Palladium−Polyimide Nanocomposite Membranes: Synthesis and

The films were characterized by both specular and diffuse reflectivity and by ... to the parent polyimide although the composite membranes are still u...
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Chem. Mater. 2005, 17, 2091-2100

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Palladium-Polyimide Nanocomposite Membranes: Synthesis and Characterization of Reflective and Electrically Conductive Surface-Metallized Films B. L. French,† Luke M. Davis,† E. S. Munzinger,† J. W. J. Slavin,† P. C. Christy,† D. W. Thompson,*,† and R. E. Southward*,‡ Departments of Chemistry and Applied Science, College of William and Mary, Williamsburg, Virginia 23185, and AdVanced Materials and Processing Branch, NASA, Langley Research Center, Hampton, Virginia 23681 ReceiVed October 25, 2004. ReVised Manuscript ReceiVed January 27, 2005

Palladium-surface-metallized polyimide films were prepared by an unusual macromolecular-matrixmediated, single-stage synthetic protocol first reported by Taylor et al. (J. Am. Chem. Soc. 1980, 102, 876). Several Pd(II) complexes, [PdCl2(SMe2)2], [PdBr2(SMe2)2], and Pd(CF3COO)2, were dissolved in the poly(amic acid)s of 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride (BTDA)/4,4′-oxydianiline (4,4′-ODA) and 3,3′-diaminobenzophenone (BPDA)/4,4′-ODA in the solvent dimethylacetamide. Films cast onto glass plates from these Pd(II)-doped resins were thermally cured to 300 °C in air, which resulted in air-side surface-metallized membranes. The films were characterized by both specular and diffuse reflectivity and by conductivity measurements as a function of the cure time and temperature. Maximum specular reflectivities of ca. 50% were observed at 530 nm. After maximum specular reflectivities were achieved in the thermal cure cycle at 300 °C from 0.5 to 2 h, the film surface quickly degraded upon further curing. The surface sheet resistivities were in the range of ca. 1-75 Ω/square. Initial Pd metal particles formed in the film were in the 3-10 nm range. The optical data coupled SEM micrographs, elemental analytical results, and X-ray diffraction observations were interpreted to support a mechanism for the formation of a reflective and conductive metallized surface which involves selective air-side surface oxidative degradation of the polyimide matrix to volatile products, which then concentrates the metal nanoparticles at the surface and increases the particle size via sintering. The mechanical properties of the metallized films are compromised with respect to the parent polyimide although the composite membranes are still useful for a range of applications. The tensile strength is reduced; the percent elongation and the tensile modulus are increased. Both BTDA/ODA and BPDA/ODA gave very similar results with the Pd(II) additives. Many Pd(II) compounds, e.g., PdBr2, were not useful in generating a metallized surface, which suggests that the pathway to a metallized surface is extremely sensitive to the experimental conditions.

Introduction In 1980 Taylor and co-workers1 reported a fascinating and seminal macromolecular-matrix-mediated, single-stage synthesis of surface-metallized palladium-polyimide films where metallized surfaces evolve from palladium nanoparticles initially formed within the polymer matrix. A portion of the initially formed nanometer-sized palladium particles always remain uniformily dispersed throughout the bulk of the film. Specifically, a four-coordinate palladium(II) complex, trans-dichlorobis(dimethyl sulfide)palladium(II),2-4 was dissolved with the poly(amic acid) derived from 3,3′,4,4′benzophenonetetracarboxylic acid dianhydride and 4,4′* To whom correspondence should be addressed. † College of William and Mary. ‡ NASA.

(1) St. Clair, A. K.; Carver, V. C.; Taylor, L. T.; Furtsch, T. A. J. Am. Chem. Soc. 1980, 102, 876-878. (2) Byers, P. K.; Canty, A. J.; Jin, H.; Kruis, D.; Markies, B. A.; Boersma, J.; Van Koten, G. Inorg. Synth. 1998, 32, 162-172. (3) Uson, R.; Fornies, J.; Martinez, F.; Tomas, M. J. Chem. Soc., Dalton Trans. 1980, 888-894. (4) Tranquille, M.; Forel, M. T. Spectrochim. Acta, Part A 1972, 28, 1305-1320.

oxydianiline or 3,3′-diaminobenzophenone, BTDA/4,4′-ODA or BTDA/DABP, in the solvent dimethylacetamide, DMAc. The palladium concentration was relatively low at ca. 5 wt % (0.6 vol %) of the final metal-polyimide system. Heating the palladium(II)-doped poly(amic acid) films cast on glass plates to 300 °C for 1 h gave imidized films with palladium(0) emerging from within the curing film to give a metallized air-side surface. The film side facing the plate did not metallize. Additional palladium(II) compounds were examined which were much less, or often not at all, effective in giving a metallized surface. The development of a metallized film surface from atom/ cluster migration to the surface within an organic macromolecular matrix is unexpected since metal atoms and nanoclusters produced in the thermal cycle must remain within the bulk of the polymer. That is, the high surface free energy of metal particles relative to organic materials (ca. 10:1) thermodynamically precludes a metal surface developing via atom/cluster migration from the bulk to the surface of the polymer film. This has been fully demonstrated by both the experimental and theoretical work of Faupel et al.5

