Polymer nanocomposites using click chemistry: Novel materials for hydrogen peroxide vapor sensors

Article abstract of DOI:10.1039/c5ra10952c

Herein, we report the synthesis of nanocomposites (NCs) prepared by azide-functionalized polystyrene (Az-f-PS) coupled with alkyne-functionalized nanographite platelets (Alk-f-NGPs) by using copper(i) catalyzed azide-alkyne click chemistry. Linear polysty

Full text of DOI:10.1039/c5ra10952c

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Polymer Nanocomposites using Click Chemistry: Novel Materials
for Hydrogen Peroxide Vapor Sensors
Received 00th January 20xx,
a
a
b,c
Payal Mazumdar, Sunita Rattan,
* Monalisa Mukherjee
Herein, we report the synthesis of nanocomposites (NCs) prepared by azide-functionalized polystyrene ( Az-f-PS)
coupled with alkyne-functionalized nanographite platlets (Alk-f-NGPs) by using copper(I) catalyzed azide–alkyne click
chemistry. Linear polystyrene (PS) was functionalized with azide moiety after bromine-termination of PS through atom
transfer radical polymerization (ATRP) using CuBr/2, 2’-bipyridyl as catalyst. The nanographite platelets (NGPs) were
formed from graphite flakes through intercalation followed by functionalized with alkyne moiety. The structure and
micromorphology of prepared NCs were confirmed by NMR, FTIR, Raman, XRD, TGA, SEM, TEM and AFM techniques. The
electrical properties of the click nanocomosites (C-NCs) were investigated and the C-NCs were found to posses the
resistance of 1.08E+08 at 1wt% filler (NGPs) loading. The C-NCs with fast response (~3s), rapid recovery (~60s) and
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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excellent repeatability at room temperature provide novel materials for chemiresistive sensors for detection of H
2 2
O
vapors.
electronics, sensor, field emission devices and many others
2
,4,10
have been extensively reported.
At present, NCs
1
Introduction
employing carbon-based reinforcement materials are
Polymer materials are widely used in industry due to their ease dominated by carbon nanotubes but the intrinsic bundling of
1
of production, lightweight and often ductile nature. However, carbon nanotubes, their intrinsic impurities from catalysts and
the disadvantages such as low modulus and strength their high cost have been hampering their application. Hence,
compared to metals and ceramics limit their applications in graphene and NGPs provide low-cost alternative to CNTs due
various fields. In this context, a very effective approach to to their excellent properties similar to CNTs and natural
1
1,12
improve electrical and mechanical properties is to add fibres, abundance of their precursor graphite.
whiskers, platelets or particles as reinforcements to the carbon nanotubes, less work has been done on NGPs although
Compared to
2
polymer matrix. A variety of fillers have been considered over they have in-plane electrical, thermal and mechanical
1
3
the years which includes metals, silica, clay, and carbon-based properties.
3
fillers as reinforcements. Among these, a range of carbon-
The main challenge in the fabrication of these NCs lies in the
establishment of homogeneous dispersion of NGPs in polymer
matrix because the NGPs have a strong tendency to
based fillers as carbon black, fullerene, carbon nanotubes
4
-6
(
CNTs), and graphene have gained prominence.
1
4
In recent years, there has been increasing interest in the field aggregate. NGPs usually form stabilized bundles due to
of polymer nanocomposites (NCs) which have been extensively vander waals interactions and therefore are extremely difficult
1
studied since the 1990s and reported to be the materials of to disperse and align in a polymer matrix. Many methods
5
2
1st century. NCs are a special class of materials having unique have been developed so far to assist dispersion of filler within
7
-9
properties and wide application in diverse areas.
NCs the polymer matrix such as high-power ultrasonic mixers,
1
6
17
18
possess nano dimension, unique designs and property surfactants,
combinations that are not found in conventional composites. polymerization,
melt mixing,
solution mixing,
in situ
grafting to nanofillers and grafting to
However, these methods for covalent
1
9,20
21
2
2
Innumerable applications of these polymers NCs in polymer.
nanoelectronic devices, optoelectrical devices, molecular immobilization suffer from certain drawbacks like low
efficiency, less control on molecular weight of polymer and
2
reaction condition.
