multiwalled carbon nanotubes nanocomposites by copolymerization of styrene and styryl-functionalized multiwalled carbon nanotubes

multiwalled carbon nanotubes nanocomposites by copolymerization of styrene and styryl-functionalized multiwalled carbon nanotubes

Materials Chemistry and Physics 137 (2013) 694e698 Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepa...

941KB Sizes 1 Downloads 98 Views

Materials Chemistry and Physics 137 (2013) 694e698

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Materials science communication

Preparation polystyrene/multiwalled carbon nanotubes nanocomposites by copolymerization of styrene and styryl-functionalized multiwalled carbon nanotubes Jing Hua*, Zhongguang Wang, Ling Xu, Xin Wang, Jian Zhao, Feifei Li Key Laboratory of Rubber-Plastics Ministry of Education, Qingdao University of Science and Technology, Qingdao, People’s Republic of China

h i g h l i g h t s < A facile and simple way to successfully prepare the polystyrene/MWNTs nanocomposites. < Characterizations show that styryl group covalently bond to the surface of MWNTs. < The solutions of p-MWNTs/PS in chloroform have the hyperchromic effect. < Thermal stability of p-tpas composites improved in the presence of MWNTs. < The performance of polymer prepared by this method have great potential for exploitation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 March 2012 Received in revised form 19 June 2012 Accepted 12 October 2012

Styryl-functionalized multiwalled carbon nanotubes (p-MWNTs) were prepared by esterification based on the carboxylate salt of carbon nanotubes and p-chloromethylstyrene in toluene. Then in situ radical copolymerization of p-MWNTs and styrene initiated by 2,20 -azobis(isobutyronitrile) (AIBN) was applied to synthesize composites of styryl-functionalized multiwalled carbon nanotubes and polystyrene (PS) (p-MWNTs/PS). Characterizations carried out by FT-IR, 1H NMR, UVevis show that styryl group covalently bond to the surface of MWNTs. The results of UV showed that the solutions of p-MWNTs/PS in chloroform have the hyperchromic effect. Transmission electron microscopy (TEM) images of p-MWNTs/PS composites and scanning electron microscopy (SEM) images of fracture surface of p-MWNTs/PS composites showed the functionalized nanotubes had a better dispersion than that of the unfunctionalized MWNTs in the matrix. The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) suggested that the thermal stability of p-MWNTs/PS composites improved in the presence of MWNTs. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.

Keywords: Composite materials Nanostructures Chemical synthesis Polymers

1. Introduction Carbon nanotubes (CNTs) as one-dimensional structures have attracted much interest due to their unique electronic, chemical, and mechanical properties, and potential applications in future nanodevices, particularly as field-effect transistors, nanoprobes, bioelectronics, and sensors [1e3]. Unfortunately, CNTs are difficult to be uniformly dispersed in the composites due to the strong van der Waals forces among the tubes stemming from their large aspect ratio and surface areas which lead to the aggregation among CNTs in composites. This disadvantage

* Corresponding author. Tel.: þ86 532 84022926; fax: þ86 532 84022725. E-mail address: [email protected] (J. Hua).

hinders its biomedical and other emerging applications [4]. Noncovalent and covalent modifications of the CNTs with polymers are commonly used to improve their dispersion and fabricate nanotube-based composites to explore new properties [5,6]. Covalent attachment of polymer chains to the surface of CNTs involves either “grafting to” or “grafting from” strategies. The “grafting to” method is based on the reaction of preformed end-functionalized polymer with reactive surface groups or radical precursor on CNTs. For instance, poly(N-vinylpyrrolidone) (PVP), polystyrene (PS) and poly(sodium 4-styrenesulfonate) (PSS) chains have been covalently linked with CNTs by a radical mechanism [7e9]. The “grafting from” approach relies on the immobilization of initiators onto the CNTs surface followed by in situ polymerization and the consequent formation of the polymer chains bound to the nanotubes. These initiators are mostly covalently attached onto the

