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J. Phys. Chem. C 2008, 112, 15956–15960

Electrical Properties of Well-Dispersed Nanopolyaniline/Epoxy Hybrids Prepared Using an Absorption-Transferring Process Cheng-Dar Liu,† Sung-Nung Lee,‡ Chao-Hsien Ho,‡,§ Jin-Lin Han,*,| and Kuo-Huang Hsieh*,†,⊥ Institute of Polymer Science and Engineering, Department of Chemical Engineering, National Taiwan UniVersity, Taipei 106, Taiwan, Department of Chemistry, Fu-Jen Catholic UniVersity, Taipei 242, Taiwan, Department of Chemical and Material Engineering, Lunghwa UniVersity of Science and Technology, Taoyuan 33306, Taiwan, and Department of Chemical and Material Engineering, National ILan UniVersity, I-Lan 260, Taiwan ReceiVed: April 21, 2008; ReVised Manuscript ReceiVed: July 1, 2008

Novel polyaniline (PANI)/epoxy hybrids have been fabricated using an absorption-transferring process in which no organic solvent is involved. An epoxy prepolymer cured by aniline monomer (DGEBA-aniline) was added to a freshly prepared PANI nanoparticles (NPs) aqueous solution with vigorous agitation and heating (ca. 90 °C). The PANI NPs were absorbed on the surface of epoxy droplet and then transferred into the whole droplet. The microstructures of the product PANI/epoxy hybrids, characterized using scanning electron microscopy, featured well-dispersed PANI NPs (ca. 40∼60 nm) within epoxy matrixes; no large aggregates were observed. The hybrids exhibited huge negative permittivities; the electromagnetic interference shielding efficiency in an electric field at low frequency (100-1000 MHz) was ca. 30-60 dB. The welldispersed PANI NPs not only provided a continuous conducting network but also a higher level of charge delocalization. Introduction Nanotechnology, which is currently one of the most active research fields, takes advantage of the fact that nanomaterials have unique mechanical, electrical, and thermal properties. For example, the high surface areas of conducting nanoparticles (NPs) provide continuous conducting pathways and extremely large interfaces when mixed with polymer matrices. Such composites are usually applied as electromagnetic interference (EMI) materials.1,2 Controlling the dispersion process remains one of the most serious challenges when preparing polymer nanocomposites because the intrinsic van der Waals attractions3 and high surface energies of NPs result in their tending to aggregate together. As a result, such NPs have very low solubilities in most solvents.4 Processes commonly used to overcome the aggregation of nanomaterials include ultrasonication,5,6 high-shear mixing, the addition of surfactants, and surface chemical modification.3,7,8 Polyaniline (PANI) has great potential for use in commercial applications because it is cheap to produce, environmentally stable, exhibits enhanced conductivity, and because its color depends upon its diverse set of redox states. The processing of PANI is difficult because (i) it possesses very low solubility in common organic solvents and (ii) its decomposition temperature is below its glass transition temperature (Tg).9,10 Thus, much effort has been exerted in attempts to modify the polymer chain with alkyl,11 ether, or hydrophilic groups12 to improve the processability of PANI; nevertheless, the crystallinity and the degree of conjugation, and hence the electronic properties,13 are * To whom correspondence should be addressed. (K.H.H.) E-mail: [email protected]. Phone: 886-2-33665314. Fax: 886-2-33665237. (J.L.H.) Phone:886-3-9357400,ext.715.Fax:886-3-9357025.E-mail:[email protected]. † Institute of Polymer Science and Engineering, National Taiwan University. ‡ Fu-Jen Catholic University. § Lunghwa University of Science and Technology. | National ILan University. ⊥ Department of Chemical Engineering, National Taiwan University.