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and Kovacs and Vincett.6-10 Thus, an alternative mechanism to metal atom/cluster migration coupled with aggregation mechanisms (e.g., sintering and ripening) must be operating to give the surface-metallized hybrid film. Such an alternative pathway will be described on the basis of the experimental results reported herein. The Taylor et al. films exhibited surface electrical conductivity (not quantitatively well-characterized) with excellent metal-polymer interfacial adhesion.1,11-14 The film bulk contained isolated palladium nanoparticles but was not conductive. Herein we present studies with palladium(II) complexes in poly(amic acid)s which elaborate the inceptive studies of the Taylor group. First, we report a facile route to preparing the dichlorobis(dialkyl sulfide)-polyimide films without having to isolate a solid palladium(II) complex, and we describe film syntheses with other palladium(II) complexes. Second, experimental data elucidating the mechanism of surface metallization are presented. Finally, characterization data extending the initial work of Taylor et al. are described including specular and diffuse reflectivity, electrical conductivity, scanning electron microscopy, X-ray diffraction, and mechanical measurements. There are several potential uses of palladium-metallized films. First, since the initial work of Taylor et al., there has been substantial interest in the use of palladium seed layers on polyimide surfaces to anchor a second metal to the film surface for circuitry applications, particularly metals such as copper and silver, which do not adhere well to polyimides.15-19 Second, there is general interest in flexible and patterned conductive and reflective hybrid films, and palladium is highly conductive and has excellent reflectivity at longer electromagnetic radiation wavelengths. For example, Seita et al.20 have photochemically developed pal(5) Faupel, F.; Willecke, R.; Thran, A. Mater. Sci. Eng., Rep. 1998, R22, 1-55. (6) Kovacs, G. J.; Vincett, P. S. J. Colloid Interface Sci. 1982, 90, 335351. (7) Kovacs, G. J.; Vincett, P. S.; Tremblay, C.; Pundsack, A. L. Thin Solid Films 1983, 101, 21-40. (8) Kovacs, G. J.; Vincett, P. S. Thin Solid Films 1983, 100, 341-353. (9) Kovacs, G. J.; Vincett, P. S. Thin Solid Films 1984, 111, 65-81. (10) Kovacs, G. J.; Vincett, P. S. Can. J. Chem. 1985, 63, 196-203. (11) Taylor, L. T.; St. Clair, A. K.; Carver, V. C.; Furtsch, T. A. Electrically conductive palladium-containing polyimide films. U.S. Patent 4,311,615, 1982. (12) Taylor, L. T.; St. Clair, A. K. Mechanical and spectroscopic properties of metal-containing polyimides; NASA Technical Memo-540891; Langley Research Center, NASA: Hampton, VA, 1983. (13) Wohlford, T. L.; Schaff, J.; Taylor, L. T.; St. Clair, A. K.; Furtsch, T. A.; Khor, E. Polym. Sci. Technol. 1981, 15, 7-22. (14) (a) Furstch, T. A.; Taylor, L. T.; Fritz, T. W.; Fortner, G.; Khor, E. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1287-1298. (b) An unreviewed preprint mentions the same film as being “approximately 0.1 µm-thick (i.e., 100 nm) ...”: Stoakley, D. M.; St. Clair, A. K. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1996, 37, 541542. (15) Viehbeck, A.; Kovac, C. A.; Buchwalter, S. L.; Goldberg, M. J.; Tisdale, S. L. Redox seeding and electroless metallization of polyimide. ACS Symp. Ser. 1990, 440 (Metallization of Polymers), 394-414. (16) Lee, K. W.; Viehbeck, A. IBM J. Res. DeV. 1994, 38, 457-474. (17) Menezes, M. E.; Birnbaum, H. K.; Robertson, I. M. Embedded cluster metal-polymeric micro interface for electronic applications. U.S. Patent 6,342,307, 2002. (18) Tokas, E. F.; Shaltout, R. M.; Chen, K. S. Formation of noble metalcontaining polyimide films for catalyzing electroless metal coatings. U.S. Patent 5,348,574, 1994. (19) Menezes, M.; Robertson, I. M.; Birnbaum, H. K. J. Mater. Res. 1999, 14, 4025-4034.

