CNTs are considered to be the ideal reinforcing nanofiller for polymer matrices because of their unique mechanical properties, thermal stability, high aspect ratio, and electrical conductivity.
CNTs have an amazing combination of excellent electronic and mechanical properties, which has drawn the attention of scientists and engineers in a range of areas. Qi et al. found that regenerated cellulose (RC) film had a high mechanical strength and Young's modulus, with values of 80.6 MPa and 3.1 GPa, respectively. There was no obvious increase in the tensile strength, and even a slight decrease in the Young's modulus with the loading of CNT in CNT/cellulose films (Table 1). However, the strain-to-failure was 11.1%, which is almost two times higher than that of the RC film (6.1%, shown in Table 1). MWCNT/cellulose composite fibers were processed from solutions in ethyl methylimidazolium acetate (EMIAc) by Rahatekar et al. They found that a moderate enhancement of tensile strength was observed with a MWCNTs mass fraction of up to 0.05. A further increase in the nanotube content (0.07 wt %) did not result in further enhancement of tensile strength, and in fact it decreased when the mass fraction of MWCNTs was 0.1. No appreciable difference in tensile modulus was observed between the cellulose control and the composite fibers, while the strain to failure at 0.05 mass fraction MWCNT was 100% higher than that for the control cellulose fiber. The enhanced mechanical properties of these fibers could be mainly ascribed to the uniform dispersion and alignment of the MWCNTs within the cellulose matrix. It is well-known that the EMIAc-treated MWCNTs have hydrophilic groups, including carboxylic and hydroxyl groups, and, as a hydrophilic biopolymer, cellulose has three hydroxyl groups in each anhydroglucose unit. Therefore, it can be expected that some interactions, such as hydrogen bonding, will occur between cellulose and the MWCNTs. The compatibility and strong interaction between the MWCNTs and the cellulose greatly enhance the dispersion, as well as the interfacial adhesion, and thus significantly improve the mechanical properties of the cellulose matrix.
Table 1. Mechanical Properties of Polymeric Carbohydrate/CNT Hybrid Nanocomposites
|Samples||Young's modulus (GPa)||Tensile strength (MPa)||Elongation at break (%)||References|
| ||3.1||2.7|| ||2.9||2.6||80.6||71.4|| ||75.7||59.4||6.1||9.5|| ||8.2||5.9||42|
| ||0.6||1.3||2.3||2.5|| ||11.6||14.5||18.4||25.4|| ||2.0||2.9||3.9||5.0|| ||46|
| ||1.1|| || || ||2.2||37.7|| || || ||74.3||49.5|| || || ||13.4||47|
| ||3.1||3.5||4.1|| || ||72.0||10.7||11.4|| || ||36||42||38|| || ||30|
| ||4.5||5.6||7.1||7.2||6.0|| || || || || ||55||53||49.0||35.0||28.0||45|
| ||0.2|| ||0.19||0.22||0.23||4.7|| ||5.4||6.1||6.8||51|| ||42.0||44.0||37.0||44|
Reinforcement effects have also been found in the matrices of thermoplastic starch. Cao et al. found that the tensile strength and Young's modulus increased significantly (from 2.85 to 4.73 MPa and from 20.74 to 39.18 MPa, respectively) with an increase in MWCNT content from 0 to 3.0 wt %. The maximum value reached was 41.99% for the sample with 1.0 wt % MWCNTs. Ma et al. found that the tensile strength and Young's modulus increased as the content of MWCNTs increased up to 4.75 wt%; however, the elongation at break decreased. Liu et al. found that the addition of CMWCNTs significantly improved the tensile strength of the thermoplastic starch (TPS) matrix as CMWCNT loading increased to 1.5 wt %, but was the reverse for elongation at break. As the CMWCNT content increased to 1.5 wt %, the tensile strength of TPS/CMWCNT nanocomposites reached 7.7 MPa, while the pure TPS was only 4.5 MPa. Ma et al. showed an increase in both tensile strength and Young's modulus with increasing MWCNT content, and Cao et al. showed a higher tensile strength and Young's modulus with the same mass fraction of MWCNTs. Liu et al. showed the highest Young's modulus of all. In their study a facile solution dispersion method was used to prepare high performance thermoplastic starch (TPS)/CMWCNT nanocomposites. Because acid-treatment incorporated polar groups with the MWCNTs, the hydrophilicity of MWCNTs was improved and agglomerations of MWCNTs reduced. Therefore, CMWCNTs benefited greatly from the enhanced hydrogen-bonding interactions as well as the dispersion in TPS/CMWCNT nanocomposites, which resulted in improved mechanical performance of the TPS/CMWCNT nanocomposite films. However, CMWCNT content above 1.5 wt % caused deterioration of the plasticization of TPS, and also destroyed the continuity of the TPS matrix; therefore, TPS/CMWCNTs with higher CMWCNT contents (above 1.5 wt %) could have inferior mechanical properties.
