A facile graft from method to prepare molecular-level dispersed graphene-polymer composites

Article abstract of DOI:10.1002/pola.26264

Graphene-polymer composites of positive-charged poly(dimethyl aminoethyl acrylate), negative-charged poly(acrylic acid), and neutral polystyrene were prepared by graft from methodology using reversible addition fragmentation chain transfer (RAFT) polymerization via a pyrene functional RAFT agent (PFRA) modified graphene precursor. Fluorescence spectroscopy and attenuated total reflection infrared (ATR-IR) evidenced that the PFRA was attached on the graphene basal planes by π-π stacking interactions, which is strong enough to anti-dissociation in the polymerization mixture up to 80°C. Atomic force microscopy (AFM) revealed that the thickness of a graphene-polymer sheet was about 4.0 nm. Graphene composites of different polymers with the same polymerization degree exhibited similar conductivity; however, when the polymer chain was designed as random copolymer the conductivity was significantly decreased. It was also observed that the longer the grafted polymer chains the lower the conductivity. ATR-IR spectroscopy and thermogravimetric analysis were also performed to characterize the as-prepared composites.

Full text of DOI:10.1002/pola.26264

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A Facile ‘‘Graft From’’ Method to Prepare Molecular-Level Dispersed
Graphene–Polymer Composites
Liang Cui,1 Jingquan Liu,1 Rui Wang,1 Zhen Liu,1 Wenrong Yang2
1
C o l l e g e o f C he m is t r y , C h e mi c a l a n d E n v i r o n me n t a l E n g i n e er i n g , L ab o r a t or y o f F ib er M a t e r ia ls a n d M o d e r n T e x t il e,
t h e G ro w i ng B a s e f o r S t a te K e y L a b or a t or y , Q i n g d ao U n i ve r si ty , Q in g d ao 2 66 0 71 , C h i n a
2
S c ho o l o f L i f e a nd E n v i r o n me n t a l S c i e n ces , D e a k in U n i v er s it y , G e el o n g , V IC 3 21 7 , A u s t r al i a
C o rr e sp on de n c e t o : J . L i u ( E -m a i l : j l i u @ q d u .e d u . c n )
R ec e i v e d
1
M a y 2 0 1 2 ; a c c e p t e d
9
J u l y 2 0 1 2 ; p u b l is h e d o n li n e
DOI: 10.1002/pola.26264
ABSTRACT: G ra p he n e –p o l y m e r c o mp o si t e s o f p o si ti v e- c h ar g ed
p o l y( di me t h yl a m i n o e t hyl a c r y l a t e) , n e g a t i v e -c h a r g ed p o ly ( a -
c r y l i c a c i d ) , a nd n e ut r a l p o l y s ty r en e w e r e p r e pa r e d b y ‘‘ g r a f t
f r o m ’’ m e t h o dolo g y u s i ng r e v e r si b l e a d d i t i o n f r a g m e n t a t io n
c h a i n t r a n s f e r ( R A F T) p o l y m e r i za tio n v i a a p y r e n e f u n c t io n al
R A F T a ge n t ( P F RA ) m od i f i e d g r a p h e ne p r e c u r s o r . F l u o r e s c en c e
s p e c t r osc o p y a nd a t t e n u at e d t o ta l r e fl e c ti o n i n f r a r e d ( A T R - IR)
e v i de n c ed t h a t t h e P F R A w a s a t t a c h e d o n t h e g r a p h e n e b a s a l
p l an e s b y p–p s t a c k i ng i n t e r a ct i o ns , w h i c h i s s tr o n g e n o u g h t o
a n t i - d i s s oc i at i on i n t h e p o l y m e r i z a t io n m i x t u r e u p t o 8 0 ꢀC .