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ladium circuit patterns on surface-modified polyimide films. Finally, there is continued interest in palladium-filled polymeric membranes21-26 and carbon molecular sieve membranes for hydrogen separation27 and catalytic hydrogenation reactions.28 Experimental Section Materials. 4,4′-ODA, BTDA, and BPDA were obtained from Wakayama via Chriskev Chemical Co. The 4,4′-ODA was used as received. BTDA and BPDA were dried under vacuum at 150 °C for 10 h prior to use. The chloride, bromide, and acetate salts of palladium(II), dimethyl sulfide, trifluoroacetic acid, and dimethylacetamide (anhydrous) were purchased from Aldrich and used as received. The poly(amic acid) solutions were prepared with a 0.5% offset of dianhydride at 15% solids (w/w) in DMAc. The resins were stirred for at least 5 h before use and were made fresh for each metallized film preparation. The inherent viscosities were in the 0.9-1.5 dL/g range at 35° C. (We have previously shown that the resin viscosity does not affect the in situ metallization of silvered polyimide films.29-32) [PdCl2(SMe2)2] was prepared by a literature method.2,13 Preparation of BTDA/4,4′-ODA and BPDA/4,4′-ODA Palladium-Metallized Films. Procedure Used To Prepare BTDA/4,4′ODA Films Using [PdCl2(SMe2)2]. The neat Pd(II) complex (0.217 g) was dissolved in 1 g of DMAc in a small vial. The Pd(II) solution was transferred (with one rinse of the vial with 0.5 g of additional DMAc) to 10.00 g of the poly(amic acid) solution prepared at 15% solids (w/w) from the two monomers by the standard protocol of dissolving at room temperature the diamine first followed by addition of the dianhydride. (All undoped initial poly(amic acid) solutions were first stirred for g5 h before proceeding further.) The resulting Pd(II)-poly(amic acid) solution was stirred for 2 h to ensure a homogeneity. Doped poly(amic acid) solutions were cast as films onto soda lime glass plates using a doctor blade set at 600 µm to obtain ca. 25 µm thick films after thermal curing. Films used for the reflectivity versus time/temperature measurements were cast on 27 × 46 mm petrographic slides. After remaining in an atmosphere of slowly flowing dry air (4% relative humidity) for 18 h, the films were thermally cured in a forced air oven with the intake vent of a GS Blue M oven closed and exit vent opened slightly to 15% of the maximum. The cure cycle involved heating to 135 °C over 20 min and holding for 1 h, heating to 300 °C over 4 h, and holding at 300 °C for varying times. Procedure To Prepare Metallized Films without Isolation of the [PdX2(SMe2)2] Complex, X ) Cl, Br. PdX2 (0.127 g of PdCl2 or (20) Seita, M.; Nawafune, H.; Nishioka, T.; Mizumoto, S.; Kanai, T. J. Appl. Electrochem. 2002, 32, 349-352. (21) Troger, L.; Hunnefeld, H.; Nunes, S.; Oehring, M.; Fritsch, D. Z. Phys. D: At., Mol. Clusters 1997, 40, 81-83. (22) Troger, L.; Nunes, S.; Oehring, M.; Hunnefeld, H.; Fritsch, D. J. Phys. IV 1997, 7, 875-877. (23) Mercea, P. V.; Mecea, V. Gas Sep. Purif. 1991, 5, 267-272. (24) Mercea, P. V.; Silipas, D.; Mecea, V. Gas Sep. Purif. 1990, 4, 137140. (25) Fritsch, D.; Peinemann, K.-V. Catal. Today 1995, 25, 277-283. (26) Baker, R. W.; Louie, J.; Pfromm, P. H.; Wijmans, J. G. Ultrathin metal composite membranes for gas separation. U.S. Patent 4,857,080, 1989. (27) Yoda, S.; Hasegawa, A.; Suda, H.; Uchimaru, Y.; Haraya, K.; Tsuji, T.; Otake, K. Chem. Mater. 2004, 16, 2363-2368. (28) Ziegler, S.; Theis, J.; Fritsch, D. J. Membr. Sci. 2001, 187, 71-84. (29) Southward, R. E.; Thompson, D. W. Metallized Plast. 2001, 7, 171188. (30) Southward, R. E. Metallized Plast. 2001, 7, 143-169. (31) Southward, R. E.; Thompson, D. W. Mater. Des. 2001, 22, 565576. (32) Southward, R. E.; Stoakley, D. M. Prog. Org. Coat. 2001, 41, 99119.

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Figure 1. Synthetic scheme for palladium metallization of BTDA/4,4′-ODA and BPDA/4,4′-ODA films.

0.191 g of PdBr2 for the BTDA/4,4′-ODA films and 0.110 g of PdCl2 for the BPDA/4,4′-ODA films) followed by SMe2 (0.178 g) was added to 1.0 g of DMAc (molar ratio PdCl2:SMe ) 1:4.) After 5 min a clear red solution of the in situ PdX2(SMe2)2 complex formed. This solution was added via syringe to the poly(amic acid) solution as described in part 1 immediately above. The resulting Pd(II)-poly(amic acid) solution was stirred for 2 h, after which films were cast and treated as in part 1. Procedure To Prepare Films with Palladium(II) Trifluoroacetate Formed in Situ from Palladium(II) Acetate and Trifluoroacetic Acid. Pd(OAc)2 (0.161 g) followed by trifluoroacetic acid (0.252 g) was added to 1.0 g of DMAc, and the resulting mixture was stirred to give a clear solution. This solution was added as above to 10.00 g of the poly(amic acid) solution with stirring. The doped poly(amic acid) solution was treated as above to prepare films. Characterization. Surface resistivities of all films were measured by the four-point probe technique whose calibration was routinely checked with 100 Ω/square indium tin oxide on poly(ethylene terphthalate). TEM was done on a Zeiss CEM-920 instrument. SEM was performed on a Hitachi S-4700 instrument on samples that were coated with ca. 5 nm of palladium/gold alloy when not conductive. Specular reflectivity measurements were made (relative to a Perkin-Elmer polished aluminum mirror with a reflectivity coefficient of 0.92 at 531 nm) with a Perkin-Elmer Lambda 35 UV/vis spectrophotometer equipped with a variable-angle specular reflectance accessory using a wavelength of 531 nm, which is near the solar maximum. Diffuse reflectance measurements were performed with a Perkin-Elmer Lambda 19 equipped with a 150 mm integrating sphere made by Labsphere and standardized with a Spectralon standard. X-ray diffraction was performed with a Philips X’pert Pro MRD diffractometer using Cu KR radiation (λ ) 1.542 Å). A grazing-incidence geometry was used with an incident angle

of 0.30°. The step size was 0.05° for all scans, but the time per step was varied according to the scattering power of each sample such that longer scans were performed on samples with weaker scattering. Background subtraction and subsequent normalization were applied to each data set. Palladium metal analyses were done gravimetrically by heating the polyimide-metal hybrid films to 650 °C in air in a platinum crucible for 30 min. The polymer was lost, leaving only Pd metal. The theoretical percent Pd in this paper is always calculated with respect to polyimide and Pd alone. The ligands of the Pd complex are assumed to be lost. This is consistent with the Taylor et al. work already cited.

Results and Discussion Film Synthesis. The films prepared and characterized in this work are summarized schematically in Figure 1. The metallized films of Figures 2-5 were synthesized by preparing, in one of two ways, a solution of the square planar dichlorobis(dimethyl sulfide)palladium(II), [PdCl2(SMe2)2], and the poly(amic acid) form of BTDA/4,4′-ODA or BPDA/ 4,4′-ODA in DMAc at Pd concentrations of 5.2 and 2.7 wt % (based only on Pd metal and the final imide form of the polymer). [PdCl2(SMe2)2] was prepared as the isolated solid complex by the recipe of Uson et al.3 and Byers et al.2 The structure of the complex is trans from infrared and Raman mutual exclusion data.4 Only the isolated solid complex was used by Taylor and co-workers, and we have prepared selected films for further characterization by their published protocol. However, we found that it is not necessary to isolate the solid Pd(II) complex to form metallized polyimide films.

