Effect of side-wall functionalisation of multi-walled carbon nanotubes on the thermo-mechanical properties of epoxy composites

Article abstract of DOI:10.1039/c6ra00080k

In this work, a novel functionalisation was carried out by grafting carboxyl-terminated poly(acrylonitrile-co-butadiene) on the side-walls of multi-walled carbon nanotubes to prepare MWCNT-g-CTBN. The functionalized nanotubes were characterized by XPS, Raman spectroscopy, and TGA and the surface morphology was analyzed by transmission electron microscopy. The dispersion behavior of the MWCNT/epoxy nanosuspension was carefully analyzed by transmission optical microscopy (TOM) and rheology. The MWCNT-g-CTBN was added to the diglycidyl ether of bisphenol A-type epoxy to prepare MWCNT-g-CTBN/epoxy composites. Incorporation of CTBN-grafted MWCNTs in epoxy matrix imparted tremendous improvement in mechanical strength as well as fracture toughness when compared with pristine MWCNT/epoxy composites. The mechanism for this improvement in mechanical properties is attributed to the increase in interfacial strength between nanotubes and the epoxy matrix through chemical bonding. The toughening mechanism that leads to the enhancement in the fracture toughness of the nanocomposites was assessed with the help of Field Emission Scanning Electron Microscopy (FESEM). Dynamic mechanical analysis of the MWCNT-g-CTBN/epoxy composite revealed an increase in the T_g of the epoxy phase as well as an increase in modulus due to the enhancement in stiffness of the material.

Full text of DOI:10.1039/c6ra00080k

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: R. Konnola and K.
joseph, RSC Adv., 2016, DOI: 10.1039/C6RA00080K.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Effect of side-wall functionalisation of multi-walled carbon
nanotube on the thermo-mechanical properties of epoxy composites
Raneesh Konnolaa and Kuruvilla Josepha∗
aDepartment of Chemistry, Indian Institute of Space Science and Technology,
Thiruvananthapuramꢀ695547, Kerala, India
* Corresponding author: Tel: +91471 2568632. Email:kuruvilla@iist.ac.in,
kjoseph.iist@gmail.com (Kuruvilla Joseph)

RSC Advances
Page 2 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Abstract
In this work, a novel functionalisation was done by grafting carboxyl terminated
poly(acrylonitrileꢀcoꢀbutadiene) on the sideꢀwall of multiꢀwalled carbon nanotube to prepare
MWCNTꢀgꢀCTBN. The functionalized nanotubes were characterized by XPS, Raman
spectroscopy, and TGA and the surface morphology was analyzed by Transmission electron
microscopic technique. Dispersion behavior of the MWCNT/epoxy nanosuspension was
carefully analyzed by transmission optical microscopy (TOM) and rheology. The MWCNTꢀgꢀ
CTBN was added to the diglycidyl ether of bisphenol Aꢀtype epoxy to prepare MWCNTꢀgꢀ
CTBN/epoxy composites. Incorporation of CTBN grafted MWCNTs in epoxy matrix imparted
tremendous improvement in mechanical strength as well as fracture toughness when compared
with pristine MWCNT/epoxy composites. The mechanism for this improvement in mechanical
properties is attributed to the increase in interfacial strength between nanotube and epoxy matrix
through chemical bonding. The toughening mechanism that leads to the enhancement in the
fracture toughness of the nanocomposites was assessed with the help of Field Emission Scanning
Electron Microscope (FESEM). Dynamic mechanical analysis of the MWCNTꢀgꢀCTBN/epoxy
composite revealed an increase in the Tg of the epoxy phase as well as an increase in modulus
due to the enhancement in stiffness of the material.
Key words: Multiꢀwalled carbon nanotube, sideꢀwall functionalisation, epoxy nanocomposite,
CTBN rubber, mechanical strength, fracture toughness

Page 3 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
1. Introduction
Owing to its outstanding thermoꢀmechanical and multifunctional properties, carbon
nanotubes (CNTs) are used as reinforcing fillers to fabricate polymer composites. It has been
shown that the inclusion of only a small amount of CNTs can significantly improve the
functional and mechanical properties of polymer composites1ꢀ4. These properties result from the
high aspect ratio, low density, exceptional strength and high surface area to volume ratio of the
5
CNTs . But the thermoꢀmechanical properties of the CNT/polymer nanocomposites were
observed to be lower than the theoretical calculations6, 7. It is attributed to the intrinsic van der
Waals force of attraction and high aspect ratio nature of the CNTs due to which they exist in the
form of bundles and ropes. Moreover, the weak interfacial bonding between CNTs and polymer
matrices may limit the load transfer. A homogeneous dispersion of the CNTs in the polymer
matrix is an essential parameter to obtain uniform properties and efficient load transfer during
mechanical loading. Surface modification of carbon nanotubes seems to be the most effective
way to overcome these problems8ꢀ10. There are two main methods for the surface
functionalisation of CNTs. One is the noncovalent attachment of molecules and the second is
covalent attachment of functional groups to the walls of the carbon nanotubes 11, 12. The
noncovalent functionalisation involves the adsorption or wrapping of functional molecules on the
surface of CNTs. But the lack of chemical bonding results in poor interfacial interaction between
CNTs and the polymer matrix, which in turn results in reduction of the effective reinforcement of
CNTs. The covalent attachment of functional groups to the surface of nanotubes offers the
opportunity for chemical interactions with the epoxy systems and improve the efficiency of load
transfer. These functional groups attached could be small molecules or polymer chain13ꢀ19
.
Polymer functionalized CNTs have attracted more attention due to the presence of multiple
anchor units for surface attachment as well as effective compatibility between the functionalised
CNTs and the hosting polymer matrix20ꢀ21. There are some previous works demonstrating the
grafting of a variety of polymers on the surface of nanotubes. Hayashida et al. covalently
anchored poly(methyl methacrylate) (PMMA)ꢀgrafted multiꢀwalled CNTs, and then dispersed
into additional PMMA matrix, yielding highly insulated PMMA–CNT composites22. In the work
of Ilcíková et al., multiꢀwalled carbon nanotubes (MWCNT) has been covalently modified by
short polystyrene chains to improve their dispersion in polystyreneꢀbꢀpolyisopreneꢀbꢀpolystyrene
(SIS) thermoplastic elastomer and prevent deterioration of its elastic properties after

