Fabrication of Highly Reflective and Conductive Double-Surface

Jun 18, 2009 - An easy technique is developed to fabricate highly conductive and reflective double-surface-silvered polyimide films at room temperatur...
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J. Phys. Chem. B 2009, 113, 9694–9701

Fabrication of Highly Reflective and Conductive Double-Surface-Silvered Layers Embedded on Polymeric Films through All-Wet Process at Room Temperature Shuaiqi Yang, Dezhen Wu, Shengli Qi, Guanghui Cui, Riguang Jin, and Zhanpeng Wu* State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R. China ReceiVed: January 26, 2009; ReVised Manuscript ReceiVed: May 14, 2009

An easy technique is developed to fabricate highly conductive and reflective double-surface-silvered polyimide films at room temperature by the incorporation of silver ions in surface-modified polyimide, and subsequently by the in situ reduction of silver ions in alkaline containing aqueous glucose solution. Surface properties of the silvered composite films were investigated as a function of treatment time and reducing environment, respectively. Sheet reflectivity and conductivity can be controlled by adjusting the potassium hydroxide (KOH) etching and reducing conditions. The excellent silver-polymer adhesive property is based on a “tree roots” like micro/nanostructure of the silver layers. The essential mechanical properties of the silvered films were maintained as their inside matrix is intact during the whole procedure. Different properties between one film’s double-side surfaces were investigated during the fabricating process. Films were characterized by inductively coupled plasma (ICP), X-ray diffraction (XRD), contact angle (CA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), four point probe instrument, and ultraviolet (UV) spectrophotometer. 1. Introduction In recent years, metal/polymer composites attract more attention due to their potential applications in numerous fields, such as flexible microelectronic circuit boards, optics, sensors, and outer space material.1-4 Polyimides (PIs), as one preferable selection of polymeric substrates, are widely used in industry where excellent thermal stability, irradiation resistance, processing performance, dielectric capability, and mechanical property are requested.5 Silver is attractive metal because of its highly electrical conductivity, excellent reflectivity, and favorable standard electrode potential (E0 ) 0.8 V) that allows silver ions to be reduced easily in the case of ultraviolet (UV), thermal, or relative weak reducing agent etc.6,7 The surface-silvered polyimide films, banding the advantages of the PI matrix and surface silver layer together, are endowed with numerous novel applications, i.e. highly thin-film reflector concentrators in space environments for solar thermal propulsion and solar dynamic power generation, optical switches, antimicrobial coating, contact devices in microelectronics, elastomeric optical mirrors, surface conductive reflective flexible polymeric tapes, and so on.7-10 The traditional methods for metallization on the polyimide surfaces mainly include sputtering, physical vapor deposition (PVD), and chemical vapor deposition (CVD).11,12 These processes are powerful methods for metallizing polyimide surfaces, but complicacy of execution, rigorous condition of manipulation and poor adhesion seriously cumber the development of these methods. In 1990s, several groups reported a new method called inverse chemical vapor deposition or in situ selfmetallization method, which provide us with a new approach to fabricate metallized polyimide films.13-15 This method refers developing a metallized film from a single homogeneous solution that contains both an organometallic silver complex * To whom correspondence should be addressed. Tel.: +86-10-64421693. Fax: +86-10-6442-1693. E-mail: [email protected].

and the desired polyimide precursor. Thermal treatment of the cast film converts the precursor into the final polyimide form with concomitant silver reduction yielding a reflective and/or conductive silvered polyimide film. However, expensive coordination fluorine containing silver(I) precursors had been used, high curing temperature was needed in the last reducing step. The high energy consumption associated tedious long thermal cycle during reducing process is a critical hindrance to industrial fabrication. As an alternative to in situ self-metallization method, several groups, including ours, reported that the chemical metallization technique could be used to fabricate the metallic layers on polymeric substrates. This methods mainly rely on chemical surface modification of polyimide to form cation exchangeable groups (i.e., carboxyl groups) and subsequent incorporation of metallic ions via ion-exchange reaction followed by reduction of these incorporated metallic ions.16,17 UV, thermal or complex organic reducing agent’s (i.e., dimethylamine borane) treatment of ion-doped precursor lead to the reduction of metal, forming metallized polyimide films with surface conductivity.18,19 In our previous study, we mainly reported on three points: (1) The chemical etching depth of the ring cleavage region on the polyimide surface by a simple potassium hydroxide (KOH); (2) diffusion and aggregation of silver nanoparticles to form reflective and conductive surfaces by thermal treatment; and (3) the pyrolysis phenomenon of the silvered polyimide films under thermal treatment.18 Herein, as a continuation to the metallization of the polyimide surfaces described in our previous paper, the present work mainly reports a room temperature etching and in situ reduction processing technique in order to avoid pyrolysis drawback of the silvered films and to continue improve the silverpolyimide adhesion properties. The procedure is outlined in Scheme 1. The silver ion containing precursor was obtained according to the typical chemical treatment which included the surface modification by aqueous KOH solution and ion

