Highly Reflective and Conductive Double-Surface-Silvered Polyimide

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Langmuir 2007, 23, 4878-4885

Highly Reflective and Conductive Double-Surface-Silvered Polyimide Films Prepared from Silver Fluoride and BTDA/4,4′-ODA Shengli Qi, Zhanpeng Wu, Dezhen Wu,* Wencai Wang, and Riguang Jin State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China ReceiVed October 7, 2006. In Final Form: NoVember 16, 2006 This work focuses on surface silver metallization on a 3,3′,4,4′-benzophenonetetracarboxylic dianhydride/4,4′oxydianiline (BTDA/ODA)-based polyimide matrix via a direct ion-exchange self-metallization technique using a simple silver salt, silver fluoride, as the silver precursor. The method involves performing an ion-exchange reaction of damp-dry poly(amic acid) films in silver aqueous solution to form silver(I)-containing precursor films. Thermal treatment under tension converts the poly(amic acid) into polyimide and simultaneously reduces the silver(I) to silver(0), yielding silver layers with excellent reflectivity and conductivity on both film sides. However, significant property differences were exhibited on the upside and underside surfaces of the metallized films and this has been discussed in detail. The variation of surface properties and surface morphologies during the thermal curing cycle was also investigated. The mechanical and thermal properties of the metallized polyimide films are essentially similar to those of the host polyimide.

1. Introduction Polyimides have found numerous applications in industries such as aerospace and electronics, where excellent dielectric properties, high-temperature stability, and chemical inertness are required.1-6 However, in certain potential applications that require low electrical resistivity and high reflectivity, polyimides have shortcomings. To address these deficiencies by combing with metal, silver is especially attractive because it has unmatched specular reflectance and the highest electrical conductivity7,8 and the silver(I)-silver couple has a favorable standard electrode potential (E0 ) 0.8V) that allows silver(I) to be readily reduced upon simple thermal treatment.9 The current widespread interest in silvered polyimide films is driven largely by their potential applications in flexible printed circuit boards (FPCB),10 magnetic data storage,11,12 highly active catalysts,13,14 optics,15-17 microelectronics devices,18-20 electromagnetic interference shielding * To whom correspondence should be addressed. Telephone: +86-106442-1693. Fax: +86-10-6442-1693. E-mail: [email protected]. (1) Ranucci, E.; Sandgren, A.; Andronova, N.; Albertsson, A.-C. J. Appl. Polym. Sci. 2001, 82, 1971. (2) Li, Y.; Lu, Q.; Qian, X.; Zhu, Z.; Yin, J. Appl. Surf. Sci. 2004, 233, 299. (3) Alegaokar, P. S.; Bhoraskar, V. N. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 225, 267. (4) Terui, Y.; Matsuda, S.-I.; Ando, S. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 2109. (5) Liaw, D.-J.; Hsu, P.-N.; Chen, W.-H.; Lin, S.-L. Macromolecules 2002, 35, 4669. (6) Huang, X. D.; Bhangale, S. M.; Moran, P. M.; Yakovlev, N. L.; Pan, J. Polym. Int. 2003, 52, 1064. (7) Southward, R. E.; Stoakley, D. M. Prog. Org. Coat. 2001, 41, 99. (8) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc. 1999, 121, 10642. (9) Southward, R. E.; Thompson, D. W. Mater. Des. 2001, 22, 565. (10) Ektessabi, A. M.; Hakamata, S. Thin Solid Films 2000, 377-378, 621. (11) Akamatsu, K.; Ikeda, S.; Nawafune, H. Langmuir 2003, 19, 10366. (12) Akamatsu, K.; Shinkai, H.; Ikeda, S.; Adachi, S.; Nawafune, H.; Tomita, S. J. Am. Chem. Soc. 2005, 127, 7980. (13) Huang, J.-C.; Qian, X.-F.; Yin, J.; Zhu, Z.-K.; Xu, H.-J. Mater. Chem. Phys. 2001, 69, 172. (14) Zhang, F.; Guan, N.; Li, Y.; Zhang, X.; Chen, J.; Zeng, H. Langmuir 2003, 19, 8230. (15) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Deki, S. Chem. Mater. 2003, 15, 2488. (16) Yen, C.-T.; Chen, W.-C. Macromolecules 2003, 36, 3315. (17) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (18) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822.

