI film doped with Pd2+ ions.
from each of the colored regions of the film in Figure 1 were also obtained. Presented in Figure 3a is an image from a sample of the black line region mentioned above. A metal layer (average thickness ) 14.8 nm) exists on the polymer surface. Directly below this surface Pd layer is a region containing a high density of small metal crystallites with an average diameter (dav) ) 5.0 nm, followed by a highly irregular and discontinuous layer (average thickness ) 44 nm) composed of large fused Pd particles. An average distance of 32 nm separates the two metal layers. The particle density decreases beneath the inner metal layer and larger spherical Pd crystallites (dav ) 13 nm) exist throughout the lower portion (bulk) of the PI film. Except for samples from the gray line region and mirrored regions of the film shown in Figure 1, all other specimens exhibit similar structures. Mirrored regions contain Pd crystallites (average diameter ) 12.4 nm) located beneath a single continuous metal layer 63.5 nm thick. Analogous results are obtained from the gray line region, but the single surface Pd layer in this region is nonuniform and discontinuous. Only the black line and the gray line regions of the film shown in Figure 1 scatter light and both possess discontinuous Pd layers either at or close to the polymer surface. In view of the similarities of the black and gray line regions, it is logical to conclude that light is scattered by their discontinuous metal layers. Figure 3b is a TEM image of a small Pd2+ doped film irradiated for 25 h and thermally cured. This film shows the same colors of the central area in Figure 1. In this film, the thickness of both continuous metal layers is similar, 21 nm for the surface layer and 26 nm for the internal layer. Metal particles located between and below the continuous Pd layers exhibit similar dav values of 7 and 10 nm, respectively; the average distance between the continuous Pd layers (interlayer distance) is 384 nm. Analysis of TEM data from the black line and graybrown regions shown in Figure 1 reveals that the images presented in Figure 3 represent extreme cases of interlayer distances and continuous Pd layer thickness. In fact, a continuous increase in interlayer distance accompanies the color progression toward the center of the film; Table 1 is a summary of these results. Also, in all highly colored regions of the films, the surface Pd layer is 14-16 nm thick and the thickness ratio between the inner metal layer and the surface metal layer is 2-3. Thus, in these colored regions, the main variable is the interlayer distance. Color changes following a progression very similar to those of the irradiated/cured Pd-doped polyimides have been shown
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Figure 4. Reflectance spectra of irradiated, thermally cured Pd-doped films obtained after different irradiation times: (a) 5 h; (b) 15 h; (c) 20 h; (d) 25 h. Inset: (e) spectrum of a nondoped, nonirradiated, thermally cured PI control film; (f) spectrum from the backside of a 25 h irradiated, thermally cured Pd-containing film.
whereas reflectance minima due to destructive interference is described by
nd ) mλ′/2
Figure 3. (a) TEM image of a microtomed sample corresponding to the black line section identified by an arrow in Figure 1b. (b) TEM image of a section from the central gray region identified by the arrow in Figure 1a.
TABLE 1: Experimental and Literature Interlayer Thickness and Resulting Color Pd-doped Irradiated polyimide film
n-butyl methacrylate film20
film color interlayer distance (nm) thickness range (nm) film color black gold purple blue green purple green
32 134 187 230 280 345 408
90-150 150-190 190-240 240-280 -
gold purple blue green -
to occur upon increasing the thickness of nanometer-thick acrylate polymers placed between two media of different refractive indices that are below that of the acrylate film.22 Interference between waves reflected from the top and bottom interfaces creates constructive and destructive interference. For light normal to the surface the condition of reflectance maxima due to constructive interference is given by23
nd ) (2m + 1)λ/4
(1)
(2)
where λ and λ′ are the wavelengths at which interference takes place, n is the refractive index of the material, d is the film thickness, and m is the order of interference (m ) 0, 1, 2, ...). Included in Table 1 are reported ranges of polymer thickness associated with the different colors.22 The interlayer distances responsible for the colors in Pd-containing films agree well with the published polyacrylate thickness values associated with the colors: gold, purple, blue, and green. The last two entries in Table 1 correspond to regions where higher order interferences occur.22 Substituting the experimental interlayer distances into eq 1 with m ) 2, yields λ values (431 and 509 nm) within the range of wavelengths associated with the observed colors.24 These results clearly demonstrate that coloration of irradiated PI films doped with Pd2+ ions arises from interference between reflected waves from the surface and internal Pd layers. Hence, the PI