Synthesis of PNIPAM polymer brushes on reduced graphene oxide based on click chemistry and RAFT polymerization

Article abstract of DOI:10.1002/pola.25036

Preparation and characterization of poly(N-isopropylacrylamide) (PNIPAM) polymer brushes on the surfaces of reduced graphene oxide (RGO) sheets based on click chemistry and reversible addition-fragmentation chain transfer (RAFT) polymerization was reported. RGO sheets prepared by thermal reduction were modified by diazonium salt of propargyl p-aminobenzoate, and alkyne-functionalized RGO sheets were obtained. RAFT chain transfer agent (CTA) was grafted to the surfaces of RGO sheets by click reaction. PNIPAM on RGO sheets was prepared by RAFT polymerization. Fourier transform-infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM) results all demonstrated that RAFT CTA and PNIPAM were successfully produced on the surfaces of RGO sheets. Nanosized PNIPAM domains on RGO sheets were observed on TEM. Micro-DSC result indicated that in aqueous solution PNIPAM on RGO sheets presented a lower critical solution temperature at 33.2 °C.

Full text of DOI:10.1002/pola.25036

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Synthesis of PNIPAM Polymer Brushes on Reduced Graphene Oxide
Based on Click Chemistry and RAFT Polymerization
Yongfang Yang,1 Xiaohui Song,2 Liang Yuan,2 Mao Li,1 Jinchuan Liu,2 Rongqin Ji,1
Hanying Zhao2
1Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China
2Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Correspondence to: H. Zhao (E-mail: hyzhao@nankai.edu.cn)
Received 9 August 2011; accepted 4 October 2011; published online 22 October 2011
DOI: 10.1002/pola.25036
ABSTRACT: Preparation and characterization of poly(N-isopropy-
lacrylamide) (PNIPAM) polymer brushes on the surfaces of
reduced graphene oxide (RGO) sheets based on click chemistry
and reversible addition-fragmentation chain transfer (RAFT) po-
lymerization was reported. RGO sheets prepared by thermal
reduction were modified by diazonium salt of propargyl p-ami-
nobenzoate, and alkyne-functionalized RGO sheets were
obtained. RAFT chain transfer agent (CTA) was grafted to the
surfaces of RGO sheets by click reaction. PNIPAM on RGO
sheets was prepared by RAFT polymerization. Fourier trans-
form-infrared spectroscopy, thermogravimetric analysis, X-ray
photoelectron spectroscopy, and transmission electron micros-
copy (TEM) results all demonstrated that RAFT CTA and PNI-
PAM were successfully produced on the surfaces of RGO
sheets. Nanosized PNIPAM domains on RGO sheets were
observed on TEM. Micro-DSC result indicated that in aqueous
solution PNIPAM on RGO sheets presented a lower critical so-
lution temperature at 33.2 ꢀC.