3
a.
Amity institute of Applied Sciences, Amity University, Noida, Uttar Pradesh,
Sector-125, Noida. Email: srattan@amity.edu
Amity Institute of Click Chemistry Research and Studies, Amity University, Noida,
Uttar Pradesh, Sector-125, Noida.
Amity institute of Biotechnology, Amity University, Noida, Uttar Pradesh, Sector-
b.
c.
Recently, click chemistry, using Cu (I)-catalyzed (3+2) Huisgen
dipolar cycloaddition has received enormous attention in
polymer and material science. In particular the
functionalization of polymer matrix and fillers using click
chemistry is the recent and potential technique to achieve
125, Noida.
Electronic supplementary information (ESI) available: ES 1: GPC curves of PS-Br
†
and PS-N3, ES 2: Tabular representation of GPC data for PS-Br and PS- N
3
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1
2,24
excellent filler dispersion.
It was postulated that the use of graphite. Chemically pure concentrated sulfuric and nitric acids
click chemistry has been successfully employed in the were used as chemical oxidizers to prepare expanded graphite.
fabrication of NCs having high reactivity (in presence of all Propargyl bromide, tin (II) chloride dihydrate, isoamyl nitrite,
functional groups), reliability (not affected by O , H O, etc) and 2,2’-bipyridyl, copper (I) bromide, sodium azide and
2
2
selectivity (affording 1,4-regioselective 1,2,3-triazoles) by bromotris(triphenylphosphine)copper(I) were purchased from
2
5-27
Shengtong Sun et. al
.
‘Click’ chemistry
is predominately Sigma Aldrich, Inc. (UK) and used as received. All reagents and
based around the Huisgen cycloaddition of azide moiety with solvents of analytical grade were used as received. All
8-30
2
alkyne moiety to the surface of matrix as well as filler.
In experiments for polymerization reaction were conducted
contrast to previously reported methods, click chemistry offers under purified nitrogen by use of standard schlenk flask
.
high yielding formations at low temperatures, more control on
reaction conditions, possible choice of dipoles/dipolarophiles
with desired side groups and the characterization of reaction
intermediates. Click reaction has been extensively studied in
recent decades with steadily increasing application in the areas
of medicinal, polymer, supramolecular, coordinational
A 500 MHz Nuclear Magnetic Resonance (NMR) Bruker Avance
III spectrometer was used for 1H NMR analysis. Spectra were
o
recorded at 28 C in deuterated chloroform. The gel
permeation chromatography (GPC), Perkin Elmer, Turbo
matrix-40 was utilized to determine the molecular weight and
poly-dispersity index (PDI). THF was used as the eluent at a
3
1,32
chemistry, chemical biology and material sciences.
-1
flow rate of 1.0 mL min , and the calibration was carried out
In the present work, polystyrene (PS) was functionalized using using low polydispersity polystyrene standards. Raman spectra
-1
ATRP technique and NCs were prepared by click reaction. The were collected in the range from 3000 to 800 cm using a
combination of surface-initiated ATRP and click reaction Renishaw inVia Reflex spectrometer (UK) with an excitation
provides a novel, well controlled and efficient process of the source of 514 nm. The laser beam was focused on the sample
synthesis of a wide variety of new materials with potential using an optical microscope. FTIR spectra were recorded on a
33
uses. To our best knowledge this is the first time click Nicolet 5700 instrument in the transmission mode in the wave
-1
reaction between azide-functionalized polystyrene (Az-f-PS) number range of 400–4000 cm at room temperature.
and alkyne-functionalized NGP (Alk-f-NGPs) was used to Spectroscopic-grade KBr pellets were used for collecting the
-1
prepare NCs to find its application in chemiresistive sensor for spectra with a resolution of 4 cm , performing 32 scans. The
detecing H O vapor.
2
nature of polymer nanocomposites was confirmed by X-ray
2
diffraction (XRD) studies carried out on a D8 Advance XRD
o
Chemiresistive sensor for detection of target gases has gained instrument (Bruker) using Cu K alpha-radiation (λ = 1.54 A ) in
o
great deal of attention in research due to its increasing the scattering range (2θ) of 10 –70 with a scan rate of 0.02
o
o
3
4,35
-1
sec and a slit width of 0.1 mm. A thermogravimetric analyzer
demand for monitoring of environmental pollution.
