Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach

Accepted Manuscript Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach Ashraful Alam, Chaoying Wan, Tony McNally PII: DOI: Reference:

S0014-3057(16)31233-2 http://dx.doi.org/10.1016/j.eurpolymj.2016.10.004 EPJ 7541

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

1 July 2016 20 September 2016 4 October 2016

Please cite this article as: Alam, A., Wan, C., McNally, T., Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/j.eurpolymj.2016.10.004

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Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach Ashraful Alam1, Chaoying Wan2*, Tony McNally2 1, 2 School of Engineering, University of South Australia, SA5095, Australia 2. International Institute for Nanocomposites Manufacturing, WMG, University of Warwick, UK, CV4 7AL *Corresponding author, E-mail: [email protected]

Abstract Composites of polymers and nanoparticles continue to find increasing applications from biomedical to electronics to transport systems. Nanostructured carbon materials (CNPs) having geometries from zero dimension (0D) to 3D are important functional additives for polymers, having great potential to produce composite materials with a range of enhanced properties including mechanical, optical, electrical and thermal. However, these possibilities have not been fully realised due to the difficulties associated with CNP dispersion in and their interaction with polymer matrices across the length scales. The surfaces of CNPs are intrinsically chemically inert and hydrophobic, and they tend to form agglomerates or bundles. Therefore, surface functionalisation of CNPs becomes a critical pre-requisite in the fabrication of polymer nanocomposites. Various functionalisation methods have been developed including, chemical, mechanochemical, electrochemical, and irritation reactions in order to activate the carbon surface, which subsequently interact with polymers through covalent bonding or non-covalent interactions. Wet-chemistry methods consume large amounts of organic solvents, hazardous chemicals, require multi-step purification with typically low yields. Mechanochemistry techniques such as ball milling can produce edge-functionalised CNPs at the expense of reduced aspect ratio. In contrast, cold plasma treatment offers a simple, clean, solvent-free, and scalable technique for modifying CNPs with variable functional groups. Recent developments in plasmatreated CNPs have driven its applications extensively in epoxy-based composites. Aminofunctionalisation of CNPs is particularly favourable, as the amine group offers a rich reaction platform to enhance the activity of the CNP as both modifier and crosslinker for epoxy resins. Research activity in this area is under development but growing rapidly. In this review, we introduce the working mechanism for plasma functionalisation of CNPs, and compare this approach with the efficiency and effectiveness of wet-chemistry methods. The discussion will focus on amine-functionalised CNPs (carbon nanotubes, graphene/GO and carbon fibre) and their use in the modification of the properties of epoxy resins.

Keywords: Plasma; surface modification; carbon nanoparticles; epoxy resin; nanocomposites

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Table of Contents Abstract ................................................................................................................................ 1 1. Introduction....................................................................................................................... 4 2. Carbon nanotubes (CNTs) and graphene (G) ..................................................................... 5 3. Surface functionalisation of CNPs ..................................................................................... 6 3.1 Non-covalent functionalisation .................................................................................... 8 3.2 Covalent functionalisation ........................................................................................... 9 3.3 Mechanochemistry .................................................................................................... 12 3.4 Plasma functionalisation ............................................................................................ 13 3.4.1 Plasma functionalisation of CNTs........................................................................ 15 3.4.2 Plasma functionalisation of graphene and graphene like materials ....................... 22 3.4.3 Plasma functionalisation of carbon nanofiber (CF) .............................................. 23 4. Performance of composites of CNPs and epoxy resins ..................................................... 24 4.1 Plasma functionalised CNT/epoxy composites ........................................................... 25 4.2 Amino-functionalised CNT/epoxy composites ........................................................... 31 4.3 Amine functionalised graphene/epoxy composite ...................................................... 33 4.4 Silane modified graphene for epoxy nanocomposites ................................................. 37 5. Summary......................................................................................................................... 41 Acknowledgements ............................................................................................................. 42 References .......................................................................................................................... 43

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1. Introduction Over the last two decades, the properties of nanostructured carbon materials (CNPs) have been studied intensively, including mechanical, electrical, thermal and optical and their use as a functional filler for polymers demonstrated [1, 2]. The challenges to realise the often promised enhancement in properties on addition of CNPs to polymers lie in their homogeneous dispersion and strong interfacial interactions with polymer matrix of interest. The unmatched surface chemistry between CNPs and polymers is a major limitation to producing high performance polymer nanocomposites [3-5]. Allotropes of carbon are known, from amorphous carbon, carbon black, carbon nanotubes (CNTs), carbon quantum dots, graphene, graphite to diamond [6] and many are important additives for modification of the properties of polymers [7-10]. However, they are intrinsically chemical-inert and not compatible with polymers. Mixing and processing techniques, such as high shear-mixing, ultrasonication and ball-milling [11, 12] are all employed in polymer nanocomposite preparation. To improve the dispersibility and miscibility of CNPs in solvents and polymers, various methods have been used to modify their surface chemistry such as mechanochemistry, physicochemistry, irradiation and plasma treatment. The aim of surface functionalisation is to activate the carbon surface and introduce functional groups to the carbon structures [4, 13, 14], therefore to facilitate CNPs dispersion and enhance interfacial interactions via, e.g. covalent-bonding or π-π stacking. A number of excellent reviews have been published in the area describing the covalent and non-covalent modification of CNTs [15, 16] and graphene [17-19] as well as their composites with polymers [20, 21], and in biomedical [15, 22] and electronic device [23] applications. Only a few research groups have reported on environmental benign functionalisation methods such as low-temperature plasma treatment of CNTs or graphene [24-27]. Plasma technology has evolved into an important technology for the surface modification of materials since the 1960s and it has shown to be very versatile. It can be effective down to depths of 10 nm below the material surface without alteration of the bulk composition [28]. It has been used in various applications from plasma cleaning, plasma sterilization, plasma coatings, fluorination and in biomedical applications [29]. Plasmas have also been used in the synthesis of CNTs and graphene-related materials for fuel cell applications [30]. Plasma treated graphene oxide (GO) materials showed enhanced properties used for removal of

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environmental pollutants [31], amine-groups can be introduced to CNT surfaces by nitrogen plasma treatment [32]. Amino-functionalisation of carbon nanomaterials can largely enrich the surface reactivity of carbon by enhancing its hydrophilicity and biological affinity. In particular, amino functional group can act as a crosslinker to form covalent bonding with epoxy. However, the current chemical functionalisation methods generally require hazardous chemicals, multi-step reactions, tedious purification and result in low yield. Herein, we critically review the plasma-functionalisation of CNPs, with a focus on plasma-amination of CNTs and graphene. The effect of plasma treatment on the surface chemistry of CNPs and their subsequent dispersion and interfacial interactions with epoxy resins is discussed. The challenges of achieving effective amination of CNPs and their composites with polymers are described. We believe this review covers most of the latest research results relating to amino- and plasma-functionalisation of CNPs and their use in epoxy composites. There are only a few reviews on the chemical functionalisation of CNPs and their use in epoxy composites [21, 33], whilst little or no attention, to the best of our knowledge, has been given to amino- and plasma-functionalisation of CNPs and their effects on the properties epoxy resins. 2. Carbon nanotubes (CNTs) and graphene (G) Carbon nanotubes (CNTs) are one-dimensional π-conjugated carbon nanostructures formed by scrolling single or multi-layered graphene [23]. Depending on the layer number of the carbon sheets, CNTs can be classified as being single-walled, double-walled or multi-walled. CNTs can be semi-conductive or metallic depending on the diameters and chiralities of the nanotubes. Multi-walled CNTs (MWCNTs) are metallic due to the chirality of each wall being different. Owing to their unique structure, CNTs have shown excellent thermal stability (up to 2800 °C in vacuum), electrical conductivity of 107 S/m [34], thermal conductivity of 3500 W/m K at room temperature [35], Young’s modulus of 0.9 TPa and tensile strength of 150 GPa [36] and optical properties [37, 38]. However, MWCNTs generally form agglomerations or tangled coils due to the strong van der Waals interactions between the nanotubes, which results in intrinsically poor solubility and dispersibility and highly limits their application and functionality [39, 40]. Graphene is a single layer of two-dimensional carbon sheet, consisting of covalent bonded sp2 hybridized carbon atoms in the form of an extended honeycomb structure [41]. The unique structure of graphene shows higher strength to weight ratio that significantly

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outperforms metal and metal composites [42], excellent electrical conductivity ~108 S/m [43], thermal conductivity ~5000 W/mK [44] and high optical transparency. The tensile strength and Young’s modulus of graphene have been reported to be 125 GPa and 1100 GPa, respectively, which is one of the stiffest and strongest materials ever measured and an ideal candidate for making super-strong and lightweight composites [45]. Pristine graphene can be fabricated by chemical vapour deposition and epitaxial growth processes, both suitable for electronics devices. However, the chemically inert nature of the carbon sheets makes pristine graphene not ideal for mixing with polymers [46, 47]. 3. Surface functionalisation of CNPs Over the past 15 years various surface methods have been developed to modify the surface of CNPs. The purpose of this is to expand reactivity and processibility, tune chemical and biological affinity, and increase the interfacial interactions between carbon nanomaterial and polymer [48-50]. Various functional groups can be attached or bound to carbon structures via covalent or non-covalent interactions through wet- or dry-chemistry processes, such as electrochemical [51], chemical and mechanochemical, fluorination, polymer wrapping [52] and plasma treatment. Fig. 1 shows the different functionalisation methods employed to date to modify CNPs.