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Rather, the complex can be prepared in situ without isolation by simply dissolving PdCl2 in a SMe2-DMAc solution. The resulting [PdCl2(SMe2)2] solution is added to a poly(amic acid)-DMAc solution. The doped poly(amic acid) is cast as a film and thermally cured. Preparing the complex from PdCl2 and SMe2 is very convenient and allows the facile preparation of additional dihalobis(dialkyl sulfide)palladium(II) complexes without isolation. Given the extensive recent work on organosulfur “metal-protected clusters”,33,34 it should be of substantial interest to investigate the effects of longer alkyl chain and also ω-functionalized dialkyl sulfides on the Pd particle size and surface aggregation in polyimide films. Palladium(II) trifluoroacetate was prepared without isolation by the reaction of palladium(II) acetate with trifluoroacetic acid in DMAc, which, when combined with BTDA/ 4,4′-ODA poly(amic acid), could be thermally cured to surface-metallized films with useful properties as demonstrated by the reflectivity and conductivity data of Figure 6. We also prepared dibromobis(dimethyl sulfide)palladium(II). Surprisingly, this dibromo analogue did not result in surface-metallized films; that is, the films were not reflective or conductive even after 5 h at 300 °C. The differing behavior of the chloride and bromide complexes is not yet clear to us. This work and that of others indicate that the formation of the metallized surface is a subtle phenomenon that is strongly dependent the Pd(II) precursor. Indeed, Taylor et al. state that “a large number of additional Pd materials were investigated ... but none had a significant on electrical conductivity except Na2PdCl4 and [PdCl2(SMe2)2]”. Compounds that were not efficacious included PdCl2, PdCl2(NH3)2, K2PdCl4, and bis(acetylacetonato)palladium(II). We found that, in addition to [PdBr2(SMe2)2], bis(hexafluoroacetylacetonato)palladium(II), a common physical vapor deposition for the preparation of thin metal films, tribenzylidenedipalladium(0), tetrakis(triphenylphosphine)palladium(0), and the mixed (dimethyl sulfide)(methyl dodecyl sulfide)palladium(II) complex did not give metallized BTDA/4,4′ODA films of acceptable quality. Often a metallized or partially metallized surface was obtained but with severe surface irregularities and seriously degraded mechanical properties for the composite films. Reflectivity and Conductivity. Neat and Nonisolated [PdCl2(SMe2)2]-BTDA/4,4′-ODA Films. The first objective of this work was to elaborate the studies of the neat/isolated [PdCl2(SMe2)2]-BTDA/4,4′-ODA system at 5.2% Pd first reported by Taylor et al.1 Most particularly, we wanted to examine by reflectivity, conductivity, SEM, and X-ray the evolution of the surface-metallized films as a function of the thermal cure conditions. Figure 2 shows the growth of specular reflectivity as a function of time and temperature. What is evident is that only upon reaching a temperature of 300 °C in an air atmosphere do we begin to see enriched surface Pd metal; that is, the specular reflectivity at 300 °C-0 h jumps suddenly to 30% although the film is not yet conductive. The reflectivity increases to a maximum of ca. (33) Brust, M.; Kiely, C. J. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 202, 175-186. (34) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42-50.

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Figure 2. Resistivity data table and specular reflectivity as a function of time/temperature for films prepared with neat PdCl2(SMe2)2 and BTDA/ 4,4′-ODA in DMAc. Time zero is after the films were heated at 135 °C for 1 h; after being at 135 °C for 1 h, the films were then heated over 4 h to 300 °C, after which the temperature was held constant at 300 °C.

37% in the range 300 °C-0.33 h to 300 °C-1 h. (The expected relative reflectivity for Pd at 531 nm is ca. 55%.) Holding at 300 °C for 2-5 h and longer results in a plummeting of the specular reflectivity. The X-ray diffraction data of Figure 10 show that face-centered cubic Pd is emerging with increasing particle size as a function of time/ temperature. Pd reflections are seen prior to the observation of metallic reflectivity and conductivity, indicating that additional time and temperature are needed after the first indication of a metallic phase in the film to realize a fully metallized surface. No unambiguous evidence was seen for any significant amount of crystalline palladium(II) oxide formation in the films. The films become electrically conductive very soon after the reflectivity becomes prominent, with the films first exhibiting conductivity after being at 300 °C for 30 min. As time at 300 °C proceeds, the sheet resistivity increases modestly while the specular reflectivity decreases dramatically. Films produced at 300 °C for 3 h and thereafter cannot be removed from the glass plates intact; that is, the mechanical properties of the polyimide matrix remaining have been severely compromised (vide infra), and one has an increasingly extensive layer of metallic Pd with polymer degradation residue on the glass casting plate. The glass side of the films of all systems studied never metallized. Figure 3 displays reflectivity curves for the essentially same system as in Figure 2 except that trans-[PdCl2(SMe2)2] was prepared without isolation in DMAc using a 4:1 molar ratio of SMe2 to PdCl2. Adding this complex solution to the poly(amic acid)-DMAc solution gives a homogeneously doped system which then leads to metallized films when cast and cured. The specular reflectivity pattern as a function of time/temperature is similar to that shown in Figure 2 using isolated, neat [PdCl2(SMe2)2]. Most particularly, the maximum reflectivity occurs at 300 °C after 1 h, and the films become conductive within the first hour at 300 °C. The reflectivity up to 300 °C is entirely specular, but after that

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Figure 3. (A) Resistivity data and reflectivity curves as a function of time/temperature for films prepared with nonisolated PdCl2 + 4(SMe2)2 and BTDA/ 4,4′-ODA in DMAc. (B) Resitivity (two-point probe), weight percent Pd metal, and specular reflectivity as a function of time/temperature for a similar (to those of (A)) set of films. Time zero is after the films were heated at 135 °C for 1 h.

