535 Journal of Applied Sciences Research, 7(4): 535-541, 2011 ISSN 1819-544X This is a refereed journal and all articles are professionally screened and reviewed

ORIGINAL ARTICLES Poly(methyl methacrylate) / Carbon Nanoparticles Cast Composites 1

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E.M. Sadek, 1N.A. Mansour, 2S.I. Shara, 1E.A.Ismail and 1A.M. Motawie

Petrochemical Department, Egyptian Petroleum Research Institute, Cairo, Egypt. Analysis and evaluation Department, Egyptian Petroleum Research Institute, Cairo, Egypt.

ABSTRACT Poly(methyl methacrylate) PMMA carbon nanoparticles CNP cast composites were prepared via in-situ polymerization of methyl methacrylate monomer with different ratios of carbon nanoparticle CNP ( viz., 0.25, 0.5, 1, 1.5, 2, 2.5 wt%) in the presence of azobisisobutyro nitrile AIBN as an initiator. pure PMMA cast and PMMA/carbon black CB composites were also prepared under the same conditions. Mechanical properties (i.e. the tensile, compressive, and flexural strengths) and morphology of the prepared composites were studied. The dielectric properties (i.e., permittivity έ and dielectric loss §) of these composites in addition to the electrical conductivity σ were investigated in frequencies of 1, 100 KHz and 4.5 MHz at room temperature. Thermal stability of the prepared samples was also evaluated by thermal gravimetric analysis. This investigated led to the conclusions that all the mechanical properties were improved by increasing CNP content up to 2wt% loading in comparison with 20wt% CB. The PMMA/CNP (2wt%) samples showed a more regular dispersion of CNP filler inside the PMMA matrix as compared with samples PMMA/CB (20%) with a coarse morphology. On the other hand, the addition of CNP by concentration up to 2wt% increased the electrical conductivity to be of the order of 10-4 S/cm, which highly recommended such composites to be used in electronic and electrical field. Also it can be used in sensors applications. The PMMA/CNP nanocomposite samples displayed higher thermal resistance with a lower weight loss than pure PMMA and its filled CB cast samples. Key words: Poly (methyl methacrylate) PMMA, nanocomposites, carbon nanopowder CNP, mechanical properties, morphology (TEM) electrical properties, thermal stability (TGA). Introduction Recently, polymer-inorganic hybrid materials (Ghoneim, 2001; Mallette, 2000; Song, 2008; Lopes, 2009) had been widely investigated because such hybrids prepared via various techniques might combine the features of inorganic and organic functions which might show controllable properties such as thermal, electrical, optoelectronic, optical and mechanical behaviors (Bruce, 2004; Mark, 1996). Poly (methyl methacrylate) (PMMA) is an important member in the family of poly (acrylic esters). PMMA has some desirable properties, including exceptional optical clarity, high strength and excellent dimensional stability. However, its biggest disadvantage is its poor heat resistance (Yan, 2003). Recent interests in MMA polymers have created new incentives in blending with fillers to form unique material systems with excellent physical properties in addition to conductive and mechanical performance (Saujanya, 2001; Zois, 1999; Mamunya, 1997). Polymer nanocomposites have received intense attention and research in the past five years, driven by the unique properties of the nanoparticles and potential to creat new material systems with superior properties (Ajayan, 2000; Bower, 1999: Jin, 2001; Lourine, 1999; Rao, 1997). A variety of nanoparticle morphologies have been considered, including spheroidal particles such as silica (Inoue, 2004; Liu, 2001) platelets such as clay (Miwa, 2006; Hwu, 2002) graphite (Miwa, 2006; Hwu, 2002; Brazhkin, 1998; Chen, 2003) and nanotubes including multiwall (MWNT) and single wall (SWNT) forms (Chowdhury, 2009; Lu, 2008; Kashiwagi, 2007; Li, 2006; Lu, 2008). Possible applications include nanoelectronics, flat-panel display devices, energy storage, and space technology (Dilbn, 1997). Corresponding Author: E.M. Sadek, Petrochemical Department, Egyptian Petroleum Research Institute,

Nasr City, Cairo, Egypt. E-mail: [email protected]

