Counterion-Induced Control of the Colloidal State of Polyamic Acid

Dec 7, 2017 - This is consistent with our previous explanation that the partially formed colloid develops at low ϕTPA (high TEOA content), and the di...
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Counterion-Induced Control of the Colloidal State of Polyamic Acid Nanoparticles for Electrophoretic Deposition Dae Ho Lee,*,†,‡ Yuri Bae,† Yu Na Kim,† Joo Yeon Sung,†,§ Se Won Han,† and Dong Pil Kang† †

Insulation Materials Research Center, Korea Electrotechnology Research Institute, Changwon 51543, South Korea Electro-Functionality Materials Engineering, University of Science and Technology, Daejeon 34413, South Korea § Department of Material Science and Engineering, Pusan National University, Pusan 46241, South Korea ‡

S Supporting Information *

ABSTRACT: Optimizing the colloidal state of polyamic acid (PAA) nanoparticles is essential for achieving a uniform and highperformance polyimide coating by electrophoretic deposition (EPD) on metal substrates of various shapes. In this paper, we report two important roles of the counterions in the formation of PAA colloids for EPD, which, to date, have not been recognized. First, when tertiary alkyl amines are used to neutralize PAA, the polarity of neutralizing counterions determines the size and stability of the PAA colloidal particles. The polarity can be finely tuned by using two different tertiary alkyl amines containing polar and nonpolar groups and adjusting the molar ratio. Depending on the polar/nonpolar ratio, various states of PAA colloids were obtained, including dissolved state, stable colloid, and aggregates. Second, we observed that the confined counterions inside PAA nanoparticles can act as an imidization catalyst during the thermal annealing process. It is revealed that some fraction of the counterion species, mostly having nonpolar groups, is not drawn toward the counter electrode and remains inside the PAA nanoparticles during the EPD process. Optimizing the polarity eventually allowed us to form uniform EPD coatings with high dielectric strengths.

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INTRODUCTION Electrophoretic deposition (EPD) is an important technology for colloidal coating and has drawn considerable interest for use with traditional ceramics because of its advantages, such as rapid formation, simplicity of its apparatus, ease of thickness control, freedom of the substrate shape, and reduced environmental issues because water is used as the solvent.1−4 The mechanism of EPD coating is considered to have two steps: the charged particles suspended in liquid (colloidal particles) move toward the oppositely charged electrode (electrophoresis), followed by accumulation at the deposition electrode (deposition), giving rise to a compact and homogeneous film. Thus, EPD produces uniform coatings on a broad range of shapes, including complex three-dimensional and porous structures.1,4−8 Recently, EPD has been recognized as a useful coating technique with various novel applications in advanced materials including functional composites, layered and functionally graded materials, thin films, biomaterials, nanoparticles, and carbon nanotubes.6−10 With the rapid development of miniaturized and highly efficient electrical/electronic devices, suitable technologies for the coating of a thin yet thermally and electrically insulating layer on fine-structured metal substrates has become increasingly important. Polyimides (PIs) have been widely used in various industrial fields, such as automobiles, microelectronics, displays, and aerospace, because of their excellent thermal © XXXX American Chemical Society

stability, chemical resistance, and electrical and mechanical properties,11−14 and there have been considerable efforts to make PI coatings by EPD15−26 to widen the applicability of PIs for specialty use, for example, uniform electrical insulation coatings on a fine rectangular structure or flat wire.19−22,25,26 PI is traditionally obtained by thermal imidization of polyamic acid (PAA), which is synthesized by stepwise polymerization of dianhydride and diamine monomers. However, this method of preparing PIs has some disadvantages, such as the use of toxic organic solvents, the need for a high imidization temperature, and the low hydrolytic stability of PAA precursors.27 Thereafter, it was reported that the rate of thermal imidization is significantly enhanced when PAA is neutralized to a salt form by the treatment of tertiary amines such as triethylamine and N,N′-dimethylethanolamine or quaternary amines such as tetraethylammonium and tetrabutylammonium.28−30 In addition, the resulting PAA salts have been found to have long-term hydrolytic stability and increased solubility in various solvents.27−30 This ionic nature of the PAA salt, which has greater solubility than PAA itself, can prevent the precipitation of PAA and yield a sufficient surface charge for colloid formation when dispersed Received: September 19, 2017 Revised: November 30, 2017 Published: December 7, 2017 A