RSC Advances
Page 4 of 31
View Article Online
DOI: 10.1039/C6RA00080K
incorporation of the MWCNTs23. Daugaard et al. prepared Poly(lauryl acrylate) and poly(stearyl
acrylate) grafted multiꢀwalled carbon nanotubes and tested in PP composites to elucidate their
efficiency for compatibilization24. Roy et al. demonstrated a novel strategy i.e., UV/O3ꢀassisted
grafting to covalently functionalize carbon nanotubes with poly(2ꢀacrylamidoꢀ2ꢀmethylpropane
sulfonic acid) (PAGꢀMWCNT). When these grafted MWCNTs were blended with a nylon 12
matrix, the resulting nanocomposites exhibited extremely high mechanical performance due to
the PAGꢀMWCNTs’ excellent dispersion and strong interfacial interactions with the nylon 12 25.
DiezꢀPascual et al. described the grafting of a hydroxylated poly(ether ether ketone) (HPEEK)
derivative onto the surface of acidꢀtreated singleꢀwalled carbon nanotubes (SWCNTs). The
HPEEKꢀgrafted SWCNTs has been used as fillers to prepare PEEK nanocomposites with
enhanced performance26. In a previous study, we modified MWCNT surface with an engineering
thermoplastic, poly(ethersulfone) and evaluated the performance of epoxy composites prepared
using the grafted nanotube27. Incorporation of PES grafted MWCNTs in epoxy matrix
composites imparted tremendous improvement in fracture toughness and tensile strength when
compared to pristine and acid modified MWCNT/epoxy composites.
Recently, the incorporation of CNTs into epoxy matrix is gaining significant interest in the
structural composite application, where strength, stiffness, durability, lightweight, design, and
process flexibility are required such as in aerospace and automobile industry28, 29. The inherent
brittle nature of the epoxy matrix can be improved by the presence of nanotube in the matrix30, 31
.
However, a better improvement in toughness and mechanical strength can be expected if the
CNTs are previously functionalized with reactive liquid rubbers which consist of multiple
functional groups. Besides homogeneous dispersion, the chemical modification of the CNTs with
reactive liquid rubbers can create a soft interface between CNTs and polymer which can result in
a better load transfer from matrix to filler. Carboxylꢀterminated poly(acrylonitrileꢀcoꢀbutadiene)
(CTBN) is conventionally used as a toughener in epoxy by making use of the carboxylꢀepoxy
reaction. The sideꢀwall functionalisation of CTBN to multiꢀwalled carbon nanotubes (MWCNTs)
ensures a uniform coating of soft liquid polymer on the CNT surface and offers the opportunity
to enhance both the mechanical and toughening properties of the epoxy system. To the best of
our knowledge, no report on the sideꢀwall functionalisation of liquid rubber on to MWCNTs is
available in the literature.

Page 5 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
In the present work, CTBN grafted MWCNTs have been proposed as an innovative filler for
the preparation of epoxy nanocomposites with enhanced thermomechanical properties. Surface
modification was characterized by XPS, Raman and TGA analysis. The effect of surface
modification on the nanotube morphology and dispersion status was then evaluated by the TOM,
SEM and TEM observations. The tensile tests were performed on the nanocomposites samples
with different amounts of nanotubes. The Halpin–Tsai model was used to study the mechanical
behavior. Change in the values of glass transition temperature and storage moduli were
determined using dynamic mechanical thermal analysis.
2. Materials and Methods
2.1. Materials
Diglycidyl Ether of Bisphenol A (Lapox ARLꢀ135) and hardener 4,4’Diamino diphenyl
sulfone (DDS) (Lapox K10) were purchased from Atul India Pvt Ltd, Gujarat. Multiꢀwalled
carbon nanotube (Nanocyl 3100) was procured from Nanocyl, Belgium. It was synthesized by
chemical vapor deposition and had an average diameter 9.5 nm, average length 1.5 micron and
purity of > 95 %. poly (acrylonitrileꢀcoꢀbutadiene) dicarboxyl terminated (CTBN, MWꢀ3600
g/mol),2ꢀ(2ꢀchloroethoxy)ethanol, sodium azide, Dicyclohexylcarbodiimide (DCC),4ꢀ
(Dimethylamino)pyridine
(DMAP),
NꢀMethylꢀ2ꢀpyrollidone
(NMP)
and
N,Nꢀ
Dimethylformamide (DMF, 99.5 %)were purchased from SigmaꢀAldrich, Bangalore, India. All
the chemicals were used as received without further purification.
2.2 Modifications on carbon nanotube
Synthesis of 2-(2-azidoethoxy)ethanol.
2ꢀ(2ꢀazidoethoxy)ethanol was synthesized from 2ꢀ(2ꢀchloroethoxy)ethanol using a procedure
similar to the one developed by Matyjaszewski et al.32. In a typical experiment,2ꢀ(2ꢀ
chloroethoxy)ethanol (124.5 g, 1 mol) and sodium azide (130 g, 2 eq.) were added to 1000 mL
water in a 2 L twoꢀnecked roundꢀbottom flask equipped with mechanical stirrer. Subsequently,
the mixture was stirred at 75 °C for 96 h. After cooling to room temperature, the mixture was
extracted with diethyl ether (5 × 100 mL). The organic mixture was dried over magnesium
sulfate overnight and concentrated on a rotary evaporator. The obtained residues were distilled
1
under reduced pressure to give colorless oil in 78 % yield. H NMR (500 MHz; CDCl3): δ 2.69

