Improved electrical and optical properties of a poly(methyl methacrylate) nanocomposite

27 May 2016
P. Maji, R. B. Choudhary, and M. Majhi
An in situ polymerization technique is used to reinforce the polymer matrix with γ-methacryloyloxy propyl trimethoxy silane-treated titanium dioxide nanoparticles.

In recent years, electrical energy storage capacity has become a scientifically challenging and industrially important area for a broad range of applications (e.g., in mobile electronic devices, stationary systems, hybrid electric vehicles, and pulse power technology).1, 2 For these purposes, dielectric materials have been used as the energy storage materials. Materials with high dielectric constants and low losses have thus attracted a large amount of attention for research into energy storage capacitors and electric stress control devices. In turn, this increasing demand for high-dielectric-constant organic materials has prompted the synthesis of organic-inorganic hybrid nanocomposites. Such organic-inorganic dielectric materials offer many advantages, such as light weight, flexibility, and good moldability. To date, research has mainly been focused on investigating how polymeric dielectric materials can be used to enhance energy storage capacity through blending of polymers, cross-linking, and incorporating the inorganic nanoparticles as fillers. Although the introduction of a plasticizer or filler (e.g., titanium dioxide) into the polymer matrix can enhance the material's dielectric properties to a great extent, these additions also tend to agglomerate because of their large cohesive strength and poor compatibility with the polymer. Polymeric nanocomposites therefore exhibit poor physical and dielectric properties.3 It is thus desirable to find a way to modify the surface of the nanoparticles to obtain a better dispersion and interfacial interaction with the polymer matrix. In this way, it should be possible to improve the physical and dielectric properties of the resultant nanocomposites.

Different methods for the surface modification of titanium dioxide (TiO2) have previously been reported. For instance, poly(methyl methacrylate)-TiO2 (PMMA-TiO2) thin films with good optical and dielectric properties can be prepared via an easy, yet cost-effective dip-coating method.4 However, there has not yet been a detailed report on the optical and electrical properties of γ-methacryloyloxy propyl trimethoxy silane (KH-570)-treated nano-TiO2 particles that have been incorporated into PMMA nanocomposites (prepared with the use of in situ polymerization).

Organic and inorganic hybrid materials are generally formed by incorporating covalent bonds, or through another type of physical interaction.5–9 In this work, we have therefore attempted to modify the surface of nanoparticles with the use of a surface modifier coupling agent (i.e., attaching the nanoparticles to a polymer chain). In particular, we have used an in situ polymerization technique to prepare PMMA-TiO2 nanocomposites. This methodology allowed the formation of covalent bonds between the PMMA and the TiO2 nanoparticles. We have also investigated the structural, morphological, optical, and electrical properties of our PMMA-TiO2 nanocomposites.

To prepare our PMMA-TiO2 nanoparticles, we first mixed 0.2g of poly(vinyl alcohol) with 100ml of water. We then added 2g of di-sodium hydrogen phosphate to the mixture. In the next step, we placed 1g of KH-570-treated TiO2 nanoparticles, 10ml of methyl methacrylate, and 0.8g of benzoyl peroxide into the solution and stirred it constantly under nitrogen flow (at 80°C) for 12 hours. We washed the resulting precipitate with water and put it into a vacuum oven for 24 hours (at 70°C). Our final polymeric sample was thus obtained for further studies.

We used x-ray diffraction (XRD) to confirm the phase and crystal structure of our samples. The XRD spectrum from our PMMA-TiO2 nanocomposite is shown in Figure 1. This spectrum reveals the presence of anatase and rutile phases of TiO2 mixed with amorphous PMMA. Using Scherrer's formula, we estimate that the average crystallite size of the nanocomposite is in the range of 19nm.


X-ray diffraction spectrum for the poly(methyl methacrylate)-titanium dioxide (PMMA-TiO2) nanocomposite. The peaks in the spectrum are labeled according to the crystal phases from which they originate (numbers in parentheses denote crystallographic planes). 2θ: Measured angle of diffraction. a.u.: Arbitrary units.

We also used field emission scanning electron microscopy (FESEM) to characterize the PMMA-TiO2 nanocomposite (see Figure 2). The FESEM images reveal the homogeneous dispersion of TiO2 nanoparticles into the PMMA matrix and the absence of agglomerations. In addition, the micrographs show that the PMMA-TiO2 nanocomposite—see Figure 2(b)—consists of spherical granules of TiO2 molecules. We calculated that the particle sizes are about 90nm. Furthermore, we obtained energy-dispersive x-ray spectra—see Figure 2(d)—that confirmed the elemental composition of the polymer composite. We did not observe any extra elemental peaks in the spectra, which confirms the purity of the material. We have thus demonstrated the successful incorporation of TiO2 into the PMMA.


Field emission scanning electron microscope images for (a) pure PMMA and (b) the PMMA-TiO2 nanocomposite. Energy-dispersive x-ray spectra are also shown for (c) the PMMA and (d) the PMMA-TiO2 nanocomposite. These spectra indicate the elements present in the samples. C: Carbon. O: Oxygen. Pt: Platinum. Ti: Titanium.