A t om i c f o r c e m i c ro s co p y ( A F M ) r e v e a l e d t h a t t h e t h i c k n es s o f
a g r a ph e ne – p o l ym e r s h e e t w a s a b o u t 4 . 0 n m. G r a p h en e c o m -
p o s i t es o f d i f f er en t p o ly m e r s w i t h t h e s a m e p o ly m er iz a t io n
d e g r ee e xh ib it e d s i m i la r c o n d u ct iv i ty ; h o w e ve r, w h e n t h e p o l y -
m e r c h ai n w a s d e s i g n ed a s r a n d o m c o p o ly m e r t h e c o n d u ct i v i t y
w a s s i g n if i c a n tl y d e c r ea s e d. I t w a s a ls o o b s e r v ed t h at t h e l o n -
g e r t h e g r af t e d p o ly m e r c h ai n s t h e l o w e r t h e c o n d u ct i v i t y .
ATR- IR s p ec t r o s c o py a n d t h e r m o gr a v im e t r i c a n a ly s i s w er e a l s o
C
V
p e r f o r m e d t o c h ar ac t er iz e t h e a s - p r e p ar e d c o m p o s i t e s.
2 0 12
W il e y P e r i o d ic al s , I n c . J P o ly m S c i P a r t A : P o ly m C h e m 0 0 0:
0 00 – 0 00 , 2 01 2
KEYWORDS: f u n c t io na li z at i o n o f p o ly m e rs ; n a n o c o mp o s it e s ;
r e v er s ib le a d d i t io n f r a g m e n tat io n c h ai n t r a n s fe r
The methods for the preparation of graphene–polymer com-
posites can be very versatile. From the point of the binding
mode between graphene and polymer they can be generally
classified as covalent and noncovalent methods. The covalent
bonding method can be used to form stable composites;
however, the covalent bonding will disrupt graphene’s conju-
gated structure, thus will compromise graphene’s natural
properties, for example, conductivity.22 Therefore, an alterna-
tive to covalent bonding could have significant potential
advantages. p–p stacking interactions usually occur between
two relatively nonpolar aromatic rings having overlapping p
orbitals. Pyrene-functional precursors have been successfully
prepared23–25 and attached onto aromatic macromolecules
such as carbon nanotubes,26–30 graphene oxide,31,32 fuller-
ene, and so forth.33 The modification of graphene using p–p
stacking should have advantages over previous attachment
strategies as the conjugated structure of graphene will be
retained after modification. p–p stacking interactions are
also strong which are comparable to covalent attachment.
INTRODUCTION In the nanocarbon family, graphene is the
most recent member but has already attracted enormous
interests due to its unique electric, optical, mechanical, and
thermal properties. Mechanical exfoliation,1 thermal deposi-
tion,2,3 oxidation of graphite,4 and liquid-phase exfoliation of
graphite5–8 are the mostly frequently used methods to syn-
thesize graphene. Nowadays, Hummers oxidation–reduction
method is the mostly adopted method to produce graphene
in large scale. Li et al.9,10 have successfully synthesized sta-
ble aqueous dispersions of graphene nanosheets via electro-
static stabilization negating the requirement for polymeric
and surfactant stabilizers. Graphene has been extensively
explored for the applications in optoelectronic devices,11,12
electrode materials,13 supercapacitors,14 sensors,15 biomedi-
cal,16 and catalytical materials.17 A lot of research also
focused on the preparation of graphene–polymer composite
materials for various purposes. One of them is to use gra-
phene as conductive fillers to afford nanocomposites with
controlled electrical conductivity. Graphene can also be used
to enhance the polymer’s mechanical,18 thermal19 and photo-
electrical20 properties. For example, a small percentage of
graphene filler may exhibit significant enhancement of the
glass transition temperature of graphene composite with
poly (acrylonitrile).21
To prepare graphene–polymer composites via p–p stacking,
two methodologies can be adopted. One is called ‘‘graft to’’
method, through which polymer chains with p-orbital rich
functional groups can bind onto the graphene basal planes.
Liu et al. have used this method to prepare thermo- and
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pH-sensitive graphene–polymer nanocomposites.34,35 The
other method is called ‘‘graft from,’’ through which polymer
chains can be directly grafted from the graphene surface.