Figure 4. Total (/), specular (O), and diffuse (2) reflectivities as a function of time/temperature for 5.2% Pd films formed with BPDA/4,4′-ODA and nonisolated PdCl2(SMe2)2 synthesized from PdCl2 + 4SMe2.

point there is significant diffuse reflectivity which may result from surface unevenness. That the total reflectivity decreases with time at 300 °C may be due to increasing absorption by the decomposing polyimide residue. Nonisolated [PdCl2(SMe2)2]-BPDA/4,4′-ODA Films. Pd films similar to the BTDA/4,4′-ODA films were prepared with BPDA/4,4′-ODA. Figure 4 displays reflectivity measurements as a function of the thermal cure cycle for [PdCl2(SMe2)2]-BPDA/4,4′-ODA, with the complex prepared without isolation. The trends in reflectivity and conductivity roughly parallel those of the BTDA/4,4′-ODA-palladium systems with the exception that the BPDA/4,4′-ODA metallized films show slightly greater maximum specular reflectivity and a more regular increase in diffuse reflectivity after the films have been at 300 °C for 1 h. Figure 5 shows specular reflectivity data for three independent preparations of 5.2% Pd BPDA/4,4′-ODA films. The observed reflectivity

Figure 5. Variation in reflectivity from run to run as illustrated by three independent preparations on different days of 5.2% Pd films prepared with BPDA/4,4′-ODA and PdCl2(SMe2)2 prepared from PdCl2 + 4SMe2 in DMAc. Time zero is after the films were heated at 135 °C for 1 h.

values range from 37% to 53%, with the maximum reflectivity occurring over a small range of times at 300 °C. Figure 5 demonstrates that it is not possible with the experimental protocol reported here to prepare identical films with respect to reflectivity from run to run. Similar variation can also be seen in the BTDA/4,4′-ODA analogues by comparing the specular reflectivity of the A and B samples in Figure 3. While Rubira et al.35 have suggested that the presence of the carbonyl group is important to the development of a silvered surface in BTDA/4,4′-ODA films, the data here for (35) Rubira, A. F.; Rancourt, J. D.; Taylor, L. T.; Stoakley, D. M.; St. Clair, A. K. J. Macromol. Sci., Pure Appl. Chem. 1998, A35, 621636.

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Figure 6. Specular reflectivity curves as a function of time/temperature for 2.7 (/) and 5.2% (O) films prepared with BPDA/4,4′-ODA and PdCl2(SMe2)2 synthesized from PdCl2 + 4SMe2 in DMAc. Time zero is after the films were heated at 135 °C for 1 h.

Pd with BPDA/4,4′-ODA do not reflect this idea. Also, our recent work36 with silvered BPDA/4,4′-ODA films does not suggest that there is anything singularly significant about the presence of the carbonyl group with regard to the development of a metallized surface. Figure 6 also shows the development of reflectivity for a 2.7% Pd film. Here the maximum reflectivity for two independent preparations (only one is shown) was 36%, lower than 53% for the 5.2% film. Consistent with this observation, we have observed that in the metallization of BTDA/4,4′-ODA with silver(I) compounds, the reflectivities at less than 4% Ag are lower than those in the range of 6-13%.37 Nonisolated Pd(CF3CO)2-BPDA/4,4′-ODA Films. Figure 7 shows reflectivity data for BTDA/4,4′-ODA metallized films prepared with palladium(II) trifluoroacetate prepared without isolation from palladium(II) acetate and SMe2 in DMAc. The specular reflectivity, measured directly and by the difference between the total and diffuse reflectivities, is greater than 50%. In contrast to the [PdCl2(SMe2)2]-BTDA/ 4,4′-ODA and -BPDA/4,4′-ODA systems, the trifluoroacetate complex achieves a maximum specular reflectivity approximately 1 h earlier in the cure cycle, i.e., at 300 °C-0 h. The reflectivity is virtually all specular to 300 °C. Thereafter, the specular reflectivity disappears catastrophically accompanied by a modest increase in the diffuse reflectivity. Thus, there is serious disruption of the quality of the reflecting surface with time at 300 °C. The first observation of conductivity coincides with the observation of the rapid increase to maximum reflectivity at 300 °C-0 h. The conductivities are more or less in the same range as for the [PdCl2(SMe2)2] films. (36) Southward, R. E.; Thompson, D. W. Chem. Mater. 2004, 16, 12771284. (37) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Caplan, M. L.; St. Clair, A. K. Chem. Mater. 1995, 7, 2171-2180.

