MWCNT-PMMA composites

Figure 7.2. SEM of 10 vol. % MWCNT-PMMA composite film showing MWCNT dispersion in PMMA. Reprinted with permission from Springer Science and Business Media (44).

Jin et al. (26) used melt blending to fabricate MWCNT-PMMA composites with different CNT loadings varying from 0 to 26 wt%. They used a laboratory mixing molder to disperse MWCNT in PMMA at 200°C followed by compression molding at 210°C. Their TEM studies revealed good dispersion even at high MWCNT concentration. The composites showed enhanced mechanical and thermal properties. 

Combination of solvent casting followed by compression molding is also one of the approaches to fabricate CNT-PMMA composites (21,24,57). Slobodian et al. (57) fabricated MWCNT-PMMA composites by this technique and studied electrical conductivity of composites obtained after two-time compression molding. Different percolation thresholds were found for different solvent systems. Mathur et al. (24) also reported improvement in the mechanical properties of the CNT-PMMA composite over the one prepared by simple solvent casting. The reason attributed is the removal of any solvent trapped in the cast films. 

Park et al. (60) studied dispersion characteristics of MWCNT-PMMA composites synthesized by in-situ bulk polymerization using AIBN as free radical initiator. In their method, CNTs in varying amounts such as 0.001, 0.01 and 0.1 wt% with respect to MMA were first dispersed in MMA monomer by ultrasonication before polymerization. Experimental evidence such as molecular weight of free PMMA prepared via in-situ polymerization with and without CNTs, diameter of pristine MWCNT and diameter of MWCNT in composite, FTIR and SEM studies confirmed the role of AIBN and MWCNT in polymerization. The induced radicals on MWCNT by AIBN were found to trigger grafting of PMMA on to CNT surface. Solvent cast film of the composite was transparent and showed a better nanoscopic dispersion without aggregates compared to the cast film prepared from direct mixing of MWCNT and PMMA. 

Shang et al. (61) used microemulsion polymerization to synthesize MWCNT-PMMA composites for gas sensor applications. Better dispersion, enhanced electrical conductivity and better sensor response was observed for in-situ fabricated composites compared to composites prepared by solution mixing. Ma et al. (62) performed in-situ polymerization of MWCNT-PMMA composites in the presence of an AC electric field to study dispersion and alignment of MWCNT in PMMA matrix induced by the electric field. Experimental evidences from in-situ optical microscopy, Raman spectroscopy, SEM and electrical conductivity showed that both dispersion and alignment qualities were significantly enhanced for oxidized MWCNT compared to pristine MWCNT. 

Figure 7.4. Microstructure of the in-situ fabricated MWCNT-PMMA composite (a) using untreated tubes, (b) treated tubes and improved in-situ process. The bright masses are CNTs wrapped with PMMA layers. Reprinted with permission from Elsevier (23).

Jin et al. (65) used poly(vinylidene fluoride) (PVDF) as a compatibilizer to assist dispersion of CNTs in PMMA. Multi-walled carbon nanotubes were first coated with PVDF and then melt-blended with PMMA. Poly(vinylidene fluoride) served as an adhesive to improve wetting of CNTs by PMMA and to increase the interfacial adhesion resulting in improved mechanical properties of MWCNT-PMMA composites. 

Kim et al. (66) used poly(3-hexylthiophene) (P3HT)-graft-PMMA as a compatibilizer to prepare MWCNT-PMMA composites by solution blending. The CNT content was varied from 0.01 to 0.1 wt%. The resulting composites showed improved tensile strength and modulus. 

Chen et al. (67) reported the use of trifluoroacetic acid (TFA) as a cosolvent with tetrahydrofuran (THF) to improve dispersion and processability of the nanocomposites. They prepared MWCNT-PMMA composite films with varying CNT content by solvent casting method using 10 vol % TFA as a co-solvent with tetrahydrofuran (THF). SEM and optical microscopy revealed a good dispersion of nanotubes in solvent and PMMA. The composites showed low percolation... 

A number of studies on CNT-polymer composites have focused on improving the dispersion and load transfer efficiency in other words the compatibility between the CNTs and polymer matrix through covalent chemical functionalization of CNT surface (12,40). Many of the studies reported above have used acid-functionalized CNTs to fabricate MWCNT-PMMA composites with improved mechanical properties using different processing methods (24,25,27,62). Yang et. al (68) modified the acid functionalized CNTs with octadecylam-ine (ODA) to obtain ODA-functionalized CNTs. These CNTs were reinforced in a copolymer P(MMA-co-EMA) to form composites with improved dispersion and mechanical properties. 

A new approach has been adopted by Yuen et al. (70) wherein they prepared silane-modified MWCNT-PMMA composites by bonding MWCNTs modified with 3-isocyanato-propyltriethoxysilane... 