DOI: 10.1021/acs.langmuir.7b03293 Langmuir XXXX, XXX, XXX−XXX

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counterion is a critical factor determining the structure and stability of the PAA colloid. Moreover, there are no published studies concerning the investigation of the colloidal structures formed in the presence of different neutralizing agents. In this study, the actual colloid structure with respect to the polarity of the counterion is presented by directly observing the frozen state of the PAA colloids using cryo-transmission electron microscopy (TEM). Furthermore, we reveal another important role of the counterion as an imidization catalyst. This catalytic role is not expected from the mechanism described in Figure 1. By optimizing the PAA colloids by adjusting the polarity of the counterions, highly electrically insulating, uniform PI coatings were obtained, and these were successfully coated on metal substrates with complicated shapes such as flat coil and micromesh.

in poor solvents such as water and alcohol. Similar to other charged particles in solution, the PAA colloidal particles have electrophoretic mobility;17,24 thus, they can be applied to the EPD coating of a metal substrate. In the EPD process, electrophoretic migration of charged particles from the bulk to the oppositely charged electrode is followed by coagulation and deposition at the electrode surface. Despite the broad application of the EPD technique, the exact mechanism of deposition at the electrode after electrophoretic migration is not entirely understood.1−4 Several mechanisms have been suggested, such as flocculation by particle accumulation,3,31 particle charge neutralization,32 electrochemical particle coagulation,33 and electrical double-layer distortion and thinning.4,34 Concerning PAA, a possible mechanism is shown in Figure 1 based on a previous study.15

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METHODS

Materials. Pyromellitic dianhydride (PMDA, purity 97%), 4,4′oxydianiline (ODA, purity 97%), and triethanolamine (TEOA, purity 99%) were purchased from Sigma-Aldrich (Yong-in, Korea). N,N′Diethylformamide (DEF, purity 99%) and tripentylamine (TPA, purity 98%) were purchased from Tokyo Chemical Industry (Seoul, Korea). Diamino-terminated siloxane (AmSO, molecular weight = 976) was purchased from DAMIPOLYCHEM (Iksan, Korea, purity 95%). PMDA and ODA were dried at 120 °C, and DEF was vacuumdistilled prior to use. Other chemicals were used as received. Considering the monomer composition of PAA, the purity of synthesized PAA is estimated to be ∼97%. Synthesis of PAA and Preparation of the PAA Colloid. PAA was synthesized by stepwise condensation polymerization using PMDA, ODA, and AmSO (molar ratio of 1/0.8/0.25) in DEF (20 wt % solid content) at 25 °C under a nitrogen atmosphere for ∼18 h. The PAA salt was prepared by adding TEOA and TPA (total 2.0 mol equivalent to PDMA) at various molar ratios at 25 °C for ∼24 h under a nitrogen atmosphere. Then, distilled water was slowly (over 1 h) added to the PAA salt (weight ratio of DEF/water = 3:7), and thus, various states of PAA salt solutions were obtained, from dissolved to stable PAA colloid to aggregates. The resulting PAA colloids were stirred gently for ∼24 h. EPD of the PAA Colloids. PAA colloids (300 g) were stirred gently in a stainless-steel container (cathode, 500 mL volume, 80 mm diameter). A Cu substrate (anode, width/length/thickness = 30/50/ 0.3 mm) was placed in the stainless-steel container. Then, a voltage of 50−200 V was applied for 1 min from a dc power supply. The coated Cu substrates were then thoroughly washed with distilled water and dried with flowing air, followed by annealing at 200 °C for 3 h. Other substrates were also tested using a Cu flat coil (width/thickness = 4.0:0.2 mm) and an SUS mesh (#200). Characterization. PAA Properties. The viscosities of PAA and the PAA salt were measured using a Brookfield DV-III Ultra viscometer (RV model) at a speed of 10 rpm and a bath temperature of 30 °C. The glass transition temperature was analyzed using a dynamic mechanical analyzer (TA, DMA Q800) in a tension mode at a frequency of 1 Hz and a heating rate of 5 °C/min from −150 to 300 °C. For this, a PI film was prepared by casting PAA onto a rubber mold, followed by heat treatment at 200 °C for 3 h. The thermal stability was investigated by thermogravimetric analysis, measuring the weight loss during heating from 100 to 800 °C under air flow at a heating rate of 10 °C/min (SDT Q600, TA Instruments). Colloid Properties. The viscosity of the PAA colloid was measured with the same method as described above. The pH was measured using a pH meter (SevenCompact, Mettler Toledo). The colloidal particle size was measured by dynamic light scattering (DLS) (BT-90 Nano Laser Particle Size Analyzer, Bettersize Instruments), and the zeta potential was measured using a Malvern Zetasizer (ZS90, Malvern). For DLS and zeta potential experiments, the colloids were diluted using the DEF/water mixture with the same ratio in PAA colloids (DEF/water = 3/7 weight ratio). See the Supporting

