Historical perspectiveFunctionalization of carbon nanotubes by combination of controlled radical polymerization and “grafting to” method
Graphical abstract
Introduction
Carbon nanotube (CNT) as one of carbon allotropes has attracted large attentions due to its special properties. CNT is imagined as a cylinder obtained from rolling-up graphene sheets around a central hollow core structure [1,2]. Single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) are two common types of CNT. The SWCNT is twisted single graphene layers, while the MWCNT is composed from two or more graphene layers packed by van der Waals forces and π-π stacking interactions. CNT has commonly been used in thermal conductors, energy storage materials, conductive adhesives, thermally-stable materials, structural materials, fibers, catalyst supports, biological applications, air and water filtration, ceramics, and other applications [3,4]. Well-dispersion of CNT in a matrix requires its functionalization with small or even long chain chemicals. Physical and chemical functionalization methods are used for modification of CNT with different molecules. Functionalization of CNT with pre-synthesized polymer chains are carried out by using coupling reactions between the functionalized polymer chains with the bear or modified CNT.
Functionalized polymer chains are commonly synthesized by controlled radical polymerization (CRP) with different mechanisms [5]. CRP methods are mainly based on reversible termination or transfer reactions, where nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer polymerization (RAFT) are the three main methods of CRP. Control of polymerization in NMP method is carried out by a dynamic equilibration between the macroradicals and nitroxide radical-terminated dormant species [[6], [7], [8], [9], [10]]. In ATRP method, transition metal complex is used as the controlling agent of its equilibrium, where the polymer chains are reversibly terminated with a halide end. In RAFT method, different chain transfer agents of dithioester, dithiocarbamate, trithiocarbonate, and xanthate are needed to achieve reversible chain transfer equilibrium between the growing macroradicals. The polymer chains with nitroxide, halide, and chian transfer agent functional groups respectively in NMP, ATRP, and RAFT systems can be grafted to the bare or modified CNT using different coupling reactions, which are commonly known as “grafting to” reactions.
In “grafting to” reactions, the bear CNT can react with polymer macroradicals with radical addition reaction. In addition, CNT can be modified with different small molecules which are needed in the coupling reaction with functinalized polymer chains. Therefore, this review has focused on small-molecule functionalization of CNT with different methods based on physical and chemical methods. After that, polymer-functionalization of the bear and modified CNT with the polymers synthesized with three common CRP methods are reviewed in detail. By using the “grafting to” method, well-defined polymers with narrow molecular weight distribution synthesized by CRP methods can form uniform layers on CNT. Moreover, synthesis of polymer brushes on CNT with different grafting densities can be achieved [[11], [12], [13], [14], [15]]. Such polymer-functionalized CNT can be used in lots of applications, such as polymer composites, Pickering emulsions, stimuli-responsive materials, etc. Functionalization of CNT with small molecules and polymer chains are investigated in this study. The most prominent studies on functionalization of CNT by small molecules using physical and chemical functionalization methods are summarized in Table 1, Table 2, respectively. In addition, the most important studies on covalent and non-covalent functionalization of CNT with polymer chains synthesized by ATRP, RAFT, and NMP methods with the focus on “grafting to” methods are presented in Tables 3 and 4, respectively.
Section snippets
Physical functionalization methods
Physical adsorption of small molecules on CNT is a promising approach for its functionalization. Physical functionalization enhances dispersibility and preserves the extended π networks of CNT. This type of functionalization is commonly carried out by some different methods such as π-π interaction and electrostatic interactions.
Physical functionalization of CNT by π-π interaction is among the most important methods, in which the π electrons on the surface of CNT interact with π electrons of
Polymer-functionalization of CNT by covalent “grafting to” method
CRP is considered as a highly applicable method to synthesize well-defined polymers with controlled molecular weight and narrow polydispersity index. Controlling reactivity of growing radicals in radical polymerization results in appearance of the CRP methods as modern polymerizations. In CRP methods, NMP, ATRP, and RAFT polymerization are highly efficient in synthesis of polymer brushes on CNT. NMP, ATRP, and RAFT polymerization rely on dissociation-combination, atom transfer, and reversible
Polymer-functionalization of CNT by non-covalent “grafting to” method
In non-covalent method, polymers could attach to the CNT surface without sharing any electrons. Hydrogen bonding and π-π interaction are the prominent kinds of non-covalent interactions. The recent and important studies of polymer grafting via “grafting to” method through the non-covalent interaction are reviewed in the following.
Applications
High surface area, high functionalization ability, enhanced cellular uptake, and possibility of conjugation of CNT with many therapeutics result in its application in enhancing storage of batteries, biomedical applications, sensors, filtration, solar cells, etc. Conductivity enhancement of CNT results in its higher practical usage in wide variety of applications like batteries. Thermal conductivity of CNT decreases in most of the chemical functionalization processes [12,15,16]. Thermal
Conclusion, outlook, and challenges
Unique structure and properties of CNT introduced it as a promising material in diverse research fields. Its potential applications in novel materials are restricted due to some drawbacks such as limited solubility in aqueous and organic solvents, limited processability, and limited compatibility with polymer matrices. Functionalization of CNT with small molecules and also polymers is considered as a major key in circumventing these issues. Covalent and non-covalent functionalization of CNT
Declaration of Competing Interest
None.
Acknowledgment
Iran National Science Foundation (INSF) is greatly appreciated for its financial support (Grant Number: 96013220).
References (177)
- et al.
Progress in polymer science RAFT/MADIX polymers for the preparation of polymer/inorganic nanohybrids
Prog Polym Sci
(2011) - et al.
Reversible addition – fragmentation chain transfer polymerization initiated with c -radiation at ambient temperature: an overview
Macromolecules
(2003) - et al.
Polymer brushes: surface-immobilized macromolecules
Prog Polym Sci
(2000) - et al.
Methane hydrate formation improved by water-soluble carbon nanotubes via π-π conjugated molecules functionalization
Fuel
(May 2019) - et al.
Experimental study on mercury ions removal from aqueous solution by MnO2/CNTs nanocomposite adsorbent
J Ind Eng Chem
(2015) Noncovalent functionalization of pristine CVD single-walled carbon nanotubes with 3d metal(II) phthalocyanines by adsorption from the gas phase
Appl Surf Sci
(Apr. 2018)- et al.
Chemical functionalization of xanthan gum for the dispersion of double-walled carbon nanotubes in water
Carbon N Y
(2013) Morphological, rheological and electrical properties of composites filled with carbon nanotubes functionalized with 1-pyrenebutyric acid
Compos Part B Eng
(Aug. 2018)- et al.
Ethylendiamine-functionalized multi-walled carbon nanotubes prevent cationic dispersant use in the electrochemical detection of dsDNA
Sens Actuators B
(2014) - et al.
Carbon nanotube-poly(methyl methacrylate) hybrid films: preparation using diazonium salt chemistry and mechanical properties
J Colloid Interface Sci
(2014)