0254-0584/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.10.020

J. Hua et al. / Materials Chemistry and Physics 137 (2013) 694e698

CNTs surfaces by acid defect group chemistry and sidewall functionalization of CNTs [10]. The advantage of this method is that polymer brushes with high grafting density can be easily prepared [11]. For better controlling the functionalization degree of the nanotubes with grafted polymer and the molecular weight distribution of grafted polymer, living radical, cationic/anionic and condensation polymerizations have been utilized in “grafting from” techniques recently [12]. As free-radical polymerization reactions are of enormous importance in academia and technology. The monomers for these reactions are available in large quantities from the petrochemicals industry, and the polymers obtained from these monomers form the foundation of much of the polymer industry. It can be carried out by the bulk, solution, emulsion, and suspend polymerizations. Free radical polymerization has been employed most to graft polymer chains from carbon nanotubes. Shaffer and Koziol successfully tested the grafting of oxidized MWNTs with PS molecules using an in situ radical polymerization reaction [13]. TEM micrographs of grafted nanotubes revealed a thin polymer coating. Poly(2-hydroxyethyl methacrylate) PHEMA was grafted to oxidized MWNTs by free radical polymerization [14]. In the work of Park et al. [15], poly(methyl methacrylate) (PMMA) covalently grafted onto MWNTs sidewalls via the methods of chemical modification followed by free radical bulk polymerization of methyl methacrylate. GPC analysis showed that the molecular weight of PMMA increased with the MWNTs contents. Recently, the dispersion of carbon nanotubes into PS matrix for the fabrication of CNTs/PS nanocomposite with various methods has stimulated interest among researchers [8,16e18]. In the present work, styryl-modified MWNTs/PS nanocomposites were prepared successfully via a simple in situ radical polymerization method for the first time. To improve dispersibility of MWNTs in PS matrix, polymerizable groups, styryl, were introduced onto the surfaces of the MWNTs treated via mixed acid oxidation process (concentrated H2SO4/HNO3, in a 3:1 ratio). Then the styryl-modified MWNTs were used as macro-comonomers for the radical copolymerization with styrene. The effect of styryl-modified MWNTs on the UV absorption and thermal properties of the MWNT/PS nanocomposites are discussed. 2. Experimental

695

grafted by CMS (MWNTseCOOCH2C6H4CH]CH2), (p-MWNTs) respectively on the basis of the previously reported method [16]. 2.2. Preparation of p-MWNTs/PS nanocomposites The p-MWNTs/PS nanocomposites were prepared via an in situ radical polymerization method as follows: certain amounts of p-MWNTs (1 wt%), 5 mL purified styrene, and 5 wt% AIBN were charged into a 10 mL flask, the mixture was dispersed under ultrasonication treatment for 45 min at room temperature, then the mixture reacted for 6 h at 70  C in water bath. N2 was present throughout the polymerization stage. The p-MWNTs/PS nanocomposites were separated from the non-grafted PS by the following procedure: the resulting products were dispersed in 30 mL toluene with ultrasonication treatment and the dispersion was centrifuged at 4000 rpm for 2 h. The precipitate was extracted with THF using a Soxhlet apparatus until no PS was eluted in the refluxing solvent, and dried under vacuum at 40  C. The PS and composites of raw MWNTs (r-MWNTs) and polystyrene (r-MWNTs/ PS) were synthesized according to similar procedures, respectively. The general strategy for grafting polymers from the MWNTs above is described in Scheme 1. 2.3. Characterizations FT-IR spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer using KBr pellets. 1H NMR was carried out on a Bruker (500 MHz) NMR instrument using DMSO (dimethylsulfoxide) as solvent and tetramethylsilane as reference. TGA was performed on a Netzsch TG209 instrument at a scan rate of 10  C min1 to 600  C in a nitrogen atmosphere. DSC experiment was measured using Netzsch 204 Phoenix with a heating rate of 10  C min1 from 80  C to 100  C. TEM investigations were performed on ultra-thin film prepared from the above sheets by cryo-ultramicrotomy using a JEOL JEM1200EX at an acceleration voltage of 100 kV. SEM was carried out using a JEOL JMS-6700F scanning microscope equipped with an energy-dispersive X-ray (EDX) Oxford ISIS 300 microanalytical system. Before the observation, the samples were cryo-fractured and the section surface was covered by a thin, deposited film of gold.