Figure 1. Schematic of EPD of the PAA colloid reconstructed based on a previous report.15

Figure 1 shows that the salt is ionized to produce a tertiary ammonium cation and the carboxylic anion of PAA, which subsequently migrate to the cathode and anode, respectively, under the influence of the electric field. Then, with the electrolysis of water, tertiary ammonium cations revert to tertiary amines at the cathode by reaction with hydroxyl ions, and carboxylic anions revert to carboxylic acids at the anode by reaction with protons; that is, the PAA salt becomes PAA near the electrode. Thus, less soluble PAA will be coagulated and deposited at the electrode. Although the above mechanism explains the PAA deposition process well, we will show that deposited films may contain some neutralizing agents; that is, deposition in the form of the PAA salt can occur, depending on the type of tertiary ammonium cations, which will be discussed in the text. Among the various parameters involved in the EPD process, such as the type and content of the neutralizing agent, polymer concentration, type of solvent, magnitude and manner of applied voltage, and deposition time,15−17,35,36 the present study focuses on the neutralizing counterions. Regarding the neutralizing agent, several types of amines such as trimethylamine, triethylamine, 1-methylimidazole, urea, and N,N′dimethylbenzylamine have been tested. Previous studies have focused on finding the optimal processing conditions by observing the variance in coating thickness and change in solution properties such as pH, ion conductivity, and colloidal stability.15−17 Here, we present a new perspective on the neutralizing agents by demonstrating that the polarity of the B

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Langmuir Information (Figure S1, Table S1) for more details. The colloidal structure was directly investigated by cryo-TEM using PAA colloids without dilution as per the standard cryo-TEM protocol.37 Briefly, 5 μL of a PAA sample was injected onto a Quantifoil grid (Cu 200 mesh, R2/2, SPI Supplies, U.S.) that had been made hydrophilic using a plasma cleaner. Then, the grid was semiautomatically vitrified using a Vitrobot Mark I (FEI, U.S.) instrument at 100% relative humidity and 4 °C and maintained at liquid nitrogen temperature during sample transfer using a cryo transfer holder (Cryo Holder-626, Gatan, U.S.). Image acquisition was carried out at 120 kV. The sample temperature was carefully monitored throughout the experiment using a SmartSet Controller (Gatan, U.S.), where the temperature was typically maintained at −177 °C. Film Properties. The thermal imidization kinetics was investigated by Fourier-transform infrared (FT-IR) spectroscopy (Jasco 4200) by analyzing the peak intensities at 1775 cm−1 arising from the CO stretching of the imide group based on the internal standard peak at 1500 cm−1 arising from the C−C stretching of the benzene ring. The remaining counterion species in the PAA film was analyzed by measuring ion conductivities of ethanol (EtOH, 30 g) solutions after coatings (∼8 μm thickness, 2 × 2 cm2) had been soaked for 1 day at 50 °C using a conductivity meter (SevenEasy, Mettler Toledo). The dielectric strengths of the PI films formed by EPD coating on Cu sheets (width/length/thickness = 30 × 50 × 0.3 mm) were investigated by measuring the breakdown voltage (BDV) as per ASTM D149 using a ball-to-plate electrode geometry in an air atmosphere at 25 °C. ac voltage (2 kV/min) was applied at 60 Hz until dielectric failure occurred, and the maximum voltage at this dielectric failure accompanying the increase in conductance was recorded as the BDV. Film Morphologies. The top and cross-sectional morphology of the EPD-coated films were investigated by a scanning electron microscopy (SEM, S-4800, Hitachi) instrument. Cross-sectional images (Figure 5c,f) were taken by observing the fractured area generated during the bending of the PI-coated Cu sheets by 180°. The morphologies at the edge corners of the PI-coated Cu substrates (Figure 7a) were investigated by focused ion beam (FIB) SEM in an FEI Scios FIB/ SEM dual beam system. A Cu flat coil (width/thickness = 4.0/0.2 mm) was used as the substrate for EPD coating. The regions of interest were preserved by the deposition of a platinum layer, and trenches were created using an etching procedure with an acceleration voltage of 30 kV and currents in the range 7−50 nA, depending on the effective area to be removed. The SEM images were collected at the center and edge regions to investigate the film uniformity over a large area.

