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Mar 5, 2013 - Controlling the Pore Structure of Polyimide Films Prepared by Exposure to High-Pressure CO2 and UV Light. Kentaro Taki , Tatsuki Isawa ...
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Rapid Production of Ultralow Dielectric Constant Porous Polyimide Films via CO2-tert-Amine Zwitterion-Induced Phase Separation and Subsequent Photopolymerization Kentaro Taki,* Kazunori Hosokawa, Shota Takagi, Hiroyuki Mabuchi, and Masahiro Ohshima Department of Chemical Engineering, A4-021, Katsura Campus, Kyoto University, Katsura, Kyoto, 615-8510, Japan ABSTRACT: Porous polymeric films are promising materials for the production of ultralow-dielectric constant materials. A high porosity polyimide thin film was prepared via the phase separation of a polyimide precursor in an N,N-dimethylacetamide (solvent)/2-(diethylamino)ethyl methacrylate/photoinitiator system. A novel technique involving high-pressure CO2 (5 MPa) gas injection was used to form CO 2 -2(diethylamino)ethyl methacrylate zwitterion salt and induce the immediate phase separation and solvent droplet formation (within 60 s) of a wet precursor film on a metal substrate. The film was exposed to UV light through quartz windows for 30 s to polymerize the 2-(diethylamino)ethyl methacrylate while maintaining a constant CO2 pressure. The cured thin film containing numerous pores with an average diameter of approximately 1 μm ± 1.0 μm was treated at 320 °C for 1 h under a continuous flow of nitrogen. The obtained film was 30 μm thick and exhibited pores with an average diameter of approximately 1 μm ± 0.9 μm. The ultralow-k level minimum relative dielectric constant for the optimal polyimide film was 1.536, and the porosity was 74% with open porous structure.

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INTRODUCTION Polyimide (PI) films and membranes have been investigated not only for the electronics industries but also for a wide range of applications, such as gas purification,1−9 the fundamental physics of diffusion and sorption in gas purification,10−16 and fuel cells.17−20 In the electronics industry, PI films are utilized in the production of flexible printed cable (FPC) in electronic devices because of the inherently high heat resistance, flexibility and chemically stability of PI.21 With the increasing transmission speed of mobile information devices, e.g., cellular and smart phones, PI FPCs require a lower relative dielectric constant to decrease the signal attenuation.22−25 There are two strategies for reducing the dielectric constant of PI. One strategy involving chemical modifications of the PI backbone and side chains has been examined in a recent review article.25 For example, it is well-known that the incorporation of fluorinated substituents into polymers decreases their dielectric constant because of the low dipole moment and the low polarizability of the C−F bond.25 The dielectric constant of the fluorinated PI is limited to 2.7−3.0 and the poor adhesion between the copper substrate and the fluorinated polymer remains a significant challenge. An alternative strategy involves the introduction of voids into PI films.25 Pioneering studies on the introduction of 10 nmsized pores into PI were conducted by Hedrick,26 who designed a block copolymer system that could be pyrolyzed at high temperatures to form voids in a PI matrix for applications such as insulator in integrated circuits. Voids were successfully formed in the PI, although the porosity was limited by the small number of pores. Another approach for forming pores involves © 2013 American Chemical Society

the physical foaming technique, wherein CO2 gas dissolved in a polymer matrix is thermally phase-separated to induce bubble nucleation in the glassy and rubbery state of the polymer.27,28 The dielectric constant achieved using this method is 1.77, which is known as the ultralow-k level.28 A supercritical fluidassisted extraction technique has been applied to remove mesoscale domains in PI matrix, while the remaining domains are allowed to become voids.29 Although the process used to create mesosized pores is highly sophisticated, it is assumed that the complete extraction of all domains is not possible. The resulting incomplete extraction may be unfavorable for the application of such materials in electronic devices. Water-borne porous PI has been prepared via the condensation of water vapor onto the PI solution spun on a substrate.24 The micrometer-seized water droplets on the substrate correspond to the porogen. This method resulted in a decrease of the dielectric constant of porous films to 1.7, although a 48 h process was required for the water droplets to form in the porous films. The formation of meso-sized pores in a PI matrix by the addition of polyhedral oligomeric silsesquioxane (POSS) as an organic porogens has been reported by several authors.30−33 Inorganic porogens such as hollow silica particles and mesoporous silica were effectively used to reduce the dielectric constant.34−39 Compounding the porogens produces films with relatively low porosity; however, it is difficult to achieve Received: November 21, 2012 Revised: February 17, 2013 Published: March 5, 2013 2275

