Advantages and disadvantages of various modification methods of CNTs .
Since Iijima’s report on carbon nanotubes (CNTs) , which consist of graphene sheets rolled up into a cylindrical shape, many researchers have focused on CNTs due to their superior mechanical, electrical and thermal properties. Depending on the arrangement of aromatic rings along the cylindrical surface, specifically for single-walled carbon nanotubes (SWCNTs), CNTs can possess two distinguished properties such as metallic and semiconducting. In spite of many advantages, the practical applications of CNTs have been limited by their poor processability and dispersability in solvents, polymers, ceramics and metallic matrices. Indeed, the pristine CNTs are insoluble in any solvent, due to strong van der Waals interactions between CNTs and lack of chemical affinity to organic solvents. To overcome this limitation, many chemical (covalent) and physical (noncovalent) modification methods to functionalize CNTs have been developed during last decades for improved compatibilities with both liquid and solid matrices [2-3]. Among them, chemical approaches using various chemical reactions are considered to be the most promising protocol for enhancing dispersability and processability of CNTs. However, CNTs are chemically inert for efficient chemical modifications, and thus reactions have to be carried out in harsh conditions, causing significant structural damages to CNT frameworks. As a results, a sharp decrease in their intrinsic properties is inevitable [2-3]. In this regard, physical modifications of CNTs have been considered to be more favorable methods for electronic applications, because electronic structures can be largely preserved due to the noncovalent approaches for modified CNTs [4-6]. However, homogeneous dispersion using the physical method accompanied with sonication often damages CNTs due to the effects of dose time and strength. Furthermore, they also have some disadvantages such as limited utilization of materials and insufficient modification levels for practical applications. Thus, the development of nondestructive and efficient chemical modification of CNTs is highly desirable.
Since the pioneering work from Baek
2. Direct Friedel-Crafts acylation of Carbon Nanotubes
2.1. Overview and mechanism
Although various chemical and physical modifications for enhancing the dispersability and processability of CNTs have been utilized for last decades, both methods have their own drawbacks depending on the platform as discussed earlier. The advantages and disadvantages of various modifications of CNTs are summarized in Table 1 .
|Chemical Method||Side wall||Hybridization of C atoms from sp2 to sp3||√||S||√|
|Polymer wrapping||van der Waals force, - stacking||√||V|
|Surfactant adsorption||Physical adsorption||√||W|
|Endohedral Method||Capillary effect||W||√|
Additionally, the most chemical modifications are initiated by chemical oxidation of CNTs in strong acids [2-3]. Therefore, dramatic structural damages of CNTs can be easily happened during harsh oxidation reaction, which results in significant weakening of many useful intrinsic properties of CNTs. To overcome these problems, the development of alternative functionalization routes, which can not only introduce homogeneous surface functional groups with high density to enhance the compatibility of CNTs and various foreign matrixes, but also minimize the structural damages of CNTs during reactions to optimize their properties in various applications, are highly demanding.
2.2.1. Functionalization of carbon nanotubes with small molecules
Recently, the functionalization of carbon nanotubes (CNTs) with small molecules containing benzoic acid [9, 13, 26]
The dispersability of MWCNTs was greatly enhanced by functionalization and debundling of MWCNTs with small molecules
Furthermore, this unique synthetic strategy can be applied to different types of CNTs like single- , few- [27-28] and multi-walled CNTs [8-13]. Recently, it has been reported that few-walled carbon nanotubes (FWCNTs), defined as nanontubes with sidewalls typically of 2 to 6 layers, diameters ranging from 3 to 8 nm, have particularly distinguished from other types of CNTs . Therefore, the functionalization of FWCNTs without structural damages to generate nanocomposites hybrid materials or even thin film has attracted great attentions for their various potentials in device applications. In this purpose, Baek
Hitherto, a various aspects of direct Friedel-Crafts acylation reaction in PPA/P2O5 between 4-substituted benzoic acids and CNTs have been discussed. Interestingly, this strategy can be expanded to 4-substituted benzamides instead of 4-substituted carboxylic acids. The benzamide could also be directly attached to the surface of CNTs. As a model compound, 4-(2,4,6-trimethylphenoxy)benzamide (TMPBA) was reacted with single-walled carbon nanotubes (SWCNTs) in PPA/P2O5 as a mild direct Friedel-Crafts acylation reaction condition to afford TMPBA functionalized SWCNTs (Figure 5a) . The covalent attachment of TMPBA onto the surface of SWCNTs was proved by elemental analysis (EA), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy and thermogravimatric analysis (TGA). In addition, the SEM image of TMPBA-g-SWCNT shows that the surface of SWCNTs is apparently decorated with covalently bonded moieties (Figure 5b). From the results, direct Friedel-Crafts acylation reaction in PPA/P2O5 could be one of powerful tools for the covalent modification of CNTs with small molecules containing various functional moieties.