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In this work, morphology study, mechanical, electrical properties and thermal stability of the prepared PMMA/CNP composites were studied and compared with those of the prepared PMMA / CB composites and unfilled PMMA cast samples. Materials and methods Materials: CNP was purchased from Aldrich, Cat # 633100, 99%Amorphous, made by Laser decomposition, with average particle size: 30 nm. MMA monomer was supplied from Sigma-Aldrich Co, Germany, Cat # M55909. AIBN, from Amerchem USA. CBwas supplied from Alex, for Carbon Black Co with particle size 325 µm. The solvents and all chemicals were used as received. Methods: Preparation of PMMA/ CNP composites: First, a dispersion was prepared by a mixing various contents of CNP ( viz., 0.25, 0.50, 1.0, 1.50, 2.0, and 2.50 w/w % ) with 100g MMA monomers and 0.1g AIBN with vigorous stirring for 30 min at 80 °C, following a procedure described by Parinya et al. (2005). The prepared samples were charged between two glass plates as a casting cell provided with a separating gasket- (made from PVA) – the cell was hold together by spring loaded clamps so the plates will come closer together. The prepared casting cell was immersed into water bath at 60oC overnight, cooled down for room temperature (25oC ±1). Finally, the sheet was removed from the cell as described by Parinya et al. and Brydson (1975). PMMA composites with CB and pure unfilled PMMA samples were also prepared by the above mentioned technique. Characterization of the prepared samples: The molecular weight of the prepared PMMA cast samples was determined by means of a Waters 600 gel permeation chromatography (GPC). The chemical structure of the prepared PMMA cast samples was characterised by FT-IR. The spectra were obtained on ATI Mattson-Genesis series FTIRTM. Its mechanical measurements (i.e. the tensile, compressive, and flexural strengths) of the prepared cast samples were determined according to the methods described in ASTMs, D5083, D256 and D570, respectively. The morphology was studied by TEM techniques. The TEM pictures were performed by TEM-1230 with an accelerating voltage of 100 KV (JEOL CO., Japan). Dielectric measurements (i.e. Permittivity (έ) and dielectric loss (ª)) for the prepared samples were measured at different frequencies (i.e. 1, 100 KHz and 4.5 MHz) by LCR meter (type Hioki 3532 Hitester). The capacitance C and the loss tangent (tanδ) were obtained directly from the bridge from which έ and ª were calculated at room temperature (25±1oC). The samples were in the form of disks of 58 mm diameter and 2 mm thickness. The electrical conductivity (σAc) at 1,100Hz and 4.5 MHz was measured by the application of Ohm's law using the NFM/5T test cell. A power supply unit (GM 45161/01) was used. The potential difference V between the plates holding the sample and the current I flowing through it was measured by a multimeter (type URI 1050). The thermogravimetric analysis (TGA) was performed with Shimadzu TGZ-50H thermal analyzer using 8-10 mg samples in nitrogen atmosphere and at a heating rate 10°C/ min from 100 to 700°C. Results and discussion In the present work, pure PMMA cast samples were prepared with average molecular weight 623284 as determined by GPC technique and the structure was elucidated and confirmed by FTIR, through the appearance of peaks at 1470, 1060, 1730 and 1360cm-1 characteristic to CH2 aliphatic, ether group, normal ester and methyl group respectively. The mechanical, morphology and electrical properties as well as thermal stability of the prepared PMMA/CNP composites were investigated through a comparison with the prepared PMMA/CB composites and unfilled PMMA cast samples. Mechanical Properties: The paragraph Fig. (1a) summarizes the effect of various CNP contents on the mechanical properties (i.e.,

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Tensile, compressive, and flexural strengths) of the prepared PMMA composites as compared with CB filled samples Fig. (1b). Generally, the incorporation of filler into PMMA cast samples improved the mechanical properties compared with unfilled ones. This is may be due to the presence of acidic groups such as hydroxyl group on the CB surface which in turn enhances formation of physical interaction force between the PMMA chains and the filler leading to an improvement in mechanical properties. The effect of filler content on the mechanical properties was drastic at 20% CB and 2% CNP, showing improved mechanical properties. Filled PMMA/CNP (2%) samples exhibited better mechanical properties in comparison with filled samples with CB (20%). This can be attributed to the fact that mechanical properties significantly depend on a good dispersion of the filler. If the filler is well dispersed and homogenously distributed into a matrix, the composite showed substantial increase in the properties (Kucierova, 2009; Gunes, 2008). In accordance with the sample morphology as show in TEM photographs, (Fig. 2), the finer particle size of CNP as compared with CB leads to better dispersion, Fig. 2c which in turn improves the mechanical properties of CNP filled samples compared to other CB filled ones (Fig. 2b). A further addition of a filler resulted in a proportionate decrease in all the tested mechanical properties.

(A)

(B) Fig. 1: Effect of filler content on mechanical properties of PMMA composites. Morphology of Prepared Composites: According to the TEM images (Fig. 2) of thin cut samples, the pure PMMA without fillers possessed a fine morphology (Fig. 2a). The fractured surfaces of the PMMA samples with CB (20%) possessed a coarse morphology (Fig. 2b) with a larger domain size in comparison with the filled ones with CNP (2%). The PMMA/CNP filled samples Fig.2c showed a more regular dispersion of CNP filler inside the PMMA matrix. A smaller nodule size and an improvement in interfacial adhesion with respect to the PMMA filled CB (Fig. 1b). Electrical Properties:

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Dielectric Measurements: The dielectric constant (permittivity) (έ) and dielectric loss (loss tangent) (ª) for the prepared samples at the frequencies 1KHz, 100KHz and 4.5 MHz were studied at room temperature (25±1°C) and the data are shown in Fig. (3a-d).