RSC Advances
Page 6 of 31
View Article Online
DOI: 10.1039/C6RA00080K
(b, 1 H), 3.40 (t, 2 H, J = 4.9 Hz), 3.57ꢀ3.60 (m, 2 H), 3.65ꢀ3.69 (m, 2 H), 3.70~3.75 (m, 2 H).
13C NMR (125 MHz; CDCl3): δ 50.7, 61.7, 70.0, 72.5 (Figure S1 and S2, ESI). MALDIꢀ
TOF/TOF MS (M)+ 131.22.
Preparation of MWCNT-OH and MWCNT-g-CTBN.
MWCNTꢀOH was prepared via the addition of nitrene formed by thermolysis of 2ꢀ(2ꢀ
azidoethoxy)ethanol to strained double bonds of MWCNTs33. A 100 mg portion of MWCNTs
were dispersed in 15 mL of NMP in an R.B flask by ultrasonication for 2 h to give a
homogeneous suspension. The functionalization was performed by the addition 5 g of 2ꢀ(2ꢀ
azidoethoxy) ethanol at room temperature and the mixture was heated at 160 °C for 18 h in a
nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a 0.22ꢀ
ꢁm Teflon membrane and washed multiple times with acetone (100 mL) and distilled water (100
mL) to completely remove the unreacted 2ꢀ(2ꢀazidoethoxy)ethanol. After being dried under
o
vacuum at 60 C for overnight, the functionalized MWNTs containing hydroxyl groups were
obtained (108 mg).
For the preparation of MWCNTꢀgꢀCTBN, 500 mg of MWNT OHs was dispersed in 125 mL
of anhydrous dimethylformamide (DMF) by sonication for 30 minutes. The MWNTꢀOH solution
was added to a twoꢀnecked flask containing a 5 gram solution of CTBN in 125 mL of DMF.
Subsequently, a solution of DCC (20.6 g, 100 mmol) and DMAP (1.2 g, 10 mmol) in DMF (250
o
mL) was added under nitrogen and the reaction was allowed to proceed at 40 C for 68 h. The
resulting product was filtered, washed with methanol, and dried under vacuum at 60°C for 24 h
to get MWCNTꢀgꢀCTBN (645 mg).

Page 7 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Figure 1: Schematic showing the preparation of (a) 2ꢀ(2ꢀazidoethoxy)ethanol and (b) MWCNTꢀ
gꢀCTBN from pristine MWCNTs
2.3 Preparation of MWCNT/epoxy nanocomposite
MWCNT/epoxy nanocomposites containing different compositions of nanotube were
prepared as follows. Epoxy resin was sonicated with required amount nanotubes in acetone for
o
30 minutes. Acetone was evaporated off by heating at 90 C for 2 hours. This MWCNT/epoxy
nanosuspension was mixed with molten hardener in the 100:35 ratio and degassed for 5 minutes
in a vacuum oven. The mixture was then transferred into pre heated mold and cured at 180 oC for
4 h. Post curing was done at 200 oC for 1 h.
2.4 Characterization techniques
Fourier transform infrared spectroscopy (FTIR)
Fourier transform infrared spectroscopy (FTIR) analysis was carried out using a Perkin
Elmer System series 100 spectrophotometer in a frequency range of 4000–500 cmꢀ1 with a
spectral resolution of 4 cmꢀ1.
Nuclear magnetic resonance spectroscopy (NMR)
NMR spectra of 2ꢀ(2ꢀazidoethoxy)ethanol dissolved in CDCl3 were recorded on a Bruker
AVANCE IIꢀ500 spectrometer. The spectra were internally referenced to a tetramethylsilane
(TMS) standard.

RSC Advances
Page 8 of 31
View Article Online
DOI: 10.1039/C6RA00080K
X-ray photoelectron spectroscopy (XPS)
XPS was carried out with a Kratos Axis Ultra DLD spectrometer, using Al Kα excitation
radiation.
Raman spectroscopy
Raman spectra were recorded from 100 to 3000 cmꢀ1 on a Raman spectrometer (INVIA,
England) with a 514 nm argon ion laser.
High resolution transmission electron microscopy (HRTEM)
The highꢀresolution transmission electron microscopy (HRTEM) was conducted using JEOL
JEMꢀ2100 with an acceleration voltage of 200 kV being equipped with an EDX spectrometer.
Sliced thin sections of MWCNT/epoxy composites with a thickness of about 60ꢀ80 nm, prepared
by ultraꢀmicrotomy, were used to take the TEM images of the composites.
Transmission optical microscopy (TOM)
Optical microscopy analysis was carried out with Leica DM1000 LED (Leica Microsystems,
Germany) in transmitted light configuration. The analysis was done on a small droplet of the
epoxy suspension placed on a microscope glass.
Rheology
Rheological analysis was done using modular rheometer (MCR102, Anton Paar, USA), using
a 50 mm parallel plate assembly at room temperature.
Scanning electron microscopy (SEM)
The morphology of a fractured surface of the sample was analyzed using Scanning electron
microscope (SEM, FEI Quanta FEG200). Prior to analysis, sample is sputter coated with gold to
make it conducting surface.
Mechanical measurements
The tensile strength of the sample was measured using universal testing machine (Instron
5984, Instron, USA) at a crosshead speed of 1mm/min (as per ASTM standard D638).Fracture
toughness of the specimens was determined according to ASTM D 5045ꢀ99. Single edge notch
specimens of 46 × 6 × 3mm3 (span length =24 mm) were used to measure the fracture toughness
of the epoxy nanocomposites. A notch of 2.7 mm was made at one edge of the specimen. A
natural crack was made by pressing a fresh razor blade into the notch. The fracture toughness
was expressed as stress intensity factor (KIC) calculated using equation

Page 9 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
L
KIC
=
f (x)
ꢀꢀꢀꢀꢀꢀ (1)
ꢀꢀꢀꢀꢀꢀ (2)
BW 0.5
6x0.5 1.99 − x 1− x 2.15−3.93x − 2.7x
2


(
)
(
)(
)


where 0 < x < 1 and f (x) =
1.5
1+ 2x 1− x
(
)
and L is the load at crack initiation, B is the specimen thickness, W is the specimen width, a is
the crack length and x = a /W.
Dynamic mechanical thermal analyzer (DMA)
The investigation of viscoꢀelastic properties was performed using dynamic mechanical
thermal analyzer (DMA 800, Perkin Elmer, USA). Rectangular specimens of 20 x 5 x 2 mm3
were used for the analysis. The analysis was done in single cantilever mode at a frequency of 1
Hz, from ambient to 250 oC and at a heating rate of 2 oC/min.
Thermogravimetric analyzer (TGA)
Thermal stability of nanocomposites was analyzed using a thermogravimetric analyzer (Qꢀ
50, TA Instruments, USA). The samples were heated from ambient to 800 oC at a ramp rate of 10
oC/ min.
3 Results and discussions
3.1 Characterization of 2-(2-azidoethoxy)ethanol
Figure 2 shows the FTIR spectra of (a) 2ꢀ(2ꢀchloroethoxy)ethanol and (b) 2ꢀ(2ꢀ
azidoethoxy)ethanol. 2ꢀ(2ꢀchloroethoxy)ethanol shows a characteristic absorption peak of CꢀCl
at around 745 cmꢀ1. Apart from this, a broad band centered around 3400 cmꢀ1, referred to the
stretching of OH groups, as well as peaks at 2870 and 2930 cmꢀ1, attributed to the C–H stretching
vibrations were also observed. The peak at 1121 cmꢀ1 of chloroethoxyethanol is due to the CꢀO
stretching. In the FTIR spectrum of 2ꢀ(2ꢀazidoethoxy)ethanol, the disappearance of the CꢀCl
stretch after the reaction with sodium azide and the appearance of the characteristic absorption of
azido groups at ca. 2094 cmꢀ1 indicates that most of the chloride groups have been substituted
with azide groups. Furthermore, a band appears at 1284 cmꢀ1 referring to the C–N stretching
vibration.