Figure 2. Characterization of synthesized PAA and PI. (a) FT-IR spectra of PAA (black) and PI (red). PAA was dried under vacuum at room temperature for ∼1 day to remove the solvent, and PI was prepared by heat treatment at 200 °C for 3 h. (b) Dynamic mechanical analysis and (c) thermogravimetric analysis of PI films.

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RESULTS AND DISCUSSION PAA Synthesis. PAA was synthesized by conventional stepwise polymerization using PMDA as the anhydride monomer and ODA and diamino-terminated siloxane oligomer (AmSO) as the diamine monomers. After reaction at 25 °C under an N2 atmosphere for ∼18 h, a viscous polymer solution (∼1400 mPa s, Brookfield Viscometer) was obtained. The FTIR spectrum in Figure 2a shows the characteristic peaks from PAA1540 cm−1 (C−N vibration) and 1655 cm−1 (−CO−N carbonyl stretching) corresponding to the amide linkage in PAA27,29,30 (PAA structure is also verified by the 1H NMR spectrum27,29,30 in Figure S5). For the characterization of the material properties after curing, synthesized PAA was thermally imidized into PI by thermal annealing at 200 °C for 3 h. It can be seen in Figure 2a that the characteristic peaks of PAA (1545, 1655 cm−1) disappear, and new peaks at 1380 cm−1 (C−N stretching) and 1775 cm−1 (CO asymmetric stretching) corresponding to the imide linkages appear.27,29,30 This confirms that synthesized PAA was completely converted into PI after curing. Figure 2b shows that the resulting PI has two distinguishable glass-transition temperatures (Tg) at about

−125 and 250 °C, indicating a block copolymer structure with siloxane-rich (lower Tg) and PDMA−ODA-rich (higher Tg) segment blocks. Between these two Tg values, an additional weak Tg peak can be observed at ∼60 °C, indicating a random copolymer structure. These results suggest that synthesized PAA is a mixture of block and random copolymers. A relatively high thermal stability against oxidative degradation was observed, as shown by the 5 wt % loss temperature under an air flow of ∼420 °C (Figure 2c). Role of the Counterion as a Critical Factor Controlling the Colloid Structure. The carboxylic acid groups of the amic acid groups of the above-mentioned PAA were neutralized using tertiary amines. In this step, the viscosity substantially increased (∼3000 mPa s), indicating the formation of a polyelectrolyte salt with carboxylate anions and tertiary ammonium cations.38 The formation of a colloidal dispersion was observed on the addition of water as a poor solvent. As C

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Figure 3. Colloidal properties of PAA with various TPA/TEOA ratios. (a) Images showing various states of PAA salt solutions after the addition of water depending on the TPA/TEOA molar ratio. The numbers in brackets in the upper part of the image represent the molar fraction of TPA [ϕTPA = mTPA/(mTPA + mTEOA), where m denotes the number of moles of each tertiary amine]. (b) Particle sizes measured by DLS, zeta potentials of the PAA salt solutions, and (c) viscosities of the corresponding samples. (d) Schematic illustrating the formation of the PAA colloid depending on the polarity of the counterions (TPA/TEOA ratio) [R1 denotes the diphenyl ether group in the dianhydride monomer (PMDA), and R2 denotes phenyl (ODA) and siloxane (diamino-terminated siloxane) groups in diamine monomer].