Neat chitosan had a tensile strength of 11.6 MPa and a Young's modulus of 580 MPa, while the tensile strength and Young's modulus of chitosan loaded with 1.5% CNT were 30.4 MPa and 2815 MPa, respectively. Both tensile strength and Young's modulus of the nanocomposites were greater than those of chitosan and increased by increasing the amount of poly(styrene sulfonic acid)-functionalized carbon nanotubes (CNT-PSSA) in the nanocomposites. Moreover, formation of nanocomposites with CNT also increased the elongation at break of chitosan from 2.0 to 6.1%. A small amount of water absorbed by the nanocomposite films may contribute to the increase in elongation. Wang et al. found that the addition of MWCNTs significantly improved the tensile properties of the chitosan matrix and that the mechanical properties increased with the increase in MWCNT loading levels. With only 0.4 wt % MWCNT filler, the tensile modulus and strength of the nanocomposite increased dramatically (by about 78 and 94%, respectively) as compared with those of its neat counterpart. As the loading level of MWCNTs increased to 0.8 wt %, the tensile modulus and strength of the chitosan/MWCNT composite were enhanced by about 93 and 99%, respectively. Cao et al. found that the tensile strength and Young's modulus increased sharply from 39.6 to 102.8 MPa and from 2.01 to 4.35 GPa, respectively, with an increase in MWCNT loading level from 0 to 3.0 wt %. More specifically, the elongation at break of the nanocomposite film CS/MWCNTs-1.0 was 11.3%, which was a decrease of 25.6% in comparison to that of the neat CS film. In contrast, the tensile strength and Young's modulus were 98.3 MPa and 3.87 GPa, dramatic increases of 148 and 92%, respectively. However, when the MWCNT loading level was higher than 1.0 wt %, the tensile strength and Young's modulus increased slightly, but the elongation at break decreased from 11.3 to 5.6%. Wu et al. found that the addition of MWCNTs significantly improved the tensile properties of the chitosan matrix, and the tensile strength and modulus increased with increasing MWCNT content. With the addition of only 0.1 wt % MWCNTs, the tensile strength and modulus of the nanocomposite increased dramatically, by about 50 and 16%, respectively. As the MWCNT content increased to 0.5 wt %, the tensile strength and modulus of the MWCNT/chitosan nanocomposite were enhanced by about 61 and 34%, respectively.
It has been generally recognized that the reinforcing efficiency of CNTs or HNTs depends strongly on their dispersion and orientation, as well as on the CNT/matrix interfacial strength. Another critical issue is the impact of the shape of the embedded CNTs on the effective mechanical properties of the nanotube-reinforced polymer. The most effective approaches towards increasing the orientation of CNTs within a polymeric carbohydrate matrix include stretching and spinning (melt-spinning, solution-spinning, and electrospinning) the composites to form films and fibers. To achieve the full reinforcing potential of CNTs in polymers, they must be well dispersed and exhibit good interfacial strength with the matrix.
Enhanced Electrical Conductivity
CNTs are very effective fillers with a 1000 times higher current carrying capacity than copper wire, which permits the movement of charge carried by the fillers through the matrix so the composite achieves a certain degree of electrical conductivity. The percolation threshold is characterized by a sharp jump in conductivity of many orders of magnitude which was attributed to the formation of a conductive network within the matrix. Thus, polymer/CNT composites show a very low percolation threshold for electrical conductivity because of the large aspect ratio and nanoscale dimension of nanotubes. The percolation threshold for electrical conductivity in polymer/CNT composites is also influenced by different nanotube characteristics such as aspect ratio, dispersion and alignment. Table 2 shows the values of electrical conductivity of polymeric carbohydrate/CNT hybrid composites. Kim and Park found that electrical conductivity increased as wt % increased. The conductivity levels were between 10−5 and 10−6 S cm−1 at wt % <1 and increased to 1.77 × 10−4 and 3.07 × 10−3 S cm−1 at 5 and 9 wt %, respectively. Piri et al. also found that with the increase in MWCNT content, the electrical conductivity increased; at 4 wt %, the electrical conductivity increased to 0.21 S cm−1. Swain et al. found that the conductivity of the composite increased with the addition of very low concentrations (0.5–3 wt %) of functionalized MWCNT. The electrical conductivity increased gradually from 0.6 × 10−9 to 1.6 × 10−9 S cm−1 due to the addition of filler. From these reported works, it is easy to determine that the dispersion of nanotubes played a key role in improving the properties and changing the structure of nanocomposites, e.g., good dispersion of MWCNT will lead to good conductivity of the nanocomposites. CNTs without polar surfaces usually do not have good interfacial compatibility with the hydrophilic carbohydrate matrix.
Table 2. Electrical Conductivity (S cm−1) of Polymeric Carbohydrate/CNT Hybrid Nanocomposites
| ||0.8 wt %||1.0 wt %||2.0 wt %||2.5 wt %||3.0 wt %||4.0 wt %||5.0 wt %||9.0 wt %||References|
|MWCNTs/cellulose||7.0 × 10−6||1.1 × 10−5||6.0 × 10−5|| ||8.0 × 10−5||1.0 × 10−4||1.7 × 10−4||3.1 × 10−3||50|
|MWCNTs/chitosan|| || || ||0.005||0.15||0.21|| || ||51|
|PS/f-MWCNTs|| ||1.38 × 10−9||1.6 × 10−9|| ||1.69 × 10−9|| || || ||23|
Theoretically, noncovalent functionalization of nanotubes normally involves van der Waals, p-p, CH-p covalent bonds, or electrostatic interactions between the polymeric carbohydrate and the CNT surface. The advantage of noncovalent functionalization is that it does not alter the structure of the nanotubes and, therefore, both electrical and mechanical properties should also remain unchanged. There are several noncovalent approaches for nanotube functionalization such as surfactant-assisted dispersion, polymer wrapping, plasma polymerization-treatment, and polymerization filling technique (PFT). A surfactant-coated species of sodium dodecyl sulfate was critical for achieving a reproducible nanotube dispersion and to obtain a homogeneous and stable solution. HNT is a polar inorganic filler full of ionic groups thus allowing it to blend directly with the polymeric carbohydrate without modification.