This method was rarely documented. One difficulty in using
‘‘graft from’’ method is what polymerization can be used and
how to make it happen. Reversible addition-fragmentation
transfer (RAFT) polymerization is one of the solutions. RAFT
polymerization is a controlled living radical polymerization
which was first proposed by CSIRO,36 and has received a lot
of attention for a large range of applications, such as in bio-
and nanotechnology.37–39 In addition, the RAFT approach has
been widely exploited for controlling the end-functionality of
polymers, by either modification of the RAFT functionality in
the polymer37,40–44 or by using pre-functionalized RAFT
agents.45,46 One recent research by Etmimi et al.47reported
the preparation of graphite oxide (GO) composite of polymer
via ‘‘graft from’’ method. By modification of GO with carbox-
ylic acid functional RAFT agent, the in situ polymerization
afforded the composite in one step.
All the samples for SEM analysis were prepared by vacuum
filtration. All the SEM images were taken using a LEO-SEM
(Supra 55VP, Zeiss) using an In-Lens detector and the accel-
erating voltage was adjusted to obtain the best image.
Transmission electron microscope (TEM) analysis was per-
formed at 100 kV using JEOL-TEM 2010. The 0.005% ethanol
solution was dropped on carbon-coated copper grid and
allowed the solution to evaporate under ambient conditions.
Attenuated total reflection infrared (ATR-IR) spectra of gra-
phene samples were recorded in the frequency range of
500–3600 cmꢁ1 using a Bruker IFS 66 spectrometer with a
FRA 106/S module. The laser source was a Nd:YAG laser.
Thermogravimetric analysis. Thermal decomposition proper-
ties of the graphene and garphene–polymer composites were
recorded on a Perkin–Elmer thermogravimetric analyzer
(Pyris 1 TGA). Analyses were conducted over the tempera-
ture range from 25 to 800ꢀC with a programmed tempera-
ture increment of 10ꢀC/min under a nitrogen atmosphere.
Pyrene functionalized polymers have been prepared by var-
ied methods, such as RAFT34,35,48 and click chemistry.23,49
Some of these polymers34,35 have been utilized to modify
graphene via ‘‘graft to’’ method, but did not study their elec-
trical and mechanical properties. However, to the best of our
knowledge, via noncovalent p–p stacking interactions to
attach RAFT agent onto graphene basal planes for directly
grafting polymers from graphene surface was not reported
yet. Herein, we adopted ‘‘graft from’’ method to prepare gra-
phene–polymer composites. A pyrene terminal RAFT agent
was synthesized and attached onto graphene surface via p–p
stacking, followed by the in situ polymerization to achieve
positive-charged, negative-charged and neutral graphene–
polymer composites. These composites were also used to
prepare composite papers, whose electrical conductivity and
mechanical properties were investigated.
The resistance of the graphene composite films was meas-
ured by a four-probe resistance analyzer.
Preparation of Chemically Converted Graphene (CCG)
Graphene was prepared with the reduction of GO which was
synthesized from natural graphite powder (KNGTM-150) fol-
50,51
lowing the methods of Hummers and Offeman.
A preoxi-
dation was required to completely oxide the graphite. The
graphite powder (2 g) was dispersed in the mixture of concen-
trated H2SO4 (10 mL) and P2O5 (1 g) at 80ꢀC. The resultant
dark blue mixture was thermally isolated and allowed to cool
to room temperature in 6 h, then diluted, filtered, and washed
with water till the filtrate became neutral. The product was
then dried and mixed with cold concentrated H2SO4 (50 mL),
followed by the addition of KMnO4 (6 g) slowly under stirring
and cooling condition to make sure the temperature below
20ꢀC. The solution was then stirred at 35ꢀC for 2 h, followed
by the addition of distilled water (400 mL). 30 % H2O2 solution
(5 mL) was added slowly, after which the mixture changed to
bright yellow. The product was dispersed in water, filtered,
washed with aqueous HCl solution (10% in volume) and then
dried under vacuum to afford GO powder for preparation of
graphene. The GO powder (15 mg) was dissolved into dioxane
(45 mL) through sonication. The resulting suspension was sub-
jected to reduction by hydrazine (60%, 20 lL) at 95ꢀC for 12 h.
The graphene dispersion was sonicated, filtered, and dried in
air and then in vacuum to afford graphene paper.