Due to the efficient modification of CNTs with a covalent attachment of small molecules with various functionalities, the functionalized CNTs are very useful for the preparation of composites
In situ grafting of linear or hyperbranched polymers onto carbon nanotubes
Due to the unique features of CNTs, they have been actively investigated for uses as reinforcing components to deliver outstanding properties to various matrixes such as polymers [38-39], ceramics  and low melting metals . The resultant nanocomposites are expected to display enhanced properties providing various potential applications for light-weight and multifunctional materials. Unfortunately, CNTs usually exist in ropes and bundles due to strong lateral interactions between the tubes, causing difficulty in homogeneous dispersing them in a multi-component system. Therefore, various physical and chemical modifications to afford homogeneous dispersion of CNTs are required for the effective transfer of their outstanding properties to the matrix materials. However, chemical approach using strong acids and physical approach with sonication treatment can easily cause significant damages such as sidewall opening and tube breakage on their structures. In addition to dispersion, the strong interfacial adhesion between CNTs and matrix is also one of crucial factors in nanocomposites. It is also well known that noncovalent interactions between CNTs and matrix in nanocomposites are not expected to have any synergic effect even after homogeneous dispersions of CNTs could be achieved. Thus, it is highly desirable to covalently link desired polymers to the surface of CNTs. As a result, the development of efficient covalent polymer grafting to the surface of CNTs without structural damages is highly demanding to meet the above mentioned two important requirements for nanocomposites,
The hyperbranched poly(ether-ketone) has been attached onto the surface of both SWCNTs and MWCNTs . The diameter range of pristine SWCNTs bundles is 40-60 nm (Figure 8a), while hyperbranched polymer grafted SWCNTs (HBP-g-SWCNTs) bundles show much smaller diameter range of 5-25 nm than that of pristine SWCNTs (Figure 8c). In addition, the shape of them resembles fractal structures. Some HBP-g-SWCNTs fibrils are stemmed out like tree branches and imbedded into hyperbranched matrix. The overall state of dispersion is homogeneous. Therefore, it could be hypothesized that once split is occurred at the edge of SWCNTs bundle when mechanical stirring shear force is applied, viscous polymeric reaction medium containing AB2 monomer, which is readily react, is penetrated in between split and finally wedged by hyperbranched poly(ether-ketone). As a result, the splits are started from the tips of SWCNTs bundles and propagated further into the bundles (Figure 6c). In case of MWCNTs, the pristine MWCNTs show the seamless and smooth surfaces (Figure 8b). However, heavy amount of hyperbranched poly(ether-ketone) attached to MWCNTs could be clearly seen from the SEM images after grafting of hyperbranched poly(ether-ketone) (Figure 8d). The resultant hyperbranched polymer grafted MWCNTs (HBP-g-MWCNTs) have the diameter range of 40-150 nm, which is strong indication that the covalent attachment of hyperbranched poly(ether-ketone) to the surface of MWCNTs. Furthermore, the surfaces of nanocomposites are appeared to be puppy and bumpy compared to pristine MWCNTs.