filters,21 and highly reflective thin film reflectors and as concentrators in space environments for solar thermal propulsion, large-scale radiofrequency antennas for space applications.22,23 In a recent communication,24 we reported the direct ionexchange self-metallization synthesis of double-surface-silvered polyimide films using silver nitrate as the silver precursor and pyromellitic dianhydride/4,4′-oxydianiline (PMDA/ODA)-based polyimide as the matrix. “Direct ion-exchange self-metallization” refers to the development of surface-metallized films from silver(I)-containing polyimide precursor films, which are obtained by directly immerging the damp-dry poly(amic acid) (PAA) films into an aqueous silver(I) solution to perform ion-exchange reactions. Since PAA is thermoplastic and dissociable,19,25,26 immersion into a metallic aqueous solution would result in polycarboxylate groups through dissociation of the carboxylic groups in the PAA molecules. In the presence of a metallic salt, the negatively charged polycarboxylate will couple to the silver cations, forming silver polyamate.19 Subsequent thermal treatment of the silver(I)-PAA films under tension induces silver ion reduction to give the metallized surfaces without the use of a discreet external reducing agent. Thermal curing also gives the imidized polyimide via cycloimidization. During the thermal cycle, metal atoms and small clusters formed in the film aggregate at the surface, giving metallic layers on both film sides. A major goal of this direct ion-exchange self-metallization technique is to produce polyimide films with high reflectivity and conductivity on both sides of the film by using a cheap and simple silver salt as the silver origin. (19) Andreescu, D.; Wanekaya, A. K.; Sadik, O. A.; Wang, J. Langmuir 2005, 21, 6891. (20) Kariuki, N. N.; Luo, J.; Hassan, S. A.; Lim, I.-Im S.; Wang, L.; Zhong, C. J. Chem. Mater. 2006, 18, 123. (21) Tachibana, Y.; Kusunoki, K.; Watanabe, T.; Hashimoto, K.; Ohsaki, H. Thin Solid Films 2003, 442, 212. (22) Ward, L. J.; Schofield, W. C. E.; Badyal, J. P. S. Chem. Mater. 2003, 15, 1466. (23) Southward, R. E.; Thompson, D. W. AdV. Mater. 1999, 11 (12), 1043. (24) Qi, S.-L.; Wu, D.-Z.; Bai, Z.-W.; Wu, Z.-P.; Yang, W.-T.; Jin, R.-G. Macromol. Rapid Commun. 2006, 27, 372. (25) Jeon, J.-Y.; Tak, T.-M. J. Appl. Polym. Sci. 1996, 61, 371. (26) Thomas, R. R. Langmuir 2003, 19, 5763.