Standard methods of gas detection are limited to solid and (Mettler Toledo TGA/SDTA 851e) was used to measure the
liquid samples. However, the issues related to available thermal stability of the material under an inert atmosphere
o
o
2
conventional gas sensors are their bulky architectures, high (flowing N gas) in the temperature range of 25 C –700 C. The
price, high power consumption, non-portability and cell pathlength was kept constant during all the experiments.
breakdown voltage which is inefficient and risky in operation. The morphology of C-NCs and NGPs were examined using
Therefore, there is an urgent need for rapid vapor phase scanning electron microscopy (SEM, Zeiss (MA EVO-18 Special
3
6
detection with a simple electronic sensor. Herein, we report Edition). The SEM samples were prepared by dispersing the
the synthesis of the C-NCs through click chemistry which can powder in ethanol using ultra-sonication and placing small
be used as chemiresistive sensors for the detection of H O
2
drops of the suspension on silicon wafers and sputter-coating
2
vapors with fast response and rapid recovery at room with gold before analysis. High-resolution transmission
temperature. So far there has been no report on the use of electron microscopy (HRTEM) was performed using a JEOL
NCs prepared by the click reaction for chemiresistive sensing 2100F instrument. The surface topography of the
and the work reported may open a promising area of exploration functionalized polymer NC surfaces was investigated by atomic
among gas sensors.
force microscopy (AFM). A multimode scanning probe
microscope equipped with a Nanoscope IIIa SPM was used to
capture the AFM images with a scan size 2µm × 2µm.
Nanoscope software was used to determine the root-mean-
square (RMS) roughness. The resistance and the sensor
properties of the C-NCs were measured by a standard two-
probe technique using a Keithley programmable current
source (model 6517 B) at room temperature. For the electrical
measurements, rectangular films of (2cm X 2cm) were
2
Experimental
Materials and Characterization
Styrene (Chemically pure) was purified by 5% sodium
hydroxide aqueous solution followed by washing with
deionized water and dried with anhydrous sodium sulphate
and finally distilled under vaccum. The natural graphite flakes
(from Asbury carbons Inc.) was used for preparing nano
2
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prepared and ohmic contacts were made on the film using Synthesis of alkyne-functionalized nanographite platlets (Alk-f-
silver paste.
NGPs)
The prepared NGPs were used for alkyne functionalization
2
.2 Functionalization of NGPs
without any further treatment. p-nitrophenyl propargyl ether
Synthesis of functionalized nanographite platlets
by reduction to aniline derivative forms p-aminophenyl
propargyl ether. To the crystals of p-aminophenyl propargyl
ether in a Schlenk flask under nitrogen environment isoamyl
nitrite (7.81 g, 66.65 mmol, 5 equiv/mol of carbon) was added
NGPs were prepared according to the modified Hummer’s
37
methods reported earlier by our group. The natural flake
o
graphite was first dried in a vacuum oven for 24 h at 80 C and
o
slowly via syringe. After stirring for 5 h at 60 C the paste was
a mixture of concentrated sulfuric acid and fuming acid (4:1,
v/v) was slowly added, under appropriate cooling and stirring,
to the graphite flakes. After 16 h of reaction, the acid-treated
natural graphite was filtered and washed thoroughly with
deionized water until the pH level of the solution reached 6.
diluted with dimethylformamide (DMF) and filtered through
PTFE (450 mm pore size) membrane. The solid was dispersed
and washed with DMF by sonication followed by filtration to
remove the DMF. The product was dried in a vaccum oven (55
o
C) for 72 h.
