High dielectric response of 2D-polyaniline nanoflake based epoxy nanocomposites

Abstract

Herein, we report the synthesis of two dimensional, flake-like nanostructures of polyaniline and their nanocomposites in an epoxy matrix with different particle loadings. An anomalous enhancement in the dielectric constant is observed for the synthesized nanocomposites due to the 2D flake-like structure of the polyaniline fillers at room temperature over the wide AC frequency range of 1 Hz to 10⁷ Hz.

---

Embedded capacitor technology (ECT) is emerging as one of the most innovative ways towards the miniaturization of electronic devices, which has the potential to improve the functionality and performance of the electronic devices of the 21^st century.^1–3 The capacitors which are printed on the circuit board are known as imbedded capacitors. Such capacitors do not require space on the surface of the printed circuit boards (PCBs) and can store electric charge. A low impedance material supports rapid charge transfer, which is needed for high speed devices like computers, and medical and telecommunication equipment. Further, a material with low impedance and large capacitance or dielectric constant allows the system to function as a power supply decoupling device, thus, the use of large external capacitors can be avoided in circuit board surfaces.^4,5 In previous studies, ceramic materials with high dielectric constants were investigated for dielectric applications but they did not have favourable mechanical properties for EC manufacturing methods.^6–9 Ceramic particle reinforced polymer composites were also proven to be inefficient as high ceramic loadings (>50 wt%) are needed to achieve the required dielectric constant value (above 100), which deteriorates the mechanical properties of the final composite.^5,10 Some researchers reported anomalous high dielectric constant values for metal doped ceramics and metal reinforced polymers.¹¹ In spite of all of these expectations and great promises, progress in the field of nanostructures reinforced composites has been slow. To achieve the desired properties for the application of the composites for ECT, high dielectric constant enhancement is needed at lower concentrations of filler materials.¹² The main challenge for the implementation of ECT is the requirement of appropriate dielectric materials, which favour PCB manufacturing and performance issues.

Epoxy resins are one of the most important engineered polymers and represent a very important class of polymeric materials due to their mechanical and dielectric properties.¹³ This results in the wide applicability of these materials in structural materials, anti-corrosion coatings,¹⁴ solid state dye lasers,¹⁵ electronic packaging¹⁶ and as flame retardant additives¹⁷ due to their easy synthesis. However, applications of epoxy at an industrial level are limited due to its low electrical conductivity. Their transparency makes epoxy resins favourable for applications in light emitting diode (LED) chip designs, which are bound to a reflector cup and standard LED technology.¹⁸ Researchers worldwide have attempted to reinforce epoxy resins with several types of materials like carbon nanostructures,¹⁹ glass fibers,²⁰ and metal nanostructures^21,22 to improve their electrical properties. However, the easy oxidation and high density of metallic powders make metal–polymer composites unstable and heavy. Agglomeration and interfacial interaction at the nanoscale present tough conditions, for the uniform dispersion of filler material within the base matrix, which results in the material having worse mechanical properties. The surface treatment of filler material is a conventional method for improving the dispersibility and polymer–filler material interfacial interactions through the use of appropriate coupling agents or surfactants. De facto, having ‘amine’ and ‘imine’ groups in its backbone, polyaniline (PANI) combines with epoxy resin very well which is necessary for thermosetting composites having better mechanical properties and isotropy in their electrical properties.²³ The effects of polyaniline nanostructures like nanospheres and nanofibers on the electrical properties of PANI–epoxy nanocomposites have been widely studied due to their exotic electrical properties as filler materials.^24,25 After the development of graphene, the 2D carbon structures of conducting polymers are one of the most exotic materials for various applications.^26,27 Polyaniline also has a benzene ring at its core, thus, it may demonstrate better conduction and electrical properties due to the presence of an electron cloud above and below its thin flake-like layer.