TEOA). As shown in Figure 3b,c, an increase in the colloid particle size, a maximum zeta potential value (Figure 3b), and a rapid decrease in the viscosity (Figure 3c) were observed with the increasing TPA concentration. All these properties reflect the structural changes in the PAA salt solutions induced by the counterions. These features are interrelated, and we suggest the following scenario based on the schematic in Figure 3d. With TEOA alone (A, Figure 3d), the PAA salt behaves as a polyelectrolyte salt that is soluble in the polar solvent [DEF/ water mixture (3/7 weight ratio) solvent in this study]. On incorporating TPA, the solubility of the PAA salt decreases, resulting in the formation of a colloid stabilized by the surface charge from the large amount of polar TEOA. In this case (low TPA/TEOA ratio), a small fraction of the PAA salt, probably polymer chains having higher molecular weights that are less soluble than those of lower molecular weights, will start forming a colloid. This gives rise to the partial development of the PAA colloid (B, Figure 3d). In this scenario, counterion binding with the PAA anion is assumed, which is commonly observed for polyelectrolyte salts.39 In this state, the viscosity will be high because of the presence of the PAA salt in the bulk solution. With further increasing TPA content (C, Figure 3d),

discussed here, we focus on the role of the counterions formed after neutralization in the formation of PAA colloids. For this purpose, two different amines, TPA and TEOA, were tested to form the counterion species. As shown in Figure 3a, PAA neutralized with TEOA became a transparent solution after the addition of water. In contrast, the PAA treated with TPA became turbid after the addition of water, indicating the formation of a colloid. However, the colloid was not stable and was highly aggregated. On the basis of these observations, various molar ratios of TPA and TEOA were tested, and indeed, various states of dispersions were formed, from a dissolved solution to a stable colloidal dispersion to aggregated precipitates. TEOA has a hydroxyl group that is polar and soluble in water, whereas TPA has only an alkyl group that is nonpolar and insoluble in water. Thus, Figure 3a indicates that the polarity of the tertiary amine is an important factor determining the PAA colloidal structure. Different solution properties were observed depending on the TPA/TEOA ratio. The pH was in the range of 6.8−7.2, increasing slightly with the increasing TPA content, which is attributed to the higher basicity of TPA compared to that of TEOA (pKa ≈ 9.9 for TPA and ∼7.8 for D

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Figure 4. Cryo-TEM images of PAA colloids with various TPA/TEOA ratios. ϕTPA = (a) 0.125, (b) 0.25, (c) 0.5, and (d) 0.75 (scale bar: 200 nm).

Figure 5. BDVs of PI coatings. BDV values with various thicknesses depending on the applied voltages during the EPD of the PAA colloids of (a) ϕTPA = 0.5 and (d) 0.125. Top (left) and cross-sectional (right) SEM images of PI films coated by EPD (applied voltage = 50 V) using PAA colloids of (b,c) ϕTPA = 0.5 and (e,f) 0.125.

aggregation of larger particles is observed and the colloid has a reduced zeta potential. To investigate the colloidal structure in more detail, PAA colloids with different ϕTPA values were observed by TEM. Here, to verify the actual structure of the PAA colloids, cryoTEM was employed, and the as-prepared samples (without further dilution) were investigated in a frozen state at about −177 °C under liquid nitrogen. Using frozen samples avoids possible factors that can affect the colloid structure such as swelling, shrinkage, or the evaporation of the diluting solvents or film formation during drying. Figure 4 shows representative cryo-TEM images taken from the frozen PAA colloidal dispersions. As shown by the DLS measurements (Figure 3b), the particle diameter increased with increasing TPA content: 55 ± 14 nm (ϕTPA = 0.125), 41 ± 11 nm (ϕTPA = 0.25), 70 ± 31 nm (ϕTPA = 0.5), and 343 ± 213 nm (ϕTPA = 0.75). The discrepancy of the particle size between DLS measurements and cryo-TEM observation may be due to several factors such as particle aggregation, adsorbed molecules on the surface, and different sensitivity to size distribution between two methods.40 DLS and zeta potential measurements can also be dependent on the

the PAA salt will become more insoluble in the DEF/water mixture, and a larger fraction of PAA salt chains will become incorporated into the colloid, increasing the size of the PAA colloidal particles. In addition, the magnitude of the zeta potential will increase because of the more developed colloid structure and the lower concentration of the PAA salt in the bulk solution. In this state, the viscosity will decrease because of the decreasing amount of the dissolved PAA salt in the solution. With TPA alone (D, Figure 3d), most of the counterion species (TPA) will reside inside the particles to minimize contact with water. Consequently, most of the ionic groups will be confined inside the PAA particles, resulting in increased hydrophobic interparticle attraction and decreased electrophoretic mobility because of the more closely bound state between ionic species. This explains the decrease in the zeta potential (less negative) and the increase in the aggregation of larger colloid particles at a higher TPA content. From the results shown in Figure 3, the critical ϕTPA for transforming most of the PAA salt into a colloid is ∼0.4, based on the substantial decrease in the viscosity, increase in zeta potential, and larger particle size. Furthermore, the region of stable colloidal dispersion is suggested to lie between 0.4 and 0.6, above which the E