RSC Advances
Page 10 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 2: FTIR spectra of (a) 2ꢀ(2ꢀchloroethoxy)ethanol and (b) 2ꢀ(2ꢀazidoethoxy)ethanol.
3.1. Characterization of grafting of CTBN on multi-walled carbon nanotube
XPS was employed to evaluate the chemical bonds formed on the surface of nanotube before
and after its functionalization with CTBN. Figure 3 shows the survey data of the pristine and
chemically modified nanotube samples. Pristine MWCNTs exhibit a strong peak at 285 eV due
to C 1s and a peak of very low intensity at 532 eV due to O1s from the defects of nanotube. After
functionalization, both MWCNTꢀOH and MWCNTꢀgꢀCTBN show a significant increase in the
O1s peak and a new peak at 400 eV is observed (N1s). This increased intensity of elements is
originated from the presence of organic moieties which confirmed the success of the
modification. The higher resolution data of C1s area of the CNTs, MWCNTꢀOH and MWCNTꢀ
gꢀCTBN are shown in Figure 4a, 4b, and 4c respectively. For pristine MWCNT, the main peak at
284.5 eV is attributed to the sp2 hybridized graphiteꢀlike carbon atoms (C=C), a peak at 285.1 eV
corresponds to sp3 hybridized carbon atoms arising from defects on the nanotube structure (CꢀC),
a peak at 286.2 eV related to carbon–oxygen single bonds in alcohols, phenols and ethers (C–O),
a peak at 288.9 eV attributed to carbon–oxygen double bonds in carboxylic acids, carboxylic

Page 11 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
anhydrides and esters (OꢀC=O) and finally a peak at 291.6 eV, the typical position of the π–π*
shakeꢀup satellite peak from the sp2ꢀhybridised carbon atoms13, 34, 35. In the C1s spectra of the
MWCNTꢀOH, the peak intensity of hydroxyl groups is much higher than that in pristine
MWCNT, indicating the reaction has occurred between nanotube and the organic moiety. The
disappearance of π–π* shakeꢀup transition can be ascribed to the increased disruption of the πꢀ
electron system, indicating a significant change in the electronic structure of the CNT sidewalls.
In the case of C1s high resolution spectrum of MWCNTꢀgꢀCTBN, the significant intensity
increment of the band at 288.9 eV (corresponds to OꢀC=O groups) suggests the covalent grafting
of MWCNT with CTBN. Deconvolution of the N 1s spectrum shown in Figure 4d displays two
contributions with binding energies of 399.6 eV and 401 eV, respectively. The former is ascribed
to the C≡N from the polymer, while the latter is due to CꢀN, suggesting the successful covalent
functionalization of MWCNT by CTBN36.
pristine MWCNT
MWCNT OH
C1s
MWCNT-g-CTBN
O1s
N1s
100 200 300 400 500 600 700 800
Binding energy (eV)
Figure 3: XPS survey spectra of pristine MWCNT, MWCNTꢀOH and MWCNTꢀgꢀCTBN

RSC Advances
Page 12 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 4: High resolution C1s spectra of (a) pristine MWCNT, (b) MWCNTꢀOH and (c)
MWCNTꢀgꢀCTBN; (d) High resolution N1s spectra of MWCNTꢀgꢀCTBN.
The FTIR spectrum of the MWCNTꢀOH shows the characteristic absorption bands due to
stretching vibration of the CꢀO at 1075 cmꢀ1 (Figure S3, ESI). In addition, the MWCNTꢀOH
shows absorption peaks between 2920ꢀ2860 cm−1 which is related to the CꢀH stretching
absorption band. On the other hand, MWCNTꢀgꢀCTBN exhibits three characteristic peaks
related to vibrations of the CTBN: the peak at 963 cmꢀ1 corresponds to =C−H out of plane
bending vibration of 1, 4 trans olefin in CTBN, peak at 1638 cmꢀ1 is due to C=C stretching and
peak at 1715 cmꢀ1 corresponds to C=O stretching of carbonyl group in CTBN. Presence of small
stretching band at 1740 cmꢀ1 clearly reveals the ester bands that came from the CTBN grafted to
the MWCNT.

Page 13 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Because of its high sensitivity to the structural change, Raman spectroscopy is commonly
used to characterize the structural and electronic properties of carbonꢀbased materials. Raman
spectroscopy of three different MWCNTs is shown in the Figure 5a. Raman spectrum of the
carbon nanotube is usually characterized by three main features. The band at around 1572 cmꢀ1 is
from the inꢀplane vibrations of sp2ꢀhybridized graphitic carbon, and hence is called the tangential
or Gꢀband. Defects and functional groups on the walls or ends of the CNTs, or amorphous
carbon give rise to the soꢀcalled disorder (D) band corresponding to sp3ꢀcarbon, which is located
around 1340 cmꢀ1. The D’ band which is a weak shoulder of the Gꢀband at higher frequencies is
also a double resonance feature induced by disorder and defects 13. The intensity area ratio of the
D to G bands, ID/IG, can serve as a standard to measure the defects of CNTs. The bigger the ratio,
the greater is the defect which in turn indicates the presence of more groups on the surface of
MWCNTs. The values of ID/IG for MWNTꢀOH (1.87) and MWCNTꢀgꢀCTBN (1.89) are greater
than that of pristine MWNTs (1.60), which indicates the increase in defects in MWCNTs due to
the chemical functionalization. The sequential functionalization of MWCNTꢀgꢀCTBN did not
result in the obvious increase in the Dꢀband intensity, which likely because the polymer was not
directly linked on the carbons of CNTs. Similar phenomena has been previously reported27.
Figure 5b shows representative TGA thermograms under nitrogen atmosphere of the pristine
MWCNT, MWCNTꢀOH and MWCNTꢀgꢀCTBN. Due to the differences in the thermal stability
of the carbon nanotube structure and the polymer moieties, by TGA analysis it is possible to gain
a reliable estimation of the relative amount of introduced functionalities. The pristine MWCNTs
o
are thermally stable up to about 550 C. For the MWCNTꢀOH, however, a slight decrease in
o
weight was observed at approximately 200 C, corresponding to decomposition of organic
groups. In the case of MWCNTꢀgꢀCTBN, the weightꢀloss region from 250 °C to 450 °C, has a
weight loss of 50 wt % that is caused by the pyrolysis of the CTBN polymer. With the weight
o
loss of the pure CTBN below 500 C as a reference, the concentration of CTBN in MWCNTꢀgꢀ
CTBN was estimated as 38 %.