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mmol) of photoinitiator were added to 6.56 g of the precursor solution and stirred until a homogeneous solution was obtained. The solution was cast on a copper substrate (4.5 × 4.5 cm2) using a wired-bar and an automated coater (Imoto Machinery, Kyoto, Japan). Wet films prepared from the solution with a thickness of approximately100 μm were placed in a preheated (40 °C) highpressure view cell with two quartz windows (Tama-Seiki, Kanagawa, Japan), as shown in Figure 1. The maximum CO2 pressure was 6.5

porosity of greater than 70% because of the presence of porogen shells. To decrease the dielectric constant by increasing the porosity further while achieving a short processing time, this study builds upon our recently reported approach where UV curable monomers and a CO2 solution were phase-separated to form a transient porous structure that was then solidified by rapid photopolymerization.40 Moreover, the photopolymerization followed by unidirectional freezing enabled the creation of honeycomb porous structures from the UV-curable monomer/ solvent/photoinitiator solid−liquid phase separation.41 This approach is useful for creating macroporous materials for systems that could be phase-separated and photopolymerized. The main advantage of this approach is the reduction in processing time and increase in porosity. The photopolymerization can create a stable porous structure instantaneously, whereas the conventional process that utilized diffusion requires significantly more time to achieve a stable porous structure. In this study, we discovered a suitable system for generating an ultralow-k PI film of high porosity using a short processing step. A mixture of PI precursor, solvent and UV curable monomer/photoinitiator, which could be a negative tone PI resist, was exposed to high-pressure CO2. Upon exposure to CO2, phase separation was immediately observed, resulting in the formation of solvent droplets in the mixture. The transient structure of the phase separated mixture consisting of droplets in a wet film was solidified by the photopolymerization of the UV-curable monomer using UV irradiation. The pore formation and solidification process was completed within 2 min, which was much faster than in previous study wherein a 2 h thermal imidization was required.24 The obtained porous PI precursor was transformed into porous PI via a heat treatment. The relative dielectric constant and loss tangent of the porous PI films were measured at 20 GHz. This manuscript describes the preparation of the porous PI precursor and PI films and then elucidated the mechanism of pore formation. In addition, the porous structure and pore diameters as a function of the CO2 pressure and UV intensity are reported. Finally, the relationships among the dielectric constant, the loss tangent and the porosity of porous PI films are described, and the tensile modulus and strength of porous PI are measured.

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Figure 1. Pore formation process from a PI precursor. MPa. The diameter and thickness of the quartz window were 65 mm and 25 mm, respectively. To visualize the phase separation, a highspeed digital camera (HAS-220, DITECT Japan) and a long-workingdistance microscope (VQ-Z50, Keyence) were used. After the introduction of the pressurized CO2 gas into the view cell, the wet film became cloudy within 60 s as a result of phase separation. The wet film was then exposed to UV light, which was transmitted from a high-pressure 200-W mercury vapor short-arc lamp using a liquid light guide (S2000, ExFo, Canada) through the quartz window. The UV-curable monomer in the wet film was polymerized for 60 s. The pressurized CO2 was decreased to atmospheric pressure for 60 s. After the film was dried at room temperature, a porous PAA was formed. The transformation from the precursor to a porous PI film was achieved using a heat treatment in a nitrogen-flushed oven (DN610I, Yamato Scientific, Tokyo, Japan) using the following temperature gradients: (1) 3 °C/min to 110 °C for 60 min, and (2) 3 °C/min to 320 °C for 60 min under 30 L/min of nitrogen. Prior to the heat treatment, the oven was flushed thoroughly with nitrogen gas for 120 min to expel oxygen from the oven. The copper substrate was removed by immersion in 30 wt % nitric acid in an aqueous solution, rinsed with distilled water and then dried in a vacuum chamber. Characterization. The structure of porous PAA and PI were observed using a scanning electron microscope (JEM-6700F) after sputtering with gold and palladium. The diameter of each pore was measured by fitting an ellipsoid model to the pore and calculating the diameter based on the area of the ellipsoid, assuming a circular pore, using Image J software (National Institutes of Health). The porosity of the porous PI was calculated in the following manner:

EXPERIMENTAL METHODS

Preparation of a Porous PI Film. Pyromellitic dianhydride (PMDA, B0040) and 4,4′-diaminodiphenyl ether (ODA, O0088) were purchased from Tokyo Chemical Industry (Tokyo, Japan). Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (photoinitiator, 326− 63232) and N,N-dimethylacetamide, anhydrous (solvent, 042− 25285) were purchased from Wako Pure Chemical (Osaka, Japan). A UV-curable monomer of 2-(diethylamino)ethyl methacrylate, which contains a tert-amine methacrylate group, was purchased from Tokyo Chemical Industry (Tokyo, Japan). CO2 gas (99.9% purity) and N2 gas (99.998% purity) were purchased from Showadenko gas products (Kawasaki, Japan). The copper substrate was supplied by Sumitomo Electric Industries (Osaka, Japan). All reagents were used as received without purification. As reported in previous studies, the PI precursor (PAA) was synthesized using a dry three-neck flask.42,43 Approximately 0.505 g (2.52 mmol) of ODA was dissolved in 5.5 g of N,N-diethylacetamide in the three-neck flask using a mechanical agitator under an atmosphere of nitrogen gas. Subsequently, 0.550 g (2.52 mmol) of PMDA was added to the flask. The polymerization was allowed to proceed for 8 h, producing a yellow-colored viscous liquid. Typically, 1.58 g (8.53 mmol) of UV curable monomer and 0.201 g (0.577

f=

LA 1 w ρs

(1)

Here, L is the thickness of the porous PI, A is the area, w is the mass, and ρs is the solid density of nonporous PI. The thickness was obtained by averaging three different measurements at different locations in the film. The area was calculated using an image of the film. The photopolymerization of the tert-amine methacrylate monomer was monitored using a real time FT-IR technique with an ex situ synchronized UV exposure system (VERTEX 70, Bruker Optics, Germany). The absorbance of the methacrylate group at 815.7 cm−1 2276

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Figure 2. Porous PAA and PI films: (a) cross-section of porous PAA, (b) magnified view of part a, (c) photograph of porous PI film, (d) cross section of porous PI film, and (e) magnified view of part d. UV intensity was 76 mW/cm2 and CO2 pressure 6.5 MPa, respectively. was deconvoluted using the method reported in a previous study.40 The conversion of the methacrylate group under UV irradiation was calculated by dividing the measured absorbance by the averaged-initial absorbance measured without UV irradiation for 3 s and subtracting from unity. The imidization of PAA was verified by the change of absorbance at 1776 cm−1 using the FT-IR. The relative dielectric constant and loss tangent of the porous PI films were measured at room temperature using the transmission technique at 20 GHz (KF604A, KEAD, Japan). The tensile modulus and strength of porous PI were measured using a tensile testing machine (AGS-1 kN, Shimadzu, Kyoto, Japan) with pneumatic clamps. Because of the thin porous film, some pieces of sandpaper were applied to the clamps so as not to collapse the porous structure.

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RESULTS AND DISCUSSION Preparation of the Porous PI Film. Micrographs of the cross-section of porous PAA film show that pores with a diameter of 1−2 μm were uniformly formed in the 50 μm-thick film (Figure 2, parts a and b). The majority of pores exhibited an interconnected morphology. A 3 μm skin layer was formed on the upper surface of the film that was exposed to the CO2 atmosphere. Figure 2c shows a porous PI film sample held with tweezers. The film is self-supporting and flexible. Parts d and e of Figure 2 provide the cross sections of the porous PI film. After the heat treatment, the thickness decreased to 30 μm whereas the pore diameter did not change dramatically. The transformation of PAA to PI decreased the pore size of the film. The observed skin layers on both surface of the porous PI film could be utilized to protect the water into the porous layer. Figure 3 shows FT-IR spectra of (a) a solution of the PAA and the monomer, (b) a PAA solution exposed to high-pressure CO2 and UV light, and (c) porous polyimide after the thermal treatment of porous PAA. A peak at 810 cm−1 in spectrum a indicates the existence of the CC bonds of the monomer (tertiary amine methacrylate). After the UV exposure, the spectrum indicates that the peak at 810 cm−1 became a shoulder peak and that its intensity decreased. The photopolymerization with the photoinitiator occurred during the UV irradiation. Spectrum c shows that the imide peak at 1776 cm−1 was formed after the heat treatment. The spectra prove that photopolymerization and imidization occurred in the PAA solution.