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Figure 6. Imidization kinetics of PAA films. (a) FT-IR spectra of spin-coated PAA (black), PAA salt (blue), and PAA by EPD (red) obtained after annealing at 120 °C for 1 h. The increasing peaks at ∼1775 and ∼1380 cm−1 from the imide group and the decreasing peak at ∼1540 cm−1 from the amide group indicate that the imidization process has occurred. The peak at ∼1500 cm−1 (C−C stretching peak from the benzene ring) was used as an internal standard for analysis.30 (b) Increase in the peak intensity at 1775 cm−1 from the imide group is clearly observed in this study, for example, for EPD−PAA with increasing annealing temperature. The degree of imidization was determined from the peak intensity at 1775 and 1500 cm−1 and calculated as (I1775/I1500)/(I1775/I1500)* × 100 (%), where the asterisk denotes the value at complete imidization achieved at 200 °C, above which the change in the peak ratio was insignificant. The calculated values of the degree of imidization with respect to the annealing temperature (c) and time at a fixed temperature of 150 °C (d) show that the imidization rates increase in the order of PAA, EPD PAA, and PAA salt. (e) Schematic illustrating that the nonpolar counterion molecules confined inside the PAA particles are forced to move together with the particle, followed by co-deposition on the Cu electrode.

the particle size with increasing TPA content. With further increasing TPA content (ϕTPA = 0.75, Figure 4d), aggregated particles with large sizes and a broad size distribution were observed. Thus, Figure 4 supports the mechanism of colloid formation proposed in Figure 3d. Therefore, the optimal conditions for PAA colloid formation can be achieved by adjusting the polarity of the counterions. Electrophoretic Deposition. Next, the PAA colloid was applied to EPD onto Cu sheets. On the basis of the above discussion, the PAA colloid at ϕTPA = 0.5 (stable colloid) was selected, although ϕTPA = 0.125 (partially formed colloid) was also tested for comparison. The samples with higher TPA content were found to be in an aggregated state and were thus not considered. For possible application to electrical insulation coatings, the dielectric strength was investigated by measuring the BDV. Figure 5a shows the film formed at ϕTPA = 0.5 (stable colloid). The film thickness increased with increasing applied

accuracy of physical parameters (relative permittivity, refractive index, and viscosity) of solvent mixtures used for data analysis (see the Supporting Information with Figure S1 for more detailed discussion). It is believed that the particle size observed by cryo-TEM is more accurate because the PAA colloids were quickly quenched and directly observed in a frozen state that is close to the actual state of colloidal dispersion. In addition, because the samples were investigated without dilution, the comparison of the relative differences in the particle concentration is meaningful. The colloid particles were rarely observed at low TPA content (Figure 4a), whereas larger particles with a larger population were observed with increasing TPA content (Figure 4c). At ϕTPA = 0.25, as shown in Figure 4b, an intermediate state between those of ϕTPA = 0.125 and ϕTPA = 0.5 was observed. This is consistent with our previous explanation that the partially formed colloid develops at low ϕTPA (high TEOA content), and the dissolved PAA salt chains become incorporated into the colloid, resulting in an increase in F

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(∼6.0 μS/cm) compared to the uncoated Cu sheets (∼2.0 μS/ cm) is attributed to the dissolved amic acid groups in the uncured PAA films. Thus, the ion conductivity should be increased if any additional ionic species (counterions) are present in the film. As shown in Table 1, the ion conductivity is