2011 Wiley Periodicals, Inc. J
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KEYWORDS: click chemistry; living radical polymerization (LRP);
nanocomposites; poly(N-isopropylacrylamide) (PNIPAM); poly-
mer brushes; reduced graphene oxide; reversible addition-frag-
mentation chain transfer (RAFT) polymerization
INTRODUCTION Graphene, a single-atom-thick sheet of sp2-
bonded carbon atoms, is the basal building block of all gra-
phitic materials.1 Since its discovery in 2004, graphene-based
materials have attracted considerable attention because of
their excellent mechanical, optical, and electrical proper-
ties.2–4 Graphene-based polymer materials also triggered
enormous interest for extremely high thermal conductivity
and strong mechanical properties.5
reactions of hydrophilic functional groups on the graphene
sheets, such as AOH and ACOOH groups.12,13 The second
approach is to take advantage of addition chemistry in gra-
phene modification, such as nitrene addition,14 diazonium
coupling,15 and 1,3-dipolar cycloaddition.16 In these years,
click chemistry, especially Cu(I)-catalyzed 1,3-dipolar azide-
alkyne cycloaddition reaction, has gained a great deal of
attention due to excellent functional group tolerance, high
specificity, and nearly quantitative yields under mild experi-
mental conditions.17 A variety of functional polymers and
materials with different structures were prepared based on
click chemistry. For example, Sun et al. used click chemistry
to functionalize graphene sheets.18 They synthesized azido-
terminated polystyrene (PS) by atom transfer radical poly-
merization (ATRP) using 3-azidoethyl 2-bromoisobutyrate as
initiator. Graphite oxide sheets were functionalized by pro-
pargyl alcohol through an acylation reaction, and the click
reaction between alkyne functionalized sheets and azido-ter-
minated PS was conducted at room temperature. He and Gao
reported a unified approach to covalently functionalize gra-
phene nanosheets based on nitrene chemistry.19 Many differ-
ent functional moieties (hydroxyl, carboxyl, amino, bromine,
Graphene sheets tend to agglomerate through van der Waals
interaction due to their high specific surface area. In order
to increase the dispersibility of the graphene sheets in or-
ganic solvents or in polymer matrix, graphene sheets are
functionalized by covalent modification or noncovalent cou-
pling modifications.6–8 Because of the inherent instability of
the resulting supramolecular systems, it is difficult to control
the structure of graphene modified by noncovalent method,9
and the chemical modification of graphene is of great impor-
tance in the preparation of graphene composites with stable
structures.10,11
There are two approaches to the covalent modification of the
graphene sheets. The first one is related to the chemical
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Outline for the preparation of PNIPAM/reduced graphene oxide (RGO) nanocomposites based on click chemistry and
RAFT polymerization.
long alkyl chain, etc.) and polymers (e.g., poly(ethylene gly-
col), polystyrene) were anchored to the graphene sheets by
this approach. Although this strategy is simple and efficient,
the reaction is time consuming and the reaction temperature
is high.
polymerized on RGO sheets via RAFT polymerization method
(Scheme 1).
EXPERIMENTAL
Combination of click chemistry and reversible addition-frag-
mentation chain transfer (RAFT) polymerization provides an
efficient method for the synthesis of new polymers and
materials.20–22 We synthesized new comb polymers and
nanohydrogels based on this method.23–25 In situ polymeriza-
tion on the surfaces of reduced graphene oxide (RGO) sheets
provides a versatile method for the fabrication of RGO/poly-
mer nanocomposites. In a previous article, we reported an
efficient process for modification of RGO sheets. ATRP initia-
tor was grafted onto the RGO sheets by reaction of 2-bromo-
2-methylpropionyl bromide with amine groups and hydroxyl
groups on the RGO sheets, and poly(2-(dimethylamino)ethyl
methacrylate) were prepared on the surfaces of RGO sheets
by in situ ATRP.26 In this article, for the first time we report
the functionalization of RGO based on click chemistry and
RAFT polymerization. RGO sheets were treated by thermal
reduction. Alkyne groups were introduced onto the RGO
sheets by a reaction of RGO sheets with aryldiazonium salts
containing alkyne groups. Azido-terminated RAFT chain
transfer agent (CTA) was synthesized by reaction of 3-azido-
1-propanol and S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic acid)
trithiocarbonate. RAFT CTA was grafted to the RGO sheets by
facile click reaction, and N-isopropylacrylamide (NIPAM) was
Materials
Natural flake graphite with an average particle size of 40 lm
(99%) was purchased from Qingdao Guangyao Graphite Co.
Fuming nitric acid (>90%) was purchased from Tianjin
FengChuan Chem. Co. Sulfuric acid (98%), potassium chlo-
rate (98%), and hydrochloric acid (37%) were purchased
from Tian Jin Institute of Chemical Agents and used as
received. NIPAM (97%) was purchased from Aldrich. Before
use, it was recrystallized from hexane and dried under vac-
uum. Copper (I) bromide (CuBr) was purchased from Guo
Yao Chemical Company and was purified by washing with
glacial acetic acid. N,N,N,N,N-Pentamethyldiethylenetriamine
(PMDETA, 99%), sodium azide (Alfa, 99%), 2-bromo-2-
methyl propionyl bromide (98%), and 3-bromo-1-propanol
(98%) were purchased from Aldrich and used as received.