Figure 3. FT-IR spectra of (a) a solution of the PAA and the monomer, (b) a PAA solution exposed to high-pressure CO2 and UV light, and (c) porous polyimide after the thermal treatment of porous PAA.

Mechanism of Pore Formation in PAA Film. The formation mechanism of the porous PI precursor is complex and difficult to analyze in the presence of high-pressure CO2. We carefully investigated the formation mechanism of the pores in the film and obtained the following step-by-step explanation. The porogen of the porous PAA film was the solvent droplets. However, the weight fraction of the solvent was a major component of the solution when the solution was cast on the substrate. The solvent could not form droplets in the solution if the composition of the solution was the same as the initial composition. When the wet PAA film was introduced into the view cell, the presence of solvent mist was observed on the upper window of the view cell. The change in the weight of the film before and after it was placed in the view cell was measured. The film weight decreased by 15 wt % of its initial weight. Assuming that the solvent mainly vaporized from the film, the concentration of the solvent decreased from 69 to 36 wt %. The solvent composition changed to that of the minor component in the solution, and the PI precursor became the major component(47 wt %). It was reasonable that the solvent could form droplets after a certain amount of solvent vaporized. The phase separation induced by the dissolution of CO2 did not resemble the nonsolvent-induced phase separation wherein 2277

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of the droplets is assumed to consist of the solvent. The zwitterions are expected to exist on the outside of the droplets. Figure 5 shows the conversion of the UV curable monomers in the mixture of the PI precursor, solvent and photoinitiator

the nonsolvent reduces the solubility of the solute and induces phase separation. We focused on the chemical reactions involving CO2 absorption in amine aqueous solution, which has been widely studied in CO2-capture processes of chemical plants.38,44−46 The tert-amine functional group of the UVcurable monomers is theoretically predicted to form a type of salt (zwitterions) with CO2 molecules in a water-free solvent according to the following equation.47,46The zwitterion is

unstable in water and readily forms carbonic acid and the amine cation. However, the PAA solution had been carefully prepared to avoid water contamination in this study, as the PAA solution is likely to degrade in the presence of water. The zwitterions should be stable in the water-free solution. Thus, zwitterions were possibly formed in the PAA solution. Figure 4a shows a photograph corresponding to the CO2dissolution-induced formation of zwitterions of CO2 and the

Figure 5. Real time FT-IR measurement of the photopolymerization of the monomer (76 mW/cm2, 60 s).

under UV irradiation of 76 mW/cm2. Up to 30% polymerization of the tert-amine monomer was obtained within 60 s. The low conversion is due to the monomer being monofunctional and the solution viscosity being high due to the solution containing PAA. These two factors limited the probability of molecular collision and reduced the conversion of the monomers. It is expected that a similar extent of photopolymerization occurred under the CO2 atmosphere. The outside of the droplets shown in Figure 4b solidified as they became photopolymerized. When the CO2 pressure was decreased to atmospheric level and zwitterions disappeared, the polymer chains could maintain the shape of the droplets if a sufficient degree of polymerization was achieved. Eventually, the pores formed when the solvent vaporized from the droplets. Figure 6 shows the porous PI film formation process from the PAA solution, CO2, and the UV-curable tert-amine monomer. The monomer and the solvent were dispersed in the PAA solution after certain amount of solvent vaporized. When the CO2 was introduced, the monomer and CO2 formed a zwitterion salt. Simultaneously, the solvent droplets were formed in domains surrounded by the salt species. The UV irradiation then induced the polymerization of the monomers under high-pressure CO2. After the CO2 pressure was decreased, the solvent droplets vaporized to form pores in the film. Effects of CO2 Pressure and UV Intensity on Porous Structure. The effect of CO2 pressure on the pore formation of porous PAA film was investigated by maintaining a constant UV intensity of 152 mW/cm2. Figure 7 shows the pore diameters of the porous PAA and porous PI films. The average pore diameters of porous PI decreased following a heat treatment of the porous PAA. The pore diameter at 0.99 MPa of CO2 decreased from 0.69 to 0.36 μm. However, the resulting nanoporous film might have decreased the void fraction of the porous PI to 0.5%. The volume of the pores was too small to reduce the relative dielectric constant. Porous PAA could not be obtained at 2.2 and 3.0 MPa of CO2, so the corresponding plots are not shown. At these CO2 pressures, the phase separation induced by the zwitterions was not as effective

Figure 4. Optical microscope images. (a) CO2-dissolution-induced formation of zwitterions of CO2 and UV curable tert-amine monomers. The UV-curable monomer was 2-(diethylamino)ethyl methacrylate. (b) Formation of solvent droplets in the film. The CO2 pressure was 6.5 MPa.