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Fig. 1. Surface functionalisation methods used to modify CNPs. The functionalisation efficiency of CNPs is generally characterised and quantified using a range of technologies [53], including X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), contact angle measurement and B.E.T surface area measurement. The grafting density of functional groups on the surface can be analysed using a combination of techniques such as XPS, acid-base titration, dye adsorption, TGA, computational modelling [25] and combined wetting and ζ-potential measurements [53, 54]. The presence of amine groups on the CNT surface can be confirmed qualitatively using chemical derivatisation reactions, such as with pentafluorobenzaldehyde followed by XPS, optical emission spectroscopy (OES) analysis of the gas phase plasma glow and FTIR analysis [32]. Due to the surface roughness and depth of penetration, XPS generally can only approach within a finite depth of about 10 nm. Angle-resolved XPS and ion scattering spectroscopy (ISS) have been combined to obtain a better understanding of the depth distribution and presence of oxygen functional groups on the carbon surface [55]. 7

Surface titrations have been extensively used to determine the concentration of functional groups on carbon surfaces [56]. 3.1 Non-covalent functionalisation Non-covalent functionalisation appears promising as it does not permanently alter the chemical structural integrity of carbon lattices. This includes hydrophobic interactions, ionic interactions, hydrogen bonding, π-π stacking, CH-π, NH-π, or van der Waals interaction. Driven by π-π stacking and hydrophobic interactions, a series of aromatic surfactants have been utilized for stabilizing graphene and CNTs, including perylene derivatives [57], pyrene moiety [58], such as the water soluble pyrene derivative 1-pyrenebutyrate [59] and pyrene butanoic succinimidyl ester [60] and polymer wrapping via CH2-π interaction [52]. Commercially available surfactants such as sodium dodecylbenzenesulfonate (SDBS), sodium cholate (SC), and sodium deoxycholate (SDC) [57] are also widely used for wrapping/coating CNTs and graphene. Conjugated and aromatic polymers such as polyaniline [61] and polystyrene [62] also interact with graphene sheets via non-covalent interactions. Some typical molecular structures of the chemicals known to interaction noncovalently with CNTs and G are shown in Fig. 2A.

Fig. 2. (A) Non-covalent and (B) covalent functionalisation methods. Adapted with permission from [57] and [19]. Copyright (2013) American Chemical Society.

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Although non-covalent functionalisation has the desirable advantage of preserving intact the electronic structure of carbon sheets, the use of aromatic surfactants or functional polymers with pyrene moieties still requires multi-step chemical synthesis and tedious purification processes [60-63]. Non-covalent interactions are much weaker than covalent bonding and a fraction of bound molecules can be cleaved during post-processing. 3.2 Covalent functionalisation Covalent chemical functionalisation has the potential to provide more diverse properties than non-covalent

modification.

Covalent

functionalisation

generally

applies

reactive

intermediates of radicals, nitrenes, arbenes and arynes to modify the carbon structure through free radical addition, CH insertion or cyclo-addtion reactions [19], some examples are shown in Fig. 2B. Oxidative chemistry can be performed by treatment of carbon nanomaterials with harsh oxidants such as nitric, sulfuric and phosphoric acids, dichromates, permanganates and ammonium bicarbonate, to replace the carbon atoms with oxygen atoms directly and results in oxidative defects in the carbon lattices. As a result, the extended π-π conjugation structure is disrupted resulting in the conversion of sp2 to sp3 hybridization [47]. Such modification is detrimental to the intrinsic optical, electrical and thermal conductivity properties of graphene or CNTs. On the other hand, oxygen-containing groups, such as carboxyl-, carbonyl- and epoxide- groups provide a versatile reaction platform. The carboxyl groups are easily to react via esterification and amination. As shown in Fig. 3a, the carboxyl groups on the CNT surface are converted firstly into acyl chlorides then react with octadecylamine to form amide bonds. Fig. 3b shows that halogenated carbon dots prepared through the solvothermal treatment of carbon tetrachloride and quinol can react with ehtylenediamine to generate an amine-modified surface [64]. As shown in Fig. 3c, the carboxylic groups on the graphene oxide (GO) surface react with amine-bearing porphyrin to provide optoelectronic properties [65]. Permanganate-based oxidization of graphene via Hummer’s method [66] can generate large amounts of epoxide- and hydroxyl groups on the graphene surface and carbonyl and carboxylic groups on the sheet edge, but also results in defects or holes inplane and smaller flake sizes [67]. A subsequent process of chemical reduction or thermal treatment can remove most of the oxygen-containing groups from the surface and partially recover the electrical and thermal conductivity properties. For example, the functional groups on the GO surface

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were reduced from an original 32% to 8.7% after reduction with hydrazine [68], but this also resulted in reduced dispersibility in polymers and a 75% decrease in mechanical stiffness of the composite relative to the unfilled polymer [69]. Other common covalent grafting reactions including hydrogenation [70], halogenation [71], carboxylic addition [72], alkyl halides [73], aryl radical additions with diazonium salts [74], disulphides [39], 1,3-dipolar cycloaddition of azomethineylides [75], coupling with amino acids and peptides [15] are promising methods. Covalent functionalisation methods via wetchemistry involve the use of hazardous chemicals, high volumes of organic chemicals and multi-step treatment and purification. A number of reviews have reported the current development of covalent functionalisation of nanomaterials [57, 76, 77]. Many of these methods have limitations in scalability, robustness, resultant conductivity, are environmental contentious and are costly. Hence, cost-effective and scalable approaches are required in order to produce high performance functional polymer nanocomposites.

Fig. 3. (a) Organic functionalisation of single-walled carbon nanotubes (SWCNTs) with aliphatic amines via amide bond formation, (b) schematic route for the synthesis of halogenated carbon dots and substitution by 1,2-ethylenendiamine [64] and

(c)

functionalisation of graphene oxide (GO) with an aminoporphyrin [65]. Amino functional groups can be introduced on the carbon surfaces through the methods depicted in Fig. 4. For example, amine-CNTs can be produced by NH3 plasma treatment [32] 10

or substitution of fluorinated SWCNTs with diamines [78]. Amination of graphene sheets were prepared through chemical reduction by hydrazine hydrate, then amination or amidation of graphite oxides, which involves reagents such as ethylenediamine, diethylenetriamine and triethylenetetramine [79]. Amino-functionalised MWCNTs were prepared by oxidation to generate carboxyl groups on the MWCNT surface (MWCNT-COOH), and then converted to acyl chloride functionalised MWCNT (MWCNT–COCl) by treating with thionyl chloride (SOCl2), and finally the MWCNT–COCl can be reacted with hexamethylenediamine to yield amine-grafted MWCNTs (MWCNT–NH2) [80]. This multi-step process is time-consuming and costly, in particular the quality and quantity of the resultant products are less controllable. A clean and facile process is urgently required for amino-functionalisation of CNP’s

Fig. 4. Methods employed for amino-functionalisation of CNTs and G The binding of amine-moiety can effectively increase the solubility of CNPs in organic solvents and water, enhancing their anchoring ability to other chemical and biological moieties [81]. Amine-functionalised CNPs hold high promise as functional nano-fillers for polymers due to the creation of interfacial covalent bonding with polymer matrices, such as epoxy, polyimide, and polyamide [82]. In particular, amine-functionalised graphene or CNTs can act as reinforcement, a crosslinker and catalyst and, play multiple functions in epoxy composites [83]. Water dispersible MWCNTs were prepared with four types of

amine groups, namely

ethylenediamine, 1,6-hexanediamine, 4,4’-diaminodiphenylmethane, and 4,4’-diaminodicyclohexylmethane following carboxylation, acylation and amidation steps [82]. Both carboxyl and epoxy groups of GO react with amine groups through amidation and nucleophilic substitution, respectively [84]. Long alkyl chains are favourable for hydrogen bonding with graphene sheets (compared to shorter alkyl chains), and longer amine chain length increases the roughness of the functionalised GO surface leading to superhydrophobic properties. The thermal stability of GO modified with different alkyl chain lengths are also 11