10.1021/la062948y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/23/2007

Highly ReflectiVe and ConductiVe Polyimide Films

This direct ion-exchange procedure was developed in our work since conventional methods, such as physical vapor deposition, chemical vapor deposition, electrodeposition, and electroless chemical reduction,22,27 which involve the external deposition of the metallic phase onto the substrate surface, are usually laborintensive and only give composite films with very poorly adhered silver layers.9,28 Another important technique for the synthesis of silver metallized polyimide films is in situ single-stage self-metallization, which was proposed in 1990s and has been steadily developed by Southward,7 Rubira and Taylor,29,30 Warner,31 Matsuda,32 Sawada,33 and our group34,35 due to its processing simplicity and outstanding adhesion at the polymer-metal interface. It refers to the development of a metallized film from a single homogeneous solution that contains both an organometallic silver complex and the desired poly(amic acid) precursor of the polyimide matrix. Thermal treatment of the cast film converts the PAA precursor into the final polyimide form with concomitant silver reduction, yielding a reflective and/or conductive silvered polyimide film. Highly reflective and conductive films have been fabricated using this in situ method. However, success was only achieved on 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride/4,4′oxydianiline (BTDA/ODA)- and 3,3′,4,4′-biphenyltetracarboxylic dianhydride/4,4′-oxydianiline (BPDA/ODA)-based polyimide matrixes36 when utilizing complex silver precursors with bridging β-diketonate ligands such as (1,1,1,5,5,5-hexafluoroacetylacetonato) silver(I) (AgHFA) and (1,1,1-trifluoroacetylacetonato) silver(I) (AgTFA).28,36 And in these cases, silver metallization was only achieved on the air side of the film. Neither reflectivity nor conductivity was observed on the glass side. Furthermore, those silver complexes are very unstable, are required to be freshly prepared, and are dramatically expensive relative to simple silver salts. Early studies of this in situ technique mainly focused on simple silver(I) salts such as nitrates, carboxylates, organosulfonates, sulfates, tetrafluoborates, oxides, and trimethylphosphineiodosilver(I).7 Only very limited success was realized; the reflectivities were 18-46% on the air side for the best films, and only a few films exhibited surface conductivity.36 In particular, films prepared with silver nitrate were often brittle and seriously degraded without any mechanical usefulness.7,37 However, success has been achieved in our previous work using silver nitrate as the precursor.24 Double-surface-silvered polyimide films were prepared on a PMDA/ODA-based matrix with optimum reflectivities of 65%/69% and surface resistivities of 0.4 Ω sq-1/ 0.5 Ω sq-1 on the upside/underside surfaces, respectively. Herein, we report our synthesis of silvered surfaces on BTDA/ ODA-based polyimide films using silver fluoride as the silver (27) Rifai, S.; Breen, C. A.; Solis, D. J.; Swager, T. M. Chem. Mater. 2006, 18, 21. (28) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Clair, A. K. S. Chem. Mater. 1999, 11, 501. (29) Rubira, A. F.; Rancourt, J. D.; Taylor, L. T.; Stoakley, D. M.; Clair, A. K. S. J. Macromol. Sci., Pure Appl. Chem. 1998, A35 (4), 621. (30) Rubira, A. F.; Rancourt, J. D.; Taylor, L. T. Met.-Containing Polym. Mater. 1996, 357. (31) 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 (7), 1847. (32) Matsuda, S.-I.; Ando, S. Polym. AdV. Technol. 2003, 14, 458. (33) Sawada, T.; Ando, S. Chem. Mater. 1998, 10, 3368. (34) Qi, S.-L.; Wang, W.-C.; Wu, D.-Z.; Wu, Z.-P.; Jin, R.-G. Eur. Polym. J. 2006, 42 (9), 2023. (35) Qi, S.-L.; Wu, D.-Z.; Wu, Z.-P.; Wang, W.-C.; Jin, R.-G. Polymer 2006, 47, 3150. (36) Southward, R. E.; Thompson, D. W. Chem. Mater. 2004, 16, 1277. (37) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Caplan, M. L.; Clair, A. K. S. Chem. Mater. 1995, 7, 2171.