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There are not great differences in the reflectivity and conductivity properties among the Pd(II) complex-polyimide systems ([PdCl2(SMe2)2], [Pd(CF3CO2)2], BTDA/4,4′-ODA, and BPDA/4,4′-ODA combinations) reported here. It is, however, imperative to emphasize that these surface-metallized Pd-polyimide films are difficult to duplicate exactly on demand! We have not found it possible to repeat faithfully the reflectivity data from run to run as we have been able to do with several silver-polyimide systems.36-43 We have found with [PdCl2(SMe2)2]-polyimide films that the reflectivity consistently begins to develop at 300 °C and reaches a maximum value at 300 °C between 0.5 and 2 h. Generally, the maximum specular reflectivity is within the 35-50% range (relative to a front-surfaced Al optical mirror). The reason for the lack of exact reproducibility among films may be due to an inability to precisely control the atmosphere (air flow and humidity) in the commercial forced air oven cited in the Experimental Section. Indeed, Taylor et al. mentioned that moist air was more effective in producing a higher quality Pd surface.44 Also, the mechanism for the development of surface metallization is a solid-state Pdcatalyzed oxidative degradation (vide infra) that may be difficult to duplicate within close tolerances due to lack of exact submicrometer homogeneity at and near the film surface. There is greater consistency in the development and magnitude of surface conductivity, which is less dependent on surface topology than reflectivity and perhaps the polyimide degradation residue. Nature of the Metallized Surface and a Mechanism for Its Development. Figure 3B presents data for a set of [PdCl2 + 4(SMe2)2]-BTDA/4,4′-ODA films at 5.2% Pd prepared via the same approach used for the films in Figure 3A. Figure 3B shows that the maximum specular reflectivity occurs after 2 h at 300 °C. However, significant specular reflectivity, 26%, has developed by 300 °C-0 h, and the percent Pd is the theoretical value of 5.2. The first conductive film appears 60 min later at 300 °C-1 h, with the reflectivity increasing to 35%. Importantly, the first conductive film is realized only when the Pd concentration increases from its theoretical value of 5.2% to 6.2%, indicating that polyimide is being lost to volatile products through oxidative degradation. The increase to 6.2% Pd means that 17% of the polyimide mass has been lost. The maximum observed reflectivity, 40% at 300 °C-2 h, occurs for a film with an increased Pd concentration at 8.1%, which corresponds to a 38% polyimide loss! Taylor et al.13 reported that, for a similar 5.2% Pd film, the metal (38) Warner, J. D.; Pevzner, M.; Dean, C. J.; Kranbuehl, D. E.; Scott, J. L.; Broadwater, S. T.; Thompson, D. W.; Southward, R. E. J. Mater. Chem. 2003, 13, 1847-1852. (39) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Clair, A. K. S. Chem. Mater. 1999, 11, 501-507. (40) Southward, R. E.; Bagdassarian, C. K.; Sudol, C. J.; Wasyk, J. L.; Sproul, S. H.; Broadwater, S. T.; Scott, J. L.; Thompson, D. W. J. Mater. Res. 1999, 14, 2897-2904. (41) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Scott, J. L.; Broadwater, S. T. Mater. Res. Soc. Symp. Proc. 1999, 551 (Materials in SpacesScience, Technology and Exploration), 163-168. (42) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; St. Clair, A. K. Chem. Mater. 1997, 9, 1691-1699. (43) Southward, R. E.; Thompson, D. W.; St. Clair, A. K. Chem. Mater. 1997, 9, 501-510. (44) Taylor, L. T.; Carver, V. C.; Furtsch, T. A.; St. Clair, A. K. Org. Coat. Plast. Chem. 1980, 43, 635-639.

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Figure 7. Resistivity data and reflectivity curves as a function of time/temperature for films prepared with BTDA/4,4′-ODA and Pd(CF3COO)2, the latter being prepared without isolation from Pd(CH3COO)2 + 3CF3COOH in DMAc. Time zero is after the films were heated at 135 °C for 1 h. Key: Total reflectivity (/); diffuse reflectivity ([); specular reflectivity from total reflectivity minus diffuse reflectivity (b); specular reflectivity measured directly (3).

concentration increased to 7.9% after being at 300 °C for 1 h; this requires that 36% of the polyimide must undergo decomposition to volatile products. Indeed, our observed thermal decomposition at 300 °C in air (i.e., an oxygencontaining atmosphere) is not unexpected since Taylor et al.13 previously reported that the “polyimide decomposition temperature (10% weight loss)” for the 5.2% doped film was ca. 320 °C in air. After 4 h at 300 °C Figure 3B shows the Pd is at 72% with 98% of the original polyimide gone. Consequently, the metallized hybrid films lose essentially all mechanical integrity after being heated at 300 °C for more than ca. 2 h. In contrast, when [PdCl2 + 4(SMe2)2]-BTDA/ 4,4′-ODA films were held at 275 °C for up to 5 h, ca. 45 °C below the 10% weight loss temperature of ca. 320 °C, no metallized surface resulted, and there was no polyimide degradation. Tough, creasable films could be removed from the casting plates even after being heated for 5 h at 275 °C. Thus, it is clear that the emergence of a Pd surface proceeds with, and is dependent on, concomitant polyimide degradation, to all or mostly volatile products, that is temperature and atmosphere (oxygen) dependent. The observation that the atmosphere-protected glass side of the films never metallized, with a 1:25 glass-side to air-side surface Pd ratio44 after 300 °C-1 h, is consistent with the necessity of oxygen in the cure atmosphere to effect a Pd surface. Ando and Sawada45 found that curing under nitrogen films of bis(hexafluoroacetylacetonato)palladium in the poly(amic acid) of pyromellitic dianhydride (PMDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) did not give an airside-metallized surface even up to 350 °C although metallic Pd was formed throughout the bulk as determined by X-ray diffraction. While no metallized surface was realized, X-ray diffraction showed that the Pd(II) in the film (heated to 300 °C) was predominately Pd metal with particle sizes from TEM of “1-10 nm”.14b A very small reflection for PdO, (45) Sawada, T.; Ando, S. Chem. Mater. 1998, 10, 3368-3378.