Fig. 2: TEM of PMMA filled samples with (a) 0 filler, (b) 20% CB 20000x).

and (c) 2%

CNP (Magnification:

For all PMMA samples loaded with CB and CNP filler, the dielectric constant (έ) was decreased and dropped down to minimum value as the frequency was increased. This may be caused by dielectric dispersion and due to the vanishing of orientation polarization at higher frequency. Thus the frequency dependent dielectric permittivity in organic polymer typically was dominated by reorientation of molecular dipoles (Malecki, 1994). Similar behaviour was noticed before in the literatures (Saad, 1997; Saad, 1998). Also, έ was affected by the filler content compared with that of pure PMMA cast samples. With an increase in the filler content (i.e., up to 20wt % CB and up to 2wt% CNP) the samples showed an improvement in the έ especially at the very low frequency region. Since, when an electric field was applied, PMMA samples will be polarized and the quantity of the accumulated charges will depend on the polarity of the polymer. Upon addition of CB and CNP, the quantity of the accumulated charges was increased as the interfacial polarization due to the filler/ PMMA interfaces made an additional contribution to the accumulated charges. This leads to the polarization effect known as the Maxwell-Wanger-Sillars (MWS) polarization (Hidvig, 1977). Therefore, the dielectric constants of the filed PMMA samples were higher than those of the pure PMMA matrix. At lower frequency, filled PMMA samples with CNP exhibited higher έ values (i.e. έ =42.5) with respect to samples with CB (έ =37.5). The addition of CB or CNP beyond these wt percentages did not improve the property any further (Fig. 3a,b). It is apparent from Fig. (3c,d) that ª data could be similarly interpreted to the data of έ. Conductivity Measurements: The Ac conductivity (σAc) for the prepared samples at various frequencies (i.e. 1KHZ- 100 KHz and 4.5MHz) was studied at room temperature (25±1°C) and the data are shown in Fig. (3e,f). It is evident that, at lower frequency, σ of the prepared samples was high and decreased as the frequency was increased. At low frequency with an applied electric field, the increase in conductance was attributed to free charges available in the composite system (hopping mechanism). At higher frequency, the conductivity was decreased due to trapped charges. This reflects the increase in insulation properties of the prepared samples at higher frequency. The effect of various filler content on sAc was also studied. A critical concentration of filler, beyond which the polymer composite becomes conductive, is referred to as the percolation threshold. At this point a conductive network is formed through the matrix. This permits the movement of charge carries of the fillers through the matrix, and also the composite achieves a certain degree of electrical conductivity. In electrical conductive polymers, the percolation threshold of fillers is closely dependent on their geometric factors (e.g. volume fraction, size, shape, and orientation) and the interaction between them (Sandler, 2003; Bryning, 2005). It is a critical issue in producing conductive composites for use in films, coatings and paints since the lower percolation threshold can reduce the loading expansive filler, leading to lighter composites (Lu, 2008). The current experiments showed that CNP-reinforced composites exhibited electrical percolation with addition of 2wt% at which electrical conductivity rises sharply by several orders of magnitude. In comparison with composites reinforced with CB, the percolation threshold of composites was 20wt%. With respect to PMMA, the electrical conductivity of the 2wt% CNP composite increased by 7 orders of magnitude up to approximately 10-4 S/cm (Fig. 3f). On the other hand, (Fig. 3e) indicates that CB composites required more carbon black (20 wt %) to reach the percolation threshold; with less sAc (i.e., 10-6 S/cm). The electrical conductivity was approximately constant for the composites with the higher filler loading. Consequently adding more CB or CNP did not significantly alter the electrical conductivity.

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(A)

(C)

(E)

(B)

(D)

(F)

Fig. 3: Effect of filler content on electrical properties of PMMA composites. Thermal Stability: The reinforcement of PMMA by CNP and CB was reflected in the data from TGA presented in Fig. (4). Initial degradation onset (i.e., initial temperature Ti) of pure PMMA, PMMA-CB and PMMA-CNP are at 300, 335 and 380°C respectively. So, relatively a small amount (20 wt %) of CNP thermally stabilized the PMMA by around 80 °C. The process of degradation of composite was delayed all the way starting from 100wt% residue to complete burning. The Tf (correspond to the temperature after which there was negligible weight loss) increased by 75°C from 400°C for PMMA to 475°C for PMMA/CNP composites suggesting a considerable increase in thermal stability. However, this enhancement in thermal stability was not observed for PMMA/CB composite. The same trend was also observed for T50 (the temperature at which 50% degradation took place), as observed from Table (1). Thus, the increase in thermal degradation temperatures of PMMA/CNP composites was likely associated with the interaction between CNP and the polymer which in turn improved the thermal stability of the samples.

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Fig. 4: Thermogravimetric analysis of PMMA and its composites. Table 1: TGA of PMMA and its composites. Sample code Weight-loss temperatures (°C) --------------------------------------------------------------------------------T50 Ti Tf PMMA 400 350 300 PMMA/CB 450 400 335 PMMA/CNP 475 430 380

Residual weight (%) 4 5 7

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