Polymethyl Methacrylate Nanocomposites Prepared by in situ Method

Polymethyl Methacrylate Nanocomposites Prepared by in situ Method

J. Mater. Sci. Technol., 2012, 28(5), 391–395. Oxygen Barrier of Multiwalled Carbon Nanotube/Polymethyl Methacrylate Nanocomposites Prepared by in si...

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J. Mater. Sci. Technol., 2012, 28(5), 391–395.

Oxygen Barrier of Multiwalled Carbon Nanotube/Polymethyl Methacrylate Nanocomposites Prepared by in situ Method Ajaya K. Pradhan1) and Sarat K. Swain1,2)† 1) Department of Chemistry, North Orissa University, Takatpur, Baripada 757003, India 2) Department of Chemistry, Veer Surendra Sai University of Technology, Burla, Sambalpur 768018, India [Manuscript received June 13, 2011, in revised form December 29, 2011]

Multiwalled carbon nanotubes (MWCNTs)/poly(methyl methacrylate) (PMMA) nanocomposites were prepared by ultrasonic assisted emulsifier free emulsion polymerization technique with variable concentration of functionalized carbon nanotubes. MWCNTs were functionalized with H2 SO4 and HNO3 with continuing sonication and polished by H2 O2 . The appearance of Fourier transform infrared absorption bands in the PMMA/MWCNT nanocomposites showed that the functionalized MWCNT interacted chemically with PMMA macromolecules. The surface morphology of functionalized MWCNT and PMMA/MWCNT nanocomposites were studied by scanning electron microscopy. The dispersion of MWCNT in PMMA matrix was evidenced by high resolution transmission electron microscopy. The oxygen permeability of PMMA/MWCNT nanocomposites gradually decreased with increasing MWCNT concentrations. KEY WORDS: Nanocomposites; Morphology; Permeability; Dispersion

1. Introduction Carbon nanotubes (CNTs) can consist of single or multiple concentric graphene cylinders. Since its discovery[1] , CNTs are ideal fillers for polymer composites due to high Young s modulus combined with low density, good electrical and thermal conductivity. The very high aspect ratio (approximately 10000) of the CNTs makes possible the addition of a small amount (≤5 wt%) of CNTs for the strong improvement of the electrical[2,3] , thermal[4] and mechanical[5] properties of the polymer matrix. However, the effective usefulness of nanotubes as fillers in polymer composites depends on the capacity to obtain a good dispersion of CNTs in the matrix[6,7] . In the preparation of good CNT/polymer composites, an appropriate surface modification of CNTs is essential for achieving a homogeneous dispersion of CNTs and strong bonding between CNTs and polymer matrices[8] . Different methods are commonly used to incor† Corresponding author. Prof.; Tel.: +91 9937082348; E-mail address: [email protected] (S.K. Swain).

porate nanotubes into polymers: (i) film casting of suspensions of nanotubes in dissolved polymers, (ii) polymerization of nanotube with monomer mixtures, (iii) melt mixing[9,10] , (iv) solution blending[11] , and (v) in situ polymerization[12,13] . Film casting was used to investigate the properties of polymers containing CNTs including the effect of dispersion and orientation[14–16] and interfacial bonding[17] . Melt mixing cannot achieve homogeneous dispersion of CNTs in polymer matrices where as solution blending does not form strong chemical bonding between CNTs and polymer. The in situ polymerization technique is a preferred method of composite formation as per earlier reports[18–20] . The mechanical properties, electrical conductivity and morphology of poly(methyl methacrylate) (PMMA)/multiwalled carbon nanotube (MWCNT) composites have been reported previously[21,22] . Schmidt et al.[23] prepared the thin transparent film of MWCNTs in PMMA matrix and the results showed increased electrical conductivity. Park et al.[24] synthesized MWCNT/PMMA nanocomposites using in situ bulk polymerization with ultrasonication and re-

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Fig. 1 Flow sheet for synthesis of PMMA/MWCNT nanocomposites