In this communication, we report the preparation of PANI–epoxy nanocomposites followed by the synthesis of 2D-polyaniline nanoflakes with different loadings. Polyaniline nanoflakes are prepared through the oxidative polymerization of aniline monomer with the help of ultrasonic high intensity waves at temperatures below 5 °C and then a rigorous cleaning of the colored emeraldine product. The synthesized product was subjected to morphological and structural characterization. A transmission electron microscopy (TEM) image (Fig. 1a) confirms the two dimensional flaky structure of the polyaniline nanostructures that forms due to the use of high intensity ultrasonic waves during nucleation and polymerization processes. The high intensity of the mechanical disturbance created by the ultrasonic waves could be a reason for the exfoliation of the spherical and fibrous structures into 2D flakes. The formation of 2D flake-like structures can be understood from a two step process. In the first step, the local hot spot generation and cavity formation and collapse processes, as a result of the ultrasound, prevent the growth of elongated polyaniline structures. Thus, emeraldine molecules of lower molecular weights are formed. In the second step, the ultrasound helps the quinoid cross-linking process among the emeraldine molecules. However, a very small amount of radicals are formed in general in the case of inorganic emulsions, but it varies for polymeric materials.²⁸ The cross-linking of quinoid rings in the emeraldine state of polyaniline can be realized with the help of the mechanism represented graphically in ESI.†

Fig. 1 (a) Transmission electron microscopy image illustrating the 2D flaky nanostructures of polyaniline; (b) SEM image of pristine epoxy sample surface; (c) SEM image of 1 wt% polyaniline nanoflake/epoxy nanocomposite; (d) SEM image of 2 wt% polyaniline nanoflake/epoxy nanocomposite; (e) UV-visible spectrum of polyaniline powder; (f) FTIR spectra of pure polyaniline, 1 & 2 wt% polyaniline nanoflake/epoxy composites and pure epoxy matrix.

The SEM images (Fig. 1b–d) clearly show the difference between the surface morphology of clean epoxy and that of the epoxy matrix after loading with 1 and 2 wt% flakes of polyaniline, respectively. From Fig. 1d, it can be concluded that the polyaniline nanoflakes are interacting well with epoxy at the surface of the flakes presumably via ‘imine’ and ‘amine’ bonds.²³ The UV-visible spectrum (Fig. 1e) confirms that the polyaniline nanoflakes are in a doped state.²⁹ Fourier transform infrared (FT-IR) spectroscopy was employed to observe the changes in the structure of the epoxy matrix due to polyaniline nanoflakes. Fig. 1f shows the FT-IR spectra of pure epoxy and 1 & 2 wt% polyaniline–epoxy nanocomposites. It can be easily observed that characteristic peaks corresponding to polyaniline are present in the spectra of the nanocomposites. The peaks at 1246 and 1300 cm⁻¹ are attributed to C–H stretching from aromatic conjugation which correspond to the characteristic peaks of polyaniline. Peaks at 1483 and 1567 cm⁻¹ also correspond to polyaniline nanostructures, which are present in the spectra of epoxy after mixing with polyaniline.³⁰ The hypothesis of quinoid ring cross-linking is also supported by the FT-IR spectra. In the polyaniline samples, the peak corresponding to a quinoid ring generally appears near 1590 cm⁻¹. However, in our case it can be observed that the signature peak of the quinoid ring is almost absent in the FT-IR spectrum corresponding to polyaniline, in Fig. 1f.³¹

Dielectric studies have been carried out in the frequency range of 1 Hz to 10 MHz with pellets of the specimens, sandwiched between brass plates as capacitors using the impedance gain phase analyzer of a Solartron model SI-1260 coupled with a Solartron dielectric interface model-1296. The dielectric constants of the samples were calculated by measuring the capacitance of the pellets and their geometry. Fig. 2a shows a variation in the dielectric constant with increasing frequency. The dielectric constants of the composites increased greatly as compared to pristine epoxy. The dielectric constants for the pristine epoxy, and the 1 wt% and 2 wt% polyaniline–epoxy nanocomposites first decrease from 1 to 100 Hz, thereafter they remain almost constant up to a frequency of 1 MHz. Above 1 MHz, the observed dielectric constants increase with increasing frequency. At low frequency, the decrease in the dielectric constant with increasing frequency is a general trend for most of the dielectric materials and it occurs due to electrode polarization capacitance.^32–34