2,20-Azobis(isobutyronitrile) (AIBN) was purchased from
Guo Yao Chemical Co. and was purified by recrystallization
from ethanol. S-1-Dodecyl-S0-(a,a0-dimethyl-a00-acetic acid)
trithiocarbonate was synthesized according to the previous
literature.27
N-(3-(dimethylamino)propyl)-N0-ethylcarbodii-
mide hydrochloride (EDC.HCl) purchased from Shanghai
Medpep Co., p-aminobenzoic acid purchased from Tianjin
Guangfu chemical Company, 4-(dimethylamino) pyridine
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(DMAP, Alfa), and nitrosyl tetrafluoroborate (Alfa) were used
as received. All the solvents were distilled before use.
dried over magnesium sulfate, and concentrated under
reduced pressure. The product was purified by column chro-
matography (eluent: petroleum ether: ethyl acetate ¼ 90:10)
to obtain yellow liquid. The yield of the product is about
75%. 1H NMR (400 MHz, CDCl3, d): 4.19 (s, 2H, AOACH2A
(CH2)2AN3), 3.36 (t, 2H, ACH2AN3), 3.28 (t, 2H, ASACH2A
(CH2)10ACH3), 1.91 (t, 2H, ACH2ACH2AN3), 1.70 (s, 6H
ACOAC(CH3)2ACOOA), 1.27–1.68 (m, 20H, AC10H20A), 0.89
(t, 3H, ACH2ACH3).
Preparation of RGO Sheets
Graphite oxide (GO) were prepared according to the Stau-
denmaier method.28,29 Graphite (5 g) was reacted with con-
centrated nitric acid (45 mL) and sulfuric acid (87.5 mL) in
the presence of potassium chlorate (55 g). On completion of
the reaction, the mixture was added to excess water, and
washed with HCl aqueous solution (5%) and water until the
pH of the filtrate was neutral. The dried sample was stored
Preparation of RAFT CTA-Modified RGO
ꢀ
in a vacuum oven at 60 C before use. Thermal reduction of
At the first step of modification, RGO sheets were modified by
diazonium salt of propargyl p-aminobenzoate,6 and alkyne-
functionalized RGO sheets were obtained. The details were
described as follows: RGO (20 mg) was dispersed in 20 mL of
doubly distilled water at pH 10, and diazonium salt was added
into the solution (0.11 mmol/mL). The solution was stirred at
room temperature overnight. The mixture was diluted with
100 mL of acetone. Modified RGO sheets were obtained after
filtration and washing with water, DMF, and acetone. The
resulting solid was dried in a vacuum oven at 50 ꢀC.
GO was conducted by placing the sample in a Muffle Furnace
ꢀ
preheated to 950 C and held in the furnace for 30 s.
Synthesis of Propargyl p-Aminobenzoate and its
Diazonium Salt
EDC.HCl (5.26 g), DMAP (0.30 g), and p-aminobenzoic acid
(3.14 g) were dissolved in 75 mL of CHCl3, and 1.70 mL of
propargyl alcohol was added droꢀpwise into the mixture at 0
ꢀC. The solution was stirred at 0 C for 2 h and at room tem-
perature for 2 days. The reaction mixture was washed with
20 mL of doubly distilled water and the organic layer was
collected and dried over magnesium sulfate, and concen-
trated under reduced pressure. The product was purified by
column chromatography (eluent: petroleum ether: ethyl ace-
tate: dichloromethane ¼ 5:1:1), and yellow liquid was col-
lected. After removal of the solvent, solid product was
obtained. The yield of the product is about 65%. 1H NMR
(400 MHz, CDCl3, d): 2.49(s,1H, AC:CH), 4.10 (s, 2H,
ANH2), 4.89 (s, 2H, AOCH2A), 6.66 and 7.90 (t, 4H, Ar H).