usually poorer than those of PANI homopolymers. An alternative approach toward increasing the solubility is to synthesize PANI NPs14-16 or oligomers,17,18 which not only improves compatibility with polymer matrices but also decreases the percolation threshold.19 Epoxy has several attractive properties, such as mechanical durability, hardness, toughness, and chemical inertness, that complement the intrinsic characteristics of PANI. Much research had been published on the preparation and properties of PANI/ epoxy composites. For example, Tiitu et al. developed several curing processes for blending epoxy with PANI,20 Wei and coworkers synthesized PANI oligomers that they fabricated into homogeneous PANI/epoxy interpenetrating networks (IPNs),19 and Jang et al. used PANI nanorods as curing agents to polymerize liquid crystal epoxy (LCE) resins.21 In this study, we developed a new blending method employing an absorptiontransferring process. An epoxy prepolymer cured by aniline monomer (DGEBA-aniline) was added to a freshly prepared PANI NPs aqueous solution with vigorous agitation and heating (ca. 90 °C). The DGEBA-aniline is insoluble in water and is viscous at this stage. The PANI NPs were absorbed on the surface of epoxy droplet and then transferred into the whole droplet. The final cured epoxy hybrid was obtained after heating at 90 °C for 4 h. A remarkable advantage of this process is that no organic solvent was used in the whole process. We studied the effect of the blending process on the morphology, conductivity, dielectric behavior, and EMI properties of the resulting PANI/epoxy hybrids. Experimental Section Materials. Aniline, dodecyl benzenesulfonic acid (DBSA), and ammonium peroxydisulfate (APS) were purchased from Acros. The epoxy resin, DER331 (DGEBA), was obtained from Dow Chemicals. All aqueous solutions were prepared using deionized water.

10.1021/jp803437v CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

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SCHEME 1: Synthesis of DGEBA-Aniline

SCHEME 2: Schematic Representation of the Absorption-Transferring Process

Preparation of DGEBA-Aniline. DGEBA was mixed with aniline through mechanical stirring for 10 min to obtain samples with aniline-to-DGEBA molar ratio of 0.5. This mixture was cured at 80 °C for 4 h to form the DGEBA-aniline prepolymers. Preparation of the Aqueous Solution of PANI-DBSA NPs. Aqueous micellar dispersions were prepared by introducing DBSA (7 g) into deionized water (100 mL) and then adding the aniline monomer (1 g) at 3 °C to form anilinium-DBSA complexes. After stirring for 2 h, a solution of APS (2.45 g) in deionized water (50 mL) was added dropwise to the mixture, which polymerized while stirring for 24 h at 3 °C to form the aqueous solution of PANI-DBSA NPs. 22,23 Preparation of PANI-DBSA/DGEBA-Aniline Hybrids through Absorption-Transferring Process. The PANI-DBSA NPs were dispersed in water and slowly heated to 90 °C. Depending on the desired concentration of PANI-DBSA in the final product, an appropriate amount of the aqueous solution of PANI-DBSA NPs was mixed with DGEBA-aniline oil droplets through mechanical stirring for 4 h. The PANI-DBSA NPs were absorbed on the surfaces of the DGEBA-aniline droplets and diffused into them through the absorption-transferring process. Most of the water was separated and removed from the mixture. The final product was washed three times with ethanol (25 vol %) and dried in an oven at 80 °C for 2 days. Preparation of PANI-DBSA/Epoxy Composites through Blending Processing. A dry PANI-DBSA powder was obtained from the aqueous solution of PANI-DBSA NPs as mentioned above. After grinding in a mortar and pestle in the presence of liquid nitrogen, this powder was mixed with DGEBA and the curing agent 2,4,6-tri(dimethylaminomethyl)phenol through mechanical stirring for 30 min. Disc-shaped samples (diameter: 15 cm; thickness: 1.5 mm) were prepared by cast molding. The composites were cured at 80 °C for 24 h. Characterization. Dielectric spectroscopy and electrical conductivity were performed using Hewlett-Packard HP4192LF

and HP4338B instruments. Scanning electron microscopy (SEM) was conducted using a JEOL 6700 instrument. The sample was cleaved in liquid nitrogen and its fresh fracture surface sputtered with gold. Infrared spectra were recorded using a BIO-RAD FTS40 Fourier transform infrared (FT-IR) spectrometer. The EMI shielding effectiveness was determined using an Advantest Corp. TR17301A instrument; the sample (size: 5 × 5 × 1.3 mm) was fixed onto the sample holder using conducting tape; the data were calculated using an Advantest Corp. R3361B instrument. Results and Discussion PANI is insoluble in common organic solvents because of its strong intermolecular hydrogen bonding. Thus, phase separation usually occurs when PANI is blended in a polymer matrix. To overcome aggregation and maintain the original morphology of PANI NPs, we wished to avoid surface modification and the use of solvents. Thus, we developed an absorption-transferring process in which a sticky polymer matrix (DGEBA-aniline, Scheme 1) would absorb the NPs by itself. The epoxy prepolymer DGEBA-aniline appeared to be a good candidate for the sticky medium for two reasons: (a) the bisphenol A structure of DGEBA imparts good mechanical properties and (b) the high mobility and viscosity of DGEBA-aniline allows adhesion of NPs when the temperature is above 70 °C. As indicated in Scheme 2, when the aniline-to-DGEBA molar ratio was 0.5, this prepolymer exhibited mobility and viscosity that were suitable for forming polymeric droplets (oil phase) at higher temperature. Because both PANI and DGEBA-aniline are insoluble in water, these two polymers exhibited a high degree of attraction in the heterogeneous system. Thus, after mechanically stirring a hot aqueous solution of PANI-DBSA NPs with DGEBA-aniline prepolymer, the PANI-DBSA NPs were first captured by the sticky surface of DGEBA-aniline