improved as a consequence of the formation of amidation bond on GO [85]. A melaminebased compound (synthesized from cyanuric chloride) with two long alkyl chains, 2-amino4,6-didodecylamino-1,3,5-triazine (ADDT), was shown to be covalently bonded to GO nanosheets. The polarity, solubility and thermal stability of the GO was increased by ADDT, although the functionalisation process took three days to complete. Long term stabilisation of graphene sheets in different solvents has been a major obstacle in fabrication of nanocomposites via solution mixing. Imidazolium derivatives were used to functionalise GO via a nucleophilic ring-opening reaction between epoxy groups on GO and the amine groups of an amine-terminated ionic liquid [86]. The cations generated from the amine-terminated ionic liquid contributed to long-term stability and homogeneous dispersion of graphene sheets in a number of solvents such as water, N,N-dimethylformamide, dimethyl sulfoxide [86], dichloromethane, dimethylformamide and methanol [87], via electrostatic repulsion. 3.3 Mechanochemistry Functional groups can be created on the carbon surface as a consequence of mechanical grinding and shearing. Ball-milling can not only exfoliate graphite into multi-layer carbon nanoplatelets [88] or break MWCNTs agglomerates under certain treatment conditions (e.g. duration, temperature, and organic modifiers), but also generate functional groups on the carbon surface. Similar levels of dispersion of MWCNTs was found for those treated by ballmilling for 20 min with those treated with concentrated acids for 120 min however, the MWCNTs were highly shortened after ball-milling [89]. It was observed that under the highenergy ball-milling process, MWCNTs were broken down and carbon based onion-like particles formed after 15 min, which were then transformed into amorphous carbon when the milling time increased to 60 min [90]. Ball milling of CNTs in the presence of ammonium bicarbonate facilitated the introduction of functional groups such as amine and amide on the surface of the MWCNTs [91], but the modified MWCNTs were more disentangled and shortened than those ball-milled in the absence of ammonium bicarbonate. The edges of graphene nanoplatelets can be selectively functionalised by simply ball-milling graphite in the presence of hydrogen, carbon dioxide, sulfur trioxide and/or carbon dioxide/sulfur trioxide [92]. The amount of functional groups formed was around 65~87 wt%, determined from TGA at 800 oC in nitrogen. From Raman spectroscopy, the intensity ratios ID/IG of the D-band (1350 cm-1) to G-band (1584 cm-1) were in the range 0.79-1.50, indicating a significant size reduction in platelet size due to mechanochemical cracking and 12

edge distortion. The resultant functionalised graphene nanoplatelets showed increased polarity and high electro-catalytic activity. Edge-carboxylated graphite (ECG) was prepared by ball-milling with dry ice (solid phase CO2) [93]. The oxygen content of ECG increased and the grain size decreased with increasing ball-milling time before levelling off after 48h. The different ECG structure obtained compared with GO are shown in Fig. 5. As characterised by FTIR, a strong C=O stretching peak at 1718 cm-1 for carboxylic acid groups was observed for both ECG and GO, a sp3 C-H peak at 2939 cm-1 associated with defects and a sp2 peak at 2919 cm-1. This suggests that reactive carbon species (radicals, anions and cations) on ECG generated by homolytic and heterolytic cleavage of the graphitic C–C bonds during ball milling in the presence of dry ice, react with CO2, followed by protonation with moisture in air.

Fig. 5. Syntheses and proposed structures of GO, reduced GO (rGO) and heat treated GO (HGO) [94, 95], (b) edge-carboxylated graphene (ECG) and heat-treated (de-carboxylated) ECG (H-ECG) [93]. 3.4 Plasma functionalisation Plasma is the fourth state of matter along with solid, liquid and gas. It is generated by energy disassociation of a gas and contains a mixture of free electrons, positively charged ions, neutral atoms and/or molecules, free radicals, and ultraviolet (UV) photons [96]. ‘Plasma Density’ is defined as the number of free electrons per cm3 and quantifies the degree of ionisation. According to the values of plasma density, plasma can be classified as nonequilibrium (or non-thermal/low-temperature/cold) and equilibrium (or thermal/hightemperature/hot) plasmas. The cold plasma (T < 102 Κ) and thermal plasma (T > 104 Κ) are regarded as low-temperature plasma (T < 106 Κ). Cold plasmas include atmospheric-pressure 13

plasma and low-pressure plasma (10-3~103 Pa), in which the low-pressure plasma can introduce functional groups to the material surface in a more controllable and reproducible manner in comparison with atmospheric pressure plasma. Plasma reactions are applied via different processes, such as by etching or ablation, sputtering, polymerisation, grafting and spraying [97]. More recently, cold plasma, especially low-pressure plasma reactions have become one of the key technologies for surface modification of materials, in addition to the wet-chemistry and mechano-chemistry methods discussed above. Plasma is typically obtained when gases are excited into energetic states by corona discharge, radio-frequency (RF), glow discharge [98], dielectric barrier discharge or microwave power [99], to the desired gas in a vacuum chamber (typically 0.1-10 Torr). The highly energised gas species of the plasma can penetrate and break covalent bonds on the material surface to a depth of several nanometres, the activated surface can then readily react with the excited gas species to form functional groups. Electrons are the main contributors to the formation of reactive species [100]. The mechanism by which oxygen plasma reacts with surfaces is shown schematically in Fig. 6.

Fig. 6. Mechanism for modification of surfaces with a plasma [100]. The level of surface functionalisation is determined by the gas type and treatment parameters such as pressure, power input, flow rate and time [29]. For surface modification, a variety of inert gases such as oxygen-containing gases including O2, CO2 and H2O; nitrogen-containing gases including NH3 and N2, as well as other gases such as H2, Ar, P and He have been investigated. Fluoro- or hydrocarbon containing gases such as BF3, CF4, styrene, allylamine, 14

acrylic acid or maleic anhydride can induce plasma polymerisation reactions to form pinhole free polymer nanocoatings on the surface. Ammonia, sometimes in in a mixture with other gases (N2, Ar, O2, CF4), are often used as precursor to introduce amine functionality to carbon and other materials to enhance hydrophilicity and biocompatibility [29]. Different plasms can introduce different functional groups on the material surface, as shown in Fig. 7. Oxygen plasma treatment can generate oxygen-containing functional groups such as carboxylic acid, peroxide, and hydroxyl groups. CO2-plasma treatment produces hydroxyls, ketones, aldehydes, and esters [97]. Ammonia, N2, and N2/H2 plasmas introduce primary, secondary, and tertiary amines, as well as amides [101, 102]. Plasma functionalisation has several advantages over traditional functionalisation techniques. Wet-chemistry functionalisation of CNPs generally requires pre-oxidation treatment with strong acids which is hazardous, complex, time-consuming and a less controllable process [23, 40, 103], especially as acid oxidation unavoidably damages the structural integrity of carbon structures [48]. In contrast, atomic scale destruction was observed for carbon nanofibers (CF) after exposure to oxygen plasma for only 30s [104]. The carbon atoms released as part of the gaseous species produced atomic vacancies on the graphitic surface, causing structural disorder on the atomic scale. This happens during the first 30s to 3min of treatment, then the ongoing removal of carbon atoms did not produce further basic structural changes on the surface, but rather resulted in a continuous reduction in the carbon fibre diameter [104], which may be detrimental to the CF intrinsic properties. O2 plasma etching comprises the disturbance of the atomic scale arrangements, the introduction of oxygen at defect sites, and the release of CO/CO2, all in a continuous and interdependent process. Various types of plasma reactors have been utilized to prevent aggregation of nanomaterials during the plasma treatment process, including rotating reactors, fluidized bed reactors, mechanical mixing plasma reactors. Mechanical mixing within a fluidized bed is a good method to minimize aggregation and to test the feasibility of plasma coating/treatment of nanomaterials [105, 106]. Magnetically assisted fluidized bed plasma reactor equipped with a radio frequency (RF) plasma power supply has been used for plasma coatings and the treatment of MWCNTs [107, 108]. 3.4.1 Plasma functionalisation of CNTs Oxygen is often used as a gas source although much plasma activation is carried out simply with ambient air. Oxygen plasma treatment can create functional groups such as -COOH, C=O, -OH, C-O-C, and -CO3- on the surface of carbon, providing a reaction platform for 15

further interaction with polymers [109]. For example, carboxyl group -COOH reacts with the amino group of epoxy resin via covalent bonding. Plasma discharge operating parameters such as gas composition, pressure, plasma power and the position of the sample inside the reaction chamber can be tuned to provide a range of oxygen-containing functional groups varying from epoxy to carbonyl [110, 111]. During oxygen plasma treatment, the formation of oxygen-containing functional groups is always accompanied with the loss of oxygen functionality by release of volatile oxygen containing products, both altering ID/IG.. An increase in ID/IG ratio is observed as the conversion of sp2 to sp3 (hybridisation) carbons prevails due to the formation of oxygenated groups on the CNT surface, while the gasification of the amorphous carbon from the surface causes a decrease of ID/IG [112, 113].

Fig. 7. Possible functional groups formed via plasma modification of CNPs. The dielectric barrier discharge (DBD) technique has advantages of simplification in experimental set-up and with standard vacuum equipment in comparison with other plasma techniques. It has been reported that functionalised CNTs by DBD plasma in air have less defects and less pollution as compared to those HNO3 oxidation functionalisation [114]. Both oxygenated and nitrogenated groups can be formed on the surface of CNTs using a combination of oxygen and nitrogen plasma treatment [115]. After RF plasma treatment with a gas mixture of O2/N2 (at gas flow rate of 75 sccm (50 sccm O2 + 25 sccm N2, where sccm donates cubic centimetre per minute at STP), for a plasma power of 300W and exposure time of 6 min), several functional groups were identified on the CNT surface, characterised by FTIR (Fig. 8a). This included -OH (3432 cm-1), carboxyl groups (C-O, C-OH at 1645 cm-1 and -COOH at 1725 cm-1), -C=O, -NH2, -CONH2, -C=NH, -C≡N, epoxide (C-O-C 1215 cm16

1

, 1055 cm-1). From XPS analysis Fig. 8b), the height of O1s peak at 532.2 eV increased

significantly and there was the evolution of a nitrogen peak at a binding energy of 400.1 eV. The C1s spectrum for both untreated and plasma treated CNTs were de-convoluted into six peaks. For the plasma treated CNTs, the sp2-hybridized C=C (284.7 eV) content decreased, while the content of sp3-hybridized carbon C-C (285.4 eV), -C-O/C-N, -C=O/C=N, and OC=O/CONH (286.4, 287.3 and 288.8 eV) increased. This suggests that the C=C bonds in the carbon lattice are oxidised and new oxygenated and nitrogenated groups form. From element analysis, the oxygen concentration increased from 3.12 to 17.71 at% and the ratio of O/C increased from 0.03: 1 to 0.23:1. The nitrogen concentration was about 5.15 at% for the plasma treated MWCNTs. The change in the structure of the carbon lattices can be characterised by Raman spectroscopy (Fig. 8c). The intensity ratio of the D (1351 cm-1, amorphous carbon) and G bands (1576 cm-1, ordered sp2 hybridized carbon structure) increased from 0.22 to 0.55 indicating the increased number of defects on the CNT surface after plasma treatment.