RAFT CTA-modified RGO was prepared by click reaction
between alkynes functionalized RGO sheets and RAFT agent.
Alkyne-functionalized RGO (16 mg) was dispersed into 5 mL
of DMF. After sonication, S-1-dodecyl-S0-(a,a0-dimethyl-a00-3-
azido-1-propyl acetate) trithiocarbonate (60 mg, 0.015
mmol), PMDETA (3.0 lL, 0.015 mmol), and CuBr (2.2 mg,
0.015 mmol) were added into the solution. The solution was
stirred at room temperature for 25 h under nitrogen atmos-
phere. The RAFT CTA-modified RGO was obtained by centrif-
ugation. After washing with DMF anꢀd CH3OH, the RGO sheets
were dried in a vacuum oven at 40 C for 2 days.
The synthesis of diazonium salt of propargyl p-aminoben-
zoate was described as follows.6 Nitrosyl tetrafluoroborate
(0.48 g) was dissolved in 10 mL of acetonitrile, and propar-
gyl p-aminobenzoate (0.6 g) in 3.4 mL of acetonitrile solu-
tion was added at ꢁ30 ꢀC. The solution was stirred at 0 ꢀC
for 0.5 h and at room temperature for 1 h. After the reaction,
30 mL of ether was added to the solution and solid product
was obtained. The solids was washed by ether and dried in
vacuum oven at room temperature.
RAFT Polymerization of NIPAM on RGO Sheets
A typical polymerization was described as follows. RAFT CTA-
modified RGO (8.8 mg, 0.0030 mmol RAFT CTA) was dispersed
in 2 mL of DMF under ultrasonication. NIPAM monomer (113.2
mg, 1.000 mmol) and AIBN (0.164 mg, 0.0010 mmol) were
added into the dispersion. RAFT polymerization was con-
ducted at 60 ꢀC for 20 h under nitrogen atmosphere. After po-
lymerization, the nanocomposite was collected via centrifuga-
tion, and was dried in a vacuum oven at 40 ꢀC for 2 days.
Synthesis of Azido-Terminated Trithiocarbonate
S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic acid) trithiocarbonate
and 3-azido-1-propanol were synthesized according to the
previous literature.27,30 S-1-dodecyl-S0-(a,a0-dimethyl-a00-3-
azido-1-propyl acetate) trithiocarbonate was synthesized by
esterification of S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic acid)
trithiocarbonate and 3-azido-1-propanol. S-1-Dodecyl-S0-(a,a0-
dimethyl-a00-acetic acid) trithiocarbonate (1.03 g, 2.75 mmol),
4-dimethylaminopyridine (35 mg, 0.27 mmol), and methyl-
ene dichloride (20 mL) were added into a round bottomed
flask. The flask was immersed in an ice bath, and EDC.HCl
Characterization
1H NMR (400 MHz) spectra were recorded on a Varian UNITY-
plus 400 spectrometer. The thermal properties of the nanocom-
posites were measured by thermogravimetric analysis (TGA).
ꢀ
The samples were heated to 800 C at a heating rate of 10 K/
min under nitrogen atmosphere on a Netzsch TG 209. High-re-
solution transmission electron microscope (TEM) observations
were carried out on a Tecnai G220 S-TWIN electron microscope
equipped with a Model 794 CCD camera (512 ꢂ 512). TEM
specimens were prepared by dipping copper grids into solu-
tions and drying in air. The TEM specimens with PNIPAM were
stained by OsO4. The apparent molecular weight and molecular
weight distribution of PNIPAM were determined with a gel per-
meation chromatograph (GPC) equipped with a waters 717
autosampler, waters 1525 HPLC pump, three waters Ultra Styr-
agel columns with 5K-600K (10,000 Å), 500-30K (1000 Å), and
ꢀ
(1.08 g, 5.49 mmol) was added. After stirring at 0 C for 0.5
h, 3-azido-1-propanol (0.56 g, 5.5 mmol) in 6 mL of dry
methylene dichloride was added dropwise. The solution was
stirred at 0 ꢀC for 2 h and at room temperature for 2 days.