2.1. Materials 3. Result and discussion Multiwalled carbon nanotubes (MWNTs) (diameter: 20e40 nm, purity >95%, prepared by chemical vapor deposition methods) were purchased from Shenzhen Nanotech Port Co., China. p-Chloromethylstyrene (CMS) (molecular weight ¼ 152) was obtained from Aldrich and used without any further purification. Styrene was dried over CaH2, and distilled under reduced pressure prior to use. 2,20 -Azobis(isobutyronitrile) (AIBN) (analytical grade, available from Chemical Factory of Nankai University) was recrystallized from methanol before use. Chloroform and toluene were analytical reagents. Distilled water was used throughout. These nanotubes were treated to obtain the MWNTs surface-grafted by carboxylic acid group (MWNTseCOOH) and MWNTseCOOH

Fig. 1aec shows the FTIR spectra of the pristine MWNTs, MWNTseCOOH and styryl-modified MWNTs respectively. The absorption peak at 1613 cm1 in FTIR spectrum of pristine MWNTs is the characteristic stretching vibrations of C]C bonds related to the expected nanotubes phonon modes [19]. Contrast to pristine MWNTs, the new peaks emerging at 1712 cm1 and 3433 cm1 after the mixed concentrated H2SO4/HNO3 (3:1, v/v) oxidation treatment corresponded to C]O and OeH bonds stretching vibrations respectively, confirming the attachment of the carboxylic groups onto the MWNTs [20]. The C]O peak at 1712 cm1 shifting to 1722 cm1 and a new peak of CeO stretching vibration at

Scheme 1. Synthesis progress of styryl-functionalized multiwalled carbon nanotubes/polystyrene composites.

696

J. Hua et al. / Materials Chemistry and Physics 137 (2013) 694e698

1.2

(a)

263nm 0.14

253nm

PS r-MWNTs/PS p-MWNTs/PS

1.0 0.12

(b)

0.10

0.8 Absorbance

Absorbance

1613

255nm

0.6

0.08 0.06 0.04

(c)

1712

3433

0.4 0.02 0.00

0.2

240

2918

1360

4000

3500

3000

2500

2000

1500

280

300

320

340

Wavelength/nm

1722 1580

2998

260

0.0 200

400

600

800

1000

Wavelength/nm 1000

500

-1

Wavenumber/cm

Fig. 1. FTIR spectra of different MWNTs: (a) the purified MWNTs, (b) MWNTseCOOH and (c) MWNTseCOOCH2C6H4CH]CH2.

1360 cm1 resulted from the substitution of carboxylic acid group by ester group during reaction with p-chloromethylstyrene [21]. In addition, the presence of new bands at 2918 cm1 and several bands between 3000 and 3100 cm1 corresponded to eCH2e stretching vibration and CeH stretching vibration of benzene ring, which is in accordance with literature [16]. Meanwhile, 1H NMR spectra of the p-MWNTs show the characteristic signals of p-chloromethylstyrene macromolecules such as those of (a, eC6H4e), (b, ]CHe), (c, CH2]) at 7.0, 6.5 and 5.8 ppm, respectively (Fig. 2) [22]. The interaction of eCl (CMS, eCH2Cl) with eH (MWNTseCOOH) affected by ester group caused a downfield shift of the new proton peak (d, eCH2e) from d ¼ 4.39 to d ¼ 4.90, which confirmed the formation of covalent bonding between p-chloromethylstyrene and MWNTseCOOH. The UVevis absorption spectra exhibit the typical absorption band of PS in the 200e300 nm which can be also observed in r-MWNTs/PS and p-MWNTs/PS samples, as shown in Fig. 3 [23]. The spectrum of p-MWNTs/PS shows the extraordinary intensity of the absorption peak at w263 nm and exhibited hyperchromic effect as compared with that of PS, r-MWNTs/PS, respectively. The reason presumably contributed to the interaction of terminal alkenes double bonds between styrene and p-MWNTs, which maybe result