Similarly, the grafting of hyperbranched poly(ether-ketone) to the surface of MWCNTs could be realized in alternative way using unique self-controlled polycondensation methodology directly from the mixture of commercially available A3 and B2 monomers in the same reaction medium of PPA/P2O5 without gelation problem . In addition, linear and hyperbranched poly(ether-ketone) containing flexible oxymethylene spacers grafted MWCNTs were also prepared by a direct Friedel-Crafts acylation reaction . The resultant nanocomposites are soluble in most strong acids such as trifluoroacetic acid, methanesulfonic acid and sulfuric acid, and they are expected to display enhanced melt processability due to the flexible spacers in structural unit. It is worth to note that the semimetallic nanocomposites, linear or hyperbranched poly(phenylene sulfide) (PPS) grafted MWCNTs, could be successfully prepared by two-step reaction sequences . Firstly, MWCNTs were functionalized with 4-chlorobenzoic acid using a direct Friedel-Crafts acylation reaction in PPA/P2O5 to afford 4-chlorobenzoyl functionalized MWCNTs (CB- MWCNTs). A subsequent nucleophilic substitution reaction between CB- MWCNTs and 4-chlrobenzenethiol as an AB monomer or 3,5-dichlrobenzenethiol as an AB2 monomer was conducted to graft the linear PPS (LPPS) or hyperbranched PPS (HPPS) in NMP/toluene in the presence of sodium carbonate to afford LPPS grafted MWCNTs (LPPS-g-MWCNTs) or HPPS grafted MWCNTs (HPPS-g-MWCNTs), respectively (Figure 9). The covalent attachment of corresponding polymers onto the surface of MWCNTs was indirectly confirmed by a model study without MWCNTs.
The SEM image of pristine MWCNTs shows that the tubes have seamless and smooth surfaces with an average diameter of 10-20 nm (Figure 10a). However, the average diameter of CB- MWCNTs is approximately 40 nm, which is 2-4 times thicker than that of pristine MWCNTs (Figure 10b). Interestingly, the shape of tube could be discerned by two parts. Opaque inner-hard core is covered by translucent outer-shadow-like part. The diameter of inner part in a rage of 10-20nm agrees well with that of the parent MWCNTs. Out-shadow-like part could be due to the 4-chlorobenzoyl moieties that have uniformly covered the surface of CB- MWCNTs. The SEM images of LPPS-g-MWCNTs reveal that the diameter approximately 100 nm, which is much larger than that of pristine MWCNTs and CB- MWCNTs (Figure 10c). Therefore, it is estimated that LPPS is heavily grafted to the CB- MWCNTs. In case of HPPS-g-MWCNTs, although the diameter dimension is close to that of CB-g-MWCNTs, the original outer-shadow-like part of CB- MWCNTs appears to be completely covered with newly attached HPPS (Figure 10d).
For the verification of structural integrity of MWCNTs during reaction sequences and the covalent attachment of the relevant polymers, transmission electron microscopic (TEM) analysis was conducted. The TEM images of LPPS-g-MWCNTs and HPPS-g-MWCNTs show that the tubes are heavily decorated with polymers (Figure 11). Furthermore, the clear wall-to-wall stripes of MWCNTs framework with its structural integrity suggest that the structural stability of MWCNTs under the two-step reaction sequence. The resultant nanocomposites show the enhanced dispersability and melt-processability, and they could be easily compression molded. Due to the synergetic effect originated from two components of MWCNTs and PPS, even without chemical doping, the surface conductivities of LPPS-g-MWCNTs and HPPS-g-MWCNTs molded samples could be reached to the semimetallic transport region at 11.76 and 3.56 S/cm, respectively .