After the reaction, the solution was diluted with 20 mL of
dichloromethane, and washed with saturated sodium bicar-
bonate aqueous solution. The organic layer was collected,
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100-10K (500 Å) molecular ranges, and a waters 2414 Refrac-
tive Index Detector. Atomic force microscopy (AFM) images
were collected on a Nanoscope IV atomic force microscope
(Digital Instruments). The microscope was operated in tapping
mode using Si cantilevers with a resonance frequency of 320
KHz. The voltage was between 2 and 3 V, and a tip radius was
less than 10 nm. A drive amplitude of 1.2 V, and a scan rate of
1.0 Hz were used. Fourier transform-infrared absorption spec-
tra (FTIR) were obtained on a Bio-Rad FTS 6000 system using
diffuse reflectance sampling accessories. The micro-DSC meas-
urements were performed on a VP-DSC microcalorimeter
(MicroCal). X-ray photoelectron spectroscopy (XPS) spectra
were recorded on a Kratos Axis Ultra delay line detector (DLD)
spectrometer employing a monochromated Al-Ka X-ray source
(hm ¼ 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a
multichannel plate and DLD. All XPS spectra were recorded
using an aperture slot of 300 to 700 microns, survey spectra
were recorded with pass energy of 160 eV, and high-resolution
spectra were recorded with pass energy of 40 eV.
RGO sheets were modified by diazonium salt of propargyl p-
aminobenzoate, and alkyne-functionalized RGO sheets were
obtained. RAFT CTA was grafted to the surfaces of RGO sheets
by click reaction. PNIPAM brushes on RGO sheets were pre-
pared by RAFT polymerization. Propargyl p-aminobenzoate
was synthesized by reaction of p-aminobenzoic acid and pro-
1
pargyl alcohol in the presence of EDC.HCl and DMAP. H NMR
spectrum of the compound was shown in Supporting Infor-
mation Figure S1. The peaks at 2.5 and 4.9 ppm are attributed
to the alkyne proton (AC:CH), and the methene protons
next to the ester group (AOACH2AC:CH). The signals at 6.7
and 7.9 ppm are related to the protons on the benzene ring.
The signal of the protons on the amino group (ANH2)
appears at 4.1 ppm. After a reaction with nitrosyl tetrafluoro-
borate, the diazonium salt of propargyl p-aminobenzoate was
synthesized. FTIR spectra of propargyl p-aminobenzoate and
its diazonium salt were shown in Supporting Information Fig-
ure S2. In FTIR spectrum of propargyl p-aminobenzoate, the
absorption peaks at 3290 cmꢁ1 and 2123 cmꢁ1 are attributed
to the stretching vibration of the unsaturated carbon–hydro-
gen bond of alkyne group and C:C bond, respectively.31 After
reaction with nitrosyl tetrafluoroborate, the absorption peaks
from ANH2 groups at 3430 cmꢁ1, 3336 cmꢁ1, and 3213 cmꢁ1
disappeared and a peak representing the N:N stretching of
RESULTS AND DISCUSSION
The synthesis of RAFT CTA-modified RGO and RAFT poly-
merization of NIPAM on RGO sheets were shown in Scheme 2.
SCHEME 2 A scheme for the preparation of RAFT CTA-modified RGO, and in situ RAFT polymerization of NIPAM on the surfaces
of RGO sheets.
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FIGURE 1 FTIR spectra of (a) RGO, (b) alkyne-functionalized
RGO.