voltage (coating time = 1 min for all cases). Accordingly, the BDV increased with increasing thickness in the range of 1.5− 3.5 kV. On the other hand, as shown in Figure 5d, for the partially formed colloid dispersion (ϕTPA = 0.125), the thickness change was not well-controlled with respect to the applied voltage. Furthermore, the BDV ranged from 0 to 0.7 kV, which is much lower compared to that of the ϕTPA = 0.5 sample. This result may be related to the morphologies of the EPD films. While the PI film formed at ϕTPA = 0.5 is uniform over a large area with the occasional observation of particles (Figure 5b), that formed at ϕTPA = 0.125 has an irregular, aggregated structure (Figure 5e) composed of small particles with diameters similar to that of the independent PAA colloid, as shown in Figure 4. In addition, large pore defects are frequent in this case (Figure 5f). On the basis of the crosssectional images in Figures 5f and S2, the areal porosity of this film is estimated to be ∼10%. In contrast, the film formed at ϕTPA = 0.5 has a much denser structure (Figures 5c and S2). In the PAA colloid of ϕTPA = 0.125, a low density of PAA particles and a large amount of PAA salt chains between particles may prevent the close packing of particles. In contrast, for the PAA colloid at ϕTPA = 0.5, a higher particle density near the electrode led to the denser packing of particles and the formation of uniform films by particle coalescence during annealing at high temperature, which is responsible for the higher degree of electrical insulation. The BDV values of the EPD PI films were similar or slightly higher than those obtained from the spin-coated films using the corresponding PAA resins (Figure S3). The BDV and dielectric strength values in Figures 5 and S1 are also in a similar range to those of PI-based polymer coatings.41 Thus, this result demonstrates that high electrical insulation coatings can be successfully achieved by EPD using the stable PAA colloids developed in this study. Role of Counterions as Imidization Catalysts. The thermal imidization behavior of EPD-coated films was investigated by FT-IR spectroscopy. For comparison, spincoated films using PAA and PAA salts dissolved in DEF (before water addition) were also investigated. Figure 6 shows the results. First, the PAA salt imidizes faster than the corresponding PAA upon heating (Figure 6c,d). This result indicates that the neutralizing molecules (counterions) act as catalysts for the thermal imidization, as in the case of other tertiary amines.28−30 In addition, note that the EPD-coated films exhibit an intermediate behavior between those of the PAA and PAA salt. Considering that the EPD-coated films were thoroughly washed right after the coating process, this result strongly suggests that some fraction of counterions remained inside the PAA colloid, even after deposition at the Cu electrode. This result is not consistent with the previous understanding of the EPD of PAA, as shown in Figure 1.15 Because the countercations are attracted to the cathode and revert to tertiary amines, the mechanism in Figure 1 does not explain the presence of counterion species in the deposited films. To further verify the presence of counterions in the EPDcoated PAA film (EPD−PAA), another experiment was conducted. The Cu substrates coated by PAA, PAA salt, and EPD−PAA dried at 100 °C were placed in vials containing EtOH at 50 °C for 1 day, and after the removal of the Cu substrates, the ion conductivities of the EtOH solutions were measured. The PAA films before imidization were found to have dissolved in EtOH, and the increased ion conductivity

Table 1. Ion Conductivities of the EtOH Solutions after the Coatings Had Been Soaked for 1 day at 50 °C samplesa

ion conductivity (μS/cm)

PAA EPD−PAA PAA salt Cu sheetc

6.0 ± 0.8 (2.0 ± 0.3)b 8.3 ± 0.5 (2.1 ± 0.4) 10.8 ± 1.1 (2.3 ± 0.3) 2.1 ± 0.3

a Samples: coated films (∼8 μm thickness) on Cu sheets (2 × 2 cm2) were placed in vials filled with EtOH (30 g). bAfter removing the PAAcoated Cu sheets, the ion conductivity of EtOH was measured. Films annealed at 100 °C were analyzed. For comparison, films annealed at 200 °C were also compared (numbers in bracket). cEtOH that contained an uncoated Cu sheet was also measured as a reference value.