Fig. 8. Characterisation of untreated and plasma treated MWCNTs by (a) FTIR, (b) XPS C1s high resolution spectra, and (c) Raman spectra [115]. A microwave excited Ar/H2O surface-wave plasma was used to treat MWCNTs in order to improve their dispersion in water [116]. The MWCNTs were pre-treated with Ar plasma for 5 min at a gas flow rate of 40 sccm, a pressure of 13.3 Pa and a microwave power of 700 W, which resulted in bond breaking due to Ar-ion bombardment and thus increased reactivity. The MWCNTs were subsequently treated with a Ar/H2O mixture gas plasma at a partial pressure of 10/3.3 Pa and a microwave power of 700 W for 10 min. XPS analysis, see Fig. 9, showed the oxygen content had increased from an original 11.3 to 31 at% after plasma treatment, indicating that most of the aromatic sites on the surface of the MWCNTs were oxidized. Additionally, the Ar/H2O plasma treatment led to a decrease in the sp2 C=C 17

component from an original 53.4 to 37.2%, and an increase in the C-O and O-C=O components from 4.8% and 1.9% to 16.9% and 13.7%, respectively. The non-saturation of C=C bonds is more active, chemically unstable and more susceptible to plasma attack [117]. Therefore, the mechanism by which surface modification is achieved is when free radicals are firstly generated on the dissociated π bonds of C=C, then further react with active OH radicals originating from the de-excitation of H2O. The OH radical created in the plasma zone as well as other fractionation products (e.g. O* and H*) of the plasma might interact with the surface, finally forming surface-bound C-O groups. The active OH radical is highly reactive and interacts with the non-activated and activated sites on the MWCNT surfaces. The OH radical might have a higher probability of forming a covalent bond upon reaction with dangling bonds or defects. The introduction of oxygen-contained functional groups to the MWCNT surfaces is in the main attributed to the presence of active OH radicals. The increase in ID/IG from 1.1 to 1.36 and the blue shift in each peak position for the plasma treated MWCNTs indicated an increase in oxygen content, disorder and defect density, relative to the untreated MWCNTs.

Fig. 9. XPS analysis of MWCNTs (a) before and (b) after plasma treatment. Adapted with permission from [117]. Copyright (1999) American Chemical Society.

Microwave generated N2 plasma has been used for surface functionalisation of CNTs for photovoltaic device application [99]. N2/Ar microwave plasma was introduced into the chamber at a constant flow rate of N2 (50 sccm) and Ar (50 sccm), respectively. After exposure for 25 min, nitrogen-containing groups were detected on the CNT surface,

18

confirmed by the increase in intensity ratio (ID/IG) and 5.23 at% nitrogen atoms determined from XPS, indicating the successful grafting of nitrogen groups onto the MWCNTs. When applied the functionalised MWCNTs improved the power conversion efficiency (0.086%) in a typical photovoltaic device due to their improved dispersion in the polymer matrix. Microwave plasma treatment using ammonia (30%) as a precursor monomer and pure hydrogen (99.99%) as a carrying gas have also been studied to modify CNTs [26], using a reactor chamber pressure of 266 Pa and a microwave power of 30W. It was found that the tubular diameter of the treated CNTs were about two times larger than the untreated CNTs, indicating homogeneous deposition of the ammonia plasma on the CNT surface. The carbon structure was well preserved after treatment. This form of plasma treatment offers CNTs with high hydrophilicity and enhanced wetting properties, which facilitates faster electron transfer kinetics for electrodes for use in fast response bio-sensors. However, ammonia plasma treated CNT modification reactions are complex because the ammonia precursor readily produces multiple reactive radicals under microwave radiation. The application of negative bias voltage can improve plasma modification efficiency. MWCNTs modified using a microwave excited NH3/Ar surface wave plasma at a bias voltage of -50V [25] yielded amino groups which the authors quantified using chemical derivatisation methods, as shown in Fig.10. Therefore, the fluorine atom content can be determined from XPS to reflect the content of primary amino groups. The authors proposed two parameters to demonstrate the grafting efficiency, amino efficiency denoted as NH2/100C and amino selectivity denoted as NH2/100N ratio. Accounting for three fluorine atoms per NH2 group, the NH2/100C and NH2/100N ratios were calculated and listed in Table 1.

Fig. 10. Chemical derivatisation of primary amino groups to imino groups [25].

19

The NH3/Ar plasma treatment leads to an obvious increase in the N:100C, NH2:100C and NH2:100N ratios, indicating the formation of nitrogen groups on the MWCNT surface via NH3:Ar plasma treatment. The NH2:100C ratio increased and the NH2:100N ratio decreased with increasing NH3 gas flow rate, and an optimum flow rate of NH3/Ar gas mixture was 90/70 sccm for surface activation. Table 1. XPS elemental analysis of plasma modified MWCNTs as a function of gas flow rate

with -50 V bias voltage for 15 min and a microwave power of 700 W.

MWCNTs

N/100C

O/C

F/100C

NH2/100C

NH2/100N

Untreated MWCNTs

0.089

0.127

0

0

0

NH3/Ar 40/70 sccm

2.360

0.134

4.50

1.50

63.73

NH3/Ar 70/70 sccm

3.087

0.134

4.80

1.60

51.70

NH3/Ar 90/70 sccm

4.304

0.136

5.31

1.77

41.09

This phenomenon suggests that there is a saturation state for surface nitrogen functionality, and a steric hindrance effect may prevent more nitrogen species attaching to the CNT surface. Furthermore, some etching and decomposition of already generated groups by ion bombardment may also lead to a saturation state for surface nitrogen functionality. The application of a negative bias voltage of -50V generates Ar ions with higher energy to penetrate a larger volume of CNTs and induce more active sites, thus more nitrogen containing groups are introduced to the MWCNT surface. Low temperature plasmas can be generated with high electron density under microwave excited surface wave plasma (MW-SWP) conditions, which allows uniform covalent functionalisation of CNTs without destroying their integrity [118, 119]. In addition, high electron energy is enough to fractionise NH3 to form metastable ions of NH2, NH, N, and H as well as free radicals. Besides ammonia gas, radio-frequency-plasma (RF-plasma) at 13.56 MHz

was

used

to

treat

MWCNTs

with

O 2,

NH3

and

CF4

to

attach

hydroxyl/carboxyl/carbonyl groups; amines/nitriles/amide groups and fluorine atoms on the surface [98]. The concentration and type of functional groups formed are closely dependent on the plasma conditions applied (power, type of gas, treatment time, pressure, position of the CNT sample inside the chamber). It was found that longer exposure times and higher power led to a greater quantity of oxygen grafted on the CNT surface, whilst lower gas pressure gave a higher oxygen concentration. The surface of MWCNTs treated with CF4 microwave 20

plasma has about 2/3 fractional surface coverage [120]. However, a treatment time of more than 300s and power above 30W destroyed the CNTs. N2, N2 and H2 or Ar have been used to generate amine- and nitrile- functional groups on the surface of CNTs [48, 49]. By tailoring reaction times, various CNTs modified with octadecylamine and carboxylate-ammonium salt were prepared. The octadecylamine modified CNTs were treated with hydrogen plasma for developing CNT field emitter displays [121]. Acetaldehyde plasma functionalised CNTs were attached with amino-dextran chains via the formation of Schiff-base linkages [120]. A two-step plasma treatment consisting of Ar plasma pre-treatment and subsequent ammonia plasma treatment is generally used for treatment of polymer [122] and nanocarbon [115] surfaces. The argon plasma pre-treatment is used to remove the contamination from and activate the surface using argon ions, which also increase surface roughness beneficial for adhesion [123]. Using a plasma exposure of 2 min with argon and 3 min with ammonia, the amino group functionalisation (-NH2:N) increased from 53.7% to 78.4%, estimated from the peak at 399.5 eV in the XPS spectrum, the latter higher than that without Ar pre-treatment (73.7%). The -NH2:N ratio can be used to measure the grafting selectivity of amino groups as the formation of both NH and hydrogen radicals plays an important role in functionalisation of amino groups. TGA is more commonly used for quantitative determination of grafting molecules on the surface of nanomaterials [124]. High residual char content is representative of a high amount of carbon skeleton while the increase in mass losses confirms the increase in grafted molecules. Ar plasma treated SWCNTs resulted in a 29 wt% loss (from TGA) confirming successful functionalisation of the CNTs by Ar-plasma [125]. The extent of functionalisation can be determined using the following equation [8]: , (1) where, R is the graft ratio, x is the weight losses of the NP, and Ma and Mc are the atomic weight of carbon and molecular weight of the grafting molecule, respectively. This formulation is based on the assumption that the main structure of CNP’s is entirely constituted of carbon. The graft ratio of carboxyl-functionalised MWCNT and aminefunctionalised MWCNT was found 1.1% and 3.6%, respectively based on XPS and TGA test results [8]. The efficiency of N2 plasma functionalisation as analysed by XPS showed NH2 content from ~1.7) [25] to a maximum NH2 ~14.9% [126].