Fig. 3. The UVevis spectra of PS, r-MWNTs/PS and p-MWNTs/PS in CHCl3 at room temperature. Inset: photos for the dispersion status of: (A) purified MWNTs; (B) MWNTseCOOCH2C6H4CH]CH2; (C) p-MWNTs/PS in CHCl3 solvent. All of concentrations of MWNTs were 0.03 mg mL1.

from its unique electroconductivity and homodisperse in matrix. The dissolvability of p-MWNTs/PS in CHCl3 increased compared to that of purified MWNTs and p-MWNTs, respectively [16] (inset of Fig. 3AeC). The morphology of the r-MWNTs/PS (a), p-MWNTs/PS (b) nanocomposites was supported by TEM, as shown in Fig. 4. CNTs in p-MWNTs/PS is apparently shorter length and thicker diameter compared with that in r-MWNTs/PS. The reason maybe result from cutting by concentrated H2SO4/HNO3 (3:1, v/v) oxidation treatment and the reaction between the styrene and p-MWNTs by the “grafting to” methods which advantage is polymer brushes with high grafting density can be easily prepared [11], respectively. Fig. 5 shows TGA diagrams of PS, r-MWNTs/PS and p-MWNTs/PS composites. The weight of PS, r-MWNTs/PS and p-MWNTs/PS composites decreases rapidly at 200e500  C with a maximum decomposition rate near 400  C. As further increasing temperature to 450  C, all samples completely decompose under the same conditions, except for a very small amount of residual carbon for r-MWNTs/PS and p-MWNTs/PS [17], because PS had a lower decomposition temperature than MWNTs [24]. The onset of the decomposition temperature (Td) of p-MWNTs/ PS is slightly higher than that of r-MWNTs/PS and PS as Fig. 5 showed. The mass residue above 500  C is higher and the decomposition rate of p-MWNTs/PS is lower than the others. p-MWNTs/PS shows better thermal stability than PS and r-MWNTs/PS.

b

a

d

c

7.0

6.5

6.0 PPM

5.5

5.0

Fig. 2. 1H NMR spectrum of styryl-functionalized carbon nanotubes.

4.5

Fig. 4. TEM images of r-MWNTs/PS (a), p-MWNTs/PS (b) composites.

J. Hua et al. / Materials Chemistry and Physics 137 (2013) 694e698

a 100

PS r-MWNTs/PS p-MWNTs/PS

Mass %

80

60

The results indicate that the thermal stability of PS can be improved by linkage with functionalized MWNTs, which introduced many unsaturated regularity double bonds that significantly enhancing the thermal stability of composites [25]. The worse thermal stability of r-MWNTs/PS composites than that of pMWNTs/PS composites may be caused by the nonuniform dispersion of the raw MWNTs in the polymeric matrix. With neat MWNTs hardly decomposing below 600  C while PS decomposing near

40

20

0 100

200

300

400

500

600

Temperature/

b

0

PS r-MWNTs/PS p-MWNTs/PS

-5

Mass %/min

-10

-15

-20

-25

-30 200

400

600

Temperature/ Fig. 5. TGA diagrams of PS, r-MWNTs/PS and p-MWNTs/PS composites.