10.1021/jp900755c CCC: $40.75  2009 American Chemical Society Published on Web 06/18/2009

Double-Surface-Silvered Layers SCHEME 1: Schematic Illustration of Present Process Involving (i) Aqueous KOH-Induced Surface Modification of Polyimide, (ii) Incorporation of Silver Ions via a Ion-Exchange Reaction, and (iii) Aqueous Alkali Glucose Solution-Induced Reduction of Doped Silver Ions

exchange process in aqueous silver nitrate (AgNO3) solution. Subsequently the precursor films were immersed into the alkali glucose solution for the reduction reaction at room temperature. The new aspect can be summarized as follows: (1) Glucose is chosen for the first time to be a reducing agent due to its weak reducing ability which is in favor of formation of the smooth and even silver layers with excellent metallic luster on the polyimide surfaces. (2) By adjusting the pH value of the aqueous glucose solution, we could control not only the micro/nanostructure of the silver layers but also properties of the highly reflective and conductive metallized films. (3) The micro/nanostructure of the silver layers presents the films with unique silver-polymer adhesive property. (4) To overcome the drawback of the catalytic and oxidative decomposition on the polyimide matrix during the high temperature processing. We expect that this fabrication method could be extended to the large scale roll-to-roll fabrication technique, and the resulting micro/nanostructure with functionalized metallized surfaces could provide potential platform for microelectronic devices, optical switches, and flexible mirrors. 2. Experimental Section 2.1. Materials. Commercial pyromellitic dianhydride oxidianiline (PMDA-ODA) polyimide films with a thickness of 75 µm were obtained from Shanghai Qian Feng Insulating Material Plant (P.R. China).The surface of the film was rinsed by ultrasonic cleaning for 15 min in deionized water and dried in an ambient environment prior to use. Glucose was offered by Sinopharm Chemical Reagent Company, Ltd. (P.R. China). Other reagents, such as KOH and AgNO3 (analytically pure, g99.8%) were purchased from Beijing Chemical Works and used as received. 2.2. Preparation of the Silver-Metallized Polyimide Films. The fabrication of the double surface silvered polyimide mainly involves three steps which are schematically shown in Scheme 1. Abluent bare polyimide films were first immersed into 4 M aqueous KOH solution for several hours and then rinsed carefully with numerous deionized water. According to previous literature, KOH-treatment of the pristine polyimide can induce imide rings cleaved to form potassium salts of carboxylic acid and amide bonds. Surface-modified polyimide films were inserted into 0.4 M aqueous AgNO3 solution for three hours to perform cation-exchange reaction, and then washed again with copious amount of deionized water. In this step, silver ions