21

Plasma treatment causes etching and grafting of functional groups on the carbon surface, which increases surface roughness. After functionalisation, the defects created and amine groups covalently attached to the CNT surface result in decreased electrical conductivity of the functionalised CNTs [127]. A reduction in the contrast and resolution of the SEM images of these functionalised SWNTs was also observed confirming surface modification [127, 128]. 3.4.2 Plasma functionalisation of graphene and graphene like materials The electronic structure of graphene can be tailored by doping the carbon lattice with different functional groups. The replacement of carbon atoms with O or N atoms through oxidation or amination reactions causes the destruction of the sp2 hybridized structure and the continuous π-networks, which generates a finite electronic band gap and therefore affects the electronic and optical properties of graphene materials. Oxygen plasma treatment has been used to induce photoluminescence properties of single- and few-layer graphene [129]. For GO thin films treated with oxygen plasma, the epoxy group converts to a carbonyl group C=O and O-C=O. The presence of isolated sp2 clusters within the C=O sp3 matrix leads to localization of electron-hole pairs, facilitating radiative recombination for small clusters in GO which results in excitation dependent emission. The conversion of epoxy group to carbonyl groups can be controlled by oxygen pressure and treatment time, which can be used to tailor photo-luminescent emission [129]. With RF plasma treatment at 0.04 mbar and 10 W for up to 6 s, the binding of oxygen atoms to carbon lattices and the conversion from sp2 carbon bonds to sp3 give rise to photoluminescence in graphene sheets [109]. The optoelectronic properties of GO can be tailored by manipulating the size, shape and relative fraction of the sp2-hybridized domains. GO was shown to be a mechanically flexible hole transporting material in organic solar cells (OSCs). Plasma treated GO improves both the short-circuit current and fill factor of the devices compared to pristine GO. Low power oxygen plasma treatment followed by heat treatment improves the ohmic contact of graphene by nearly 6000 times compared to untreated graphene [130]. The treatment time of the cold oxygen plasma dictates the transformation from semi-metallic to semi-conducting behaviour of graphene. Xiao et al. measured the thermopower (opening of band gap) of few-layer graphene films and found that the thermopower of graphene films after oxygen plasma treatment could be greatly enhanced by 775% in the temperature range 475-575 K, as compared to pristine graphene [130]. In the case of functionalisation of graphene using a CF4 RF-plasma source and maintaining an RF 22

bias voltage fixed at -250 V at room temperature [132], fluorine atoms were attached to the graphene surface through C-F covalent bonding which led to a stable dispersion of individual graphene sheets

in non-aqueous media. Room temperature mild ammonia plasma edge

functionalisation of graphene has been reported [133]. Hydrogen plasma has also been used for edge functionalisation at around 300 °C [134]. N2 plasma-treated graphene can be used in electrochemical applications [135]. N content can be controlled by changing the plasma strength and/or exposure time. N-graphene exhibits much higher electro-catalytic activity compared to unmodified graphene. Besides nitrogen functional groups, structural defects and oxygen-containing groups also contribute to enhanced electro-catalytic activity. N2 plasma treatment of chemically functionalised graphene film is useful in improving hydrophilicity and biocompatibility [27]. In this process, N2 gas was directly injected into the reactor through a quartz injector for 20s and 100s. 3.4.3 Plasma functionalisation of carbon nanofiber (CF) Different types of low power plasma treatments (NH3, N2, and Ar) can generate hydroxyl and carbonyl groups on CF surfaces without significantly etching or pitting [136]. Both amines (NH2) and C-NH groups were formed with either nitrogen or ammonia plasma treatment. Comparing the XPS spectra of the immediate surface with those taken at bulk sensitive angles, it confirmed that the chemical change only occurred in the first few atomic layers of the CF surface. As the result of air plasma treatment, the amount of oxygen containing groups, especially -COH and/or –C-O-C–, as well as -C=O functionalities were increased, as characterised by XPS [137]. There is little difference observed between the CF surfaces treated for less than 2.5 min and those over 20 min. From ζ-potential measurements, a shift in the iso-electric point towards lower pH values was observed, indicating the CF surface became more acidic after plasma treatment [138]. Similarly, the morphology of the CF surface changed little for different exposure times between 1 and 20 min when treated with oxygen plasma for a chamber pressure of 1.50 ±0.05 mbar, plasma resonance frequency of 27.12 MHz, reactor power of 16.5 W, a gas flow rate of 4.5 ± 0.6 mL(STP)/min and a temperature of 20 ± 1°C [54]. The contact angle continuously decreased (water) with increasing treatment time, which is associated with a uniform chemical surface composition of the oxygen-plasma treated carbon fibres. The surface tension increased with increasing treatment time up to 20 min. A remarkable decrease of the main graphitic peak was observed in the XPS [137], which was

23

due to the disruption of the graphitic surface structure by the bombarding species in the plasma. As characterised by scanning tunnelling microscopy (STM), under oxygen plasma treatment at 1.0 mbar pressure, carbon atoms are extracted from random locations from the CF surface, giving rise to surface defects, which allows the oxygen to be introduced during and after plasma etching. The surface structural modification (disordering) and uniform introduction of oxygen were achieved only after treatment for a few (~3) minutes. The whole surface became structurally disturbed suggesting oxygen was introduced and reached its maximum concentration on the surface. Longer treatment times do not produce further structural changes on the atomic scale to the CF surface. 4. Performance of composites of CNPs and epoxy resins Uniform dispersion of CNPs and strong interfacial interactions of the constitutive phases of epoxies with CNPs influence the macroscopic properties of composites of CNPs and epoxy resins. To date, polymer nanocomposites are mainly produced using three methods: in-situ polymerisation, solution blending and melt processing [103, 139, 140]. The unmatched surface chemistry between CNPs and polymers make it difficult to achieve homogeneous dispersion and promote strong interfacial interactions in these composites. In this section, we critically review epoxy-based nanocomposites reinforced with amino- and plasmafunctionalised CNPs, focusing on the efficiency of grafting of functional groups and the effects of functionalised CNPs on macroscopic properties. Chemical and plasma functionalisation of CNPs influences the thermo-mechanical properties of composites [47, 139, 141-145]. Epoxy resin, i.e. the diglycidyl ether of bisphenol A (DGEBA) having one or more epoxide groups in one molecule, has been widely used in various industrial applications such as for coatings [146], adhesives [147] and structural composites [148]. The highly crosslinked epoxy resin has high mechanical strength but low toughness and poor surface resistivity [120]. A large range of co-reactants/curing agents/hardener, such as anhydrides, amines, and amides are used for crosslinking of epoxy resins [149]. The incorporation of CNPs to epoxy resins [150] has shown great potential for simultaneously enhancing the mechanical, thermal and electrical properties of the epoxy, to develop high performance composites for automotive [8], aerospace, tissue engineering [151], and electronics [152, 153] applications.

24

4.1 Plasma functionalised CNT/epoxy composites Plasma functionalisation has been considered as a promising approach for the modification of functional nanofillers due to distinct advantage that the process is operated at relatively low temperatures, low pressure, short exposure times and at low cost [154]. The enhancement of the interaction between polymers and plasma-modified fillers is caused by both physical (surface roughness) and chemical treatment (active polar groups). Williams et al. developed an upscale process for plasma-treatment of CNTs in the presence of oxygen and ammonia [53]. The relationship between the process parameters utilised , i.e. treatment time, process pressure and gas composition (mixtures of Ar, O2, H2O, and H2) on the composition of the functional groups generated was investigated, and about 6-fold enhancement in the amount of functional groups were formed after switching the process gas from Ar/O2 to Ar/EO2 (enhanced treatment).In enhanced treatment, processing time was longer that Ar/O2 to increase the carboxylic content. Helium/air plasma treated MWCNTs were prepared for reinforcement of an epoxy resin [114]. The surface chemistry of the plasma treated MWCNT’s were compared with MWCNT’s chemically treated. As listed in Table 2, the oxygen atom content was increased from 0.4% for untreated MWCNTs to 5-6% for those plasma treated. The intensity ratio of the D and G bands (ID/IG) increased for the modified MWCNTs relative to the untreated MWCNTs, indicating more oxygen-containing groups were introduced to the carbon lattice, and chemical modification resulted in more disordered structure than plasma-treatment. Air plasma treatment led to the highest number of acidic groups on the MWCNT surface, which also led to the best reinforcing effect when the modified MWCNTs were added to an epoxy/CF composite. The reinforcement is due to the increased interfacial compatibility between the MWCNTs and the epoxy matrix. Table 2 Physico-chemical properties of plasma etched MWCNTs [114].