ported the increase in electrical conductivity of the composites. Pande et al.[25] reported the improved mechanical properties of MWCNT/PMMA nanocomposites prepared by in situ polymerization method. Ormsby et al.[26] prepared the polymethylmethacrylate bone cement/MWCNT nanocomposites in different methods. The improved mechanical properties of the nanocomposites cement and the decrease in thermal properties were attributed to the method of incorporation of MWCNT. Jin et al.[27] prepared the thin films by compressing the well dispersed MWCNT/PMMA nanocomposites through miniature mixture-molder and reported the increase in electrical conductivity of the composites. Valentino et al.[28] studied the influence of polymer structure and nanotube concentration on conductivity of polyethylene/MWCNT nanocomposites by melt mixing process and found the enhanced electrical properties of the composites. The MWCNT/PMMA nanocomposites may be applicable for electronic packaging material and corrosion resistance materials in engineering applications[27,28] . In this work, the PMMA/MWCNT nanocomposites were synthesized in aqueous medium through in situ polymerization under ultrasonication technique for good dispersion and debundling of the MWCNTs. The morphology and dispersion of MWCNTs were investigated by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The influence of the MWCNT concentrations on thermal properties and oxygen permeability were studied as compared to the virgin PMMA. 2. Experimental The MWCNTs that have been used during this experiment were purchased from Sigma Aldrich, USA and they had diameters ranging from 10 to15 nm and length ranging from 0.1 to 10 μm. Methyl methacrylate (MMA) was purchased from Merck, Germany and used after purification with solution of phosphoric acid, sodium hydroxide and double distilled water. Potassium persulphate (KPS) was obtained from Merck, Germany and used as received. All other reagents such as H2 SO4 , HNO3 , and ammonium ferrous sulphate were of analytical grade and used as received. The MWCNTs were cut and functionalized by treatment in a mixture of concentrated H2 SO4 and HNO3 in a volume ratio of 3:1 and sonicated by an ultrasonic cleaner (120 W/60 kHz) for 24 h at about

40 ◦ C in a flask. The solution was then diluted with double distilled water and filtered. The residue was washed with distilled water. Further the open end tubes were polished with hydrogen peroxide and H2 SO4 in a volume ratio of 1:4 with stirring at 70 ◦ C for 30 min. The resultant solution was diluted with distilled water and centrifuged to get the functionalized multiwalled carbon nanotubes (f-MWCNT) and continued till no acid remain in the f-MWCNT. The f-MWCNTs with distilled water were sonicated in a two necked flask for 20 min. Then MMA monomer was added into the flask and stirred for 10 min, which were further sonicated for 10 min. The stock solution of KPS of 0.1 mol/L as initiator was added and nitrogen gas was purged into the flask to remove oxygen. It was allowed to polymerize for 3 h with constant stirring at 63 ◦ C. Ammonium ferrous sulphate of 0.1 mol/L solution was added to arrest the polymerization process. The precipitate was filtered to get PMMA/MWCNT nanocomposites and dried in hot air oven for characterization of its properties. The concentration of MMA and KPS were optimized by weight percent conversion of polymerization and [MMA]=1.41 mol·dm−3 , [KPS]=1× 10−2 mol·dm−3 were kept constant for variation of MWCNT. The schematic representation of synthesis of PMMA/MWCNT nanocomposites is illustrated as Fig. 1. The chemical structure and interaction of MWCNT with polymer matrix were detected with a Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer Paragon 500) in the range of 400 to 4000 cm−1 using KBr powder. The morphology and dispersion of MWCNT in PMMA matrix were investigated by using SEM (JEOL, JSM-5800, Japan). The high resolution transmission electron microscopy (Tec-nai 12, Philips), operating at 120 kV was used to study the dispersion of MWCNT in PMMA matrices. Thermogravimetric analysis (TGA) of powdered composites was performed for the samples in nitrogen flow at a heating rate of 10 ◦ C/min using a TGA apparatus (Model DTG-60, Shimadzu Corporation, Japan). Oxygen permeability of the nanocomposites were measured with ASTM F 316-86 by using oxygen permeation analyzer (PMI instrument, GP-201A, NY, USA) 3. Results and Discussion 3.1 FTIR analysis The FTIR spectra of MWCNT, f-MWCNT,

A.K. Pradhan et al.: J. Mater. Sci. Technol., 2012, 28(5), 391–395.

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(Fig. 2 (inset)) tion bands at 3500 and 1710 cm associating with O–H stretching and C=O stretching in carboxyl group, respectively. The results indicate that MWCNTs bonded to –COOH groups are obtained. The peak at 1735 cm−1 shows the characteristics C=O stretching from the carboxyl and carbonyl groups introduced by the acid treatment on the MWCNT surface by which PMMA macromolecules are grafted onto the surface of functionalized MWCNT. 3.2 Morphology study

Fig. 2 FTIR spectra of MWCNT, f-MWCNT, PMMA and PMMA/f-MWCNT nanocomposites

PMMA and PMMA/f-MWCNTs were studied to identify the functional groups proving interaction of MWCNT with PMMA in Fig. 2. MWCNT before functionalization exhibits as O–H stretching band at 3410 cm−1 which is induced by the carboxyl and hydroxyl group attached to the open ends of MWCNT. After being sonicated in the H2 SO4 /HNO3 (3:1) by volume mixture solvent, MWCNTs show two absorp-