RSC Advances
Page 14 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 5: (a) Raman spectra and (b) TGA of pristine MWCNT, MWCNTꢀOH and MWCNTꢀgꢀ
CTBN
The microstructures of pristine MWCNTs and MWCNTꢀgꢀCTBN were further characterized
by HRTEM. Figure 6a is the image of the pristine MWCNTs, in which the tube surface is
relatively smooth and clean. The TEM microphotograph of MWCNTꢀgꢀCTBN shows that the
surface of MWCNT is fully enwrapped with the amorphous polymer which is quite continuous
all along the CNTs (Figure 6b), implying that the covalent attachment of CTBN had been
achieved. The thickness of the wrapped polymer layer is about 10 to 20 nm.
Figure 6: TEM images of (a) pristine MWCNTs and (b) MWCNTꢀgꢀCTBN

Page 15 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
3.2 Dispersion behavior of nanosuspension and composite
3.2.1 Transmission Optical Microscopy (TOM)
TOM observation of dispersion level of CNTs in epoxy is useful to examine changes in
dispersion state and interfacial bonding affected by the surface modification. TOM observations
of the epoxy suspensions revealed the presence of MWCNTs aggregates. There were some zones
with very high local MWCNT concentrations, as shown in Figure 7. In fact, the pristine
MWCNT samples were very viscous, and the homogenization process was difficult. The
agglomerates that result from the existence of strong van der Waals interactions between
individual tubes together with large aspect ratio of CNTs is due to which nanotubes exists as a
highly entangled network. By contrast, after the surface functionalization with CTBN molecules,
the dispersion of MWCNTꢀgꢀCTBN becomes relatively better although some small agglomerates
are still observed. The reason for the differences in the MWCNT dispersion between both
pristine and MWCNTꢀgꢀCTBN was the uncoiling of nanotube during functionalisation and
enhanced interfacial interaction between the nanotube and resin. The functionalisation seems to
be favorable for the dispersion for the MWCNT in epoxy matrix.

RSC Advances
Page 16 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 7: TOM images of epoxy nanosuspensions containing (a) 0.2 wt % MWCNT, (b) 0.2 wt
% MWCNTꢀgꢀCTBN, (c) 0.4 wt% MWCNT and (d) 0.4 wt% MWCNTꢀgꢀCTBN. Insets show
high magnification images
3.2.2 Rheology
The rheological properties of the MWCNT/epoxy dispersions play a crucial role in the
processing and mechanical features of epoxy nanocomposites. Plots of viscosity vs shear rate of
the pure epoxy, the pristineꢀMWCNT/epoxy composites, and the surface modified MWCNT/
epoxy composites are shown in Figure 8a to demonstrate the significance of viscosity build up
and shear thinning in the epoxy nanosuspension system. The addition of pristine MWCNTs to
epoxy resulted in an abrupt increase in viscosity at low shear rates. The pristineꢀMWCNTs were
composed of bundles of nanotubes which are highly entangled. These nanotubes exist as many
aggregates of different sizes in the epoxy resin. The aggregates would be obstacles to uniform
dispersion of the MWNTs and were hardly broken into individual tubes in the epoxy resin. The
dispersion and homogenization was very difficult. The viscosity of MWCNTꢀgꢀCTBN epoxy
suspension is observed to be much lower than that of MWCNT/epoxy suspension at low shear

Page 17 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
rates. This arises from a better dispersion of functionalized MWNTs within the epoxy resin
suspensions. At high shear rates, both nanoꢀfillers show good dispersion due to disentanglement
of nanotubes at very high shear forces.
PristineꢀMWCNT/epoxy and MWCNTꢀgꢀCTBN/epoxy nano suspension showed a shear
thinning effect while neat epoxy resin showed a near Newtonian behavior. The epoxy composites
filled with the poorly dispersed CNTs exhibit stronger nonꢀNewtonian behavior than ones with
the well dispersed CNTs. The shear thinning effect of the suspensions were quantified by
calculating the pseudoꢀplasticity index n. The shear dependency of the viscosity of solutions is
usually given by the power law37.
τ
=
ηγ n
ꢀꢀꢀꢀꢀꢀ (3)
Experimental data were fitted by the power law equation for different epoxy nano suspension
systems. A typical plot between shear rate (log γ) and shear stress (log τ) is presented in Figure
8b for epoxy resin, pristine MWCNT/epoxy nanosuspension and MWCNTꢀgꢀCTBN/epoxy
nanosuspensions. The slope of the plots of shear rate (log γ) vs shear stress (log τ) provided the
pseudo plasticity index (n). Usually, n is in the range of 0 to 1. At n=1, the equation reduces to
the constitutive equation of a Newtonian fluid. When n<1, shear thinning behavior is observed.
The lower the n value, the more pronounced is the shear thinning behavior. As seen from data in
Table 1, the pseudoꢀplasticity index n of the solution decreases by the addition of CNTs. On
adding pristine CNTs, the n value shows an abrupt decrease. After chemical modification of
CNTs, shear thinning behaviour is observed but it is less pronounced; in other words, after
chemical modification of CNTs, the extent of Newtonian plateau is greater than with the pristine
CNT epoxy suspension.

RSC Advances
Page 18 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Table 1: Rheological data of MWCNT/epoxy nanosuspension
Sample
Viscosity at shear Flow index (n)
rate of 1 s-1 (Pa*s)
4.07
17.6
10.3
174
0.99
0.73
0.84
0.62
0.73
Epoxy resin
0.2 wt % MWCNT
0.2 wt % MWCNT-g-CTBN
0.4 wt % MWCNT
34.7
0.4 wt % MWCNT-g-CTBN
Figure 8: Variation of (a) viscosity with shear rate and (b) shear stress with shear rate for epoxy
nano suspension of MWCNT and MWCNTꢀgꢀCTBN
3.2.3 TEM micrographs of epoxy composites
TEM images of MWCNT modified epoxy and MWCNTꢀgꢀCTBN modified epoxy
composites are shown in Figure 9. In the pristine MWCNT/epoxy composite, agglomerates of
MWCNTs were observed. On the other hand, a significant improvement in the dispersion was
observed by the grafting of CTBN with MWCNTs.