2.3. Other carbon-based nanomaterials: fullerene (C60), carbon nanofiber, and graphene
In addition to CNTs, the covalent modification method of direct Freidel-Crafts acylation reaction in a mild PPA/P2O5 medium can be expanded to other carbon-based nanomaterials such as fullerene (C60) , carbon nanofiber [7, 15-17] and graphene [19-22]. Buckminster fullerene, C60, which is of the most abundant carbon sphere, is generally considered as a stable electron deficient material. Due to the electron affinity, C60 is considered as to be more susceptible to nucliophilic reaction than to electrophilic one. However, Baek
In comparison to CNTs, vapor-grown carbon nanofibers (VGCNFs), which are structurally hollow and multi-walled but several orders of magnitude larger in diameter and length than those of CNTs, are more attractive from a standpoint of practicality in terms of their relatively low cast and availability in larger quantities as a result of their more advanced stage in commercial production. These carbon nanofibers (CNFs) are typically produced by a vapor-phase catalyst process in which a carbon-containing feedstock (e.g. CH4, C2H4, etc.) is pyrolyzed in the presence of small metal catalyst (e.g. ferrocene, Fe(CO)5, etc.) and have an outer diameter of 60-200 nm, a hollow core of 30-90 nm, and length in the order of 50-100 μm [15-16]. Furthermore, VGCNFs have been widely used for tailoring properties in their polymer composites via cost-effective way, because of their inherent electrical and mechanical properties. To enhance compatibility and dispersability of VGCNF in polymeric matrix, various covalent grafting methods including ring-opening, atom-transfer radical and self-condensing polymerizations have been developed . However, these approaches generally require multi-step synthetic procedures and limited species of materials can be utilized. To overcome these problems, Baek
Similarly to CNTs, the functionalization of VGCNF
Graphene as a one of carbon-based nanomaterials, is currently the focal point for research into condensed matter due to its promising properties such as exceptional mechanical strength (~ 1100 GPa), high thermal conductivity (~ 5000 Wm-1K-1), large specific surface area (~ 2630 m2g-1) and ultra high electron transport properties (200,000 cm2V-1s-1) . There are two major approaches used in the preparation of graphene. The first method is the exfoliation of pristine graphite into graphene, which involves physical and chemical methods [47-48]. The second method is where graphene can be directly grown using chemical vapor deposition (CVD) on a metal substrate  or from single crystal carbide . For mass production, the chemical methods belong to the first approach is more preferred, but they still need to be optimized. In this regard, graphene oxide (GO) are widely investigated for the various applications of graphene, however GO has larger structural damages during the harsh preparation methods using strong acids and requires reduction, which has a limited conversion to reduced graphene oxide (rGO). Hence, the original graphitic structures cannot be efficiently restored in final graphitic structure, when GO is used as a starting material.
Therefore, the development of less destructive and highly efficient method to exfoliate graphite into two-dimensional graphene and/or graphene-like sheets is highly required for the graphene research community. To meet this strong demand, Baek
The resultant edge-selectively functionalized graphene (EFG) becomes dispersible without damaging the inner crystalline graphitic structure. The TEM image for EFG dispersion in NMP and dip-coated on an aperture carbon-grid, along with the corresponding selected-area electron diffraction (SAED) pattern is shown in Figure 16a. The graphene sheet is wrinkled due to its flexibility, and its surface is clean without noticeable flaws. Most of EFG consists of less than five graphene sheets. AFM images obtained from EFG on a silicon wafer clearly show EFG with approximately ~ 2 μm width and a few micron lengths (Figure 16b). Many bright spots on the edges of graphene are seen due to the covalent attachment of organic wedges. The thickness of graphene is 0.8 nm, whose value indicates single layer graphene. All topological height profiles clearly show that the interior (basal plane) are lower than the edges, implying that edge-functionalization is exclusively occurred at edges, where presumably sp2C-H defects are located . Thus the efficient exfoliation of graphite and edge-selective functionalization of graphene for improving dispersability and processabiliy have been successfully achieved by simple one-pot reaction using a direct Friedel-Crafts acylation reaction in a mild PPA/P2O5 medium. In addition to small molecular wedges, various macromolecular wedges using linear  or hyperbranched  polymer have also been introduced to graphene. Due to the enhanced dispersability and compatibility without structural damages, the resultant EFG has huge potentials in various applications such as polymer nanocomposites [51-52], fuel cells  and optoelectronic devices .