The morphology and the degree of dispersion of raw MWCNT (Fig. 3(a)), f-MWCNT (Fig. 3(b)) and PMMA/f-MWCNT composites (Fig. 3(c)) have been analyzed by SEM. The SEM micrograph in Fig. 3(c) of PMMA/f-MWCNT nanocomposites at concentration of 0.70 wt% of MWCNT shows homogeneous dispersion of the carbon nanotubes throughout the polymer matrices and nanotubes have been interconnected at this concentration and form a nanotube network. Carbon nanotubes are observed as bright spots lying over the dark PMMA matrix and explain the good dispersion of MWCNT in PMMA matrix. Fig. 3(d) indicates the HRTEM image of PMMA/f-MWCNT nanocomposite sample which clearly shows a uniform

Fig. 3 SEM images: (a) MWCNT, (b) f-MWCNT, (c) PMMA/f-MWCNT nanocomposites with 0.70 wt% of MWCNT; (d) HRTEM image of PMMA/f-MWCNT nanocomposites

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Fig. 4 TG curves of PMMA, PMMA/f-MWCNT nanocomposites with 0.70 wt% of MWCNT

PMMA coating around the tube core. This is considered as a clear evidence for the interaction of fMWCNT with PMMA matrix. The MWCNT overlaid with PMMA confirms that the interaction between MWCNT and PMMA is not a physical contact but strong adhesion due to chemical bond. 3.3 TG analysis The thermal stability of PMMA, f-MWCNT and PMMA/f-MWCNT composites are shown in Fig. 4. It is observed that in comparison to PMMA the decomposition of PMMA/f-MWCNT composites are shifted towards a higher temperature. The thermal decomposition of PMMA occurs in three steps reaction with maximum decomposition at 270 ◦ C, the first step due to water loss, second step due to degradation of polymer chain and third step due to oxidation of PMMA macromolecules above 400 ◦ C. Two step change of MWCNT is observed because of water loss from 80 to 130 ◦ C and dehydroxylation of functionalized MWCNT from 200–400 ◦ C. From TG analysis it is observed that PMMA macromolecules decompose almost completely, whereas for wrapped PMMA/fMWCNT composites, a residue of about 50% is obtained due to MWCNT content. The thermal stability of PMMA/f-MWCNT composites is more than that of the PMMA due to grafting of MWCNT with PMMA macromolecules. Further it is seen from Fig. 4 that the plot of virgin polymer is sharper than that of its composites, thus the addition of f-MWCNTs can slower the degradation rate of PMMA matrix. This indicates that the dispersion of the MWCNT into the polymer matrix also improves the thermal stability of the composites. 3.4 Oxygen permeability The oxygen permeability of virgin PMMA and PMMA/f-MWCNT nanocomposites has been studied as shown in Fig. 5. At 3.447 kPa (0.5 Psi)

Fig. 5 Oxygen permeability of PMMA/f-MWCNT nanocomposites at constant pressure of 3.447 kPa (0.5 Psi)

pressure, the oxygen permeability of PMMA/fMWCNT nanocomposites with 1.75 wt% of MWCNTs loading is about eight times less than that of virgin PMMA. The reduction of permeability arises from the longer diffusive path of the penetration of the oxygen in the presence of MWCNTs. The incorporation of MWCNT in PMMA matrix is particularly efficient at maximizing the path length due to the high aspect ratio. Further, the presence of MWCNT introduces a torturous path for which the oxygen travels longer diffusive path. Hence oxygen permeability of PMMA/MWCNT nanocomposites is less than that of PMMA matrix. The tremendous decrease in oxygen permeability with increasing MWCNT (wt%) is due to good dispersion of MWCNTs in polymer matrix of PMMA/f-MWCNT composites which is supported with Fig. 3(c). The substantial reduction in oxygen permeability of PMMA/f-MWCNT may enable the composites for coating industry. 4. Conclusion PMMA/f-MWCNT nanocomposites were synthesized by emulsion polymerization process providing good dispersion of carbon nanotubes in the polymer matrix. Dispersion and morphology of MWCNT in the polymer matrix were explained by HRTEM and SEM micrographs. The decrease in oxygen permeability of the PMMA/f-MWCNT nanocomposites with addition of the MWCNT would make the materials be more applicable in the industrial coatings. Acknowledgements The authors are thankful to Department of Atomic Energy, BRNS, Government of India for providing financial support under Grant OM # 2008/20/37/5/BRNS/1936. REFERENCES [1 ] S. Iijima: Nature, 1991, 354, 56.

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