Morphology and Properties of Aminosilane Grafted MWCNT/Polyimide Nanocomposites

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2008, Article ID 786405, 15 pages doi:10.1155/2008/786405 Research Article Morphology and Properties of Aminosilane Grafted MWCNT/Polyimide Nanocomposites Siu-Ming Yuen,1 Chen-Chi M. Ma,1 Chin-Lung Chiang,2 and Chih-Chun Teng1 1 Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan 2 Department of Industrial Safety and Health, Hung Kuang University, Salu, Taichung 443, Taiwan Correspondence should be addressed to Chen-Chi M. Ma, Received 2 April 2007; Revised 2 September 2007; Accepted 4 October 2007 Recommended by Jun Lou This investigation presents a novel method for modifying multiwalled carbon nanotubes (MWCNTs). The morphology, electrical resistivity, and percolation threshold of MWCNT/Polyimide nanocomposites were studied. Acid-modified MWCNTs reacted with (3-aminopropyl)triethoxysilane by ionic bonding, and were then mixed with polyamic acid via imidization. TEM microphotographs reveal that silane-grafted MWCNTs were connected to each other. The electrical resistivity of silane-grafted MWCNT/polyimide decreased substantially below than that of acid-treated MWCNTs when the silane-modified MWCNT content was lower than 2.4 wt%. The percolation threshold of the MWCNT/polyimide composites is 1.0 wt% for silane-modified MWCNT and exceeds 7.0 wt% for acid-modified MWCNT. The acid-modified MWCNT/polyimide composites possess slightly higher glass transition temperatures than that of pure polyimide. The glass transition temperature of the polyimide increased significantly with silane-modified MWCNT content. Tensile properties of the polyimide have been improved with the MWCNTs content. Copyright © 2008 Siu-Ming Yuen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Polyimide is a high-performance polymer with high thermal stability, favorable dielectric properties, and chemical resistance. It has been found applications in the microelectronics and composites industries [1]. Carbon nanotubes (CNTs) have attracted much research interest in various areas since their structures were identified in 1991 [2]. They exhibit excellent mechanical and electrical properties, low density, high-surface area, and high chemical resistance [3–8]. CNT/polymer composites are interesting materials whose mechanical properties and electrical conductivity can be improved by the addition of CNT [9– 13]. CNT/polyimide composites are interesting materials and have been extensively studied [14–17]. In our previous investigation, [17] unmodified-, acid-modified-, and amine-modified-MWCNT/polyimide nanocomposites were prepared and their morphology and electrical, thermal, and mechanical properties were examined . CNT can be modified by refluxing with strong acid or a strong oxidizing agent. Carboxyl and hydroxyl functional groups are formed on the surface of CNTs during acid modification [18]. Acid-modified MWCNT can be modified with silane [19–23]. Shanmugharaj et al. [20] grafted 3-aminopropyltriethoxysilane (APTES) to acid-modified MWCNT and prepared silane-modified MWCNT/natural rubber composites. They suggested that silane can be reacted with the hydroxyl groups (−OH) on the surface of MWCNTs [19–23]. The oxidation of MWCNT may generate carboxylic groups (−COOH) rather than hydroxyl groups. Valentini et al. [23] modified SWCNTs using CF4 plasma to obtain fluorinated SWCNT (f-SWCNT). The f-MWCNT then reacted with APTES and the amine functional group of APTES was grafted on the f-MWCNT. Our previous study has successfully modified MWCNT with silane [24–26]. The silane-modified MWCNTs/Poly (urea urethane) composites have been prepared. The molecular structure and molecular mobility of the carbon-nanotube/PUU nanocomposites have been investigated [26]. These references indicated that carbon nanotube may be dispersed effectively. Increase the dispersion does not improve the electrical conductivity nor decrease the percolation threshold effectively. 2 Journal of Nanomaterials In this study, acid-modified MWCNT was mixed with (3-aminopropyl)triethoxysilane(APTES). Silane functional groups are grafted on the acid-modified MWCNT (APTESMWCNT) by ionic bonding or amide bonding, and some ungrafted APTES may react with polyamic acid to form a complex [1]. After modification of the MWCNT, silane functional groups were remained. The APTES-MWCNT was dispersed in the polyamic acid which was imidized at 300◦ C. When the APTES-MWCNT/polyamic acid was heated to 300◦ C, the silane reacted on the MWCNT surface, and this reaction was examined. This work studies the electrical resistivity, the percolation threshold, and the thermal properties of the MWCNT/polyimide. Table 1: The ratios of APTES to acid-modified MWCNT for APTES-MWCNT. 2. 2.5.1. Fourier transform infrared spectroscopy EXPERIMENTAL SECTION 2.1. Materials Multiwalled carbon nanotubes were obtained from the Nanotech Port Company, Shenzhen, China. The diameters of the MWCNTs were 40–60 nm; their lengths were 0.5– 40 μm, and their special surface areas were 40–3000 m2 /g. Both 4,4 -oxydianiline (ODA) and 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride (BTDA) were obtained from Chris KEV Company, Inc. Terrance Leawood, KS, USA. (3Aminopropyl)triethoxysilane (APTES) was obtained from Lancaster Synthesis Co., Morecambe, England. N,N-Dimethylacetamide (DMAc) was obtained from Tedia Company Inc., Fairfield, OH, USA. 2.2. Synthesis of polyamic acid The precursor of polyimide (polyamic acid) was prepared by reacting 4,4 -oxydianiline (ODA) with 3,3 ,4,4 benzophenone tetracarboxylic dianhydride (BTDA) in N,NDimethylacetamide (DMAc). The mole ratio of ODA to BTDA was 1 : 1. 4,4 -Oxydianiline (ODA) was dissolved in DMAc and then 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride (BTDA) was added to the 4,4 -oxydianiline (ODA) solution with an ice bath. The polyamic acid was imidized at 300◦ C to yield polyimide (as shown in Scheme 1). 2.3. Modification of MWCNT Pristine MWCNTs were functionalized by refluxing with a mixture of H2 SO4 and HNO3 (weight ratio of H2 SO4 to HNO3 is 3 : 2) at 50◦ C for 24 hours. After acid treatment, the MWCNTs were washed using deionized water, filtered and dried at 100◦ C. Then, the modified MWCNT was dispersed in DMAc and then (3-aminopropyl)-triethoxysilane (APTES) added to the mixture and was stirred for 1 hour (as presented in Scheme 2). Table 1 presents the mass ratio of APTES to MWCNT. The APTES-treated MWCNT is denoted APTES-MWCNT. 2.4. Preparation of carbon nanotubes/polyimide nanocomposites APTES-MWCNT was added to polyamic acid then put on to a plastic plate and heated to 60◦ C to remove the solvent ID APTES-MWCNT-1 APTES-MWCNT-2 APTES-MWCNT-3 APTES : MWCNT (in weight) 1:1 2:1 3:1 (DMAc). The mixture was then heated to (...truncated)