Property

45 min

90 min

45 min

90 min

air

air

helium

helium

plasma

plasma

plasma

plasma

215

236

248

222

229

251

0.41

4.74

5.75

0.36

0.31

5.12

0.46

—

2.34

—

0.33

3.49

Untreated CNTs

BET surface area (m2 g−1)

Elemental analysis (oxygen %) CO (mmol g−1)

25

HNO3CNT

CO2 (mmol g−1) Total (CO + CO2) (mmol g−1)

0.52

—

4.10

—

0.41

2.84

0.98

—

6.44

—

0.74

5.33

954(±24)

971(±30)

915(±23)

921± (24)

959(±28)

862(±31)

883(±25)

822(±20)

819(±22)

872(±27)

885 Tensile strength (MPa)* Flexural strength (MPa)

(±22) 789(±20)

*mechanical properties of MWCNT modified epoxy/CF composites with additional 0.5 wt% MWCNTs (values in the parentheses are standard deviations).

Acid oxidised MWCNTs were treated with octadecylamine at 120 oC for 3 days to produce amine-CNTs. The acid oxidised CNTs were also irradiated with Ar plasma containing 1% of oxygen at 200 W for 1 min [143]. The characteristic oxygen peak of the carbonyl group was observed by XPS on the surface of the plasma-treated CNTs. For various modified CNTs reinforced epoxy composites, it was found that the acid oxidised CNTs showed better dispersion than amine-CNTs in epoxy matrix, but the latter showed higher interfacial bonding with the epoxy matrix, as demonstrated by the different fracture behaviour of the composites, see Fig. 11a. The acid-CNTs were pulled out of the epoxy resin while the amine-CNTs were broken during tensile testing due to the stronger interfacial adhesion. The plasma-treated CNTs showed similar behaviour as the amine-CNTs. Both tensile strength and elongation at break of the composites containing 1 wt% plasma treated CNTs was increased by 124% compared with the pristine epoxy, as shown in Table 3 and Fig. 11b.

Fig. 11. (a) FESEM images of the fracture surface of composites of an epoxy containing 1.0 wt% plasma treated CNTs (scale bar is 1 µm) and (b) stress-strain curves of the cured epoxy and composites of the same epoxy containing 1.0 wt% CNTs [143].

26

The composites containing modified CNTs showed strong shear-thinning behaviour and higher shear viscosity than the epoxy matrix and epoxy composites containing unmodified CNTs. The plasma-treated CNTs filled epoxy composites showed the highest shear viscosity due to good dispersion and strong adhesion between both components. Table 3. Mechanical properties of unfilled epoxy and composites of the same epoxy with plasma modified CNTs [155]. Fillers

Tensile strength (MPa)

Young’s modulus (GPa)

Elongation at break (%)

No filler (Epoxy)

26

1.21

2.33

Untreated CNTs

42

1.38

3.83

Acid treated CNTs

44

1.22

4.94

Amine treated CNTs

47

1.23

4.72

Plasma treated CNTs

58

1.61

5.22

Amine-CNTs were prepared by a plasma nanocoating technique using a mixture of allylamine vapour and Ar gas using a pressure of 100 mtorr, an allylamine/Ar mixture at a 1:1 ratio, 6 W of RF power input and a 30 min plasma exposure treatment [155]. The plasma treatment provided an amine coating of about 1nm on the CNT surface, transforming the surface from hydrophilic to hydrophobic with a contact angle of 137 o. The tensile strength and Young’s modulus of the epoxy composites containing 1 wt% amine-CNTs increased by 50% and 13.2%, respectively, relative to the pristine epoxy, without a decrease in ultimate strain. The interfacial bonding thickness was estimated to be 8 nm for the plasma-treated CNTs in the epoxy composites. With addition of 1 wt% of the plasma treated CNTs, the glass transition temperature (Tg) increased from 68 oC for the unfilled epoxy to 70 oC, and the specific heat capacity decreased from 2.4 to 1.74 J/kg/oC, further supporting enhanced interfacial interactions between plasma-treated CNTs and the epoxy matrix. Ar-plasma treated CNTs were further grafted in a controlled process with maleic anhydride (MA)(6.4%), the reaction mechanism is shown in Fig. 12 [48]. The amino groups attached to the CNT-MA which can function as a curing agent, which led to the formation of a highly crosslinked structure via covalent bonding between the treated CNTs and the epoxy matrix. With 1.0 wt% addition of the CNT-MA, tensile strength, ultimate elongation at break and tensile modulus increased by 50%, 380% and 100%, respectively, compared to the pristine epoxy. This improvement was attributed to the enhanced load transfer from the epoxy matrix to the CNT-MA reinforcement through strong chemical interfacial bonding. For the 27

unmodified CNTs, any increase in tensile strength, ultimate elongation at break and tensile modulus were only achieved at lower CNT content (0.1 wt %) but these properties diminished when the CNT addition was above 0.1 wt %, due to the aggregation of the CNTs in the epoxy matrix. The good dispersion of CNT-MA led to a lower electrical percolation threshold at around 0.1~0.2 wt%, compared with the composite with unmodified CNTs 0.5~0.6 wt%. The maximum electrical conductivity achieved was 2.6× 10-4 S/m for a CNTMA addition of 1 wt%, 2 orders of magnitude higher than that measured for the composites filled with unmodified CNTs - 3.5× 10-6 S/m. The Tg and thermal decomposition temperature of the CNT-MA based composites also increased markedly.

Fig. 12. Schematic representation of the preparation of composites of CNT -MA and epoxy resin [48]. Chemical functionalisation largely improves the compatibility of graphene sheets (GNs) with epoxies, thereby, forming a thicker sheathe (hundreds of nm thick) enwrapping the GNs, in contrast to thinner polymer sheaths (50 nm) coating MWCNTs [156]; both are required for stress transfer and crack inhibition/deflection upon loading [157]. Pull out of CNTs from the polymer matrix is a critical concern for carbon-based composites. Without functionalisation, CNTs are usually easily pulled-out from the matrix, see Fig. 13, due to the cluster formation and poor wetting between both components. However, plasma coating on the surface of MWCNTs improves their dispersion in the matrix material and that the presence of covalent bonding between the amine on the plasma coated MWCNTs and the epoxy resin via interfacial interaction significantly reduces MWCNT cluster formation. [53, 155].

28

Fig. 13. SEM images of MWCNT powder; (a) as received, (b) oxygen-, (c) ammonia-, and (d) enhanced oxygen- treated [53].

Table 4 summarises the mechanical and thermal properties of composites of epoxy and CNTs where the CNTs have been modified using different techniques. It shows that plasma treated CNTs when added to epoxy result in improvements in tensile strength and stiffness of the composite compared to traditional CNT functionalisation methods.

29

Table 4 Different methods employed to modify CNTs and its effect on epoxy composite propertiesa

Filler SWCNTs

Modification Method Oxidation and fluorination

SWCNTs

Chemical functionalisation (diamine) MWCNTs Non-covalent functionalisation MWCNTs Amino functionalised MWCNTs Amine treated MWCNTs Ar plasma functionalised MWCNTs Plasma (allylamine) MWCNTs Plasma treatment a

Dispersion technique

Solvent DMF

wt% of filler 1.0

% increase in σ 18

% increase in E 30

Bath sonic + mechanical mix Bath sonic + mechanical mix Bath sonic + mechanical mix mechanical mix, 3-roll machine Direct mixing without any solvent Direct mixing without any solvent Bath sonic + mechanical mix Bath sonic + mechanical mix5

Chloroform

1.0

25

30

[148]

Acetone+DMF 1.0

13

25

[159]

-

0.5

0.73

8.52

[160]

-

1.0

80

1.65

[143]

-

1.0

124

33

[143]

-

1.0

54

15.23

2.66

[155]

-

1.0

50

100

18.18

[48]

Blue shaded area: amino functionalised; gray shaded area: plasma functionalised

30

Increase in Tg (°C)

Ref. [158]

4.2 Amino-functionalised CNT/epoxy composites Amino-functionalised CNP’s [139, 161, 162] are desirable additives for epoxy resins given their reactivity in forming strong covalent bonds with epoxies via ring-opening esterification and crosslinking reactions [48, 142]. Amino-CNTs

prepared

(triethylenetetramine) agent

by

refluxing

oxidised

CNTs

with

a

multi-functional

have been shown to be less agglomerated and with strong

covalent-bonding between the CNTs and epoxy resin [162]. The effects of

modified

MWCNTs on the thermo-mechanical properties of the epoxy composites have been studied previously [139], using a range of techniques, including dynamic mechanic thermal analysis (DMTA) and differential scanning calorimetry (DSC). Amino-functionalised CNTs (aminoMWCNTS) with a diameter of about 15nm and a length of approximately 50 µm were mixed with long-chain hardener separately to facilitate CNT dispersion. The modified CNTs increased the storage modulus in the rubbery region compared to the glassy region of the epoxy. The loss modulus increased for 0.05 wt% filler loading, but decreased at higher loading (0.75 wt%) due to CNT agglomeration. The Tg of the composites increased by 30% also confirming that the amino-MWCNTs reacted with the epoxy resin via covalent bonding, thereby improving the thermo-mechanical properties. Functionalised MWCNTs can readily react with the epoxy and act as curing agents for the epoxy matrix. Shen et al. modified MWCNTs using ethylene diamine followed by carboxylation, acylation and amidation reactions [160] and, the functionalised MWCNTs used to modify epoxy resins. For a 1 wt% loading of unmodified MWCNTs, the flexural strength of the composites was approximately doubled, while the flexural modulus has slightly increased. By adding 1 wt% of amino-functionalised MWCNTs, Tg decreased approximately by 20 °C due to the non-stoichiometric balance of MWCNTs and the curing agent. The decrease in Tg of the composite could have also originated from the reduced crosslink density of the composite. Wang et al. [164] used the two amino-group containing curing agent for epoxy resins (EPIW) to directly functionalise SWCNTs through diazotization and the degree of functionalisation was estimated to be 1 in 50 carbons. The reaction occurred in two steps (Fig. 14); diazotization followed by amino-grafting through electronic extraction.