EXPERIMENTAL
Materials
N, N0-Dicyclohexyl carbodiimide (DCC, 99%) and 4-dimethy-
laminopyidine (DMAP) (99%) were purchased from Adamas-
beta. 1-Pyrenebutyric acid was purchased from Sigma
Aldrich. Dimethyl aminoethyl acrylate (DMAEA) was from
Aladdin. Acrylic acid (AA) and styrene (St) were purchased
from Tianjin Damao Dioxane and 2,20-azobis(2-methylpropio-
nitrile) (AIBN) and hydrazine (60%) were from Tianjin
Regent H2SO4 (98%), HCl (37%), KMnO4, P2O5, and K2SO4
were purchased from Qingdao Laiyang Technology Develop-
ment Region. Ethylene glycol (EG), ethyl acetate, dichlorome-
thane (DCM), tetrahydrofuran (THF), and n-hexane were pur-
chased from Tianjin Fuyu.
Synthesis of 3-(Benzylsulfanylthiocarbonylsulfanyl)-
propionic Acid (BSPA)
The RAFT agent, BSPA was synthesized using published
procedures.40,52
Measurements
1H NMR spectra of RAFT agents were obtained using a JNM-
ECP600 spectrometer. AFM samples were prepared by drop-
casting modified graphene on mica surface. AFM images were
obtained by a Molecular Imaging Picoscan II in tapping mode
and the analysis of the AFM images was performed using the
WSxM software (version 3, Nanotec Electronica S.L., Spain).
Synthesis of 1-(3-(Benzylsulfanylthiocarbonylsulfanyl)-
propionic) Glycol Ester
To the solution of EG (0.570 g, 9.190 mmol), DCC (0.454 g,
2.206 mmol) and DMAP (0.027 g, 0.220 mmol) in THF (5 ml)
was dropwise added BSPA (0.5 g, 1.838 mmol) solution in THF
(5 mL). The mixture was kept stirring at room temperature for
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The resulting suspension was mixed with the pyrene-func-
tionalized RAFT agent (PFRA; 58 mg), followed by mild soni-
cating for 10 min and stirring overnight at room tempera-
ture to afford a graphene/PFRA mixture. The monomer (St,
AA, or DMAEA) was added in the above mixture in a molar
ratio of monomer: PFRA:AIBN at 400:1:0.33. The solution
was deoxygenated with nitrogen for 30 min, and then stirred
in oil bath for 12 h at 80ꢀC for St, 70ꢀC for AA, and 70ꢀC for
DMAEA. Another system for random copolymerization of AA
and
DMAEA
monomers
(AA:DMAEA:PFRA:AIBN
¼
200:200:1:0.33) was incubated at 70ꢀC for 12 h.
The polymer generated by the free PFRA and residual mono-
mer was removed by centrifugation of the above mixture at
12,000 rpm for 30 min. The precipitate was collected and
redispersed in dioxane (40 mL) by mild sonication, and this
process was repeated for another two times to completely
remove the free polymer. The obtained homogeneous disper-
sion was filtered with Teflon membrane (U ¼ 0.22 lm) and
dried overnight at 60ꢀC and in vacuum for 48 h to afford the
composite films which were peeled off from the Teflon mem-
brane for analysis.
1
FIGURE 1 ( a ) T he H N MR s p e c tr a o f B S P G E a n d ( b ) P F R A .
Polymerization Without Free PFRA Agent
12 h, followed by filtration to remove the solid byproduct. The
filtrate was concentrated under vacuum and purified via silica
gel column chromatography using n-hexane/ethyl acetate (3/1)
as eluent to yield an orange oily product, which was character-
ized by 1H NMR [Fig. 1(a)] in CDCl3 (0.279 g, 57%).
PFRA (58 mg) was added to the graphene (9.7 mg) suspen-
sion in dioxane (5 ml), followed by mild sonicating for 10
min and stirring overnight at room temperature to afford a
graphene/PFRA mixture. The mixture was centrifugated at
10,000 rpm, the precipitate was collected and redispersed in
dioxane. This procedure was repeated for three times to
completely remove the free PFRA to afford pure graphene–
polymer composite. The final product was dried in vacuum
at room temperature and weighed to obtain the grafted den-
sity of PFRA (10 wt%) and the polymerization was designed
based on the actual amount of PFRA attached on graphene.