“Direct” Friedel-Crafts acylation reaction of electron-deficient CNTs in a mild PPA/P2O5 medium is a simple but less destructive functionalization method. Numerous results envision that various functional materials such as small molecules, linear and hyperbranched polymers could be covalently attached to the surface of CNTs without or with minimal damages to their carbon framework. The dispersability and compatibility of the functionalized CNTs have been greatly improve keeping their intrinsic properties, which could be regarded as a feasible approach to hybridization of CNTs and organic materials such as polymers. Furthermore, this nondestructive synthetic strategy can be expanded to other carbon-based nanomaterials such as fullerene, carbon nanofiber and graphene. Therefore, a direct Friedel-Crafts acylation reaction in a mild PPA/P2O5 medium possesses indeed significant potentials for the development of functional materials in various fields from all types of carbon-based nanomaterials.
This research work was supported by World Class University (WCU), US-Korea NBIT, and Basic Research (MCR) programs through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (MEST) and the U.S.Air Force Office of Scientific Research (AFOSR).
Iijima S. 1991 Helical microtubules of graphitic carbon 354 56 8
Karousis N. Tagmatarchis N. Tasis D. 2010 Current progress on the chemical modification of carbon nanotubes 110 5366 97
Tasis D. Tagmatarchis N. Bianco A. Prato M. 2006 Chemistry of carbon nanotubes 106 1105 36
Bahr J. L. Tour J. M. 2002 Covalent chemistry of single-wall carbon nanotubes 12 1952 8
Chen R. J. Zhang Y. Wang D. Dai H. 2001 Noncovalent sidewall functionalization of single-walled carbon nanotubes for protein immobilization 123 3838 9
Zhao Y. L. Stoddart J. F. 2009 Noncovalent functionalization of single-walled carbon nanotubes 42 1161 71
Baek J. B. Lyons C. B. Tan L. S. 2004 Covalent modification of vapour-grown carbon nanofibers via direct Friedel-Crafts acylation in polyphosphoric acid 14 2052 6
Choi J. Y. Oh S. J. Lee H. J. Wang D. H. Tan L. S. Baek J. B. 2007 In-situ grafting of hyperbranched poly (ether ketone) s onto multiwalled carbon nanotubes via the A3+ B2 approach 40 4474 80
Han S. W. Oh S. J. Tan L. S. Baek J. B. 2008 One-pot purification and functionalization of single-walled carbon nanotubes in less-corrosive poly (phosphoric acid) 46 1841 9
Han S. W. Oh S. J. Tan L. S. Baek J. B. 2009 Grafting of 4-(2, 4, 6-Trimethylphenoxy) benzoyl onto Single-Walled Carbon Nanotubes in Poly (phosphoric acid) via Amide Function 4 766 72
Jeon I. Y. Kang S. W. Tan L. S. Baek J. B. 2010 Grafting of polyaniline onto the surface of 4‐aminobenzoyl‐functionalized multiwalled carbon nanotube and its electrochemical properties 48 3103 12
Jeon I. Y. Lee H. J. Choi Y. S. Tan L. S. Baek J. B. 2008 Semimetallic transport in nanocomposites derived from grafting of linear and hyperbranched poly (phenylene sulfide) s onto the surface of functionalized multi-walled carbon nanotubes 41 7423 32
Lee H. J. Han S. W. Kwon Y. D. Tan L. S. Baek J. B. 2008 Functionalization of multi-walled carbon nanotubes with various 4-substituted benzoic acids in mild polyphosphoric acid/phosphorous pentoxide 46 1850 9