1
FIGURE 2 H NMR spectrum of S-1-dodecyl-S0-(a,a0-dimethyl-
a00-3-azido-1-propyl acetate) trithiocarbonate in CDCl3.
the diazonium group appeared at 2,280 cmꢁ1 (Supporting In-
formation Fig. S2b), which confirmed the successful synthesis
of diazonium salt.6
ing the trithioester groups at 1,072 cmꢁ1 were observed.30
Elemental analysis was used to determine the sulfur content
and the amount of the RAFT CTA grafted to the RGO sheets.
The elemental analysis result shows that the sulfur content
is 3.27 wt %, so the RAFT CTA in the nanocomposite is
about 0.34 mmol/g. XPS spectrum provides information on
the type and number of different species of a given atom in
the molecules. The RAFT CTA-modified RGO was also charac-
terized by XPS. Figure 3 shows wide XPS spectra of unmodi-
fied and RAFT CTA-modified RGO. On the XPS spectrum of
RAFT CTA-modified RGO, a peak at 162.5 eV corresponding
to the binding energy of S2p of the trithiocarbonate was
observed. However, on the spectrum of RGO, no peak
appeared at this position, which proved successful function-
alization of RGO by RAFT CTA. PNIPAM on RGO sheets was
prepared by in situ RAFT polymerization. Because of the
Alkyne-functionalized RGO was prepared by treatment of
RGO dispersion with diazonium salt of propargyl p-amino-
benzoate at room temperature overnight. FTIR spectra
of RGO and alkyne-functionalized RGO are presented in
Figure 1. After modification, the absorption peaks at 3290
cmꢁ1 and 2,125 cmꢁ1 representing the characteristic valence
vibration of C:CH, and the peak at 1,720 cmꢁ1 from the
C¼¼O stretch were observed.30
S-1-Dodecyl-S0-(a,a0-dimethyl-a00-3-azido-1-propyl acetate) tri-
thiocarbonate was synthesized by esterification of S-1-do-
decyl-S0-(a,a0-dimethyl-a00-acetic acid) trithiocarbonate and 3-
azido-1-propanol. 1H NMR spectrum of the azido-terminated
trithiocarbonate was shown in Figure 2. The peaks at 4.19
ppm [ACOOACH2A (CH2)2AN3] and 3.36 ppm (ACH2AN3)
can be observed, which indicates the successful esterification
of S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic acid) trithiocarbon-
ate and 3-azido-1-propanol. On the spectrum the peaks from
1.27 ppm to 1.68 ppm are attributed to the 20 methylene
proton [CH3A(CH2)10ACH2A], and the peak at 3.28 ppm
(peak c) is related to the methylene protons [AC(S)SACH2A]
next to the trithiocarbonate group. FTIR spectrum of azido-
terminated trithiocarbonate is shown in Supporting Informa-
tion Figure S3a. On the spectrum a strong absorption at
2,112 cmꢁ1 representing the valence vibration of azide
group, and an absorption peak at 1080 cmꢁ1 representing
the trithioester groups were observed.32
RAFT CTA-modified RGO was prepared by click reaction
between azido-terminated trithiocarbonate and alkyne-func-
tionalized RGO. FTIR spectra of azido-terminated trithiocar-
bonate and RAFT CTA-modified RGO are shown in Support-
ing Information Figure S3b. On the spectrum of RAFT CTA-
modified RGO, an absorption peak at 1724 cmꢁ1 represent-
ing the C¼¼O stretch, and the characteristic peaks represent-
FIGURE 3 Wide XPS spectra of (a) RGO, (b) RAFT CTA-modi-
fied RGO, and (c) RGO with PNIPAM on the surfaces.
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FIGURE 5 TGA curves of (a) RGO, (b) RAFT CTA-modified
RGO, and (c) RGO sheets with PNIPAM on the surfaces.
FIGURE 4 FTIR spectrum of PNIPAM on RGO sheets.