o
After being dried at 100 C overnight, the resulting graphite
intercalation compound (GIC) was subjected to a thermal
o
shock at 1050 C for 15s in a muffle furnace to form exfoliated
2
.3 Functionalization of polymer
graphite (EG). 1g EG was mixed and saturated with 400 ml Atom transfer radical polymerization of styrene
alcohol solution consisting of alcohol and distilled water with a
Atom transfer radical polymerization (ATRP) method was
ratio of 65:35 for 12 h. The mixture was subjected to ultrasonic
irradiation for various times and after hours of sonication, EG
performed to get high yield and reactive functional groups to
PS which will act as highly active binding sites for
particles were effectively fragmented into foliated graphite
o
FG). The FG dispersion was then filtered and dried at 80 C to
3
3
biomolecules. In this method a dry Schlenk flask equipped
with a magnetic bar was evacuated and refilled with nitrogen
three times. Then the mixture of deoxygenated styrene (10.40
g, 0.10 mol, 75 equiv), solvent DMF, ligand 2,2’-bipyridyl
(
remove residue solvents. The as-prepared FG powder, called
functionalized NGPs, was then kept in a dry dessicator for
testing and further use.
(
416mg, 2.66 mmol), catalyst (CuBr, 191 mg) and initiator
Synthesis of p-nitrophenyl propargyl ether
(ethyl 2-bromoisobutyrate, 260mg) was degassed by freeze-
pump-thaw cycles. After the reaction of 4 to 6 h the mixture
was diluted with THF and then passed through a column of
neutral alumina and precipitated in methanol to give bromo-
terminated PS.
The solution of p-nitrophenol (55.65 g, 400 mmol) and
tetrabutylammonium bromide (12.89 g, 40 mmol) was heated
o
at 60 C in 0.8 N sodium hydroxide solution. The propargyl
bromide was added dropwise in the mixture with stirring for
24 hrs. By cooling the mixture to room temperature a yellow
white precipitate was formed which was further washed with
distilled water (3 × 100 ml). This forms crude pale yellow
crystals which were dissolved in dioxane (ca. 300 ml) and were
precipitated in distilled water (1000 ml) until a white powder is
formed
.
Synthesis of p-aminophenyl propargyl ether
o
At 10 C, the solution of p-nitrophenyl propargyl ether (31.90
g, 180 mmol) in dioxane (500 ml) was added dropwise to the
solution of stannous chloride dihydate (203.00 g, 900 mmol) in
concentrated HCl. After complete addition the mixture was
stirred for 48 h at room temperature, until it became yellow in
color. The reaction mixture is neutralized with aqueous sodium
hydroxide and then by using dichloromethane (400 ml) the
solution was extracted. The separated organic layer was dried
over anhydrous sodium sulphate and then the solvent was
evaporated in vaccum. This yields brown viscous oil which was
purified under pressure to form highly viscous oil, which was
then crystallized.
Scheme 1 Schematic representation of formation of NGPs
from natural graphite flakes through acid intercalation and
o
thermal shock exposure at 1050 C.
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Scheme 2 Schematic representation of the functionalization of NGPs with PS by click reaction.
Synthesis of azide-functionalized polystyrene (Az-f-PS)
The conductivity of the film was measured for the films
prepared by the click chemistry method (C-NCs) and solution
3
8
The prepared bromo-terminated PS by ATRP was dissolved in
DMF. The substitution of bromine-end with azide-end was
blending method (S-NCs) at room temperature within the
potential window of -10 V to 10 V. The two point probe
Keithley 6517B electrometer was used for calibration. The
principle operation of the chemiresistive sensors is based on
the measurement of resistance change associated with the
3
installed on polymer with NaN (676 mg, 10.40 mmol, 10.00
equiv). The complete transformation takes overnight stirring
and washing with distilled water and methanol to remove
3
9
3
NaBr and NaN . Then the azide-modified PS was dried under
adsorption of analyte by the NCs material. The sensing
measurements were conducted under a static condition in a
sealed chamber with a total volume of 5 L. The temperature
vaccum at room temperature, yielding a white amorphous
solid.