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Figure 1. FTIR spectra of hybrids prepared using the absorptiontransferring process.

Liu et al.

Figure 3. SEM micrographs of hybrids prepared using the absorptiontransferring process: (a) DGEBA-aniline; (b) 12.8, (c) 28, and (d) 38 wt % PANI-DBSA.

Figure 4. SEM micrographs of the composites prepared using the blending process at PANI-DBSA contents of (a) 12 and (b) 38 wt %.

Figure 2. Electrical conductivity of the epoxy composites plotted with respect to the PANI-DBSA concentration.

droplet and then diffused into the whole droplet (oil phase). When the PANI-DBSA NPs encountered the DGEBA-aniline droplets, the PANI component reacted further with the epoxy rings of the prepolymer to form a hybrid. Figure 1 presents FTIR spectra of the hybrids prepared by this absorption-transferring process. The intensity of the signal for the epoxide group (910 cm-1) decreased after PANI had been added, that is, the amino groups on the surface of PANI could reacted with the epoxide rings. These spectra revealed that PANI was also a curing agent of epoxy. Figure 2 displays the conductivities of the PANI-DBSA/epoxy samples prepared using both the absorption-transferring and blending processes. As shown in Figure 2, the conductivity of the hybrid prepared using the absorption-transferring process was higher than that from the blending process regardless of the content of PANI-DBSA. In addition, the percolation threshold was also lower (absorption-transferring, 28 wt %; blending, 38 wt %) because the PANI particles aggregated readily during the latter process. In the absorption-transferring process, when the content of PANI-DBSA reached a threshold, the conductivity of the hybrid decreased thereafter because the NPs aggregated into secondary particles. Not only did this process cause the PANI chains to entangle readily, it also interrupted the conducting pathway. Figure 3a indicates that the morphology of the pure epoxy matrix (DGEBA-aniline) was that of a continuous phase. In

Figure 3b, after blending this matrix with the PANI-DBSA solution in the absorption-transferring process, the PANI primary particles exhibited excellent dispersion. The inset of Figure 3b reveals that the average diameter of PANI NPs, estimated at a magnification of 50 k, was 40-60 nm. When the content of PANI-DBSA was above the percolation threshold (28 wt %, Figure 3c), the primary particles further clustered into small agglomerates (38 wt %, Figure 3d). Figure 4a displays the morphology of 12.6 wt % PANI-DBSA/epoxy prepared using the blending process; the micrograph indicates that the PANIDBSA NPs aggregated into secondary particles (diameter: ca. 0.3-0.4 µm). Upon increasing PANI-DBSA content, the secondary particles underwent macroscopic phase separation in the epoxy matrix (Figure 4b). These SEM images indicate that the microstructures of the hybrids obtained from the absorption-transferring process had no similarities with those prepared through the blending process. Therefore, we can conclude that the hybrids prepared by absorption-transferring process would exhibit superior electrical properties, due to the well-dispersed PANI-DBSA NPs. The chain arrangement of a conducting polymer is an important factor affecting its electrical behavior. To improve the conductivity, it is necessary to increase the crystallinity. For conducting polymers such as PANI, the crystalline region contains a well-ordered polymer chain structure (metallic island). In this domain, interchain interactions are sufficiently strong to allow the charge to delocalize.24 Each metallic island disperses in the amorphous phase of PANI and connects with a single chain. Figure 5 depicts the variation in the dielectric constant (ε′) of the two types of PANI-DBSA/epoxy hybrids, plotted with respect to their PANI-DBSA content and the applied AC frequency. In the blending process (Figure 5a), the value of ε′

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Figure 5. Frequency dependence of the dielectric constant (ε′) for PANI-DBSA/epoxy hybrids prepared using the (a) blending and (b) absorptiontransferring processes.