Fig. 2 (a) Evolution of the dielectric constant with the reinforcement of polyaniline nanoflakes (1 & 2 wt%) in epoxy matrix at room temperature in the frequency range of 1 to 10⁷ Hz; (b) dependence of the dielectric loss in epoxy reinforced with polyaniline nanoflakes (1 & 2 wt%) at room temperature in the frequency range of 1 to 10⁷ Hz.

The effect of electrode polarization capacitance is more noticeable for the materials having higher ionic conductivity. The ionic conductivity value will be discussed later. The increase in the dielectric constant above 1 MHz is because of a parasitic effect due to the connecting wires. The increase in the dielectric constant for 1 wt% flakes was 10 fold whereas it was 25 fold for the 2 wt% flake loading with respect to pristine epoxy in the whole range of frequency studied. This effect is more prominent particularly in the low frequency range of 1 to 100 Hz. In the frequency range of 1 to 100 Hz, the dielectric constants were found to increase more than 15 fold and 40 fold for 1 and 2 wt% flake loadings, respectively.

The enhancement in the dielectric constant for the polyaniline nanoflakes as compared to that for the pristine epoxy might be due to the following reasons:

(1) The formation of a thin layer between the insulating base matrix close to the interface. Formation of depletion layers is also probable due to a partial overlap of the nanoscale flaky structures with each other.

(2) The enhancement in the dielectric constant of the composites depends on the morphological structure of the filler materials due to network formation and percolation phenomena. Nanoflakes have a higher network formation probability due to their elongated two dimensional structure, compared with fibres and spheres, that facilitates the percolative cluster networks or percolation network at lower filler loadings.

(3) The electronic distribution on the surfaces of the two dimensional structured PANI affects the electric field distribution in the matrix; it supports local field enhancements, compared with fibril and spherical filler materials which strengthen the matrix polarization.³⁵

(4) The 2D morphology of the filler PANI, which supports the formation probability of percolative clusters inside the matrix at far below the value of the percolation threshold concentration of fibril and spherical filler structures.³⁶

(5) The larger surface to volume ratio escalates the interaction of highly conductive PANI nanofillers within the epoxy matrix. At the nanoscale, interfacial polarization becomes prominent, which is the result of short range dipole interactions (i.e. exchange coupling mechanism) and space charge accumulation (i.e. Maxwell–Wagner–Sillers mechanism) at two phase boundaries.³⁷

According to the phenomena discussed above; the high dielectric enhancement in the synthesized 2D flake-like PANI reinforced epoxy nanocomposites is observed as expected.

Fig. 2b shows a variation in the dielectric loss as a function of frequency which is a function of dielectric absorption. The dielectric absorption in terms of the ionic conductivity can be written as^32,33