o
inside the chamber remained constant at 25 C, monitored by
2
.4 Click Reaction: Coupling of alkyne-f-NGPs to azide-f-PS by
a mercury thermometer. The films of C-NCs were prepared
Huisgen cycloaddition
from the powder sample using hydraulic press at
a
compression pressure of 5 tons. Two ohmic contacts were
made on each end of the film using silver paste with 0.1 mm
spacing between the two contacts. The prepared film was
placed in a closed chamber and exposed to the vapors of
hydrogen peroxide. When the electrical resistance of the
composite approached its equilibrium value, the vapors of
The alkyne-functionalized NGPs (5 mg) with DMF (15 ml) were
dispersed by sonication. After the dispersion, azide-
functionalized polystyrene (500 mg) with catalyst (PPh
65 mg, 0.070 mmol) were mixed. The mixture was evacuated
and refilled with nitrogen three times followed by stirring for
3 3
) CuBr
(
o
2
4 to 65 h at 100 C. The mixture after cooling to room
H
2
O
2
was removed from the closed vessel with a flow of air in
temperature was diluted with THF and dispersed by sonication
and filtered through PTFE membrane. The product was
washed with THF, methanol, aqueous ammonium hydroxide
solution (28%) and CH Cl and dried under vaccum for 24 h.
order to clean the sensor environment as shown in scheme 4.
3
Results and discussion
2
2
The ATRP of styrene was performed resulting in bromo-
functionalized-PS depicting monomodal and symmetric peak
2.5 Preparation of sensor device and sampling system
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−
1
mg ml loading and allowed to stand for 24 h. Clearly NGPs
and Alk-f-NGPs are completely insoluble in THF and (all other
organic solvents). Alkyne functionalization has tuned the
material to exhibit higher density then its precursor (NGPs).
However C-NCs exhibited high solubility forming a clear yellow
colour solution without any dissercinable particulate materials
and can remain stable for three weeks due to the
4
0
functionalization through hydrophobic polymeric chain.
1
H NMR was used to detect and quantify the polystyrene
samples. A bromo-terminated PS sample was prepared using
Scheme 3: Scheme representing the nanocomposite (triazole
conjugate) formation by click chemistry.
n w n
with M and PDI (M /M ) of 2360 and 1.07 as shown in Fig
ES1. After the nucleophilic substitution, the GPC curve of the
Azide-functionalized-PS shifted to higher molecular weight
n
with M and PDI of 2770 and 1.06. The Az-f-PS and Alk-f-NGPs
were coupled via [3 + 2] Huisgen cycloaddition between the
alkyne and azide end groups using either CuI or the
3 3
organosoluble Cu(I) species, [(PPh ) CuBr] as catalyst, as
depicted in Scheme 2. The successful click coupling resulting in
formation of triazoles as shown in Scheme 3. Here, all
reactions were performed in DMF, yielding NCs by click
reaction C-NCs. In this study, excess PS was used in an attempt
to drive the click reactions for high conversion, and this excess
polymer was easily removed after each reaction by ultra-
filtration and prolonged washing with THF. In addition, trace
amounts of copper salts in the products were removed by
washing with an aqueous ammonium hydroxide solution.
1
Fig. 2 H NMR spectrum (zoom of the region 4.8 – 2.6 ppm) of
bromo-terminated polystyrene. Spectra was recorded at room
3
temperature in CDCl .
Fig. 1 represents dispersion images of NGPs, Alk-f-NGPs and C-
NCs in THF. All three materials look different because of their
respective structural and physicochemical properties. The
samples were dispersed by sonication for 60 min in THF at 1
1
Fig. 3 H NMR spectrum (zoom of the region 3.85- 4.0 ppm) of
azide-terminated polystyrene. Spectra was recorded at room
Fig. 1 Dispersion of three separate samples in THF (a) NGPs (b)
3
temperature in CDCL .
Alk-f-NGPs (c) C- NCs.
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-1
Fig. 4 (a) IR spectra of NGPs, Az-f-PS and C-NCs (b) Expanded region between 2000 and 2200 cm of the three samples,
-1
highlighting the appearance and disappearance of the azide strech at 2100 cm . (c) Raman spectra of NGPs, Alk-f-NGPs and C-
NCs. (d) XRD spectra of NGPs, Alk-f-NGPs and C-NCs
ATRP in the presence of the catalyst (CuBr) and initiator (ethyl transformation of ATRP bromide chain end into azide moiety
a
2-bromoisobutyrate). The quartet at 3.4-3.8 ppm, (2H ) is was found to be quantitative.