UV curable tert-amine monomer binary system. The black band extending from the top right to the bottom left of the image represents the interface between the monomer (left) and the CO2 atmosphere (right). The CO2 molecules diffused into the monomer droplet and formed the zwitterions, which appear as black dots to the left of the interface. The zwitterions immediately formed after the dissolution of CO 2 and disappeared when the CO2 pressure was decreased to atmospheric pressure. The formation and the disappearance of zwitterions could be repeated more than 10 times by the change of CO2 pressure. When 5 wt % water was added to the solution prior to exposure to CO2, the zwitterions did not form. In addition, our preliminary experiments revealed that the monoalkyl and the secondary amines did not form the zwitterions in the solvent and CO2. These results support the theoretical prediction regarding the formation of zwitterions between the tert-amine and CO2. The PAA/solvent/UV curable monomer/photoinitiator film exposed to CO2 was observed using the optical microscope; the resulting image is provided in Figure 4b. The black band represents the interface between the mixture (top) and the CO2 atmosphere (bottom). A liquid crystalline-like phase separation formed in the mixture. As the UV curable monomer and the CO2 form solid particles composed of zwitterions, each domain 2278

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Figure 6. Formation of porous PI films from a PAA solution, CO2, and a UV-curable tert-amine monomer.

Figure 8. Effect of UV intensity on the pore diameters of porous PAA and PI. Solid squares: porous PAA. Open circle: porous PI. The error bars indicate the standard deviation of pore diameters.

Figure 7. Effect of CO2 pressure on the pore diameters of porous PAA and PI. Solid squares: porous PAA. Open circles: porous PI. The error bars indicate the standard deviation of pore diameters.

resulted in more zwitterions in the solution and the formation of larger number of solvent droplets. Assuming that the amount of solvent in the solution was same for all CO2 pressure conditions, the larger number of solvent droplets reduced the size of droplets due to the mass balance of solvents. The effect of the UV intensity on the structures of porous PAA and PI was investigated by maintaining a constant CO2 pressure of 6.5 MPa. The pore diameter of porous PI became lower than that of PAA with increasing UV intensity. The pores in porous PAA shrunk after undergoing a heat treatment. Without UV exposure, the average pore diameter was 0.69 μm even though the pores disappeared after the heat treatment. A UV intensity of 38 mW/cm2 was also applied; however, the pores could not be preserved by the UV irradiation and did not survive the heat treatment. Nonexposure and lower intensity of the UV light could not cause sufficient polymerization of the monomer in the solution. A lower degree of polymerization resulted in lower viscosity and made it difficult to maintain the shape of solvent droplets as porogen species.

because the concentration of CO2 was not sufficiently high and solvent droplets were not generated. However, at 3 MPa of CO2, a porous PI was obtained by the heat treatment of nonporous PAA film that was subjected to CO2 and UV irradiation (Figure 8). The formation of pores occurred as a result of the vaporization of residual solvent in the film. The pore diameter and the porosity were 0.47 μm and 8%, respectively; however, the porosity was insufficiently low for use as a low-dielectric constant film. Using CO2 pressures of 4−6.5 MPa, we successfully created porous PAA and PI films with a high porosity and 1 μm pores. The porosities at 4, 5, and 6.5 MPa were 52%, 57%, and 65%, respectively. Thus, a minimum of 4 MPa of CO2 was required to create a high volume fraction of pores in the PAA film. As shown in Figure 7, the average pore diameters of porous PAA decreased slightly with increasing CO2 pressure. An increase in CO2 pressure corresponds to an increase in the concentration of CO2 in the solution. The increased concentration of CO2 2279

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Figure 9. Relative dielectric constant (a) and the loss tangent (b) of porous PI film as a function of the porosity.

Figure 10. (a) Tensile modulus and (b) strength of porous PI films as a function of porosity.