where σ is the ionic conductivity, k is a constant and ε₀ (= 8.85 pF m⁻¹) is the free space permittivity (usually, k = 1). As the contribution of ions in the dielectric measurements is more influential particularly in the low frequency range, we determined the ionic conductivity in pristine epoxy and polyaniline–epoxy nanocomposites by fitting eqn (1) in the frequency range of 1 Hz to 100 kHz. The ionic conductivities for pristine epoxy, and 1 wt% and 2 wt% polyaniline–epoxy nanocomposites were found to be 1.03 × 10⁻¹⁰ S m⁻¹, 1.14 × 10⁻⁹ S m⁻¹ and 3.96 × 10⁻⁸ S m⁻¹, respectively. Hence, the ionic conductivities of 1 & 2 wt% polyaniline–epoxy nanocomposites compared to that of pristine epoxy increased by ∼11 and ∼384 times, respectively. Because of the increase in the ionic conductivity, and hence the ions in the polyaniline–epoxy nanocomposites, the dielectric absorption values in the polyaniline–epoxy nanocomposites were found to be relatively higher than that of pristine epoxy. Although the calculated tangent loss (tan δ = ε′′/ε′) for the polyaniline–epoxy nanocomposites was greater than 0.5 in the frequency range of 1–100 Hz due to ionic effects, nevertheless the tangent loss above 100 Hz is less than 0.5, and hence is a favourable condition for the materials to be used in EC technology.

Fig. 3 shows the frequency dependent electrical conductivity behaviour of the nanocomposites and pure epoxy matrix at room temperature. The net AC conductivity (σ_ac) of the synthesized nanocomposites at room temperature is the linear superposition of the DC conductivity and the AC conduction component (Aω^s; A is a constant and s is the power law exponent), which is called the universal dynamic response (UDR).³⁸ The second component (Aω^s) is representative of the electrons tunnelling and hopping among 2D nanoflake-like structures which contribute to the electrical conductivity. In the present case, it is clear that the DC component of electrical conductivity (ω → 0) is less dominant than the AC part. This increase in the electrical conductivity at higher frequencies is attributed to space charge polarization removal.³⁹ Electrical conductivity raises to an order of 10⁻⁸ in nanocomposites with 2 wt% PANI from 10⁻¹¹ S cm⁻¹ in the epoxy matrix at 1 Hz. At 1 kHz, the increase in the electrical conductivity is 10 and 100 fold, respectively, for 1 and 2 wt% nanoflake loadings compared to value for the epoxy matrix. At 100 kHz the increase in the electrical conductivity is again of the same order as stated above. The fluctuations in the electrical conductivity near 10 MHz are due to the effect of the brass plate resistance, in which the samples were sandwiched in the form of a parallel plate capacitor. The conductivity increases with the 2D-nanofiller content in the epoxy matrix, with a linear behavior.

Fig. 3 Behavior of the electrical conductivity in epoxy reinforced with polyaniline nanoflakes (1 & 2 wt%) at room temperature in the frequency range of 1 to 10⁷ Hz.

Conclusions

In summary, we have synthesized novel two dimensional nanoflake-like polyaniline fillers via sonochemistry and reinforced them in an epoxy matrix to prepare the desired nanocomposites. The two dimensional nanoflakes have good interfacial interactions with the epoxy matrix as confirmed using FTIR spectroscopy. It can be concluded from SEM images of the different nanocomposites that the nanofillers are uniformly distributed in the epoxy matrix. The observed remarkable enhancement in the dielectric constant over that of the pristine epoxy matrix is due to the 2D geometry of the nanofillers, which is higher than for nanofibers and nanospheres at low concentrations. The high dielectric constant and relatively low dielectric loss at lower nanofiller loadings infer the potential applications of these nanocomposite materials in embedded electronics and artificial muscles.

Experimental

All chemicals, aniline monomer (1 M), potassium biiodate, sodium hypochlorite, HCl (12 M), and ammonium hydroxide (1 M), were analytical grade and were purchased from Merck & Co., Inc; these chemicals were used without further treatment. Deionized water (Merck & Co., Inc.) was used in all experiments.

For the synthesis, 1 ml of 0.1 M aniline monomer was dissolved in 100 ml of HCl (1 M) at room temperature by magnetic stirring for up to 30 minutes. After the mixing of the aniline monomer in HCl, a solution of potassium biiodate was added. 100 ml of potassium biiodate (0.015 M) was mixed and exposed to ultrasonic radiation of low intensity for 20 minutes. A green suspension product of polymerized aniline was formed after approximately half an hour. The product was separated followed by a centrifugation process and then washed with HCl (N/10) and acetone many times. The resulting suspension was again centrifuged gently and separated, continuously washed with distilled water until the filtrate became colorless and then dried in a 60 °C oven overnight. The dried product was subjected to characterization. UV-visible and FTIR spectroscopy and electron microscopic techniques were used to characterize the product samples. Micro-structural and optical techniques confirmed the formation of flaky nanostructures of polyaniline.