Fig. 5). Wide XPS spectrum of RGO/PNIPAM nanocomposites
is presented in Figure 3. On the spectrum a peak at 399.9 eV
corresponding to the N1s binding energy of PNIPAM can be
observed, which proves successful grafting of PNIPAM on
RGO sheets. AFM is a direct method to observe the structure
of the original RGO sheets. Figure 6 shows a tapping mode
difficulty in the direct measurement of the molecular
weight of polymer chains prepared on the surfaces of RGO
by living/controlled free radical polymerization, one way to
determine the molecular weight of the grafted polymer
chains was to add sacrificial initiator (or RAFT CTA) mole-
cules into the polymerization system and measure the mo-
lecular weight of the free polymer produced in the solution,
because it was reported that free polymers produced in the
solution have almost the same molecular weight as those
prepared on the solid substrates.26,33,34 In this research,
trace amount of free RAFT CTA was added to the reaction
system, and the molecular weight of the free polymer pro-
duced in the solution was determined by GPC. The apparent
molecular weight (Mn) and polydispersity index of PNIPAM
were 19K and 1.77, respectively. 1H NMR spectrum of the
free PNIPAM in CDCl3 was presented in Supporting Infor-
mation Figure S4.
FTIR spectrum of RGO/PNIPAM nanocomposite is presented
in Figure 4. A broad band at about 3500 cmꢁ1 is attributed
to the NAH stretching vibration of PNIPAM and the
hydroxyl stretching vibration of the hydroxyl groups on
RGO sheets. On the spectrum, the typical absorption peaks
of PNIPAM include an absorption at 1643 cmꢁ1 due to
C¼¼O stretching and an absorption at 1551 cmꢁ1 from NAH
stretching.35
RGO, RAFT CTA-modified RGO and RGO/PNIPAM nanocom-
posites were analyzed by TGA (Fig. 5). TGA of the original
RGO was found to have 13 wt % weight loss in the range
between 100 and 750 ꢀC (curve a in Fig. 5), which was
attributed to the loss of the functional groups such as COOH
and OH groups on the sheets. Upon grafting of RAFT CTA,
the composite was found to have 24 wt % weight loss (curve b
in Fig. 5), and so approximately 11 wt % of the modified
RGO was the RAFT CTA and the diazonium salt of propargyl
p-aminobenzoate. After in situ RAFT polymerization, RGO/
PNIPAM nanocomposite was found to have 68 wt % weight
losses in the range between 280 ꢀC and 750 ꢀC (curve c in
FIGURE 6 AFM image and height profile of RGO.
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FIGURE 7 (a) A TEM image of RGO observed at low magnification; (b) a magnified TEM image showing the blank RGO surface;
(c) a TEM image of RGO/PNIPAM nanocomposite. The dark dots on the surfaces of nanosheets represent PNIPAM domains.
AFM image of the original RGO sheets. The sample was pre-
pared by depositing RGO aqueous dispersion (0.1 mg/mL)
onto a new cleaved mica surface and dried under vacuum
at room temperature. The cross-sectional view of the AFM
image indicated that the height of the sheets ranged from 1.7
nm to 3 nm, or the flake of RGO had two or three layers.
The morphology of RGO and RGO/PNIPAM nanocomposite
was studied by TEM. RGO or RGO/PNIPAM nanocomposite
was dispersed in H2O under sonication. TEM specimens
were prepared by dipping copper grids into the solution and
drying in air. Before TEM observation, the specimens were
stained in OsO4 atmosphere. Figure 7(a,b) are TEM images
of RGO at different magnifications. On the two images blank
RGO surfaces were observed. Figure 7(c) is a TEM image of
RGO/PNIPAM nanocomposite. The dark dots on the surface
of RGO sheet represent PNIPAM nanosized domains. In aque-
ous solution, the grafted PNIPAM chains are extended due to
the solubility of the polymer chains in water; however, after
FIGURE 8 Microcalorimetric endotherm recorded for aqueous
dispersion of RGO/PNIPAM nanocomposites. In the measure-
ments, the heating rate and cooling rate were both set at 1 ꢀC/
min, and the nanocomposites concentration was 2 g/L.
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