material between the surface and inner Pd layers must have a higher refractive index than the PI material beneath the inner metal layer, presumably due to the higher crystallite density in the former. Presented in Figure 4 are diffuse reflectance spectra from small doped films irradiated for 5, 15, 20, and 25 h followed by thermal curing. Curve a displays the reflectance across the UV-visible spectrum for the 5 h irradiated, slightly mirrored sample. The spectrum of the 15 h irradiated blue-green sample is presented in curve b and exhibits two reflectance maxima that are associated with the colors orange and magenta.24 Although this 15 h irradiated small film exhibits an overall bluegreen color, closer inspection of the sample revealed the presence of numerous small multicolored domains, of which, the orange and magenta dominate in the optical reflectance spectrum. Curves c and d correspond to films irradiated for 20 and 25 h, respectively, which exhibit features typical of FabryPerot filters (or etalons),25 such as the constant frequency difference between crests in the spectra (∆ν ) 7.2 × 1013 and 8.7 × 1013 s-1, respectively) arising from multiple internal reflections.23 Etalons consist of two partially transmitting parallel mirrors of similar thickness separated by a fixed distance. As shown in
17382 J. Phys. Chem. B, Vol. 108, No. 45, 2004
Figure 5. Load versus time curves for the 25 h irradiated, thermally cured PI film doped with Pd2+ ions under three different initial load forces (25, 45, and 75 g), and a control 25 h irradiated, nondoped PI film. The time profiles include responses determined before, during, and after illumination of the samples with white light.
Figure 4b, such an arrangement is achieved after extensive irradiation and subsequent thermal curing of Pd2+ doped polyamic acid films (the 20 h sample has two 28 nm thick continuous metal layers). Included in the inset of Figure 4 are spectra of a nonirradiated film free of Pd (e), and of the 25 h photolyzed sample when measured from the brown substrate side of the film (f). Similar spectra were acquired for all doped, irradiated, and cured polymers when the measurements were performed on the side originally in contact with the substrate. The different optical behavior between colored and substrate sides is a consequence of the anisotropic distribution of Pd metal in the cured PI films. The reflectivity is low in curve (f) because most of the light is absorbed by Pd crystallites present in the bulk film region (several microns thick) existing beneath the continuous Pd metal layers. The most striking property of these etalon-type PI films is their ability to transform electromagnetic radiation into mechanical energy. For instance, a vertically placed film is deflected by 90° upon exposure to intense white light. This lightinduced motion is completely reversible and reproducible. Presented as Supporting Information are images illustrating the motions experienced by a film folded into an “L-shape” under exposure to light. Qualitative tests indicate that the optically activated motion is induced by near-UV and visible light, and that the magnitude of the response is a function of the light intensity. Experiments aiming at evaluating this phenomenon included subjecting the films to a force while simultaneously exposing them to white light. Displayed in Figure 5 are load versus time curves for a Pd2+-doped PI film, which was irradiated for 25 h prior to thermal cure, when subjected to several initial forces in the presence and subsequent absence of a white light source. Included in Figure 5 is a curve of an analogous experiment performed using an undoped, nonirradiated PI blank. The response of the blank is similar to but less pronounced than that of PI films that were metallized only on the polymer surface.26 Initially, the PI blank elongates slowly under the constant applied force and undergoes a small further elongation when exposed to white light due to a temperature increase. The heating effect of light ceases once illumination is stopped and the load increases as the polymer contracts. In contrast, a large (∼8 g) decrease (elongation) in the force acting on the irradiated doped PI film is observed during the
Gaddy et al. initial exposure to white light. The decrease in load is faster and more pronounced as compared to the changes observed with the undoped PI and the surface-metallized polymers.26 Additional unique changes occur under continuous illumination; a few seconds after the initial elongation the force exerted by the 25 h irradiated sample increases (signifying film contraction) until a constant value above the initial load is reached after ∼5 min of illumination. Another rapid increase of the force occurs when illumination is stopped, which is followed by a gradual decrease to the initial load value. The same response is observed irrespective of illumination direction (multicolored side or substrate side). According to the data of Figure 5, the response of the etalon-type PI film to light is independent of the initial applied load (25, 45, or 75 g). The surface temperature of the sample reaches ∼80 °C during