J. Phys. Chem. B, Vol. 113, No. 29, 2009 9695 exchanged with potassium ions were incorporated into the precursor layers. Finally, the silver-ion-doped polyimide films were immersed into alkali aqueous glucose solution for certain minutes to reduce the doped silver ions. The resulting silvered films were obtained by deionized water washing and exposing to flowing dry air to evaporate surface-rudimental liquid. Every step was performed at the ambient temperature. 2.3. Characterization. The films were analyzed by the measurement of contact angle (CA) between the surface and deionized water with each drop on the order of 2 µL using an OCA20 type contact angle system (Data Physics Instruments GmbH), and each surface was analyzed with ten drops. Ion loadings in the modified layers were determined by a Seiko Instruments SPS 8000 inductively coupled plasma (ICP) atomic emission spectrometer. The samples were cut into a square with each side being 1 cm and dissolved into concentrated nitric acid solution. X-ray diffraction (XRD) was performed on the surface of silvered polyimide films using an X-ray diffractometer (D/ Max2500 VB2+/PC, Rigaku, Japan). The X-ray beam was generated by a KR target, and the diffractograms were recorded in the 5°-90° region. The silvered polyimide surface morphology was observed by scanning electron microscopy (SEM, Britain Cambridge MK3-250) and atomic force microscopy (AFM, Digital Instruments Inc. of Santa Barbara, California, U.S.A.), AFM images were collected using a Nanoscope IIIa in tapping mode in air with a scanning rate of 1.0 Hz. The cross-sectional microstructure of the film was investigated by transmission electron microscopy (TEM) which was done on an H-800 type Hitachi instrument at accelerating voltage of 200 kV. For cross-sectional TEM observation, the samples were sectioned into ca. 80 nm thick slices with an ultramicrotome. An SDY-4 four point probe instrument (Guangzhou, China) was employed to measure electrical resistivity of two surfaces of the silvered PI films. Reflectivity spectra (relative to a BaSO4 mirror set at 100% reflectivity) were scanned on a Shimazu 2501 PC UV/vis spectrophotometer in the 200-800 nm wavelength range. The values at 531 nm were chosen to represent the films’ reflectance in the visible region. The optical photo of largesize silvered polyimide film was taken by Canon IXY 5.0. 3. Results and Discussions 3.1. Surface Modification and Ion Exchange. Many researchers have investigated the hydrolysis mechanism of PI films including ring-cleavage of the imide bonds and formation of amide bonds.16,20,21 Because of the side-to-side near-surface difference in the commercial PI films, the modified thickness layers between the two sides should be different. For clarity, we define the surface of the film in contact with the substrate in the commercial procedure as the downside, whereas that exposed to the atmosphere is referred to the upside. According to Mahendra Dabral’s report, the upside surface region had order and orientation in the film plane; in contrast, the downside region undergoes imidization in a disordered, entangled network state. Surface roughness of the downside is usually larger than that of upside surface.22 The phenomena could be detected by our CA measurements on both sides of the bare polyimide film. Our selected commercial polyimide film was ca. 80° on the upside surface and 70° on the downside surface, respectively. Thus, the downside surfaces of pristine polyimide film are in possession of better hydrophilicity, which makes it easier to be etched than that of upside surfaces. Figure 1 shows the surface modified thickness of the PMDA-ODA type polyimide films

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Figure 3. X-ray diffraction patterns for the PI-Ag film immersed into reducing solution for different time. The reduction was performed using 0.1 M KOH and 0.22 M glucose solution at room temperature. (a) curve, 5 s; (b) curve, 0.5 min; (c) curve, 15 min; and (d) curve, 30 min.

Figure 1. TEM images of cross-sectional polyimide films after 4 M KOH treatment for 7 h and subsequent ion exchange in 0.4 M aqueous AgNO3 solution for 3 h: (a) upside of the modified layer and (b) downside of the modified layer.

Figure 2. Loadings of adsorbed silver ions in a polyimide film after KOH treatment (4 M, room temperature) and after subsequent ion exchange in AgNO3 aqueous solution for 3 h (0.4 M, room temperature) as a function of initial KOH treatment time (9) downside, (b) upside.

after it was treated with 4 M aqueous KOH solution for 7 h at room temperature. TEM results showed that a different thickness of the double modified layers was observed for the film initially immersed in aqueous KOH solution. That is, the modified layer on the cast-side surface is thicker than that of the air-side. The different thicknesses of the side-to-side of the modified layers would cause a different silver ion loading. We will discuss it more detail below. Figure 2 shows the content of adsorbed silver ion loadings on the double surface for the modified polyimide film immersed in aqueous AgNO3 solution for 3 h (0.4 M, room temperature) as a function of initial KOH treatment time. At the fixed KOH treatment time, the amount of silver ions loaded on the downside is higher than that of the upside. No potassium ions could be detected, suggesting that 3 h of ion exchange is enough for silver ions to exchange with potassium ions completely. The thickness of the hydrolytic layers of the polyimide films linearly depend on immersion time in the aqueous KOH solution with respect to the certain surface, conformal diffusion of KOH leads to the uniform reaction of alkali-induced cleavage of the imide rings

