Improvement of mechanical and thermal properties of carbon nanotube composites through nanotube functionalization and processing methods
Introduction
Since the carbon nanotubes (CNTs) were reported by Iijima in 1991 [1], they have attracted as ideal reinforcing fillers in high strength, light weight polymer composites due to their unique structural, mechanical, electrical, and thermal properties [2], [3], [4], [5]. The impressive and unique properties of CNTs make them promising materials for a wide variety of applications such as nanoelectronic and photovoltaic devices [6], [7], superconductors [8], electromechanical actuators [9], electrochemical capacitors [10], nanowires [11], and composite materials [5], [12].
However, the pure CNTs are generally insoluble in common solvents and polymers, and they usually form stabilized bundles due to van der Waals force. So, it is extremely difficult to align and disperse the CNTs in a polymer matrix.
A significant challenge for developing high performance polymer/CNT composites is to introduce the individual CNTs in a polymer matrix in order to achieve better dispersion, possible alignment and stronger interfacial interactions, which improve the load transfer across the CNT–polymer matrix interface. The functionalization of CNT is an effective way to prevent nanotubes from aggregation, which helps to achieve better dispersion and stabilize the CNTs within a polymer matrix. There are several approaches for functionalization of CNTs, including defect functionalization, covalent functionalization, and noncovalent functionalization [13].
Noncovalent functionalization of nanotubes is of particular interest because it does not spoil the physical properties of CNTs but improves solubility and processability. This type of functionalization mainly involves surfactants or wrapping with polymers. In the search for nondestructive purification methods, nanotubes can be transferred to the aqueous phase in the presence of surfactants such as sodium dodecylsulfate (SDS) [14], [15]. In this case, the CNTs are surrounded by hydrophobic moieties of the corresponding micelles of surfactants. The main potential disadvantage of noncovalent attachment is that the forces between the wrapping molecule and the nanotube might be weak, thus as a filler in a composite the efficiency of the load transfer might be low.
The covalent functionalization of CNT can improve solubility as well as dispersion in solvents and polymer. The functional groups at the surface of CNT make the strongest type of interfacial bonding with the polymer matrix. The covalent functionalization can be accomplished by either modification of surface bound carboxylic acid groups on the nanotubes or direct reagents attached to the side walls of nanotubes.
Currently various techniques are used to incorporate CNTs into a polymer matrix, e.g., solution casting, melt mixing, electron spinning, and in situ polymerization [16], [17], [18], [19], [20], [21], [22], [23]. Melt mixing is a common and simple method, which is particularly useful for thermoplastic polymers. In melt processing, CNTs are mechanically dispersed into a polymer matrix using a high temperature and high shear force mixer or compounder [24]. This approach is simple and compatible with current industrial practices.
Various polymer matrices are used for making composites, such as thermoplastics [25], [26], thermosetting resin [27], [28], liquid crystalline polymers [29], [30], water-soluble polymers [31], conjugated polymers [7], and so on. In this research, we have used nylon-6 [polyamide 6 (PA6)] and multi-walled carbon nanotubes (MWCNTs) to prepare high performance polymer composites through a melt mixing process. Nylon-6 has been widely used as an important engineering plastic due to its excellent chemical and abrasions resistance, toughness, and low-coefficient of friction. Various researchers incorporated CNT into nylon-6 in order to study their effect on the properties such as thermal, mechanical, electrical, and crystallization behavior of nylon-6 [19], [32], [33].
The objective of the present study is to investigate in detail the thermal, dynamic mechanical, morphological and electrical properties of MWCNT/PA6 composites. The MWCNTs have been functionalized covalently and noncovalently for better dispersion in the matrix. The effects of processing methods and nanotube content on the properties of the composites have also been examined.
Section snippets
Materials
The matrix polymer, PA6 (Ultramid® B36 LN 01) used in this study, was purchased from BASF, Singapore. Its density is about 1.12–1.15 g cm−3. SDS was purchased from Aldrich. The MWCNTs were purchased from Bayer Materials Science. The diameter and length of the MWCNT were 13–16 nm and 10 μm respectively. Two types of methods were used to functionalize the raw MWCNTs. First, MWCNT–COOH was prepared by oxidation of raw MWCNTs with concentrated H2SO4/HNO3 (volumetric ratio 3:1) at 90 °C for 10 min with
Functionlization of MWCNTs
The FTIR and Raman spectra for the functionalized MWCNTs and raw MWCNTs are shown in Fig. 1, Fig. 2. The FTIR spectra of the raw MWCNTs showed the peaks with very low intensity at 3440, 1640, and 1182 cm−1, corresponding to OH, CO, and CCO stretching, respectively. In the case of our modified MWCNTs, these characteristic bands appeared with significantly higher intensity, according to the degree of modification. This was attributed to the increased number of carboxylic acid groups generated at
Conclusions
The raw MWCNTs, which were treated by an anionic surfactant (SDS) or carboxylically functionalized, have been incorporated into PA6 by two methods of mixing in order to examine their effects on the crystalline, thermal, morphological, and mechanical properties of MWCNT-reinforced composites of PA6. The PA6 containing functionalized MWCNTs showed the improved mechanical and thermal properties over the composites filled with the raw MWCNTs. The composites fabricated by method I exhibited the
Acknowledgment
This work was supported by the A*STAR SERC Grant (0721010018), Singapore.
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