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origin via a direct ion-exchange self-metallization approach. This work aims at fabricating metallized polyimide films with more desirable performances. A BTDA/ODA polyimide matrix was selected since very excellent surface performances have been achieved on it via the in situ technique. Silver fluoride was chosen due to its good water solubility. The chemistry involved in our present procedure closely resembles that of the in situ technique but has a distinct silver-adding protocol and a different thermal cure mode; that is, silver(I) is added via ion exchange, and then the film is cured under tension. The uniqueness of this approach lies in the direct use of damp-dry PAA films to perform an ion-exchange reaction and in the achievement of reflective and conductive silver layers on both film sides using a simple silver salt as the silver origin. The surface properties of the resulting films were investigated using a UV-vis spectrometer and a four point probes meter, while the structural and morphological characteristics were determined by attenuated total reflectionFourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. 2. Experimental Section 2.1. Materials. 3,3′,4,4′-Benzophenonetetracarboxylic dianhydride (BTDA) was purchased from Acros Organics and used without further purification. 4,4′-Oxydianiline (4,4′-ODA) was obtained from the Shanghai Research Institute of Synthetic Resins and recrystallized in ethyl acetate prior to use. Dimethylacetamide (DMAC, analytical pure, e0.1% water) was purchased from Tianjin Fu Chen Chemicals Reagent Factory and used after distillation. Silver fluoride (AgF, analytical pure, g98% content) was obtained from Zhejiang Dongyang Galt Fine Chemical Co., Ltd. and used as received. 2.2. Synthesis of the Metallized Polyimide Films. The BTDA/ ODA poly(amic acid) resin employed in this study was prepared with a 1% (mol/mol) offset of dianhydride at 20 wt % solid content in DMAC. The resin solution was synthesized by first dissolving the diamine in DMAC and then adding the dianhydride gradually. After stirring at room temperature for 2 or 3 h, a light yellow viscous solution was obtained. The inherent viscosity of the resulting solution applied in our present work was 1.77 dL g-1 at 35 °C. The films were next prepared by spreading the homogeneous poly(amic acid) solution onto a clean and dust-free glass plate. After holding these wet films at ambient atmosphere or in a vacuum oven for some time, most of the solvent was evaporated and damp-dry poly(amic acid) films were produced. These films were then peeled from the glass substrate and immersed into a 0.1 M aqueous silver fluoride solution to perform an ion-exchange process. Films with thicknesses of 4045 µm and DMAC contents in the range of 35-38 wt % were utilized in the present work. For comparison, the surface of the damp-dry PAA film in contact with the glass substrate is referred to as the underside, while that exposed to the atmosphere is referred to as the upside. The absence of light is necessary during the ionexchange process since the silver fluoride aqueous solution is sensitive to photolysis. Following ion exchange, the silver(I)-containing films were rinsed with distilled water and then thermally cured under tension in a forced-air oven. The cure cycles employed are as follows: heating over 1 h to 135 °C and holding for 1 h, heating to 300 °C over 2 h, and remaining constant at 300 °C. After thermal treatment, polyimide films were formed and silver metallic layers were achieved on both film surfaces. A synthetic diagram is shown in Scheme 1. 2.3. Film Characterization. Silver(I) loadings in the PAA films were quantified by a Seiko Instruments SPS 8000 inductively coupled plasma (ICP) atomic emission spectrometer. The measurements were performed after dissolving the ion-exchanged PAA films in a 5 wt % nitric acid solution. Attenuated total reflection-Fourier transform infrared (ATRFTIR) spectra of the PAA films were collected using a Nicolet Nexus670 IR spectrometer with an ATR attachment. Contact angle

4880 Langmuir, Vol. 23, No. 9, 2007 Scheme 1. Illustrative Protocol for the Preparation of Silvered BTDA/ODA-Based Polyimide Films via the Direct Ion-Exchange Self-Metallization Process