representative of the methylene-oxy protons from the initiator
ethyl 2-bromoisobutyrate moiety (R functionality) which is
partially overlapped with the solvent peak (THF). The complex
asymmetric shape of these signals is due to polymer tacticity,
the restricted motion of the end groups, and conformation
effects. To perform azide-alkyne click reaction, the bromo-
terminated PS (H
b
) was transformed in an azide-terminated PS
(H
c
) by nucleophilic substitution. This reaction was quantitative
1
as verified by H NMR. In Fig. 2 signal at 4.35-4.65 ppm
showing triplet is due to the methine proton neighboring the
bromine chain end (H
with NaN , it was completely shifted to upfield (3.85-4.0) (H
due to the quantitative substitution of bromine into azide
b
). After twenty four hours of reaction
3
c
)
4
1-43
moiety.
The Integration of the new signal in Fig. 3 confirms
the chain end transformation was quantitative. Hence, the
Fig. 5 TGA spectra of C-NCs, NGPs and PS
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-
1
-1
methylene groups at 2920 cm , 2850 cm and 1650 cm
-1
4
6
apparently arises in the C-NCs spectrum after click reaction.
Raman scattering is strongly sensitive to the electronic
structure and proves to be an essential tool to characterize
graphite and graphene materials. The Raman spectrum of
graphene is characterized by two main features, the G mode
2
2
arising from the first order scattering of the E g phonon of sp C
atoms and the D mode arising from a breathing mode of k-
1
point photons of A g symmetry. It is observed from Fig. 4 (c)
47
−1
that the peak of NGPs around 1321 cm (D-band for defect) is
related to the defects and disorder in the hexagonal lattice,
−1
2
while the 1587 cm (G-band) is due to the vibration of sp -
4
8,49
bonded carbon atoms in the 2D hexagonal lattice.
The
calculated I /I ratio for pristine NGPs is 0.80, which
G
D
2
3
represents the atomic ration of sp /sp carbons and express
the extent of disordered NGPs. Upon reaction of NGPs with
alkyne moiety the intensity of D band increases relative to G
Fig. 6 Sem image of (a) Petal-like structure of NGPs at 2 µm (b)
Bare and flat surface NGPs at 2 µm (c) Sem image of C-NCs at 2
µm (d) at 200 nm.
2
band, which indicates the convertion of sp hybridized carbon
3
to sp hybridized carbon along with blue shift of G position.
This was due to the presence of unrepaired defects that
remain after functionalization with alkyne moiety. In contrast
to Alk-f-NGPs the I /I ratio of C-NCs was seem to decrease to
Fig. 4 (a) depicts the IR spectra of unmodified NGPs, the Az-f-
PS and the C-NCs. However the NGPs and C-NCs shows a broad
-1
D
G
peak at 3450 cm due to the presence of O-H stretching
-1
D G
0.83 after click reaction, this decrease in I /I ratio attributed
vibration. A small but clearly discernible signal at 2100 cm is
present in the spectrum of Az-f-PS which represents the
characteristic signal of azide functionality, indicating that the
2
to the defect healing effect along with restoration sp network
2
8,45,50
during click reaction as depicted in Fig. 4(c).
-1
PS has been grafted. Although the azide signal at 2100 cm is
Fig. 4 (d) exhibited XRD patterns of NGPs, Alk-f-NGPs and C-
NCs. The interlayer distance can be easily calculated from the
diffraction angle of the corresponding material according to
weak, magnification of the three spectra in the region
-1
between 2000 and 2200 cm was depicted in Fig 4 (b). After
the Huisgen cycloaddition click reaction, the disappearance of
the azide stretch in C-NCs indicates that most of the azides
Bragg's equation: nλ = 2
d
sin
diffraction peak at 24.1 with the corresponding d-spacing of
.37 nm. In addition, after alkyne functionalization of NGPs,
θ. NGPs shows a typical sharp
o
4
4
must have consumed during the reaction. The triazole bands
0
-1
-1
in the C-NCs spectrum at 1457 cm and 1636 cm confirms
o
the Alky-f-NGPs exhibit an diffraction peak at 16.4 with d-
4
the successful “click” reaction. The characteristic bands of
5
spacing
~
0.54 nm suggesting the successful grafing of alkyne
polystyrene for C–H stretching and bending vibrations of
Fig. 7 Surface topography of C-NCs by TEM at (a) 20 nm (b) 5 nm.