during initial KOH treatment and ions exchanged are uniformed distributed in the modified layer.20,21 Thus, the silver ions loading content in the films linearly depend on the thickness of modified layer. 3.2. In Situ Reduction Process. Treatment of the silver iondoped polymeric films in alkali aqueous glucose solution leads to reduction of the silver ions inside the modified layer of the polyimide films. The hybrid solution of KOH and glucose was used as a reducing agent in this study because it is relatively weak reducing agent as compared to several others, such as formaldehyde, which would avoid unhomogeneous growth of silver nanoparticles. Possible reaction mechanisms for the formation of Ag nanoparticles are

The formation of well-defined double-surface-silvered polyimide surfaces is attributed to the diffusion of newly born silver particles. The appearance of the films surface exhibited different colors, which were yellow, dark black, and silver-white successively when the immersion time in alkali aqueous glucose solution was increased within 15 min. The color changes of the film suggested the continuous size change of the silver nanoparticles as the treatment time increased. When immersion time was 5 s, the silver(I) films had a yellow appearance. The XRD pattern shows no reflection for face-centered cubic crystalline silver in Figure 3. After 30 s of immersion, the XRD (Figure 3, curve b) pattern displayed a weak peak at 2θ ) 38.1° referring to the dark color of the film indicating formation of silver nanoparticles. After treatment for 15 min, one can clearly observe four obvious peaks at 2θ ) 38.1°, 44.3°, 64.4°, and 77.5°, which correspond to the crystal faces of (111), (200), (220), and (311) of silver, consistent with the formation of facecentered-cubic silver crystallite. The full width at half-maximum (fwhm) of the strongest characteristic peak (111) is used to estimate the average crystallite size which is approximately 14.29 nm. 3.3. Reflectivity and Conductivity of Silvered Films. We carried out a series of experiments using reducing hybrid solution with different values of pH ranging from 11 to 14 to fabricate the double-surface silvered polyimide film. Table 1 presents the surface resistance characterization data for the specimens undergoing the same initial treatment. When setting pH value at 11, there is no electrical conductivity for the

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TABLE 1: Sheet Resistance of Films’ Double Surfaces as a Function of pH/Timea surface resistance (Ω/sq) pH ) 14 treatment time (min) 1 3 5 15 30 45 60 90 120

U

1

8 11 2 1 2 2 2 2 2

D

2

72 12 22 21 25 26 21 25 31

pH ) 13

pH ) 12

U

U

40 5 5 3 3 3 3 3 3

D 140 6 6 3 3 3 3 3 3

D

4 895 2 4 2 2 1 0.6 1 0.6 1.2 0.8 0.6 0.6 1 0.7 0.8 0.6

pH ) 11 U

D 3

NC NC NC NC NC NC NC PC4 PC

NC NC NC NC NC NC NC NC NC

a All of the samples were immersed into 4 M aqueous KOH solution for 7 h and ion-exchanged in AgNO3 solution for 3 h before reduction. 1U and 2D are used to refer to the upside and downside of the films, respectively. 3NC is not conductive. 4PC is partially conductive.

Figure 4. Reflectivity (at 531 nm) of the upside and downside surfaces of silvered polyimide film as a function of reducing environment and time, respectively: (a) silver ion loaded film reduced in pH 14 glucose solution, (b) silver ion loaded film reduced in pH 13 glucose solution, and (c) silver ion loaded film reduced in pH 12 glucose solution (9) downside, (b) upside.