Page 19 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Figure 9: TEM images of epoxy nanocomposites containing (a) 0.2 wt % MWCNT and (b) 0.2
wt % MWCNTꢀgꢀCTBN
3.3 Tensile strength of epoxy composites
The tensile properties of the composites as a function of nanotube concentrations are plotted
in Figure 10 and also summarized in Table 2. Representative tensile stress versus strain curves
are shown in Figure 10a. The tensile strength of nanocomposites increased initially with
increasing the nanotube content and attained the maximum value at the 0.2 wt % nanotube
content corresponding to an increase of 11 % compared with the pure epoxy. The decreased
tensile strengths beyond 0.2 wt % might be probably due to the aggregation of the nanotube
which can result in the stresses concentrating at the aggregation points. By comparison, the
composites containing the MWCNTꢀgꢀCTBN exhibit higher strength and modulus values than
the pristine MWCNTs over the whole filler contents studied. For epoxy composite with 0.3wt%
MWCNTꢀgꢀCTBN, the tensile modulus and strength increased by 17 % (2.21 ± 0.02GPa)
and25% (85.4 ± 3.1 MPa), respectively. In order to compare this experimental modulus with
theoretical value, a modified HalpinꢀTsai model was used. The predicted modulus of the
MWCNT/epoxy composite using modified HalpinꢀTsai equation given by10,
1+ϕηυf
1−ηυf
Ef / Em −6
E =
Em
η =
;
ꢀꢀꢀꢀꢀꢀ (4)
Ef / Em + 6ϕ

RSC Advances
Page 20 of 31
View Article Online
DOI: 10.1039/C6RA00080K
where E, Em and Ef are moduli of composite, matrix and carbon nanotube, respectively; υf is the
volume fraction of the filler, φ is the shape factor which is equal to twice the aspect ratio of the
filler.
Figure 10c shows a comparison of the variation in theoretical and experimental tensile
modulus values of composites with weight fraction of MWCNTs. It is found that the results
obtained from the modified Halpin–Tsai equation on Young’s moduli fit successfully the
experimental ones at low concentration and the predicted values are slightly higher than the
experimental ones for some compositions of MWCNT/epoxy composites. The discrepancy is
caused by the aggregation of MWCNTs. However, the experimental tensile modulus of the
MWCNTꢀgꢀCTBN/epoxy composite is found to be comparable to the modulus predicted using
the HalpinꢀTsai modeling. It means that a high reinforcing effectiveness is achieved by
introduction of MWCNTꢀgꢀCTBN as reinforcement. In this case, more homogeneous dispersion
and a better interface between the nanotubes and the epoxy matrix resulted in better mechanical
properties. In the MWCNTꢀgꢀCTBN/epoxy composites, terminal carboxyl groups of CTBN react
with epoxy matrix during curing reactions, which provide interfacial adhesion for load transfer
between epoxy and nanotubes, and therefore, mechanical properties of the composites are
enhanced. Another reason for the observed behavior is the presence of voids which are
developed during the mixing of hardener with the MWCNT/epoxyꢀsuspension via stirring (see
the blue circles in Figure 12e). The high viscosity disabled degassing of the nanocomposite
samples with voids remaining in the matrix.

Page 21 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Figure 10: (a) Representative stressꢀstrain curves (b) tensile strength and (c) tensile modulus of
epoxy nanocomposites containing pristine MWCNTs and MWCNTꢀgꢀCTBN.
3.4 Fracture toughness of epoxy composites
The ability of a material to resist fracture is described as fracture toughness and is expressed
in critical stress intensity factor (KIC). KIC values of neat epoxy, pristine MWCNT, and
MWCNTꢀgꢀCTBN epoxy nanocomposites are given in Table 2. The addition of MWCNT and
MWCNTꢀgꢀCTBN into the epoxy matrix resulted in an improvement in fracture toughness
(Figure 11). Neat epoxy showed the fracture toughness of 0.74 ± 0.8 MPa.m1/2, which increased
up to 1.35 ± 0.11 MPa*m1/2 (82% improvement) by adding 0.2 wt.% pristine MWCNTs.
Composites with MWCNTꢀgꢀCTBN improved the fracture toughness of epoxy significantly,
achieving an 117% improvement at 0.3 wt. % filler loading. This is followed by a decrease in the
value of KIC at higher MWCNT concentrations due to the presence of agglomerates and defects.
The decrease in value of KIC at 0.4 wt % in the case of MWCNTꢀgꢀCTBN modified epoxy might
be due to the presence of agglomerates.

RSC Advances
Page 22 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 11: Fracture toughness of epoxy nanocomposites containing pristine MWCNTs and
MWCNTꢀgꢀCTBN.
Table 2: Fracture toughness and tensile properties of epoxy nanocomposites
SI
Sample ID
KIC
Tensile
Tensile
Modulus at break
(GPa) (%)
Elongation
No
(MPa*m1/2) strength
(MPa)
Neat epoxy
0.74 ± 0.08 68.4 ± 1.7
1.14 ± 0.07 69.4 ± 3.0
1.35 ± 0.11 76.1 ± 2.8
1.15 ± 0.01 71.6 ± 1.8
1.12 ± 0.05 68.4 ± 3.7
1.39 ± 0.08 74.1 ± 4.1
1.47 ± 0.01 79.9 ± 2.6
1.62 ± 0.06 85.4 ± 3.1
1.24 ± 0.07 76.5 ± 2.5
1.89 ± 0.04 5.66 ± 0.63
1.92 ± 0.02 6.01 ± 0.88
1.98 ± 0.01 6.53 ± 1.07
2.01 ± 0.03 6.09 ± 0.99
1.93 ± 0.03 5.75 ± 0.41
1.96 ± 0.02 6.12 ± 0.11
2.05 ± 0.03 7.05 ± 0.40
2.21 ± 0.02 6.76 ± 0.31
2.07 ± 0.03 6.96 ± 0.88
1
2
3
4
5
6
7
8
9
0.1wt% MWCNT
0.2wt% MWCNT
0.3wt% MWCNT
0.4wt% MWCNT
0.1wt% MWCNTꢀgꢀCTBN
0.2wt% MWCNTꢀgꢀCTBN
0.3wt% MWCNTꢀgꢀCTBN
0.4wt% MWCNTꢀgꢀCTBN