The feed ratio of monomer (St, AA, or DMAEA):PFRA:AIBN
was designed at 400:1:0.33 and other conditions for poly-
merization was the same as that for the polymerization in
the presence of free PFRA as described above. The obtained
homogeneous dispersion was filtered with Teflon membrane
(U ¼ 0.22 lm) and dried overnight at 60ꢀC in vacuum for
48 h to afford the composite films which were peeled off
from the Teflon membrane for further analysis. The same
procedure was performed to prepare graphene–polystyrene
(PS) composites with different PS chain length by controlling
the monomer/PFRA molar ratio at 100, 200, 300, 500, and
600, respectively. The graphene–PS composite papers were
obtained and their tensile strength (TS) and conductivity
characterized.
1H NMR (CDCl3 298 K, 600 MHz), d (ppm from TMS): 2.80–
2.84 (t, 2H, CACH2ACO), 3.62–3.65 (t, 2H, CACH2AOH),
3.81–3.84 (t, 2H, SACH2AC), 4.21–4.25 (t, 2H, OACH2AC),
4.60–4.61 (s, 2H, CACH2AS), 7.25–7.34 (m, 5H, aromatic
protons of benzene).
Synthesis of Pyrene-Functionalized RAFT Agent
1-Pyrenebutyric acid (0.123 g, 0.428 mmol) was added to a
solution of 1-(3-(benzylsulfanylthiocarbonylsulfanyl)-propi-
onic) glycol ester (BSPGE) (0.136 g, 0.428 mmol), DCC
(0.106 g, 0.514 mmol), and DMAP (0.006 g, 0.051 mmol) in
THF (5 mL) and DCM (2 mL). The resulting mixture was
kept stirring at room temperature overnight. The orange-
brown mixture was filtered to remove the solid byproduct.
The filtrate was then concentrated under vacuum and puri-
fied via silica gel column chromatography using DCM/hexane
(50/20) to afford the light-orange oily product, which was
1
analyzed by H NMR [Fig. 1(b)] in CDCl3 (0.121 g, 48%).
1H NMR (CDCl3 298 K, 600 MHz), d (ppm from TMS):
2.192–2.275 (m, 2H, CACH2AC), 2.478–2.511 (t, 2H,
COACH2AC), 2.788–2.817 (t, 2H, CACH2ACO), 3.814–3.464
(t, 2H, CACH2AC), 3.588–3.644 (t, 2H, SACH2AC), 4.287–
4.386 (m, 4H, OACACH2AO), 4.557–4.594 (s, 2H,
CACH2AS), 7.25–7.35 (m,5 H, aromatic of benzene), 7.883–
8.358 (m, 9H, aromatic of pyrene).
RESULTS AND DISCUSSION
Modification of Graphene with PFRA and the In Situ
Polymerization
The synthesis of PFRA was started from the synthesis of the
hydroxyl-functionalized RAFT agent via the condensation
reaction between a RAFT acid, BSPA with excess EG in the
presence of DCC and DMAP to afford the hydroxyl terminal
RAFT agent, BSPGE. The same condensation method was
subsequently used to synthesize the PFRA via the reaction
‘‘Graft from’’ Method to Prepare Graphene–Polymer
Composite Papers Polymerization in the Presence of
Free Pyrene-Functionalized RAFT Agent
The freshly prepared graphene (9.7 mg) was redispersed in
5 mL dioxane with the assistance of Ultrasonic Cell Crusher.
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(inset b of Scheme 1). It is believed that the attachment of
PFRA onto graphene basal planes is via the p–p stacking
interactions. It is well known that pyrene is a fluorophore
and its fluorescence will be quenched when it is attached
onto graphene due to photoinduced electron transfer.53,54
Therefore, the successful attachment of PFRA onto graphene
via p–p stacking can be demonstrated by monitoring the flu-
orescence of pyrene groups. As shown in Figure 2(a), the flu-
orescence spectrum reveals that the fluorescence of pyrene
group was significantly quenched after being attached onto
graphene.
The RAFT controlled polymerization using PFRA as chain
transfer agent was performed. As shown in Figure 3(a), the
monomer conversion increased concomitantly with polymer-
ization time and the radical concentration remained constant
with conversion as indicated by the pseudo-first order plot.