101 planes, was observed. Thus, oxygen in the cure atmosphere is not needed to reduce Pd(II) to Pd metal; that is, the reducing agent must be the Pd(II) ligands or the polymer. Also, Taylor et al. found that films cured in oxygenfree atmospheres such as nitrogen or argon did not develop conductive surfaces; oxygen was said to be essential to the realization of a conductive air-side surface.13 While the specific role of oxygen was not addressed, the implication of the Taylor et al. observations is that the metallized surface is brought forward via oxidative degradation of the surface polyimide. Of course, as mentioned in the Introduction, we knew before any characterization data were collected that the only way to bring the metal to the surface was polyimide ablation since the high surface energy of the metal preludes its migration/phase separation to the air-side surface. The SEM micrographs of Figure 8 for a series of neat [PdCl2(SMe2)2]-BTDA/4,4′-ODA films reveal the pathway to a conductive surface which leaves the bulk of the hybrid material as a dielectric medium with embedded metal nanospheres. At 275 °C the SEM micrograph of the surface is relatively smooth with small (ca. 5-10 nm) isolated Pd particles which are clearly visible at high magnification. (Gaddy et al., in a [PdCl2 + 4(SMe2)2]-BTDA/4,4′-ODA film at 5% Pd which was first photolyzed at 350 nm followed by curing in air to 300 °C-1 h, observed particle sizes in the bulk of the film in the 3-10 nm range as ascertained from the shown TEM micrograph.46) When 300 °C is first reached, one sees Pd “patches” being formed which are not in continuous contact with one another. These “patches” contribute to a developing specular reflectivity which is 30% at 300 °C-0 h with no noticeable polyimide degradation as shown by the analytical data in Figure 3B. Films can be reflective without exhibiting long-range electrical conductivity as we have shown in numerous similar studies with (46) Gaddy, G. A.; Locke, Edward P.; Miller, M. E.; Broughton, R.; Albrecht-Schmitt, T. E.; Mills, G. J. Phys. Chem. B 2004, 108, 1737817383.

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Figure 8. SEM micrographs of a series of neat [PdCl2(SMe2)2]-BTDA/4,4′-ODA films (5.2% Pd) withdrawn from the cure cycle at the indicated temperatures. The thermal cycle was 20 min to 135 °C, from 135 to 300 °C over 4 h, and hold at 300 °C. The scale bar at 1000 nm applies to all figures. The inset in the 275 °C micrograph is at a magnification of 500K×; the scale bar is 10 nm. (These are the same films as in Figure 2.)

silver-polyimide systems. The islandlike “patches” grow in size from 300 °C-0 h to 300 °C-0.33 h, and the reflectivity increases to 37%, but the film remains nonconductive. By 300 °C-0.67 h (micrograph not shown) the film becomes continuously surface conductive. A connected network of Pd has now formed and is illustrated by the micrographs for the 300 °C-1 h and -2 h films. As seen in Figure 3B the development of conductivity correlates with a slow degradation of the polyimide. The surface of the 300 °C-2 h film is more uneven than that at 300 °C-1 h and may account for the decrease in specular reflectivity to 15% coupled with an increase in the diffuse component. There may also be increased absorption from nonvolatile polyimide degradation residue. The Pd surface particle size where spherical or nearspherical particles can be delineated is ca. 50-60 nm. These particles and aggregates of such particles extend with an irregular topology over the film surface. A Pd surface layer thickness of ca. 100 nm for a 5.2% neat [PdCl2(SMe2)2]BTDA/4,4′-ODA film by Auger depth profiling was reported by Taylor et al.14 This is consistent with our SEM data. The SEM surface scans for 5.2% films made from [PdCl2 + 4SMe2] in DMAc, i.e., without isolation of neat [PdCl2(SMe2)2] complex, and BTDA/4,4′-ODA are essentially the same as those of Figure 8. SEM micrographs (not shown) for the 5.2% Pd films with [PdCl2(SMe2)2]-BPDA/4,4′-ODA films as a function of time/temperature are essentially the same as those for the BTDA/4,4′-ODA analogue. Surface micrographs for selected 5.2% Pd films with Pd(CF3CO2)2-BTDA/4,4′-ODA films, which develop good

specular reflectivity 1 h sooner than the [PdCl2(SMe2)2] analogues, are shown in Figure 9. They again reveal that at 275 °C only very small isolated Pd particles (ca. 5-10 nm range) are formed. Increasing the temperature to 300 °C-0 h increases the particle size to give a continuous conductive network. It is simply not clear from the micrographs why the specular reflectivity essentially vanishes over the next 60 min at 300 °C. There is again some increase is the diffuse reflectivity as the specular reflectivity diminishes. Mechanical Data. Table 1 displays mechanical data for a 5.2% film heated at 100, 200, and 300 °C for 1 h, which is the Taylor et al. thermal protocol. The tensile strength of this film is much less than that of the parent polyimide with a much greater standard deviation. This is not surprising for a film heated to 300 °C-1 h (Figure 3B). Such a film suffers significant polyimide mass loss (ca. 17% in our work), and it is likely that some of the remaining polyimide is in a partially damaged, but still useful, state. Taylor et al. observed an even greater polyimide degradation of 36% after 1 h at 300 °C as cited above; they did not report mechanical measurements on their 5.2% BTDA/4,4′-ODA metallized films. Film (i.e., polyimide) damage/degradation may result because of a small amount of oxygen diffusing into the bulk of the polyimide and/or because the curing poly(amic acid) is participating as the reducing agent, i.e., providing electrons, toward Pd(II) in the complex. (We have found strong evidence in analogous silver(I)-polyimide systems that the reducing agent is the curing polymer itself.) While the Pdmetallized films appear uniform to the eye, the lowering of

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Figure 9. SEM micrographs as a function of time/temperature for films prepared with BTDA/4,4′-ODA and Pd(CF3COO)2, the latter being prepared without isolation from Pd(CH3COO)2 + 3CF3COOH in DMAc. The 500 nm scale bar applies to all figures. The inset in the 275 °C micrograph is at a magnification of 500K×; the scale bar is 10 nm. (These are the same films as in Figure 7.) Table 1. Selected Mechanical and Thermal Data for a [PdCl2 + 4SMe2]-BTDA/4,4′-ODA Film at 5.2% Pda

sample

Tg(DSC) (°C)