Page 23 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
The toughening mechanisms can be understood by observing the representative HRSEM
images of the fracture surface of the neat epoxy and MWCNTs/epoxy composite with 0.3 wt %
MWCNTs resulting from the fracture testing of the samples. Neat epoxy resin as shown in
Figure 12a exhibits a relatively smooth fracture surface, which indicates a typical fractography
feature of brittle fracture behavior, thus revealing low fracture toughness of the neat epoxy.
Compared to the case of neat epoxy, the fracture surfaces of the nanocomposites show
considerably different fractographic features. The rough surface of the composites containing
MWCNTs as seen in the Figure 12b is most likely the result of crack deflection produced by the
interaction of nanofillers and the epoxy matrix. Well dispersed nanotubes with proper adhesion
to the epoxy could resist crack propagation and lead to crack deflection. Apart from crack
deflection, pullout of nanotubes, debonding of MWCNTs and bridging mechanism also plays a
significant role in improving the fracture toughness of composites. At higher magnification,
some pulled out CNTs can be observed on the fracture surface (see Figure 12d and 12f) in the
MWCNTꢀgꢀCTBN/epoxy composites. These agglomerations can be responsible for reduction of
tensile strength and KIC of nanocomposite in 0.4 wt% of MWCNTs which was mentioned earlier.

RSC Advances
Page 24 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 12: FESEM images of fractured surface of sample: (a) neat epoxy (b) 0.3 wt %
MWCNT/epoxy (c) 0.3 wt % MWCNTꢀgꢀCTBN/epoxy (d) 0.3 wt % MWCNTꢀgꢀCTBN/epoxy
at high magnification (e) 0.4 wt % MWCNTꢀgꢀCTBN/epoxy and (f) 0.4 wt % MWCNTꢀgꢀ
CTBN/epoxy at high magnification

Page 25 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
3.5 Dynamic mechanical analysis of epoxy composites
The effect of surface MWCNT functionalization on the viscoelastic performance of
nanocomposites was investigated by DMA analysis by comparing the storage modulus, and tan δ
values with those of neat epoxy. The storage modulus (E’) and tan δ were obtained by DMA in a
range between 30 and 250 oC for all samples, and the results are shown in Figure 13a, b, c and d.
Addition of pristine nanotubes into epoxy resulted in an improvement in the storage modulus. In
the glassy state, a maximum improvement of 14 % from 1995 MPa to 2269 MPa in the storage
modulus is observed by the addition of 0.2 wt% pristine MWCNTs to epoxy matrix. The epoxy
nanocomposite with MWCNTꢀgꢀCTBN nanofiller shows an enhanced storage modulus
compared with pure epoxy resin and pristine MWCNT/epoxy nanocomposite with the same filler
o
content. At 30 C, the storage modulus of the epoxy nanocomposite with 0.3 wt% MWCNTꢀgꢀ
CTBN is 2417 MPa, which is around 21% larger than that of neat epoxy resin (1995 MPa). The
storage modulus in the rubbery region is also significantly increased for epoxy/MWCNTꢀgꢀ
CTBN, which is believed to be due to improved dispersion and the strong interfacial adhesion
with epoxy which ensures an efficient load transfer at the interface. A reduction in Tg was
observed by the addition of pristine nanotubes. The lowered Tg of MWCNT/epoxy composite is
possibly due to either lower crosslink density of epoxy or poor adhesion between MWCNTs and
epoxy, or both. After adding the MWCNTꢀgꢀCTBN, CTBN grafting provides carboxylic acid
groups that can react with the oxirane rings of epoxy resin to form a strong bonding between
matrix and MWCNTs, leading to a higher crossꢀlinking density of the composites. This results in
a confinement of the fillerꢀmatrix interface and reduce the chain mobility38, 39. As a result, the
increase of Tg occurs.

RSC Advances
Page 26 of 31
View Article Online
DOI: 10.1039/C6RA00080K
Figure 13: (a) and (b) Storage modulus curves for pristine MWCNT and MWCNTꢀgꢀCTBN
epoxy composites respectively and (c) and (d) tan delta versus temperature curves for pristine
MWCNT and MWCNTꢀgꢀCTBN epoxy composites
3.6 TGA of epoxy composites
Thermal stability of the epoxy composites was traced using TGA. The thermal stability of
epoxy composites with 0.3 wt % loading of pristine MWCNTs and MWCNTꢀgꢀCTBN were
compared with neat epoxy system in Figure 14. The thermal stability of epoxy matrix was not
affected by the addition of nanotubes. As shown in the figure, the main weight loss for the
o
composites takes place at around 320 C, which is attributed to the degradation of the epoxy
network. This means that all the composites prepared are very stable and show little degradation
below 320 oC and therefore can be used for many highꢀtemperature applications31,40
.

Page 27 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
Figure 14: TGA curve of epoxy nanocomposites
4. Conclusion
Multiꢀwalled carbon nanotubes were surfaceꢀmodified with CTBN by sideꢀwall
functionalisation and dispersed homogeneously throughout the epoxy resin system. A systematic
study has been conducted to investigate the thermoꢀmechanical and viscoelastic properties of
epoxy nanocomposites prepared by the introduction of MWCNTꢀgꢀCTBN into epoxy resin. The
study indicates that the side wall functionalisation of nanotube is a good route to achieve higher
mechanical properties of the composites. It has been possible to obtain an improvement of
~117% in fracture toughness and 25 % improvement in tensile strength in the CTBN
functionalized MWCNT based epoxy composites with only 0.3 wt. % loading. The mechanism
of fracture behavior was also studied using SEM, which reveals that crack deflection, pullout of
nanotubes, debonding of MWCNTs and bridging mechanism plays an important role in
improving the fracture toughness of composites. The surface morphology reveals improved
interfacial bonding between the filler/matrix. Therefore, MWCNTꢀgꢀCTBN modified composites
are able to carry higher level of loading during mechanical testing. The nanocomposites also
exhibited an increase in the storage modulus and glass transition temperature compared to those
of the neat epoxy resin. This improvement in Tg is due to the hindered polymer chain mobility
near the filler/matrix interface. TGA analysis indicates that the presence of nanotubes has not