Figure 6. EMI shielding effectiveness toward electric fields plotted with respect to frequency (100 MHz-1 GHz) for PANI-DBSA/epoxy hybrids prepared using the (a) blending and (b) absorption-transferring processes.

Figure 7. EMI shielding effectiveness toward magnetic fields plotted with respect to frequency (100 MHz-1 GHz) for PANI-DBSA/epoxy hybrids prepared using the (a) blending and (b) absorption-transferring processes.

increased upon increasing the PANI content. A high, positive dielectric constant (ε′) means that the charge carriers are no longer free, but rather are localized. The secondary particles of PANI-DBSA would form many metallic grains dispersed in the matrix. At each AC frequency, the mobile charges were localized in each metallic region because the PANI particles were surrounded by disordered regions (epoxy or an amorphous phase of PANI). The charges could not hop to adjacent metallic regions, but rather remained localized within the domain to form mini capacitors. On the other hand, all the hybrids prepared using the absorption-transferring process exhibited a negative dielectric constant (Figure 5b). We infer that the PANI-DBSA NPs were dispersed well and formed a continuous conducting pathway. The charge was delocalized in a macroscopic scale. Thus, the dielectric constant became negative.25,26 The hybrid

containing 28 wt % of PANI-DBSA possessed the lowest negative dielectric constant (i.e., the highest absolute value) because its PANI content reached the percolation threshold, providing the highest level of delocalization. Adding any more PANI-DBSA resulted in a slight degree of aggregation, which interrupted the charge transport. Figures 6 and 7 display the values of the EMI shielding effectiveness (SE) for the PANI-DBSA/epoxy hybrids. The main ranges of shielding frequency are 100-500 and 700-1000 MHz in the electric field and 800-1000 MHz in the magnetic field. Because of their higher levels of charge delocalization as well as high conductivity, the hybrids obtained using the absorptiontransferring process exhibited greater values of SE, in both the electric and magnetic fields, than did those obtained through blending. The highest values of SE were observed for the 38

15960 J. Phys. Chem. C, Vol. 112, No. 41, 2008 wt % PANI-DBSA/epoxy: ca. 30-60 dB in an electric field of 100-1000 MHz and ca. 35 dB in a magnetic field of 800-1000 MHz. Electrons move differently under the influence of electrical conduction and EMI. For electrical conduction, the applied voltage prompts the delocalized electron transporting along the polymer chain or hopping to adjacent metallic grains. For this reason, the continuous conducting pathway is important for longdistance electron transport. The factors affecting EMI are more complicated because it involves both conduction and polarization mechanisms.27 The doped PANI-DBSA can form two types of charge species: polarons/bipolarons and dipoles. In the former, charge is delocalized and free to move along the polymer chain (via a conduction mechanism); in the latter, charge is restricted to various metallic domains evenly distributed within the matrix (via a polarization mechanism). When more PANI-DBSA NPs were added in the epoxy, both the conductivity and dipole density increased accordingly. An excess of PANI-DBSA (i.e., beyond the percolation threshold of 28 wt % PANI-DBSA) did not benefit the electrical conduction, because of NP aggregation, but it did result in more dipolar and interfacial polarization in the hybrid material.28 Therefore, 38 wt % PANI-DBSA/epoxy improved the EMI efficiency but not the conductivity. Conclusion We have synthesized well-dispersed nanoPANI/epoxy hybrids using an absorption-transferring process. Morphological analysis revealed that this process led to a effective absorption of PANIDBSA NPs, with no apparent aggregation, because of the hinderence of the chain structure of epoxy. No organic solvent is involved in the whole process. The conducting NPs were highly dispersed in the matrix, thereby establishing a continuous three-dimensional conducting pathway. This morphology not only decreased the percolation threshold but also improved the level of charge delocalization; thus, the material exhibited a huge negative dielectric constant and effective EMI shielding. Acknowledgment. We thank the National Science Council, Taipei, Taiwan, for providing financial support through Grant NSC 94-2216-E-002-024.

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