PS r-MWNTs/PS p-MWNTs/PS

Heat Flow(mw/mg) exo

0.30

96.5

87.3

0.25

98.2 77.4 0.20

0.15 40

60

80

100

120

140

160

180

Temperature/ Fig. 6. DSC curves of PS, r-MWNTs/PS and p-MWNTs/PS nanocomposites.

697

Fig. 7. SEM images of PS (a), r-MWNTs/PS (b) and p-MWNTs/PS (c), 50,000.

698

J. Hua et al. / Materials Chemistry and Physics 137 (2013) 694e698

400  C, the increased thermal conductivity of grafting samples can attribute to the addition of MWNTs, especially for the slowed PS decomposition with MWNTs uniform distributed in PS matrix [24]. To evaluate the effect of the different MWNTs on the morphological characteristics of PS materials, PS, r-MWNTs/PS and pMWNTs/PS composites were subjected to DSC analyses. As shown in Fig. 6, the Tg of the PS increased with the addition of the MWNTs, specially for that of p-MWNTs/PS composite [18], which contributed to that the MWNTs tend to restrict the polymer chain mobility through the MWNTs-matrix interaction [25]. In consideration of the fact that styrene structural units interact more strongly with styryl-modified MWNTs resulting in more physical crosslinking and constraining the motion of the molecular chains of PS, the Tg of the p-MWNTs/PS is expected to increase more drastically. The other reason, as discussed by TGA studies, the thermal stability of the polymer matrix can be enhanced by the addition of MWNTs. Specially, the p-MWNTs are mostly dispersed in the matrix of PS due to the similarity in structure and the covalent bonding between them, and then promote the heat flow and dissipate the thermal, thus the Tg of p-MWNTs/PS increased and its range of Tg decreased. However, it should be noted that r-MWNTs/PS composites had two transition temperatures locating on 77.4  C and 96.5  C, which were near those of PS and p-MWNTs/PS composites, 87.3  C and 98.2  C, respectively. The reason for two Tg indicates two kinds of PS molecules exist in the r-MWNTs/PS composites. Because r-MWNTs was physically mixed in PS matrixes, its dispersion and interaction with PS matrix were nonuniform. In the case of its loose packing with PS matrix, the consequent increasing of the free volume of polymer chains [26] resulted in the decreased Tg of 77.4  C. In the other case of tight packing with PS matrix, r-MWNTs obstructed the motion of PS resulting in the increased Tg of 96.5  C. The SEM images for the fractured surfaces of PS, r-MWNTs/PS and p-MWNTs/PS composites are included in Fig. 7. Only a few carbon nanotubes are observed (Fig. 7b small white-ellipse). It suggests that the MWNTs are not well dispersed in the PS matrix and most of the MWNTs are aggregated together by its characteristic [18]. The bright dots that the ends of broken MWNTs can be seen everywhere in Fig. 7c, which indicated styryl-modified MWNTs with structure of styrene promoting the compatibility with PS and uniformly distributing in PS matrix. 4. Conclusions Polystyrene/MWNTs nanocomposites were synthesized by copolymerization with styrene and p-chloromethylstyrene grafted MWNTs initiated by AIBN via an in situ bulk polymerization. We determined that PS is covalently linked to the MWNTs during the proposed synthetic procedure via FT-IR, 1H NMR and UVevis. The TGA and DSC analysis also revealed that the increased thermal conductivity and Tg due to the uniform dispersion of MWNTs in PS matrix and covalently link between them. Based on the comparison