measurements were performed using an OCA contact angle system (Data Physics Instruments GmbH) with an input power of 55 W. A minimum of seven points were measured for each surface. Reflectivity spectra (relative to a BaSO4 mirror set at 100% reflectivity) were scanned using a Shimazu 2501PC UV-vis spectrophotometer with an incidence angle of 8° in the 200-800 nm wavelength range. The values at 531 nm were selected to represent the film’s reflectance in the visible light region. Surface electrical resistivities were measured with a RTS-8 four point probes meter produced by Guangzhou Semiconductor Material Academe in China. Surface morphologies were characterized on a HITACHI S-4300 field emission scanning electron microscope (FE-SEM) operating at 15 kV. All the samples were coated with an ∼5 nm platinum layer prior to measurements. X-ray diffraction (XRD) of the silvered polyimide films was performed on the underside of the silvered films using an X-ray diffractometer (D/Max2500VB2+/PC, Rigaku, Japan). The X-ray beam was generated by a copper (KR) target, using a tube voltage of 40 kV and a current of 200 mA. The diffractograms were recorded in the 20°-90° region with a scanning rate of 0.18° per second. X-ray photoelectron spectra (XPS) were collected using an ESCALAB 250 spectrometer (Thermo Electron Corporation) in the fixed analyzer transmission mode. The instrument is equipped with a monochromatic Al KR X-ray source and a magnetic lens system that yields high spatial resolution and high sensitivity. A takeoff angle of 45° was used to obtain most of the spectra. The pressure in the analysis chamber was maintained at 2 × 10-10 mbar or lower during each measurement. Surface elemental compositions were calculated from the peak area ratios on the XPS spectra. Differential scanning calorimetry (DSC) measurements were carried out using a NETZSCH4 differential scanning calorimeter with a heat rate of 5 K min-1. Thermogravimetric analysis (TGA) was performed using a NETZSCH TG 209 system with heating at 10 K min-1. The mechanical properties were evaluated using an Instron-1185 system.

3. Results and Discussion 3.1. Consideration of the Ion-Exchange Process. The ideal synthetic protocol for the metallized films has been illustrated

Qi et al.

in Scheme 1. Compared to the imide form, the PAAs have significantly higher cation-complexing properties due to the presence of carboxylic groups. We supposed that an ion-exchange reaction would occur between the active carboxylic groups and the free silver positive ions when the PAA films were immersed into the aqueous silver fluoride solution. And ideally, the silver ions were loaded into the precursor films by forming a silverpolycarboxylate salt, silver polyamate. Contact angle measurements have been carried out on the PAA films before and after ion exchange. Prior to ion exchange, the water contact angles of the upside/underside were 66.8°/54.1°, respectively. However, following ion exchange in a 0.1 M aqueous silver fluoride solution for 20 and 40 min, they increased to 90.9°/77.8° and 92.9°/80.9°, respectively, which indicates the generation of more hydrophobic surfaces. This could be used as effective evidence to prove the formation of the silver-poly(amic acid) salt, which usually possesses very bad water solubility. XPS analysis was also applied on the ion-exchanged films. It was found that silver ions were loaded on both film surfaces. However, no signals of a fluoride element were detected, further confirming the formation of silver polyamate. ICP measurements suggest that silver ions loaded into the PAA films after 20 and 40 min of ion exchange were only 4.7 wt % and 5.7 wt %, respectively. The low silver contents indicate that the ion-exchange reaction occurred only partially along the polymer chains, which is necessary to retain the integrity of the PAA molecule structures. However, due to the existence of the weak and hydrophilic amide groups in PAA, ion exchange would be inevitably accompanied by hydrolysis, especially in the presence of reactive silver ions. Fortunately, both ion-exchange and hydrolysis could be controlled by time. In the present study, ion-exchange times no more than 40 min were employed. It is assumed that, under the present experimental conditions, the destructive effect of hydrolysis could not play a significant role because no distinct changes were observed on the films after ion exchange, and the final metallized films maintained mechanical properties similar to those of the pure polyimide film (shown later). 3.2. Reflectivity and Conductivity of the Metallized Films. Following ion exchange, double-surface-silvered polyimide films could be fabricated by thermal curing of the silver(I)-containing precursor films. Since the PAA films have been peeled from their glass substrate prior to ion exchange, they must be thermally treated under uniform tension. Otherwise, curl, shrinkage, and deformation would occur due to the thermoplastic characteristics of the uncured PAA molecules. The variation of the reflectivity (at 531 nm) and conductivity on both film surfaces during thermal treatment was traced by measuring the surface properties of individual films as they were withdrawn from the oven at selected times and temperatures. Figure 1 shows the development of reflectivity as a function of cure time and temperature for the 20 and 40 min ion-exchanged films. As can be seen, both film surfaces have been well-metallized with reflectivities at levels of more than 80% on the upside surface and more than 100% on the underside surface. The 40 min ion-exchanged films only metallized a little earlier (∼1 h) than the 20 min ion-exchanged films. This earlier metallization behavior is suggested to be attributed to the acceleration effect of water molecules on silver reduction, as reported by Akamatsu.11 Compared to the 20 min ion-exchanged film, it is probable that more water molecules were introduced into the 40 min film due to the hydrolysis effect during the ion-exchange process. Consequently, more water molecules would be released during the thermal treatment process and then their acceleration effect