CTE (ppm/K)

parent filmb 5.2% Pd film 13% Agd film

275 >300 276

43 not measured 33

tensile strengthc modulusc (MPa) elongationc (Gpa) (at 25 °C) (25 °C) (%) 145 ( 7 84 ( 14 136 ( 5

8.3 ( 0.5 3.1 ( 0.2 48 ( 40 4.0 ( 0.8 10 ( 0.4 3.1 ( 0.15

a Cure cycle: held at 100, 200, and 300 °C for 1 h each with 30 min ramp times between temperatures. b Data from R. E. Southward et al.42 c The film thickness was ca. 40 µm with measurements at 25 °C. d Data from R. E. Southward et al.39

the break strength with a larger standard deviation implies that on molecular and microscopic levels the films are nonuniformly compromised relative to the parent polyimide structure. The large increase and range in the percent elongation is particularly interesting. Elongations greater than 100% were observed. We have never observed elongation for the parent BTDA/4,4′-ODA greater than ca. 10% (Table 1). When the mechanical test strips were examined after analysis, it was observed that the metal at the air-side surface had separated and the stretched polyimide between the separated metal junction was yellow in color, appearing quite similar to the undoped parent BTDA/4,4′-ODA polyimide and consistent with very small nanometer-sized particles in the bulk. We have never observed such behavior in the case of surface conductive silver-metallized films reported earlier by us (Table 1) even at concentrations as high as 13%, that is, much higher than the 5.2% Pd films of this study. The

modulus (initial slope) increases, which is very common for inorganic additives in polyimides. We tentatively suggest that the increase in percent elongation may be due to very low density cross-linking that occurs during the reduction of Pd(II) to Pd(0). This two-electron reduction may well give rise to radicals which can then couple to give a lightly crosslinked system, giving an enhanced effective chain length which favors greater elongations before break. X-ray Diffraction. Figure 10, left, depicts the diffraction patterns of the 250-300 °C-0.67 h samples over a limited range of 2θ. Additionally, diffraction data obtained from a 1 mm thick rolled Pd sheet are plotted for reference. As the temperature is increased to 300 °C, there is a clear evolution of a face-centered cubic Pd phase, as evidenced by the Pd(111) and Pd(200) reflections. This phase evolves further with increasing cure time at 300 °C. Another trend evident in these data is the decreasing peak breadth with increasing temperature and time. The decrease in peak breadth is likely a result of an increase in the average Pd crystallite (or grain) size. Exact quantification of size is difficult due to the effects of the X-ray optics employed, but a qualitative comparison between scans is valid. The narrowest Pd(111) peak breadth, in sample 300°C-2/3 h, is still 40% greater than that of the reference specimen, indicating a substantially smaller grain size than that present in a rolled Pd sheet. Additionally, there is a steady shift of Pd reflections to higher 2θ with increasing temperature and time, which results

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Figure 10. (Left) X-ray patterns for emerging palladium metal in [PdCl2(SMe2)2]-BTDA/ODA films as a function of time/temperature. The reference is rolled Pd metal. (Right) Diffraction pattern for the [PdCl2(SMe2)2]-BTDA/ODA film cured at 300 °C for 2/3 h.

from a decrease in plane spacing. Such a decrease can result from a change in the average composition of the Pd, or the presence of strain. The former possibility is doubtful, since the shape of the Pd reflections continues to approach that of the reference sample with increasing temperature and time. Additionally, the relative intensity of these reflections increases to the point at which no other phases can be observed in the view of Figure 10, right. This leaves the presence of strain as the probable cause of the observed peak shift. One possible origin of this strain is the coefficient of thermal expansion mismatch between the Pd phase and the surrounding polymer (RPd ) 11.8 ppm °C-1 and Rpolymer ≈ 42 ppm °C-1). With this mismatch, upon cooling from an elevated temperature, a large compressive strain would be introduced to the Pd phase, consistent with the decrease in plane spacing. From the Pd(111) peak position in Figure 10, left, the 300 °C-2/3 h sample possesses a strain of -0.45% relative to the reference specimen. This is a rough estimate derived from a single peak; a more thorough X-ray diffraction investigation would need to be carried out to determine the strain tensor of the Pd phase with any degree of certainty. Conclusion The conclusion to be drawn from the collage of data in this paper and in the work of others is that Pd in the 2+ oxidation state begins to undergo reduction to Pd metal in the vicinity of 200 °C as shown by X-ray photoelectron spectra.1 X-ray diffraction data (Figure 10, left) indicate Pd metal is present by 250 °C, with the reflections substantially broadened due to the very small particle size. This is

consistent with the SEM data taken at 275 °C referred to above (Figure 8). These nanometer-sized Pd particles then catalyze oxidative degradation of surface/near-surface polyimide beginning near 300 °C to volatile products and grow into larger particles via sintering and aggregation to form a network. The film surface does not have a continuous layer of metal in the manner that occurs with physical vapor deposition; that is, the surface has nanoporous voids of irregular and random shapes. Only a relatively narrow processing window exists in the thermal cure protocol within which to generate the optimum film with regard to specular reflectivity, conductivity, and mechanical integrity. One approach to improving the mechanical properties of surfacemetallized Pd films would be to cast thin Pd(II)-doped poly(amic acid) films on top of thicker parent polyimide films. We have found this useful with silver-BTDA/4,4′-ODA composite films.42 Recently, Gaddy et al. reported an extraordinary observation of a photochemical-thermal protocol for Pd(II)-poly(amic acid)/polyimide films which led to double metallic Pd layers with a layer separation of ca. 400 nm. Thus, further interesting materials from the Pd(II)polyimide system may yet be uncovered. Acknowledgment. We acknowledge the Jeffress Memorial Trust and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for significant support of this work. D.W.T. expresses gratitude for Professor Brian Holloway’s generosity with regard to use of the scanning electron microscope. CM040378M