between the TEM and SEM micrographs of polystyrene/MWNTs nanocomposite prepared with p-chloromethylstyrene grafted MWNTs and raw MWNTs respectively, the p-chloromethylstyrene grafted MWNTs were found to provide better compatibility with PS matrix than raw MWNTs. MWNTs were sufficiently wetted in the PS matrix for the polystyrene/MWNTs nanocomposites by copolymerization with styrene and p-chloromethylstyrene grafted MWNTs. Acknowledgments Financial support from the National Science Foundation of China (No. 50603009 and No. 51073082) and National Natural Science Foundation of Shandong Province, China (No. ZR 2011 EL008) is gratefully acknowledged. References [1] W.A. Heer, A. Châtelain, D. Ugarte, Science 270 (1995) 1179. [2] H.J. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [3] E. Katz, I. Willner, ChemPhysChem 5 (2004) 1084. [4] S. Niyogi, M.A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen, M.E. Itkis, R.C. Haddon, Acc. Chem. Res. 35 (2002) 1105. [5] K.T. Lau, D. Hui, Composites Part B 33 (2002) 263. [6] Y.C. Jung, N.G. Sahoo, J.W. Cho, Macromol. Rapid Commun. 27 (2006) 126. [7] S.H. Qin, D.Q. Qin, W.T. Ford, J.E. Herrera, D.E. Resasco, Macromolecules 37 (2004) 9963. [8] S.H. Qin, D.Q. Qin, W.T. Ford, D.E. Resasco, J.E. Herrera, Macromolecules 37 (2004) 752. [9] S.H. Qin, D.Q. Qin, W.T. Ford, J.E. Herrera, D.E. Resasco, S.M. Bachilo, R.B. Weisman, Macromolecules 37 (2004) 3965. [10] Y.L. Liu, W.H. Chen, Macromolecules 40 (2007) 8881. [11] A.L. Martínez-Hernández, C. Velasco-Santos, V.M. Castaño, Curr. Nanosci. 6 (2010) 12. [12] Z. Spitalsky, D. Tasis, K. Papagelis, C. Galiotis, Prog. Polym. Sci. 35 (2010) 357. [13] M.S.P. Shaffer, K. Koziol, Chem. Commun. 18 (2002) 2074. [14] N.A. Kumar, H.S. Ganapathy, J.S. Kim, Y.S. Jeong, Y.T. Jeong, Eur. Polym. J. 44 (2008) 579. [15] S.J. Park, M.S. Cho, S.T. Lim, H.J. Choi, M.S. Jhon, Macromol. Rapid Commun. 24 (2003) 1070. [16] X.H. Chen, F. Tao, J.F. Wang, H.J. Yang, J.G. Zou, X.H. Chen, X. Feng, Mater. Sci. Eng., A 499 (2009) 469. [17] H.X. Wu, R. Tong, X.Q. Qiu, H.F. Yang, Y.H. Lin, R.F. Cai, S.X. Qian, Carbon 45 (2007) 152. [18] S.T. Kim, H.J. Choi, S.M. Hong, Colloid Polym. Sci. 285 (2007) 593. [19] C. Velasco-Santos, A.L. Martínez-Hernández, V.M. Castaño, J. Phys. Chem. B 108 (2004) 18866. [20] J.N. Deng, X.Q. Zhang, K. Wang, H. Zou, Q. Zhang, Q. Fu, J. Membr. Sci. 288 (2007) 261. [21] Z.M. Wang, X.X. He, D.Q. Sun, Applied Infrared Spectroscopy, second ed., Petroleum Industrial Press, Beijing, 1990. [22] S.C. Lin, J.P. Chen, J.N. Guan, J.D. Wu, Y. Lu, Acta Polym. Sin. 2 (1991) 238. [23] W. Zhao, Y.T. Liu, Q.P. Feng, X.M. Xie, X.H. Wang, X.Y. Ye, J. Appl. Polym. Sci. 109 (2008) 3525. [24] Z. Zhou, S.F. Wang, L. Lu, Y. Zhang, Y.X. Zhang, Compos. Sci. Technol. 67 (2007) 1861. [25] H. Kong, C. Gao, D.Y. Yan, Macromolecules 37 (2004) 4022. [26] S.J. Park, S.W. Chae, J.M. Rhee, S.J. Kang, Bull. Korean Chem. Soc. 31 (2010) 2279.