Lim D. H. Lyons C. B. Tan L. S. Baek J. B. 2008 Regioselective chemical modification of fullerene by destructive electrophilic reaction in polyphosphoric acid/phosphorus pentoxide 112 12188 94
Baek J. B. Lyons C. B. Tan L. S. 2004 Grafting of vapor-grown carbon nanofibers via in-situ polycondensation of 3-phenoxybenzoic acid in poly (phosphoric acid) 37 8278 85
Wang D. H. Baek J. B. Tan L. S. 2006 Grafting of vapor-grown carbon nanofibers (VGCNF) with a hyperbranched poly (ether-ketone) 132 103 7
Wang D. H. Mirau P. Li B. Li C. Y. Baek J. B. Tan L. S. 2008 Solubilization of carbon nanofibers with a covalently attached hyperbranched poly (ether ketone) 20 1502 15
Wang D. H. Tan L. S. Huang H. Dai L. Osawa E. 2008 In-Situ Nanocomposite Synthesis: Arylcarbonylation and Grafting of Primary Diamond Nanoparticles with a Poly (ether−ketone) in Polyphosphoric Acid 42 114 24
Bae S. Y. Jeon I. Y. Yang J. Park N. Shin H. S. Park S. et al. 2011 Large-Area Graphene Films by Simple Solution Casting of Edge-Selectively Functionalized Graphite 5 4974 80
Jeon I. Y. Bae S. Y. Lee H. J. Shin H. S. Dai L. Baek J. B. 2010 High-yield exfoliation of three-dimensional graphite into two-dimensional graphene-like sheets 46 6320 2
Jeon I. Y. Choi H. J. Bae S. Y. Chang D. W. Baek J. B. 2011 Wedging graphite into graphene and graphene-like platelets by dendritic macromolecules 21 7820 6
Jeon I. Y. Yu D. Bae S. Y. Choi H. J. Chang D. W. Dai L. et al. 2011 Formation of Large-Area Nitrogen-Doped Graphene Film Prepared from Simple Solution Casting of Edge-Selectively Functionalized Graphite and Its Electrocatalytic Activity 23 3987 92
Ma Siddiqui P. C. Marom N. A. Kim G. J. K. 2010 Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review 41 1345 67
Jeon I. Y. Choi E. K. Bae S. Y. Baek J. B. 2010 Edge-Functionalization of Pyrene as a Miniature Graphene via Friedel-Crafts Acylation Reaction in Poly (Phosphoric Acid) 5 1686 91
Jeon I. Y. Tan L. S. Baek J. B. 2008 Nanocomposites derived from in situ grafting of linear and hyperbranched poly (ether‐ketone) s containing flexible oxyethylene spacers onto the surface of multiwalled carbon nanotubes 46 3471 81
Jeong J. Y. Lee H. J. Kang S. W. Tan L. S. Baek J. B. 2008 Nylon 610/functionalized multiwalled carbon nanotube composite prepared from in‐situ interfacial polymerization 46 6041 50
Jang S. S. Kumar S. Baek J. B. 2010 Sponge Behaviors of Functionalized Few-Walled Carbon Nanotubes 114 14868 75
Kumar N. A. Jeon I. Y. Sohn G. J. Jain R. Kumar S. Baek J. B. 2011 Highly Conducting and Flexible Few-Walled Carbon Nanotube Thin Film 5 2324 31
Hou Y. Tang J. Zhang H. Qian C. Feng Y. Liu J. 2009 Functionalized few-walled carbon nanotubes for mechanical reinforcement of polymeric composites 3 1057 62
Tan L. S. Baek J. B. 2005 In situ synthesis of poly (ethylene terephthalate)(PET) in ethylene glycol containing terephthalic acid and functionalized multiwalled carbon nanotubes (MWNTs) as an approach to MWNT/PET nanocomposites 17 5057 64
Lee H. J. C. J. Cho S. H. Kim D. Tan L. S. Beek J. B. 2006 Multi-walled carbon nanotubes/termoplastic polyester nanocomposites 47 400 1