illumination, and bringing the film in close proximity to a hot surface induces a motion similar to that resulting from exposure to white light. On the other hand, all the PI films experience small, continuous elongations with increasing temperature during thermo-mechanical experiments. Thus, the simplest explanation for the macroscopic motions is that the films behave as bimorphs in which thermal changes of the two metal layers occur at different speeds. If this rationalization is correct, then the films possess an unusual ability of transforming efficiently light into thermal energy. Earlier studies have suggested that metal generated during thermal curing of polyamic acid films doped with metal complexes migrates to the film surface forming a top metallic layer, but the mechanism of this process is not yet known.7,8 The results presented in this study demonstrate that photochemical treatments can induce profound effects on the self-metallization of polyimides. Photoreduction of the Pd2+ ions seems a logical explanation for the effect of the photochemical step on the generation of metal layers, but no Pd particles form when DMAc solutions of Pd[S(CH3)2]2Cl2 are irradiated. Another possibility involves the benzophenone (BP) groups present in BTDA, which generate benzophenyl ketyl (BPK) radicals upon illumination in the presence of hydrogen-atom donors.27 In fact, PdCl42- ions are reduced to metal particles by BPK radicals generated via irradiation with 350 nm photons of polymeric films containing BP groups, such as sulfonated poly(ether etherketone).16a However, no metal particles or layers were detected in TEM images from doped films of the BTDA/ODA polyamic acid that were exposed to 350 nm photons but not cured. This means that reduction of the Pd2+ ions occurs mainly while the polyamic acid is curing. BTDA-based polyimides are known to photocrosslink as a result of reactions of the BPK radicals.27 Crosslinking is not possible in the absence of H-atom donors, but solvent molecules (such as N,N′-dimethylformamide, DMF) entrapped in the polymer films can fulfill that role.28 Although ODA is unsuitable as a donor of hydrogen atoms, DMAc molecules present in the BTDA/ODA polyamic acid films could serve as crosslinking agents in way analogous to DMF. Photocrosslinking of our polyamic acid films offers a reasonable explanation for the rather long illumination times required to produce materials with different properties because the quantum yield of the solventinduced reaction is very low.28 Formation of metal in the polyimide films probably involves diffusion of Pd atoms during the curing step, but diffusional processes are anticipated to be restricted in the crosslinked polymer regions. The mobility of palladium atoms will decrease when they reach the boundary between crosslinked and non-crosslinked regions, which favors metal nucleation and formation of internal layers. Because crosslinking proceeds from the surface to the interior of the
Generation of Pd Crystallites in Polyimide Films films, the resulting inward movement of the boundary with increasing illumination time lengthens the distance between polymer surface and internal Pd layer. Photocrosslinking of the polyamic acid also explains the differences between the Pd crystallites generated between surface and internal metal layers and those formed in the film bulk. The former are produced within the region where the polymer is crosslinked, which results in smaller and more abundant crystallites as compared with the larger particles found in film regions free of crosslinking. An important result of the present investigation is that incoherent light can be used to control the thickness of both internal and surface metal layers, the interlayer distance and the diameters of the generated Pd crystallites. As a result, PI films with a variety of internal metallic structures and a range of properties are accessible by combining photochemical and thermal processes. The characteristics of the polymeric etalons are particularly intriguing because Fabry-Perot resonators are of interest as natural laser cavities. The novel photomechanical properties of these films are currently under further investigation. Acknowledgment. We are grateful to D. M. Stoakley, G. A. Miner, and L. Forbes for their help and encouragement through many stages of this work, and to R. Blumenthal for helpful discussions. Work at NASA was supported by the Advanced Aircraft Program and by the DARPA sponsored MetaMaterials program, whereas work at Auburn was partially supported by DOC/NTC. Supporting Information Available: Images of polymer film motion upon exposure to white light. This material is available free of charge via the web at http://pubs.acs.org. References and Notes (1) (a) Mittal, K. L., Susko, J. R., Eds. Metallized Plastics 1; Plenum Press: New York, 1989. (b) Mittal, K. L., Ed. Metallized Plastics 2; Plenum Press: New York, 1991. (2) Feger, C.; Franke, H. In Polyimides Fundamentals and Applications; Gosh, M. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1996; p 759. (3) Mallory, G. O., Hajdu, J. B., Eds. Electroless Plating; AESF: Orlando, 1990.