Epoxy LY 556 resin (bisphenol-A-diglycidyl-ether) was employed as a matrix resin. The epoxy resin and polyaniline nanoflakes were dispersed in acetone in weight ratios of 1 and 2 wt% and the mixtures were ultrasonicated for a uniform dispersion of the fillers in the base matrix. After sonication, the corresponding hardener (HY 951) was added to the suspensions and the samples were again ultrasonicated with 400 watt intensity at 20 kHz ultrasonic frequency (Sonics VC 505). The solvent, acetone, was evaporated at 60 °C for 5 hours. Samples of pure epoxy and composites of 1 and 2 wt% PANI–epoxy were cured at 120 °C for 6 hours. The surface areas and thicknesses were measured before the dielectric measurements with a precision of 0.001 cm.

The prepared samples were subjected to morphological characterizations using scanning and transmission electron microscopic techniques. The high resolution transmission electron microscopy images were recorded using a HRTEM (FEI Tecnai G2 F30 STWIN). The surface morphologies of the pure epoxy and composite samples were measured using a scanning electron microscope (Zeiss EVO MA-10 SEM operating at 10.0 keV). The UV-vis spectra of all of the samples were measured using a UV-vis spectrometer (UV-2401 PC, Shimadzu Corporation Japan). The FT-IR spectra were recorded with a single beam Perkin Elmer (Spectrum BX-500) spectrophotometer. An impedance gain phase analyzer, a Solartron model SI-1260 coupled with a Solartron dielectric interface model-1296, was employed for dielectric studies in the frequency range of 1 Hz to 10 MHz with pellets of the specimens sandwiched between brass plates as capacitors.

Acknowledgements

Meher Wan is grateful to CSIR, New Delhi for a senior research fellowship and financial support.