RSC Advances
Page 28 of 31
View Article Online
DOI: 10.1039/C6RA00080K
deteriorated the thermal stability of the epoxy matrix. To conclude, MWCNTꢀgꢀCTBN is found
to be an effective reinforcement for epoxy system for high performance applications.
Acknowledgements
R. K wishes to hereby thank IIST for the research fellowship. The authors would like to
acknowledge Dr. C. P. Reghunadhan Nair, PSCG, Vikram Sarabhai Space Centre,
Thiruvananthapuram for valuable discussions and suggestions. The authors also acknowledge
Sophisticated Analytical Instrument Facilities (SAIF), IIT Madras for the SEM analysis and
Vikram Sarabhai Space Centre, Thiruvananthapuram for mechanical testing.
References
1.
N. George, J. Chandra, A. Mathiazhagan and R. Joseph, Compos. Sci. Technol., 2015,
116, 33ꢀ40.
2.
3.
S. Gong, Z. Zhu and S. Meguid, Polym. J., 2015, 56, 498ꢀ506.
R. M. Kumar, S. K. Sharma, B. M. Kumar and D. Lahiri, Composites Part A, 2015, 76,
62ꢀ72.
4.
5.
6.
Y. Kuang and B. Huang, Polym. J., 2015, 56, 563ꢀ571.
E. T. Thostenson, Z. Ren and T.ꢀW. Chou, Compos. Sci. Technol., 2001, 61, 1899ꢀ1912.
A. Montazeri, J. Javadpour, A. Khavandi, A. Tcharkhtchi and A. Mohajeri, Mater.Des.,
2010, 31, 4202ꢀ4208.
7.
J. N. Coleman, U. Khan, W. J. Blau and Y. K. Gun’ko, Carbon, 2006, 44, 1624ꢀ1652.
K. Yang, M. Gu, Y. Guo, X. Pan and G. Mu, Carbon, 2009, 47, 1723ꢀ1737.
F. H. Gojny and K. Schulte, Compos. Sci. Technol., 2004, 64, 2303ꢀ2308.
L. Sun, G. Warren, J. O’reilly, W. Everett, S. Lee, D. Davis, D. Lagoudas and H.ꢀJ. Sue,
Carbon, 2008, 46, 320ꢀ328.
8.
9.
10.
11.
Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis, Prog. Polym. Sci., 2010, 35, 357ꢀ
401.
12.
13.
D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006, 106, 1105ꢀ1136.
V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis and
C. Galiotis, Carbon, 2008, 46, 833ꢀ840.

Page 29 of 31
RSC Advances
View Article Online
DOI: 10.1039/C6RA00080K
14.
15.
M. Yoonessi, M. LebrónꢀColón, D. Scheiman and M. A. Meador, ACS Appl. Mater.
Interfaces, 2014, 6, 16621ꢀ16630.
J. M. GónzalezꢀDominguez, M. Gonzalez, A. AnsonꢀCasaos, A. DiezꢀPascual, M. Gomez
and M. Martinez, J. Phys. Chem. C, 2011, 115, 7238ꢀ7248.
16.
17.
J. L. Bahr and J. M. Tour, Chem. Mater., 2001, 13, 3823ꢀ3824.
M. Lavorgna, V. Romeo, A. Martone, M. Zarrelli, M. Giordano, G. Buonocore, M. Qu,
G. Fei and H. Xia, Eur. Polym. J., 2013, 49, 428ꢀ438.
18.
H. Gu, S. Tadakamalla, X. Zhang, Y. Huang, Y. Jiang, H. A. Colorado, Z. Luo, S. Wei
and Z. Guo, J. Mater. Chem. C, 2013, 1, 729ꢀ743.
19.
20.
21.
22.
23.
O. Zabihi, M. Ahmadi and M. Naebe, RSC Adv., 2015, 5, 98692ꢀ9.
B. Wang, Z. Wang, F. Jiang, H. Fang and Z. Wang, RSC Adv., 2014, 4, 26468ꢀ75.
B. Massoumi, V. BadrꢀValizad, M. Jaymand, RSC Adv., 2015, 5, 40840ꢀ8.
K. Hayashida and Y. Matsuoka, Carbon, 2015, 85, 363ꢀ371.
M. Ilčíková, M. Mrlík, T. Sedláček, D. Chorvát, I. Krupa, M. Šlouf, K. Koynov and J.
Mosnáček, Polym. J., 2014, 55, 211ꢀ218.
24.
25.
A. E. Daugaard, K. Jankova and S. Hvilsted, Polym. J., 2014, 55, 481ꢀ487.
S. Roy, T. Das, Y. Ming, X. Chen, C. Y. Yue and X. Hu, J. Mater. Chem. A, 2014, 2,
3961ꢀ3970.
26.
A. M. DíezꢀPascual, G. Martínez, J. M. GonzálezꢀDomínguez, A. Ansón, M. T. Martínez
and M. A. Gómez, J. Mater. Chem., 2010, 20, 8285ꢀ8296.
27.
28.
R. Konnola, C. Nair and K. Joseph, Polym. Adv. Technol., 2016, 27, 82ꢀ89.
M. B. Jakubinek, B. Ashrafi, Y. Zhang, Y. MartinezꢀRubi, C. T. Kingston, A. Johnston
and B. Simard, Composites Part B, 2015, 69, 87ꢀ93.
29.
30.
31.
32.
L. Guadagno, M. Raimondo, V. Vittoria, L. Vertuccio, C. Naddeo, S. Russo, B. De Vivo,
P. Lamberti, G. Spinelli and V. Tucci, RSC Adv., 2014, 4, 15474ꢀ88.
Y. MartinezꢀRubi, B. Ashrafi, J. Guan, C. Kingston, A. Johnston, B. Simard, V. Mirjalili,
P. Hubert, L. Deng and R. J. Young, ACS Appl. Mater. Interfaces, 2011, 3, 2309ꢀ2317.
P. Jyotishkumar, E. Abraham, S. M. George, E. Elias, J. Pionteck, P. Moldenaers and S.
Thomas, J. Appl. Polym. Sci., 2013, 127, 3093ꢀ3103.
B. S. Sumerlin, N. V. Tsarevsky, G. Louche, R. Y. Lee and K. Matyjaszewski,
Macromolecules, 2005, 38, 7540ꢀ7545.