Pande et al. (25) also reported significant improvements in the flexural properties of MWCNT-PMMA composites prepared by in-situ polymerization method. They observed a maximum reinforcing effect of CNTs at 3 wt% for a-MWCNT and at 1.8 wt% for f-MWCNT. The flexural strength for the two cases was about 90 MPa as compared to about 64 MPa from two step method... 

Figure 7.9. FTIR spectra of MWCNT-PMMA composites. Reprinted with permission from John Wiley Sons, Inc (25).

Tensile mechanical properties of MWCNT-PMMA composites were shown to be improved by using P3HT-g-PMMA as a... 

Figure 7.12. (a) TEM images of a MWCNT-containing PMMA thin film taken at different times (i) t = 0, (ii) t = 4, and (iii) t=10 min and (b) SEM image of the g-MWCNT-PMMA composite. Reprinted with permission from Wiley VCH (69). 

Velasco Santos et al. (27) measured both the dynamic mechanical behavior and tensile mechanical properties of MWCNT-PMMA composites. They observed 1135% increase in storage modulus at 90°C and increase in glass transition temperature by 40°C over neat PMMA with only 1 wt% functionalized nanotubes. The tensile... 

In another study Slobodian et al. (57) found that the percolation threshold for electrical conductivity of MWCNT-PMMA composites depends on the solvent used. The lowest percolation threshold was achieved for toluene where percolation was found to be at 4 wt% of MWCNT, for chloroform at 7 wt% and for acetone at 10 wt%. The highest conductivity was obtained at 20 wt% of MWCNT at values around 4x 10 5 Sc nr1 for composite prepared from toluene solution. They observed that the Hansen solubility parameters of individual solvent play an important role in the dispersion of MWCNT in PMMA. 

Mathur et al. (24) and Pande et al. (44) studied the effect of MWCNT content on the EMI shielding properties of MWCNT-PMMA composite films in the X-band (8.2-12.4 GHz). The MWCNT-PMMA nanocomposites with higher MWCNT content exhibit greater EMI shielding effectiveness (SE) (Figure 7.14). 

Figure 7.16. (a) Variation in the SEA of multi layered MWCNT-PMMA composite films in the X-band, (b) variation in the SER of multi layered MWCNT-PMMA composite films in the X-band and (c) variation in the SETota of multi layered MWCNT-PMMA composite films in the X-band. Reprinted with permission from Springer Science and Business Media (44). 

Kim et al. 2004 (42) MWCNT CVD Iljin Nanotech Co. Ltd. Raw and Purified Solvent Casting CNT loading levels 0.1 to40wt% Film Electrical Conductivity increases with increasing CNT content EMI SE (50 Mz to 13.5 GHz) increases with CNT content Highest EMI SE for raw MWCNT-PMMA composite was 27 dB at 40 wt% MWCNT EMI SE for purified MWCNT-PMMA at 20 wt% loading was 7 dB Use of raw MWCNT-PMMA composites is good for EMI SE for far field and near field applications. ... 

Shang et al. 2009 (61) MWCNT Purified Microemulsion polymerization CNT Loading levels 1 tol5wt% MWCNT-PMMA composites prepared by microemulsion polymerization at 8 wt% loading showed high sensor responses to different organic vapors such as acetone, toluene, THF, choloroform, acetonitrile, benzene They suggested the use of these composites for gas sensor applications ... 

Park et al. [144] conducted a study in which PMMA/MWCNT nanocomposites were prepared via both in-situ bulk polymerization and suspension polymerization, using the radical initiator 2,2-azobis(isobutyronitrile) (AIBN). The electrical and electrorheological (ER) properties of the nanocomposites were investigated. The conductivity of pure PMMA and MWCNT/PMMA nanocomposites were measured, and it was shown that the conductivity of MWCNT/PMMA composites rapidly increased when MWCNTs were added to the PMMA matrix, i.e., 3.192 x 10 , 2.163 X 10 2, and 1.693 x 10 Scm for 1.5, 5 and 10wt% of MWCNT in the composites, respectively. The conductivity of insulating PMMA was about 1 X 10 2Scm [144]. 

Yuen, S.M., Ma, C.C.M., Chiang, C.L., Chang, J.A., Huang, S.W., Chen, S.C., et al., 2007. Silane-modified MWCNT/PMMA composites-preparation, electrical resistivity, thermal conductivity and thermal stabihty. Composites Part A 38, 2527—2535. 

Pande, S., Singh, B.P., Mathur, R.B., Dhami, T.L., Saini, P., Dhawan, S.K., 2009. Improved electromagnetic interference shielding properties of MWCNT-PMMA composites using layered structures. Nanoscale Research Letters 4, 327—334. 

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