In addition, we conducted optical studies on our samples with the use of a UV-visible (UV-VIS) spectroscopic technique (see Figure 3). We found that the light absorption was higher for the PMMA-TiO2 nanocomposite than for the pure PMMA. Moreover, our results show that the absorption of photon energy by organic molecules in the UV-VIS spectral region caused an upward transition of electrons in the n, σ, and π orbitals (as has previously been predicted4). We also observed various electronic transitions that correspond to the different bonds present in the samples (listed in Table 1).


UV-visible (UV-VIS) absorption spectra for the pure PMMA and the PMMA-TiO2 nanocomposite.

Electronic transitions (and equivalent bonds) that correspond to various peaks in the UV-VIS absorption spectra of the PMMA and the PMMA-TiO2 nanocomposite. n, π, and π* denote orbitals.

SampleWavelength (nm)Electronic transition
PMMA<250π–π* (carbonyl group)
276n–π* (aldehydic carbonyl group)
PMMA-TiO2 nanocomposite< 250π–π* (carbonyl group of PMMA)
276n–π* (aldehydic carbonyl group of PMMA)
350Transition of electron from valence band to conduction band of TiO2

In the final part of our work, we conducted a dielectric study to measure the energy storage capacity of our samples. The variation in dielectric constant with temperature, at different frequencies, is shown in Figure 4(a). We find that the dielectric constant increased as a function of temperature because of the increased polarization and segmental mobility of the polymer. Furthermore, we observe a high dielectric constant at low frequency, but which decreases with increasing frequency. This behavior is caused by dipolar and space-charge polarization.10–13 We also show the variation of AC conductivity as a function of temperature, for different frequencies in Figure 4(b). The results indicate that the AC conductivity increased with both temperature and frequency because of the thermally activated transport properties of the material. Similar behavior has been reported in the past for a PMMA-TiO2 thin film.14


(a) Dielectric constant (ε ′) and (b) AC conductivity (σac) of the PMMA-TiO2 nanocomposite as a function of temperature and frequency.

In summary, we have synthesized a PMMA-TiO2 nanocomposite via a free-radical polymerization technique. The incorporation of the surface-modified TiO2 nanoparticles greatly improved the structural, morphological, and optical properties of the material. We also found that the nanocomposite's dielectric constant and AC conductivity were strongly dependent on temperature and frequency. Moreover, the dielectric constant (about 14) and AC conductivity (about 3.9×10−6Sm−1) of the PMMA-TiO2 nanocomposite exhibited better characteristics than the pure polymer matrix. In our future work we will study the dielectric relaxation of this polymeric nanocomposite.


Authors

P. Maji
Department of Applied Physics, Indian School of Mines

R. B. Choudhary
Department of Applied Physics, Indian School of Mines

M. Majhi
Department of Applied Physics, Indian School of Mines


References

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  2. Energy Storage Systems in Electronics, pp. 604, CRC Press, 2000.

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  4. B. M. Krasovitskii and B. M. Bolotin, Organic Luminescent Materials, pp. 340, VCH, 1988.

  5. Y. Wei, W. Wang, J.-M. Yeh, B. Wang, D. Yang and J. K. Murray, Photochemical synthesis of polyacrylate-silica hybrid sol-gel materials catalyzed by photoacids, Adv. Mater. 6, pp. 372-374, 1994.

  6. Q. Wang, N. Liu, X. Wang, J. Li, X. Zhao and F. Wang, Conductive hybrids from water-borne conductive polyaniline and (3-glycidoxypropyl)trimethoxysilane, Macromolecules 36, pp. 5760-5764, 2003.

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  8. Y. Chujo, H. Matsuki, S. Kure, T. Saegusa and T. Yazawa, Control of pore-size of porous silica by means of pyrolysis of an organic-inorganic polymer hybrid, J. Chem. Soc. Chem. Commun. 5, pp. 635-636, 1994.

  9. R. Tamaki, K. Samura and Y. Chujo, Synthesis of polystyrene and silica gel polymer hybrids via π-π interactions, Chem. Commun. 10, pp. 1131-1132, 1998.

  10. P. Maji, R. B. Choudhary and M. Majhi, Structural, optical, and dielectric properties of ZrO2 reinforced polymeric nanocomposite films of polymethylmethacrylate (PMMA), Optik Int'l J. Light Electron Opt. 127, pp. 4848-4853, 2016.

  11. M. Majhi, R. B. Choudhary and P. Maji, TiO2 reinforced polymeric nanocomposites of HCl-doped polyaniline and their properties, Polym. Compos., 2016. First published online: 20 March

  12. P. Maji, P. P. Pande and R. B. Choudhary, Effect of Zn(NO3)2 filler on the dielectric permittivity and electrical modulus of PMMA, Bull. Mater. Sci. 38, pp. 417-424, 2015.

  13. M. Majhi, R. B. Choudhary and P. Maji, CoCl2 reinforced polymeric nanocomposites of conjugated polymer (polyaniline) and its conductive properties, Bull. Mater. Sci. 38, pp. 1195-1203, 2015.

  14. S. Sugumaran and C. S. Bellan, Transparent nano composite PVA–TiO2 and PMMA–TiO2 thin films: optical and dielectric properties, Optik Int'l J. Light Electron Opt. 125, pp. 5128-5133, 2014.

DOI:  10.2417/spepro.006492



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