resulting films. The unchanged yellow appearance of the films also suggested that the pH value is too low to meet the request for formation of a surface silver layer. The silvered films could be bound conductive for adjusting its pH values from 12 to 14. When the pH value is set at 12 and the treating time is kept for about only 15 min, silvered films were found with surface resistance at ca. 1 Ω/sq on the upside surface and at ca. 0.6 Ω/sq on the downside. If the reducing treatment time is prolonged and/or the pH value is further increased, the surface conductivity of the silvered films would not continuously improve because of the side effect of KOH etching the sliver ion loaded polymeric films. Figure 4 shows the development of the reflectivity as a function of treatment time and pH values in glucose solution for the same samples in Table 1. The reflectivity of the resulting films increased with decreasing pH value in the range from 14 to 12. When setting the pH value to 14, the surface reflectivity could be detected only about 40% on both sides of the surface. When the pH value is set to 12, the surface reflectivity could be detected up to 80% on the downside and up to 100% on the upside surface. It was also found that the reflectivity on the upside surfaces was universally better than that on the downside surfaces. The different reflectivity of the two sides of silvered layers was most probably because of the different roughnesses of the two sides. We will discuss this in more detail below. Figure 5 presents reflectivity (at 531 nm) of the upside and downside surfaces of the resulting silvered films versus initial KOH treatment time for the sample immersed in alkali, followed by Ag+ ion exchange for 3 h, and finally reduced in the pH 12

Figure 5. Reflectivity (at 531 nm) of the upside and downside surfaces of the resulting silvered films versus initial KOH treatment time: after initial KOH modification, the films were immersed into 0.4 M aqueous AgNO3 solution for 3 h, and finally reduced in a pH 12 glucose solution for 15 min (9) downside, (b) upside.

glucose solution for 15 min. Compared with the downside surfaces, the reflectivity of the upside surfaces increased significantly with increasing etching time. The upside surface showed maximum reflectivity of about 40% when the etching time was 1 h, which suggested that the silver content in the polymeric matrix was too low to form a thicker and continuous silver layer. As partial light was trapped in and/or penetrated into the silvered films, the silvered film presented a lower reflectivity. For alkali treatment time that have been prolonged to 6 and 7 h, upside surfaces of the silvered films could be detected at about 75.7% and 90.9%, respectively. On the contrary, the alkali etching time have little influence on the reflectivity of the downside surface. For example, for alkali treatment time of 2 and 6 h, the downside surfaces of the silvered films could be detected to have a similar reflectivity, which was about 58.6% and 55.9%, respectively. The reason that double silvered surfaces of one film present different reflectivities during the same etching process will be discussed in the AFM measuring part below. The effect of the concentration of the glucose solution on the properties of silvered polyimide films is also investigated during the reducing reaction. We found nearly no change on the reflectivity and sheet resistance of the resulting films when varying the concentration of the aqueous glucose solution, indicating that only little influence existed. 3.4. Surface Morphology and Cross-Sectional Profile. The formation process of the silvered polymeric films was traced by SEM observation. Figures 6 and 7 show SEM images for the upside and underside surfaces of the silvered films treated in pH 12 glucose solution. It is clearly shown that the silver particles altered with various reducing time. After only a 20 s immersion in the hybrid reducing solution, the color of the silver ion loaded film varied from transparent light-yellow to darkblack in appearance; the SEM image in Figure 6B shows that silver nanoparticles formed on the modified films. No conductivity in the film was found in the film because of the discontinuous silver layer. As the silver particles grew large enough, continuous silver layers had formed with the reducing time up to 1 min. When reducing reaction time was prolonged, the silver particles grew larger and larger, and they were closely compacted with each other. The color of the film slowly changes to white and finally becomes luster silver-white after 5 min of treatment (Figure 7B), which was consistent with its metal-like conductivity and reflectivity. The corresponding TEM images in Figure 8B also show that the continuous layers have been formed which directly leads to dramatically high reflectivities of 90.6% and 78.5% and low sheet resistances of 2 and 2 Ω/sq for the upside and downside surfaces, respectively. For the reducing time prolonged to 15 min, the silver particles interconnect to each other closely and the layers become more compact (Figure 7C). Figure 8C shows that the silver layers become

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Figure 6. SEM images of silvered polyimide films with an etching time of 7 h in the 4 M aqueous KOH solution, an ion exchange time of 3 h in a 0.4 M aqueous AgNO3 solution, and finally treated in pH 12 reducing agent for (A) 0 s, (B) 20 s, and (C) 1 min (U, upside of film, D, downside of film).