31

Fig. 14. Proposed functionalisation scheme: (a) diazotization through EPI-W and isoamyl nitrite and (b) amino-grafting through electronic extraction [164]. As-prepared composites showed that the strong interfacial interactions promoted the highest level of exfoliation and dispersion of the CNTs in the matrix, and achieved the largest increase in storage modulus and Tg of the matrix. For a 0.5 wt% loading of modified SWCNTs, the storage modulus of the epoxy was increased by 25%, suggesting efficient load transfer at the interface between matrix and filler after functionalisation. The authors reported a reduction in the Tg of the composites in conflict with that reported previously. In this case, it was suggested the presence of SWCNTs interfere the curing reaction resulting in a lower conversion. However, the strong interfacial bonding between the amino groups grafted onto the SWCNT surface and the epoxy resin still led to enhanced storage modulus. The effects of interfacial interactions between CNP’s and polymer matrices can be investigated by AFM [164] and TEM [106]. It many instances the surface of functionalised CNTs is completely covered by the matrix, see Fig. 15a. Without functionalisation, CNTmatrix forms weak interactions and therefore, stress transfer from the matrix to the tube is low and pull-out of the CNTs from the surrounding matrix results (Fig. 15b).

32

Fig. 15. TEM image of functionalised MWCNTs: a) matrix covers the surface of the CNTs and (b) a weak interaction between the epoxy matrix and CNTs leads to CNT pull-out [163].

4.3 Amine functionalised graphene/epoxy composite Compared to CNTs, lateral graphene nanosheets (GN) with a higher surface-to-volume ratio and specific surface area, are expected to be more efficient at reinforcing polymers [13, 20, 67, 165-167]. Apart from the dimensions and intrinsic properties of GN’s, the extent of GN dispersion and interfacial interactions with polymers play critical roles in the physical properties of their composites with polymers [168-170]. Amine functionalisation of graphene is preferable for modification of epoxy resins since covalent bonding with epoxy and curing agents are possible, thereby enhancing many properties of the neat epoxy[157, 171-174]. Thermally expanded graphene nanoplatelets (EGNPs) were refluxed in 9M nitric acid to carboxylate the graphene sheets. The carboxylic EGNPs were the treated with thionyl chloride to convert the surface –COOH groups to acyl chloride groups. The EGNPs was further reacted with dodecylamine (DDA) to obtain amine functionalised EGNPs [175]. The fracture toughness of the modified epoxy composites increased by 66% on addition of 0.1 wt% EGNPs. The hardness and modulus of the composites increased steadily with the incorporation of EGNPs up to 1.5 wt%. Agglomeration of EGNPs was also observed when the filler content was above 0.5wt%, but the thermal conductivity of the composite was 36% greater than the unfilled epoxy at 2 wt% loading. The curing agent poly(oxypropylene)diamine (D2000) with molecular weight of 2000 g/mol was used to modify GO to introduce amine groups to the GO surface to obtain amine modified GO (DGO) [174], see Fig. 16. The formation of a CO-NH bond between D2000 and the –COOH group on GO was confirmed by the evolution of a new peak at 1628 cm-1 in the FTIR spectrum. The thickness of the GO platelets increased from 0.94nm to 2.66nm for 33

DGO, as characterised by AFM. With 0.2 wt% DGO, the resultant epoxy composites showed a large increase in Tg from 155 oC for the neat epoxy to 174 ºC for the composite. The addition of 0.3 wt% DGO resulted in increased tensile strength, flexural strength, elongation at break and toughness of the epoxy resins by 20%, 40%, 90% and 145%, respectively. The unreacted amino groups of poly(oxypropylene)diamine may form covalent bonding with the epoxy resin, increasing interfacial interaction between DGO and epoxy matrix.

Fig. 16. Reaction scheme for grafting poly(oxypropylene)diamine on to GO surface [174].

The crosslink density is usually recognized as a critical factor influencing thermal and mechanical properties of epoxy composites [83]. Diamine functionalised GO was used as cocuring agents for epoxy composites. GO was modified with 4, 4’-diaminodiphenyl sulfone (DDS) or hexamethylenediamine (HMDA). The strong absorption band at 1647 cm-1 detected in the FTIR spectrum of DDS-GO, was assigned to amide groups [176] . The level of DDS or HMDA grafting to GO was determined to be 7 wt% by TGA. From the XPS spectra of GO and DDS-GO shown in Fig. 17a–b, an additional peak at 288.4 eV was assigned to O=C-NH [177]. These results show that the DDS was grafted to the GO surface via the amide bonds between the reaction of amine and the -COOH groups on GO. For the GO modified epoxy composites, as shown in Fig. 17c, the Tg decreased from 160.7 to 136.8 oC with 1.0 wt% GO addition, while the addition of 1.0 wt% DDS–GO increased Tg from 160.7 to 183.4 oC. This was a consequence of the improved interfacial interactions between the amine functionalised 34

GO with the epoxy resin. The amine functional groups on the GO surface formed covalent bonds with epoxy groups, and act as a crosslinker for the composites. This effect is also shown by the increased crosslink density from 0.028 to 0.069 mol cm−3 with addition of 1.0 wt% DDS-GO (Fig. 17d). Accordingly, the tensile strength of the composites was enhanced by 26%.

Fig. 17. XPS C 1s spectra of (a) GO and (b) DDS-GO, (c) Tg of EP/DDS/diamine-GO nanocomposites and (d) crosslink density of the EP/DDS/diamine-GO composites, calculated from storage modulus and tan δ data in the rubbery region of EP/DDS/diamine-GO composites [83]. Polymer molar mass or chain length also influences the physico-chemical properties of composites of amine-functionalised GO and epoxy resins [178]. GO was modified with linear poly(oxyalkylene) amines (two different molar masses, D400 and D2000) via the reaction of amines with alkylcarboxyl groups. The DGO NP’s could integrate into the epoxy matrix via the reactions between the amine and epoxy groups, thereby acting as crosslinker for the composite formation. The free amino-terminated groups of the attached poly(oxyalkylene) amine chains could react with the epoxy resin during the curing reaction, as shown in Fig. 18. The D2000-GO/epoxy composites showed higher Tg than D400-GO/epoxy composites, which may be due to more entanglement of the longer chains with the epoxy resin.

35

Fig. 18. Synthetic scheme for amine-functionalised GO nanosheets [178]. Covalent anchoring arisen from imidazole (cure accelerators) functionalisation of graphene promoted organic compatibility and homogenous dispersion of f-GO in an epoxy matrix [179]. GO sheets were modified with 1-(3-aminopropyl) imidazole and homogeneously mixed with epoxy resin using dimethylformamide (DMF) as a solvent. The nanocomposites were cured by an anhydride–epoxy system without adding any catalyst. The 1-(3aminopropyl) imidazole was covalently bonded to graphene via the amide linkage. The tensile strength and Young’s modulus of the composites prepared were enhanced by 97% and 12%, respectively, for only a 0.4 wt% f-GO loading compared with neat epoxy resin. However, Tg decreased from 148 °C (neat epoxy) to 132 °C (1.5 wt% composites) due to the incorporation of short flexible alkyl chains. A local amine-rich environment around the graphene sheets and volume exclusion effects of the grafting chains on the graphene surface can create a hierarchical and flexible interphase structure in the epoxy matrix, thus contributing to improved interfacial adhesion and load transfer, resulting in enhanced mechanical and thermal properties [157]. Aminefunctionalised GN’s were prepared through the reaction of 4,4’-methylenebis(phenyl isocyanate) (MDI) and hydroxyl groups of GO. Then the curing agent 4,4’-methylene dianiline (MDA) was introduced to react with isocyanate groups to obtain NH2-GN. The thickness of the GN increased from an original 0.97nm to about 4.83nm after reaction, confirming successful grafting of the MDI on the GN surface. The level of MDI grafting was 36