J. Phys. Chem. B, Vol. 108, No. 45, 2004 17383 (4) Southward, R. E.; Stoakley, D. M. Prog. Org. Coatings 2001, 4, 99. (5) Mazur, S.; Manring, L. E.; Levy, M.; Dee, G. T.; Reich, S.; Jackson, C. E. In Metallized Plastics 1; Mittal, K. L., Susko, J. R., Eds.; Plenum Press: New York, 1989; p 115. (6) Krause, L. J.; Speckhard, T. A. In Metallized Plastics 2; Mittal, K. L., Ed.; Plenum Press: New York, 1991; p 3. (7) (a) Rubira, A. F.; Rancourt, J. D.; Taylor, L. T.; Stoakley, D. M.; St. Clair, A. K. J. Macromol. Sci.sPure Appl. Chem., A 1998, 35, 621. (b) Caplan, M. L.; Stoakley, D. M.; St. Clair, A. K. J. Appl. Polym. Sci. 1995, 56, 995. (c) Caplan, M. L.; Stoakley, D. M.; St. Clair, A. K. Polym. Mater. Sci. Eng. 1993, 69, 400. (d) Carver, J. C., Taylor, L. T., Furtsch, T. A., St. Clair, A. K. J. Am. Chem. Soc. 1980, 102, 876. (e) Furstch, T. A.; Taylor, L. T.; Fritz, T. W.; Fortner, G.; Khor, E. J. Polym. Sci.: Polym. Chem. Ed. 1982, 20, 1287. (8) Southward, R. E.; Thompson, D. S.; Thompson, D. W. AdV. Mater. 1999, 11, 1043. (9) Feldheim, D. L., Foss Jr., C. A., Eds. Metal Nanoparticles Synthesis, Characterization and Applications; Marcel Dekker: New York, 2002. (10) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (11) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362. (12) Link, S.; El-Sayed M. A. J. Phys. Chem. B 1999, 103, 8410. (13) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, 1995. (14) Caseri, W. Macromol. Rapid Commun. 2000, 21, 705. (15) (a) Belloni, J. In Homogeneous Photocatalysis; Chanon, M., Ed.; John Wiley: New York, 1997; p 169. (b) Sahyun, M. R. V. J. Imaging Sci. Technol. 1998, 42, 23. (16) (a) Korchev, A. S.; Bozak, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10. (b) Malone, K.; Weaver, S.; Taylor, D.; Cheng, H.; Sarathy, K. P.; Mills, G. J. Phys. Chem. B 2002, 106, 7422. (c) Weaver, S.; Taylor, D.; Gale, W.; Mills, G. Langmuir 1996, 12, 4618. (17) Gaddy, G. A.; McLain, J. L.; Steigerwalt, E. S.; Broughton, R.; Slaten, B. L.; Mills, G. J. Cluster Sci. 2001, 12, 457. (18) Boitsova, T. B.; Gorbunova, V. V.; Volkova, E. I. Russ. J. Gen. Chem. 2002, 72, 642. (19) Kauffman, G.; Cowan, D. Inorg. Synth. 1960, 6, 211 (20) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (21) de Witte, A. J. Am. J. Phys. 1967, 35, 301. (22) Peachey, L. D. J. Biophys. Biochem. Cytol. 1958, 4, 233. (23) Hecht, E. Optics, 2nd ed.; Addison-Wesley: Menlo Park, CA, 1988; Chapter 9. (24) Overheim, R. D.; Wagner, D. L. Light and Color; John Wiley: New York, 1982; p 42. (25) Rancourt, J. D. Optical Thin Films; MacmillanPublishing: New York, 1987; p 104. (26) Sarbolouki, M. N.; Fedors, R. F. J. Polym. Sci.: Polym. Lett. Ed. 1979, 17, 629. (27) Hasegawa, H.; Horie, K. Prog. Polym. Sci. 2001, 26, 259. (28) Higushi, H.; Yamashita, T.; Horie, K.; Mita, I. Chem. Mater. 1991, 3, 188.