Figure 7. SEM images of silvered polyimide films with an etching time of 7 h in the 4 M aqueous KOH solution, an ion exchange time of 3 h in a 0.4 M aqueous AgNO3 solution, and finally treated in pH 12 reducing agent for (A) 3 min, (B) 5 min, and (C) 15 min (U, upside of film, D, downside of film).

thicker to about 500-700 nm which results in a further decrease of the sheet resistance with 1 and 0.6 Ω/sq for upside and downside surfaces. The gradual increase in the thickness of the silvered layers suggests that the silver(I) ions migrate in significant quantity to the film surface from the inside matrix of the modified layer. The mechanism of the formation of the silver layer on the polyimide surface is probably that silver ions close to the film’s surface first were reduced to form silver clusters in aqueous glucose solution. Then, the decline of the amount

of surface silver ions would be formed. The silver(I) concentration gradient presents, which would drive silver ions in the bulk of the modified layer to migrate to its surface. The silver ions spontaneously aggregate to produce a thermodynamically stable micrometric silver layer after reduction.23 Moreover, the presence of the poly (amic acid) in the modified layer enhances water absorption during the ion exchange reaction, which facilitates silver(I) ions mobility and permits the quick formation of a well-defined silver layer within only several minutes.16

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Figure 8. Cross-sectional TEM images of silvered polyimide films finally treated in pH 12 reducing agent for (A) 20 s, (B) 5 min, and (C) 15 min (U, upside of film, D, downside of film). Each scale bar is 1 µm.

3.5. Property Differences between the Two Sides. As we mentioned above, the properties of the two surfaces are not identical to each other during the whole etching and reducing process, which can be observed from the modified layer in Figure 1, silver ion loading content in Figure 2, surface resistance in Table 1, the reflectivity in Figures 4 and 5, and SEM images in Figures 6 and 7. It seems that the upside surfaces are always superior to the downside, especially regarding the reflectivity and surface resistance at the final reducing stage, though the downside was modified thicker and reflectivity are higher to the upside at the early reducing process. Mahendra Dabral et al.22 described that the upside surface region was ordered and oriented in the PI film plane; in contrast, the downside region undergoes imidization in a disordered, entangled network state. As shown in TEM images in Figure 1, the upside of PI films would be more difficult to modify by KOH than that of the downside. The SEM images in Figure 7 show that the spherical silver particles on the upside of the films are more compact to each other than that of the downside. To further inspect the surface topography of the resulting PI films, tapping mode atomic force micrograph (AFM) was applied for the same samples in Figure 7C. AFM images in Figure 9 show that the sheet reflectivity is tremendously dependent on the smoothness of the surface. The upside surface of the silvered film was relatively smoother compared to the downside surface, which directly resulted in the upside surface holding the better reflectivity. On the contrary, Figure 9b shows that there are amounts of regular, orientable “dongas” on the downside surface. The AFM measurement of our silvered films offers a reasonable explanation for experimental results in Figure 5. Different reflectivity between the double surfaces was the synergism of both contents of silver(I) loading and roughness of the film surfaces. For the upside of the films with low roughness, fabrication of the silvered films demonstrating high reflectivity would need a thicker modified layer and sufficient silver ion loading. As shown in Figure 5, for shorter KOH etching time, the downside surfaces usually present higher reflectivity because their thicker modified layers owned higher contents of silver(I) loading than that of the upside surfaces. When the etching time was prolonged to ensure enough silver(I) loading contents for forming continuous silver layers, downside

Figure 9. AFM 3D images of double surfaces of silvered polyimide films for the same sample in Figure 7C: (a) upside surface of the films with R > 100%, surface resistance ) 1 Ω/sq, (b) downside surface of the films with R > 80%, surface resistance ) 0.6 Ω/sq.

TABLE 2: Sheet Resistivity Data of One Film’s Two Surfaces before and after Ultrasonic Cleaning for 15 min surface resistance (Ω/sq) upside

downside

KOH etching time (h)

before ultrasonic

after ultrasonic for 15 min

before ultrasonic

after ultrasonic for 15 min

1 3 5 7

1.6 1.6 1.4 1.1

0.9 1.5 0.7 0.9

1.2 1.1 0.84 1.06

NC 0.88 0.82 0.95

surfaces of the films usually present relatively lower reflectivity because of their relatively rougher surfaces. 3.6. Adhesive and Mechanical Property. The adhesive property of prepared PI films is of concern in this research. Table 2 shows surface resistance of the silvered PI surfaces before and after the ultrasonic cleaning for 15 min as a qualitative evaluation of bonding strength between silver particles and the polyimide substrate. Before ultrasonic cleaning, the sheet resistance maintains ca. 1.5 and 1.1 Ω/sq on the upside and downside surfaces, respectively. After ultrasonic cleaning, sheet