highest for the PAA salt, lowest for PAA, and intermediate for EPD−PAA, which is indeed consistent with the imidization rates derived from the FT-IR measurements. Thus, these results strongly indicate the presence of counterions in the EPD−PAA film. Additional important information was further extracted from the FT-IR results. A large and broad peak around 3300 cm−1 was observed and is attributed to the hydroxyl group of TEOA (Figure S4a). This peak is also observed for the PAA salt film dried at low temperature (100 °C). In contrast, this peak was not observed for the EPD−PAA films (dried at 100 °C). Thus, together with the results shown in Figures 6 and S4a and Table 1, it is likely that the remaining counterions in the EPD−PAA films are mostly TPA, as postulated in Figure 3d. Because the existence of TAA in the EPD−PAA films could not be directly observed by FT-IR, a separate experiment was performed by using a 1H NMR spectrometer. The NMR spectrum was obtained for the EPD−PAA film that was dried at 100 °C for 1 h and dissolved in DMSO-d6. In Figure S5, among the characteristic peaks from TPA and TEOA, only those from TPA were mainly observed in the NMR spectrum of EPD− PAA, which evidences the presence of TPA in the electrodeposited films. Thus, we suggest the following mechanism using Figure 6d: the hydrophilic TEOA, which is soluble and able to freely move in solution, is attracted toward the cathode, whereas the hydrophobic TPA, which is preferably confined inside the particles because of its insolubility in water, is not fully repelled by the anode and is forced to move together with the particle, followed by co-deposition on the Cu electrode. As shown in Table 1, the ion conductivities in EtOH for the annealed samples (200 °C, 1 h) are similar to those of the uncoated substrates, irrespective of the sample type (PAA, PAA salt, or EPD−PAA). This indicates that the counterions were evaporated during the thermal imidization, which is advantageous for obtaining pure PI films after the annealing process. This is in accordance with the FT-IR results, which showed that the intensity of the broad peak at ∼3300 cm−1 (characteristic of TEOA) substantially decreased after annealing at ∼150 °C (Figure S4b), and this temperature is also coincident with the nearly complete imidization (Figure 6c). Thus, our results reveal another important aspect regarding the counterions; that G

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Langmuir

Figure 7. Uniform and contour coatings by EPD on fine-structured substrates. (a) Cross-sectional SEM images of the EPD coatings at the center region and (upper) near the corner edge (lower) cut by the FIB method. (b) Top (upper) and side (lower) views of the EPD coating on the SUS mesh (prepared at an applied voltage of 50 V).

colloid, uniform and contour coatings on fine-structured substrates such as flat coil and micro-mesh were achieved.

is, they are a controlling factor determining the colloidal structure and act as catalysts for the thermal imidization after film formation. Uniform and Contoured EPD Coating on MicroComplicated Structures. As noted in Figure 5, the films obtained using the optimized PAA colloid were uniform over a large area. To check the film uniformity around the edges, a Cu substrate with a flat-coil structure (width/thickness = 4.0/0.2 mm) coated with a PAA colloid (ϕTPA = 0.5) was cut by an FIB, and the cross-sectional images were investigated. As shown in Figure 7a, the EPD films around the corner edge were relatively uniform with similar thicknesses in the central region of the film. To further test the applicability of the EPD process for more complicated structures, the SUS mesh (#200) was tested. As shown in Figure 7b, an excellent contour coating with a uniform thickness of ∼10 μm was achieved, and the original mesh structure was maintained. This result demonstrates that the EPD coating using the optimized PAA colloid can be widely applied to specialized areas such as miniaturized devices and micro-fabric structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03293. Detailed information on the DLS and zeta potential measurement (Table S1, Figure S1); image analysis on the cross-sectional SEM images (Figure S2); dielectric strengths of spin-coated PI films and EPD-coated films (Figure S3); further details of FT-IR spectra of PAA, PAA salt, and EPD−PAA (Figure S4); and 1H NMR spectra of PAA and EPD−PAA (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+82)55-280-1688.

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ORCID

CONCLUSIONS We provide a new method of using counterions to prepare PAA colloids for application to EPD coating. We have demonstrated that the polarity of the counterions is a critical factor determining the particle size and the stability of PAA colloids. By using two counterions with different polarities (polar TEOA and nonpolar TPA) and adjusting the ratio of these molecules, various states of PAA colloids were formed, from dissolved to stable colloid to aggregates. The optimized state of the PAA colloid, which was determined by characterization using DLS, zeta potential, viscosity, and TEM measurements, eventually led to a high-performance electrically insulating film. We found that the nonpolar, hydrophobic TPA counterions were confined to the PAA nanoparticles, acting as catalysts and accelerating the imidization rate during film drying. Therefore, the counterions play an important role both in the formation of a stable PAA colloid, which is necessary to obtain a uniform film during EPD, and in increasing the imidization rate. By using the stable PAA

Dae Ho Lee: 0000-0001-7971-4441 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the KERI primary research program (no. 17-12-N0101-40). We also express our thanks to the Korea Basic Science Institute (KBSI, Ochang) for technical assistance during the cryo-TEM measurements.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b03293 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b03293 Langmuir XXXX, XXX, XXX−XXX