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moiety onto NGPs and partial reduction of NGPs during the
functionalization process. The increase in the interlayer
distance depicts the exertion of the alkyne moiety on the
5
1
o
platlets. For the C-NCs a broad peak at 19.8 with d spacing
.44 nm was observed. The d-spacing of C-NCs is larger than
0
that of pristine NGPs, illustrating that the PS chains attached
on the NGPs by click reaction disrupts the van der waals
interactions and enlarges the interlayer distance of the
5
2,53
platlets.
Moreover it confirms the expected coexisting
structures of both NGPs and PS demonstrating that the NGPs
are exfoliated and dispersed uniquely in the polystyrene
5
4
matrix.
We further investigated the thermal stability using
thermogravimetric analysis (TGA) under N atmosphere. Fig. 5
2
shows that the NGPs are not thermally stable, a slight weight
o
loss started below 100 C, then it begins to decompose and
o
degrades completely at 450 C which is presumably due to the
gradual removal of labile oxygen containing functional groups.
o
Pristine PS begins to decompose at 320 C and degrades at 440
o
C, which is attributed to main-chain pyrolysis. But after click Fig. 8 Surface topography determined by AFM images of the
reaction, the C-NCs exhibits good thermal stability with no C-NCs. Total scales of images vary from (16 – 22 nm) a, b, c and
o
weight loss below 340 C and the pyrolysis shifts towards a d.
higher temperature range compared to that of neat PS. The
alkyne click reaction results in uniform dispersion of NGPs in
shift in the temperature is due to the interfacial strong
PS matrix as seen in Fig. 6c. This is attributed to the enhanced
interaction existing between the PS and NGPs which can
compatibility between polystyrene and NGPs through click
restrict the mobilization of polymer chains and effectively
5
protect against thermal degradation. However, to a certain
5
reaction, leading to strong interaction between the filler and
the polymer matrix. The Fig. 6d feature individual platlets of
nanographite well dispered throughout the polymer matrix at
extent the presence of PS in the composites slightly affects the
5
6
reduction degree. The C-NCs exhibit higher thermal stability
2
00nm. The dispersion is further observed using TEM images.
due to the phenyl-triazole groups formed in the click
57
product. Additionally, a higher residue (20% ) of C-NCs at higher
temperature can be attributed to the presence of NGPs covalently
linked to PS which are sufficient to block the premature
evaporation of the decomposed molecular fragments of the
High-resolution transmission electron microscopy (TEM)
analysis of the C-NCs was performed by placing a single drop of
chloroform solution onto holey carbon-coated copper grid.
The Fig. 7 represents interconnected structure established by
the homogeneous dispersion of NGPs and polystyrene. The
inset shows a particular well-resolved feature in which
polymer is well dispersed on platlets of nanographite. To
complement the TEM data, atomic force microscopy (AFM)
was also performed.
5
4,58
polymer and increase the residence time.
Scanning electron microscopy (SEM) analysis of pristine NGPs
and C-NCs by click reaction was performed by placing the
sample on the slide. Fig. 6 shows micrographs of NGPs
obtained after ultrasonication in water/ethanol solution. A
droplet of the dispersed NGPs in water/ethanol was poured
into the specific support and allowed to evaporate to obtain
the micrographs. The micrographs revealed that the stacked
graphite flakes are separated into thinner petal-like structure,
after ultrasonication in Fig. 6a. Bare and flat surface of NGPs
without any curvature is clearly observed in Fig. 6b. The azide–
The C-NCs was analyzed on the basis of surface morphology
using current AFM images. AFM provides a measure of the
thickness of the interface layers of polystyrene on NGPs. Upon
click reaction, the soluble material of Az-f-PS and Alk-f-NGPs
exhibits relative uniform dispersion. The PS coated on NGPs,
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shows a minimum height profile of approximately 20 nm as
depicted in Fig. 8 The effect of incorporating NGPs in the
.
organization of PS matrix could be correlated with the increase
in the density of pleats with the percentage of NGPs, which
could be verified by the increase in surface roughness of 1.207
nm. The C-NCs shows an obvious increase in the root mean
square (rms) surface roughness of 2.069 nm.