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bare PI prepared prepared prepared prepared

Figure 10. TEM images of cross-sectional silvered polyimide films after 4 M KOH treatment time for 7 h and then by Ag+ exchange and reduction treatment in glucose solution for different pH value: (a) pH 12; (b) pH 14.

resistance of prepared films does not increase but decrease to some extent (upside: 0.9 Ω/sq, downside: 0.9 Ω/sq) except for the downside surface of the sample which was modified by KOH for 1 h. At the same time, the reflectivity which is measured on the same samples (except NC sample) almost remains unchanged. The strong adhesion property between silver layers and polymeric interfaces can be ascribed to their mechanical interlocking mechanism.4,9,24 Through reduction of the prepared silver(I)-doped hydrolyzed Kapton films in aqueous sodium borohydride solution, the silver “finally localized only at the top surface” of the film. That is, silver atoms and clusters do not form in the bulk of the films.25,26 However, our observation showed that the silver layers’ roots could be rooted in the bulk of the films by adjusting the alkalinity of the reducing environments. For example, when the glucose reducing solution was set at pH 12, TEM images in Figure 10a show that a continuous silver layer formed on the polymeric surface. Meanwhile, the silver layers “root” into the bulk of the polymer. When the reducing solution was set at pH 14, TEM images in Figure 10b clearly showed that “tree root” like micro and/or nanostructures of the silver layers rooted into the polyimide matrix. The “tree root” like microstructures/ nanostructures of the silver layers definitely ensure the excellent adhesive properties of silver-polymer interfaces. Thus, we suggest that formation of the “tree root” like silver layer is because glucose diffused into the bulk of the polymeric films. The glucose in situ reduced silver ions into silver atoms and near-atomic silver clusters, which diffused and aggregated to give “tree root” like silver layers. The mechanical performance of the resulting PI films is important for practical application. Table 3 displays the characterization data for four silvered films that have undergone a different etching time, together with the data of an undoped parent film. The silvered films still show acceptable mechanical properties compared to those of parent polyimide films as the main part of the PI film remained undisturbed during the whole alkali treatment in our experiments.

PI/Ag PI/Ag PI/Ag PI/Ag

(KOH-1 (KOH-3 (KOH-5 (KOH-7

h) h) h) h)

tensile strength (MPa)

elongation (%)

modulus (MPa)

165.30 156.74 149.63 146.89 142.64

84.03 62.99 47.87 42.06 37.24

1020.4 1229.7 1599.6 1534.6 1626.9

Figure 11. Outdoor photograph of the resulting surface-silvered polyimide films for about 27 × 18 cm2 in size; the mirror image in the silvered films shows shadows of trees and blue sky.

As we mentioned above, this technique could provide a platform for those applications where the silvered films with large scale roll-to-roll fabrication are needed. Thus, we choose a large scale parent polyimide film with about A4 sheet paper size to fabricate the highly reflective and conductive films. Figure 11 shows a photograph of the prepared 27 × 18 cm2 silvered polyimide films. The large scale film’s properties are very similar to those with smaller sizes. The technique is readily extended to large scale fabrication of several other noble metal layers on polymeric films for both scientific and industrial applications. 4. Conclusion In conclusion, large scale silvered polyimide nanocomposite films can be easily fabricated with double-sided excellent reflective and conductive surfaces, outstanding metal-polymer adhesion, and controllable microstructures/nanostructures of silver layers, through surface modification, silver ion exchange, and in situ reduction all in aqueous solution at room temperature. The reflectivity and surface conductivity of the metallized films can be controlled by adjusting the etching time and the reducing conditions. It is expected that the current approach based on ion exchange and in situ reduction in glucose solution of metallic nanoparticles at room temperature can provide a simple and effective nanoscale patterning tool for a variety of functional nanoparticles and enable large scale roll-to-roll manufacturing on flexible polymer substrates aimed at electronic and space applications. Acknowledgment. The authors acknowledge the support of the program for the National High Technology Research and Development of China (863 Program) administered by the Ministry of State Science and Technology (Project No. 2007AA03Z537).

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