estimated by comparing the weight loss of NH2-GN before and after functionalisation, and a grafting density of one NH2 per 69 carbon atoms was determined. The thickness of the interfacial layer, of about hundreds of nanometers was identified using TEM. A 93.8% increase in fracture toughness and a 91.5% improvement in flexural strength were achieved with the addition of only 0.6 wt% f-GNs. These results indicate that the f-GNs may inhibit crack propagation and thus increase the strain energy. However, the above approach required multiple reactions and a tedious preparation process. The same research group further studied the effect of hardeners on the structure and properties of epoxy composites [142]. Two types of hardeners, polyoxypropylene (J230, Mw 230) and 4,4’-diaminodiphenyl sulfone (DDS, Mw 248) were used as a hardener at weight ratios of 100:30 and 100:33, respectively. Curing temperature was different for two curing systems: J230 cured at 80 °C for 3 hours and at 120 °C for 12 hours and, DDS cured at 140 °C for 14 hours. DDS cured composites showed a higher degree of GNP dispersion and exfoliation since DDS contains a benzene ring which more readily intercalated in to the GNP inter-layer spacing. Both fracture toughness and critical strain energy increased steadily with increased GNP fraction, reaching a maximum at 0.984 vol%, before both properties then diminished. This behaviour may be explained by the GNP hindering crack propagation and absorbing the fracture energy. EP are electrically insulative having electrical resistivity values typically of 2-10×1015Ω m, but after producing a percolating network with GNP, a percolation threshold was obtained for 0.736 vol% and 0.984 vol% DDS-cured and J230-cured systems, respectively. 4.4 Silane modified graphene for epoxy nanocomposites Silane has also been used for modification of GNP’s. The grafting of organo-silane through hydrolysis to GO can provide additional functional groups, such as amine, epoxy or carboncarbon double bonds, depending on the silane type used. Fig. 19 shows three types of silane modified GO. The grafted silane chains on the graphene surface can prevent stacking and aggregation of the platelets and improve their dispersion in the epoxy matrix [180]. The amine functional groups can also form covalent bonds with epoxy molecules and act as crosslinkers. The addition of 1 wt% amine silane functionalised GNP’s increased the tensile strength and elongation at break of an epoxy resin by 45% and 133%, respectively [181]. Various silane functionalised GOs were used to bond joints in carbon-epoxy composite system. With amine functionalised GO, the bonding strength of a CNP-epoxy composite increased by 53% [182].

37

Fig. 19. Possible silane modification reactions of GO. The structure of 3-(trimethoxysilyl)propyl methacrylate (MPTS) modified GO was characterised, see Fig. 20[180]. As shown in Fig. 20a–b, the emergence of two characteristic peaks at Si2p and Si2s in the XPS spectrum of MPTS-GO, the decrease of the C1s peak (COH), increase of C=C bonding and the appearance of C-Si for MPTS-GO clearly indicated covalent-bonding of MPTS to GO surface. In Fig. 20c, the diffraction peak at 2θ=10.78o for GO indicates a large interlayer spacing of 0.802nm due to the presence of functional groups on the GO surface. After MPTS modification, the characteristic peak is weakened due to the intercalation of the MPTS. In Fig. 20d, the ID/IG ratio of GO was increased from 1.07 to 1.11 after modification, indicating grafting of MPTS caused a slight structure disorder of the GO.

38

Fig. 20. High resolution C1s XPS spectra of (a) GO, (b) MPTS-GO; and (c) XRD pattern and (d) Raman spectra of GO and MPTS-GO [180]. The mechanical and thermal properties of functionalised graphene epoxy composites have been studied. Selected experimental results based on amino-functionalised and plasmafunctionalised graphene/epoxy composites are presented in Table 5.

39

Table 5 Different methods used to modify graphene and representative property enhancements achieved when the functionalised graphene is added to epoxy resinsa. Filler m-GO

Modification Protocol Silane modification

m-G m-G GO m-G

Surfactant modified Polybenzimidazole Polyvinylpyrrolidone (PVP) Spirocyclic phosphazene

m-G

Diazonium addition

EGNPs

Dodecylamine

m-GO m-GO

4,4’- diaminodiphenyl sulfone (DDS) (through amide bond) Polyetheramine (short chain)

m-GO

Polyetheramine (long chain)

EGO m-G

Ethylenediamine (EA), 4,4’ oxydianiline (ODA) poly(oxyalkylene)amines

m-G

poly(oxypropylene)diamine

Dispersion method

Solvent Acetone

wt% filler 1.0

% increase in σ 18.8

% increase in E 42.2

% increase in KIC 85.7

Bath sonic + mechanical mix mechanical mix Bath sonic Bath sonic Bath sonic + mechanical mix Bath sonic + shear mix High-pressure processor and 3-roll mill Bath sonic

THF DMF DMF Acetone

4 0.3 0.13 1.0

-11.1 46.2 24.4 31.8

22 31.7 14.4 34.1

103 127.2

DMF

0.6

91.5

95.6

Dichloromethane

1.0

Bath sonic + mechanical mix+ ball mill Bath sonic + mechanical mix+ ball mill Bath sonic + mechanical mix Bath sonic + mechanical mix Bath sonic + mechanical mix

Acetone

0.5

63

12

Acetone

0.5

51

10

DMF, THF, water, alcohol Acetone

0.2 0.1

21.6

67.6

5.6

Acetone

0.3

20

145

10.32

0.1

Increase in Tg (°C) 2.55

Increase in Td (°C)

75.3

4.9 9.3 7.6

4.8 4 -2

93.8

-7.6

[183] [184] [185] [186] [187] [157]

66

26

Ref.

[175]

14.1

[150]

90

0.27

[172]

119

0.6

[172]

6

[188]

4.1

[178] [174]

a Abbreviations in table: σ: tensile strength, E:elastic modulus, KIC:fracture toughness, Tg:glass transition temperature, Td:thermal degradation temperature, G:graphene, GO:graphene oxide, m-G:modified graphene, m-GO:modified graphene oxide, rGO:reduced graphene oxide, EGNP:Expanded graphene nanoplatelets, m-Gi:modified graphite, EGO:expandable graphite oxide, bath sonic:bath sonication, tip sonic:tip sonication

40

5. Summary Surface functionalisation of CNPs involve covalent and non-covalent modification of carbon structures, which is generally achieved via wet- or dry-chemistry processes. Non-covalent modification is achieved via weak attractive forces between molecules and the CNP surface, such as van der Waals, π-π stacking and hydrophobic interactions. Covalent modification is obtained by directly binding heteroatoms or functional moieties to the carbon lattices by removal of carbon atoms. The functionalised carbon surface can be further modified with polymers via ‘grafting to’ or ‘grafting from’ strategies. Oxidation and amination reactions are generally used to modify CNPs via wet-chemical processes, which offer high flexibility for tuning surface activity and grafting density. However, wet-chemistry process involve multiple reaction steps, such as acid oxidation, condensation or free radical polymerisation, centrifuging/filtration, washing and drying, which consume large amounts of organic solvents and energy. In addition, the corrosive oxidation treatment unavoidably causes severe damage to the graphitic basal plane by changing the hybridization state from sp 2 to sp3, subsequently greatly diminishing the electrical, thermal and mechanical properties of the CNPs. In contrast, plasma techniques provide a facile, efficient, non-destructive and more environmentally friendly alternative for the functionalisation of CNPs. With several nanometers of etching depth possible, various functional groups can be easily bound to the carbon surface by varying the plasma gas used. In this review, we described recent research progress on amino- and plasma-functionalised CNPs and their application as functional fillers for epoxy resins reviewed. Amine groups, especially primary amines produced by plasma can directly react with epoxy resins via amide bonds, enhancing mechanical and functional properties of epoxy resins. However, the effects of plasma parameters on the surface structure and grafting density still need to be quantified and studied in more detail. Large-scale and uniform plasma treatment of CNPs remains a technical challenge. Despite the achievements in the development of novel CNPs for modification of the properties of epoxy resins, limitations still exist in materials selection and fabrication methods to fulfil the potential of CNP/epoxy composites (e.g. in applications where high thermal conductivity is required) and improve the performance of composites for advanced industrial applications.

41

Acknowledgements The authors thank the International Institute for Nanocomposites Manufacturing (IINM), WMG, University of Warwick for the provision of research facilities. The first author is grateful to the University of South Australia for awarding him a University President's Scholarship and to the Australian government for awarding an Endeavour Postgraduate Scholarship.

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Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach Ashraful Alam1, Chaoying Wan2*, Tony McNally2 1, 2 School of Engineering, University of South Australia, SA5095, Australia 2. International Institute for Nanocomposites Manufacturing, WMG, University of Warwick, UK, CV4 7AL *Corresponding author, E-mail: [email protected]

Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach Ashraful Alam1, Chaoying Wan2*, Tony McNally2 1, 2 School of Engineering, University of South Australia, SA5095, Australia 2. International Institute for Nanocomposites Manufacturing, WMG, University of Warwick, UK, CV4 7AL *Corresponding author, E-mail: [email protected]

Highlights -

Critical review the latest development of plasma-functionalisation of carbon nanoparticles, including working mechanism and modification strategies.

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In comparison with other covalent surface functionalisation methods including chemical, mechanochemical, electrochemical, and irritation reactions.

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Amino- and plasma-functionalised carbon nanoparticles (carbon nanotubes, graphene/GO and carbon fibre) and their recent progress in the modification of epoxy resins.