At a UV intensity greater than 76 mW/cm2, the porous structures were successfully obtained in the PAA and PI films. The average pore diameters of both the PAA and PI decreased as the UV intensity increased. Sufficient intensity of UV light resulted in higher degrees of polymerization of the monomer, i.e. 30%, and allowed the droplets to maintain their shape after heat treatment. Dielectric Constant and Loss Tangent. The relative dielectric constant and loss tangent of the porous PI films are shown in Figure 9. Figure 9a shows the relationship between the relative dielectric constant and the porosity. The dielectric constant and the porosity exhibit an inverse relationship. The line in the plot is the theoretical prediction of a parallel model, extending from a relative dielectric constant of 3.4 for the nonporous PI to the value of 1 for air.28 The measured relative dielectric constant data fit this line. It is reasonable that the relative dielectric constant of the porous PI film decreases with an increase of the porosity. The relative dielectric constant was less than 1.5 at a porosity of greater than 70%. The films with a relative dielectric constant of less than 1.5 have an uneven surface and large voids and thus cannot be used for flexible printed circuit materials. Figure 9b shows the loss tangent of the porous PI film. The horizontal line in the plot represents the loss tangent of solid PI. The loss tangent of porous PI is greater than or equal to that of solid PI. Although the five data points for porous PI for a porosity of greater than 70% exhibited lower relative dielectric constant values than those of the solid PI, these films have an uneven surface and large voids. With respect to reducing the signal attenuation in an electrical circuit, a reduction in the loss tangent is more important than the relative dielectric constant. The polymers formed by the photopolymerization of the monomer may affect the PI chain during the heat treatment, or

the residues of the pyrolyzed polymer and the copper substrate etched by nitric acid may affect the loss tangent as an impurity. Furthermore, to minimize the loss tangent, the temperature profile of the heat treatment to remove the polymer prior to the imidization of the PAA and the etching process of copper substrate should be optimized. Tensile Modulus and Strength. Figure 10 shows the tensile modulus and tensile strength of the porous PI films. The tensile modulus linearly decreased with increasing porosity. The porous polyimide was barely applied to the current specification of flexible cable and it exhibited a low dielectric constant, as shown in Figure 9, which is an unavoidable trade-off of materials engineering. To achieve low-loss data transmission in a low-dielectric-constant flexible print cable, a support material should be adhered onto the porous film without increasing the film’s dielectric properties.

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CONCLUSION A porous PI film was prepared from a porous PI precursor in which pores were formed via a CO2 dissolution-induced phase separation and photopolymerization of tert-amine monomer in a PI precursor solution. The pore formation in the PI precursor required only 2 min, which is significantly less time than that the time reported in previous studies. The porosity of the porous PI film exceeded 70%, the relative dielectric constant decreased to 1.5, and the film maintained high flexibility. The mechanical properties of solid films are obviously diminished when pores are introduced into the films. Although we could reach the ultralow-k level, after the pores were introduced, the degradation of the mechanical properties was unavoidable. In the near future, an increase in data transmission speeds will be indispensable in enhancing communications through mobile devices. Strong demand exists for the 2280

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development of a “new” type of low-k material. An opportunity now exists to reinvent the porous low-k film with a special advantage of a shortened processing time. The use of high-pressure CO2 is both an advantage and a disadvantage. The current disadvantage is that the process is a batch process. The construction of a roll-to-roll continuous process has thus far proven difficult. The advantages lie on the formation of zwitterions and in phase separation. Phase separation can be induced through the addition of highpressure CO2, which quickly vaporizes after the pressure is released. This approach dramatically shortens the processing time compared to that of the conventional nonsolvent-induced phase separation process. The high-pressure CO2 is indispensable in conducting this process. The developed process will facilitate the manufacture of high-end electronic devices that require a low-k film at the initial stage of application. Moreover, the developed process has considerable potential; specifically, the process could be used for the fabrication of a porous PI film in a high-throughput process for the semiconductor industry.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-75-383-2696. Fax: +81-75-383-2696. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Shinsuke Nagamine for helpful discussions and Mr. Akira Mizoguchi for attracting me to the field of porous PI film and flexible print cables for highthroughput signal transmission. The dielectric properties of porous PI film were measured with Mr. Ken Tahara (KEAD) with his great helps. This study was supported by the New Energy and Industrial Technology Development Organization, Japan (NEDO, 09A16003d).

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dx.doi.org/10.1021/ma302406m | Macromolecules 2013, 46, 2275−2281