References

1. R. Snogren, Printed Circuit Fabr., 2005, 25, 26.
2. J. Prymark, S. Bhattacharya, K. Paik and R. R. Tummala, Fundamentals of microsystems packaging, McGraw-Hill, New York, 2001.
3. R. Funer, Printed Circuit Des., 2002, 19, 8.
4. R. Ulrich, Circuit World, 2004, 30, 20.
5. Y. Bai, Z. Y. Cheng, V. Bharti, H. S. Xu and Q. M. Zhang, Appl. Phys. Lett., 2000, 76, 3804.
6. Z. H. Ren, P. Jin, X. M. Cao, Y. G. Zheng and J. S. Zhang, Compos. Sci. Technol., 2015, 107, 129.
7. L. Zhang and Z.-Y. Cheng, J. Adv. Dielectr., 2011, 1, 389.
8. Z. M. Dang, J. K. Yuan, J. W. Zha, T. Zhou, S. T. Li and G. H. Hu, Prog. Mater. Sci., 2012, 57, 660.
9. C. W. Nan, Y. Shen and J. Ma, Annu. Rev. Mater. Res., 2010, 40, 131.
10. R. Popielarz, C. K. Chiang, R. Nozaki and J. Oberzut, Macromolecules, 2001, 34, 5910.
11. L. Zhang, P. Bass and Z.-Y. Cheng, Appl. Phys. Lett., 2014, 105, 042905.
12. H. Windlass, P. M. Raj, D. Balraman, S. K. Bhattacharya and R. R. Tummala, IEEE Trans. Adv. Packag., 2003, 26, 10.
13. K. Wang, L. Chen, J. Wu, M. L. Toh, C. He and A. F. Yee, Macromolecules, 2005, 38(3), 788.
14. M. R. Bagherzadeh and F. Mahdavi, Prog. Org. Coat., 2007, 60, 117.
15. M. J. Cazeca, X. L. Jiang, J. Kuma and S. K. Tripathy, Appl. Opt., 1997, 36(21), 4965.
16. S. Rimdusit and H. Ishida, Polymer, 2000, 41, 7941.
17. R. J. Jeng, S. M. Shau, J. J. Lin, W. C. Su and Y. S. Chiu, Eur. Polym. J., 2002, 38, 683.
18. A. K. Mohammad, Electrical-Optical Devices and Systems, PWS-KENT Publishing, Boston, MA, 1990, ch. 4.
19. H. Gu, S. Tadakamalla, Y. Huang, H. A. Colorado, Z. Luo, N. Haldolaarachchige, D. P. Young, S. Wei and Z. Guo, ACS Appl. Mater. Interfaces, 2012, 4(10), 5613.
20. H. Gu, S. Tadakamalla, X. Zhang, Y. Huang, Y. Jiang, H. A. Colorado, Z. Luo, S. Wei and Z. Guo, J. Mater. Chem. C, 2013, 1, 729.
21. M. Paligova, J. Vilčákova, P. Sáha, V. Křesálek, J. Stejskal and O. Quadrat, Phys. A, 2004, 335, 421.
22. L. Qi, B. I. Lee, S. Chen, W. D. Samuels and G. J. Exarhos, Adv. Mater., 2005, 17, 1777.
23. A. Pron and P. Rannou, Prog. Polym. Sci., 2002, 27, 135.
24. X. Zhang, Q. He, H. Gu, H. A. Colorado, S. Wei and Z. Guo, ACS Appl. Mater. Interfaces, 2013, 5, 898.
25. J. Lu, K. S. Moon, B. Kim and C. P. Wong, Polymer, 2007, 48, 1510.
26. Z. Yin, J. Zhu, Q. He, X. Cao, C. Tan, H. Chen, Q. Yan and H. Zhang, Adv. Energy Mater., 2014, 4, 1300574.
27. K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim, Nature, 2012, 490, 192.
28. G. J. Price, Ultrason. Sonochem., 2003, 10, 277.
29. L. Dennany, P. C. Innis, S. T. McGovern, G. G. Wallacea and R. J. Forster, Phys. Chem. Chem. Phys., 2011, 13, 3303.
30. M. Trchová, Z. Morávková, M. Bláha and J. Stejskal, Electrochim. Acta, 2014, 122, 28.
31. W. L. Zhang, B. J. Park and H. J. Choi, Chem. Commun., 2010, 46, 5596.
32. A. K. Srivastava, V. K. Agrawal, R. Dabrowski, J. M. Otón and R. Dhar, J. Appl. Phys., 2005, 98, 013543.
33. A. K. Srivastava, V. K. Agrawal, R. Dhar, S. H. Lee and R. Dabrowski, Liq. Cryst., 2008, 39, 1101.
34. A. K. Srivastava, M. Kim, S. M. Kim, M. K. Kim, K. lee, Y. H. Lee, M.-H. Lee and S. H. Lee, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2009, 80, 51702.
35. J. Y. Li, Phys. Rev. Lett., 2003, 90, 217601.
36. J. Y. Li, C. Huang and Q. M. Zhang, Appl. Phys. Lett., 2004, 84, 3124.
37. I. Webman, J. Jortner and M. H. Cohen, Phys. Rev. B: Solid State, 1975, 11, 2885.
38. A. K. Jonscher, Nature, 1977, 267, 673.
39. K. Suelho, N. F. Dinant, S. Magana, G. Clisson, J. Francois and C. Dagron-Lartigau, Polym. Int., 2006, 55, 1184.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05660h

‡ Authors contributed equally.

---