Ahn S. N. Lee H. J. Kim B. J. Tan L. S. Baek J. B. 2008 Epoxy/amine‐functionalized short‐length vapor‐grown carbon nanofiber composites 46 7473 82
Lee D. H. Baek J. B. Cho G. W. 2008 Enhancement of the field-effect mobility of poly (3-hexylthiophene)/functionalized carbon nanotube hybrid transistors 9 317 22
Jeon I. Y. Tan L. S. Baek J. B. 2010 Synthesis and electrical properties of polyaniline/polyaniline grafted multiwalled carbon nanotube mixture via in situ static interfacial polymerization 48 1962 72
Choi H. J. Jeon I. Y. Kang S. W. Baek J. B. 2011 Electrochemical activity of a polyaniline/polyaniline-grafted multiwalled carbon nanotube mixture produced by a simple suspension polymerization 56 10023 31
Jeon I. Y. Choi H. J. Tan L. S. Baek J. B. 2011 Nanocomposite prepared from in situ grafting of polypyrrole to aminobenzoyl‐functionalized multiwalled carbon nanotube and its electrochemical properties 49 2529 37
Tan L. S. Baek J. B. 2011 Preparation and Electrocatalytic Activity of Gold Nanoparticles Immobilized on the Surface of 4-Mercaptobenzoyl-Functionalized Multiwalled Carbon Nanotubes 115 1746 51
Cai L. Bahr J. L. Yao Y. James M. 2002 Ozonation of single-walled carbon nanotubes and their assemblies on rigid self-assembled monolayers 14 4235 41
Mitchell C. A. Bahr J. L. Arepalli S. James M. Krishnamoorti R. 2002 Dispersion of functionalized carbon nanotubes in polystyrene 35 8825 30
Wang H. Fan Y. 2004 Carbon‐Nanotube‐Reinforced Polymer‐Derived Ceramic Composites 16 2036 40
Cha S. I. Kim K. T. Arshad S. N. Mo C. B. Hong S. H. 2005 Extraordinary Strengthening Effect of Carbon Nanotubes in Metal‐Matrix Nanocomposites Processed by Molecular‐Level Mixing 17 1377 81
Park S. Y. Baek J. B. 2006 Multiwalled carbon nanotubes and nanofibers grafted with polyetherketones in mild and viscous polymeric acid 47 1132 40
Choi J. Y. Han S. W. Huh W. S. Tan L. S. Baek J. B. 2007 In situ grafting of carboxylic acid-terminated hyperbranched poly (ether-ketone) to the surface of carbon nanotubes 48 4034 40
Kim Y. H. 1998 Hyperbranched polymers 10 years after 36 1685 98
Voit B. 2000 New developments in hyperbranched polymers 38 2505 25
Dai L. Chang D. W. Baek J. B. Lu W. 2012 Carbon Nanomaterials for Advanced Energy Conversion and Storage 8 1130 66
Loh K. P. Bao Q. Ang P. K. Yang J. 2010 The chemistry of graphene 20 2277 89
Dreyer D. R. Park S. Bielawski C. W. Ruoff R. S. 2009 The chemistry of graphene oxide 39 228 40
Ahn J. H. Hong B. H. 2009 Large-scale pattern growth of graphene films for stretchable transparent electrodes 457 706 10
Poon S. W. Chen W. Tok E. S. Wee A. T. S. 2008 Probing epitaxial growth of graphene on silicon carbide by metal decoration 92 104102
Choi E. K. Jeon I. Y. Oh S. J. Baek J. B. 2010 “Direct” grafting of linear macromolecular “wedges” to the edge of pristine graphite to prepare edge-functionalized graphene-based polymer composites 20 10936 42
Kim K. S. Jeon I. Y. Ahn S. N. Kwon Y. D. Baek J. B. 2011 Edge-functionalized graphene-like platelets as a co-curing agent and a nanoscale additive to epoxy resin 21 7337 42