Electrical conductivity of polymer NCs
Measurement of electrical conductivity is an important step to
evaluate the polymer NCs. Significant difference of resistance
was observed for NCs prepared by the click method and
38
solution blending method. Fig 9a shows the electrical
resistance for C-NCs is 1.08E+08 with 1wt% NGPs, which is
three order lower than that for polymer-functionalized
nanocomposite prepared by solution blending method S-NCs
having 9.1E+11. The interconnected network and uniform
dispersion of NGPs in the PS matrix is responsible for the
increase in the properties at filler loading. This resulting in an
increase in an electrical conductive pathway within the
4
8,59
Fig. 9 a) Resistance of C-NCs b) Response of C-NCs to a) H O2
2
nanocomposite system.
b) xylene c)toulene d) benzene c) Repeatibility of C-NCs to
H
2
O
2
at every 5 seconds d) Response and recovery of the C-
Response of C-NCs to H
2
O
2
vapors
NCs to H O with time (s).
2
2
6
0
2 2
The response of C-NCs films for H O vapor was measured
using conventional resistance measurement methods by
placing the sensor inside the gas chamber and exposing to the
vapor. Here, the response time is the time taken for the sensor
to reach the saturation value after the sensor was exposed to
H O vapor and the recovery time is the time in which the
2
2
resistance of the sensor returns to 90% of the initial value after
removal of the vapor. As seen in Fig. 9b, the response of
sensing film C-NCs to H O vapor is considerably high. The
2
2
initial resistance of C-NCs before exposure to H
measured as 1.08E+08 and the resistance decreases to
.29E+06 on exposure and this indicates the immediate
response of C-NCs. To certain the specificity of C-NCs films to
vapor it was tested against the vapors of other solvents
2 2
O vapor was
1
2 2
H O
of PS. The responses of the C-NCs films to solvents like
benzene, toluene and xylene does not give detectable
response as shown in Fig. 9b. Moreover, other solvents of PS
like dichloromethane, acetone and chloroform were also
unable to illustrate any detectable response for C-NCs films. As
compared to conventional gas sensors, C-NCs shows rapid
Scheme 4: Schematic diagram of experimental set-up for
sensing chemical vapors (H ) using C-NCs film.
2 2
O
2 2
response within 3 seconds of exposure to H O vapor. In The sensitivity of H O vapor was calculated by using:
2
2
addition, the C-NCs demonstrates excellent reproducibility and
reversibility (Fig 9c). In Fig 9d, the minimum response time of
C–NCs is 3 seconds and the recovery time is not more than 60
Here, R
0
and R are the original resistance and the maximum
vapours
vapors at 120 seconds was
seconds for H
films goes on decreasing with increase in the exposure time
2 2
with (3, 5, 10, 20, 30, 40, 50, 60, 100, 120) seconds of H O
2 2
O vapors. Here, the resistance of the C-NCs
resistance of the films upon exposure to H
2 2
O
respectively. The sensitivity to H
found to be 91.1%.
2 2
O
vapor but after 120 seconds of exposure, the resistance
becomes stable. This confirms the stability, reproducibility and
Sensing mechanism of the chemiresistor
2 2
repeatibility of the C-NCs film to H O vapor.
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NGPs provides an additional surface area for adsorption sites One of the authors is grateful to the Amity University Uttar
Pradesh for providing financial support in the form of JRF. We
thank Dr John E. Moses, University of Nottingham for
providing technical support.
and reaction with target gases, which may lead to high sensor
response and response speed since the surface of NGPs is
known to be active to various reducing and oxidizing gases.
2 2
The decrease in resistance on exposure to H O vapors was
Notes and references
because of the formation of densely conductive network which
leads to a large increase in the electrical conductivity of the
film. The conducting paths are formed inside the C-NCs
material is due to the quantum mechanical tunneling effects.
Here, the distance between the conducting elements is such
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