Highly ReflectiVe and ConductiVe Polyimide Films

Langmuir, Vol. 23, No. 9, 2007 4881

Figure 1. Plots of reflectivity (at 531 nm) on the upside and underside surfaces versus cure time and temperature for PAA films with ionexchange times of (A) 20 min and (B) 40 min in a 0.1 M aqueous silver fluoride solution. (Time zero is at 135 °C after 1 h.) Table 1. Surface Resistivity of the Two Sides of the 20 and 40 min Ion-Exchanged BTDA/ODA-AgF (0.1 M) Films Heated at 300 °C for Different Times surface resistivity (Ω sq-1) 20 min 40 min film no. in Figure 1 thermal history upside underside upside underside 4 5 6 7 8 9 10

300 °C for 1 h 300 °C for 2 h 300 °C for 3 h 300 °C for 4 h 300 °C for 5 h 300 °C for 6 h 300 °C for 7 h

>106 27 3.4 0.6 0.8 0.5 0.7

>106 18 1.6 0.2 0.3 0.3 0.2

94 1 0.3 0.7 0.7 0.9 0.7

13 0.4 0.5 0.4 0.8 0.7 0.5

would induce earlier metallization on the 40 min film. However, optimum reflective films were achieved in both cases after being cured at 300 °C for no more than 3 h. Table 1 presents the surface resistance characterization data for the same samples shown in Figure 1. It is exciting to note that surface electrical conductivity has been realized on both film surfaces as early as 1 h at 300 °C for the 20 min films and 2 h at 300 °C for the 40 min films with values less than 100 Ω sq-1. Further heat treatment at 300 °C yields more conductive film surfaces with surface resistivities on the order of 0.6/0.2 Ω sq-1 and 0.3/0.5 Ω sq-1 on the upside/underside surfaces for the 20 min films (300 °C for 4 h) and the 40 min films (300 °C for 3 h), respectively. Both the reflectivity and conductivity data suggest that silvered BTDA/ODA polyimide films with high reflectivity and conductivity on both film sides have been fabricated using silver fluoride as the silver origin via our direct ion-exchange selfmetallization technique. However, such reflective and conductive films have never been produced with the in situ single-stage self-metallization technique utilizing a simple silver salt as the silver precursor.7,37 Herein, the success of a direct ion-exchange self-metallization approach is suggested to originate from the different chemical state of silver(I) in the precursor film. That is, in the in situ method, silver(I) mainly existed as the original silver complex, which was only physically blended with the precursor matrix, while, in our ion-exchange technique, silver(I) was present in the form of silver polyamate, which would be favorable to silver reduction since the coordination sites on silver(I) involve the donor groups from the macromolecules.19 Thus, silver metallization was realized. Furthermore, it is exciting that the reflectivity of more than 80%/100% and the surface resistance of less than 0.6/0.2 (or 0.3/0.5) Ω sq-1 on the upside/underside are superior to those of the optimum films prepared from BTDA/ODA-AgTFA (98% reflectivity,