The experimental (measured from GPC) and theoretical mo-
lecular weights (MWs) were found to be proportional to the
monomer conversion. The theoretical MW values were
slightly higher than the experimental ones and the polydis-
persity index (PDI) of the purified PS was less than 1.17,
indicating a well-controlled polymerization consistent with
the known traits of living radical polymerization [Fig. 3(b)].
SCHEME 1 T he m o d i f i c at i o n o f g r a ph e n e w i t h P F R A v ia p–p
s t a c k i ng i n t e r ac t i ons a n d t h e s u bs e qu e n t i n s i t u p o l y m e ri za t io n
t o d i re c t l y g ra f t i n g p o l y m e r o n g r a ph e ne . I n s e t a : f r e s h l y p r e-
p a r e d g ra ph e ne i n d i o x a n e a f t e r 2 4 h ; I n s e t b : g r a ph ene m o d i -
f i e d w i t h P FR A i n d i o x a n e a f t e r 1 w e e k ; I n s e t c : g r ap h e ne –
p o l ym e r c o m p o si t e g e n e r a t e d b y in situ p o l y m er iz a t io n i n
d i o xa n e a f t er o ne w e e k .
Among the enormous research on preparation of graphene–
polymer composites most of them using physical mixing
methods, where the molecular level homogeneous compo-
sites are difficult to be achieved.55 In recent studies by Liu
et al. functional polymers with pyrene terminal groups have
been synthesized and used to modify graphene via p–p
stacking interactions and the polymeric properties were suc-
cessfully imparted to graphene–polymer composites.34–56
The in situ polymerization method present in this paper will
simplify the composite preparation. Using the same RAFT
agent modified graphene precursor different polymer compo-
sites can be directly generated via RAFT-controlled polymer-
ization. The length of the polymer chain can also be freely
tailored just by controlling the monomer feed. Furthermore,
because the RAFT agent is much smaller than the polymer
between pyrene butyric acid and BSPGE. As shown in Scheme
1, the PFRA was then utilized to modify the graphene basal
plane via p–p stacking to afford the graphene–RAFT composite.
The in situ polymerizations can be used to directly generate
polymer brushes on both sides of graphene to yield sandwich-
like graphene–polymer composites, where polymer layers are
on both sides of graphene sheets (Scheme 1).
The successful synthesis of hydroxyl and PFRA were con-
firmed by 1H NMR, respectively (Fig. 1). It is noteworthy
that pure graphene sheets are relatively hydrophobic and
easy to proceed irreversible aggregation (inset a of Scheme
1). However, when the graphene sheets were modified with
PFRA a stable graphene suspension in dioxane was formed
FIGURE 2 ( a ) F l uo r es ce n ce e m i s s io n s p e c tr u m o f P F R A b e f o r e a n d a f te r b e i n g a t ta c h e d o n t o g r ap h e ne a t e xc it a ti o n w a v e le n g t h o f
3 5 0 n m . ( b ) G ra p he ne –p o l y m e r c o mp o si te p a p e r .
4
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FIGURE 3 P ol ym e r i z a t io n o f s ty r e n e u s i n g P F R A i n d i o x a n e a t 8 0 ꢀC ( [ M ] /[ R AF T ] / [A I B N] ¼ 4 00 : 1: 0. 3 ). ( a ) M o n o m er c o n ve rs i o n a t
v a r y i ng p ol ym e r i za t io n t i me s. ( b ) M W a n d P D I o f t h e P S a g a in s t m o n o m er c o n v e r s i on [ f il le d a n d e m p t y d i a m o n d s r e p r e s e nt t h e
e x p e ri me nt al ( o b t a in e d f r o m G P C ) a n d t h e o r e ti c a l M W v al u e s , r e s p ect iv e ly , w h il e f il l ed t r i an g l es r e p r e s e nt P D I ] .
chains the density of the RAFT molecules attached on gra-
phene will be much higher. Therefore, the density of the
polymer chains via in situ polymerization will be much
higher comparing to the postpolymerization grafting
methods.
1354 cmꢁ1 peaks can be attributed to the CAH band and
CAC stretch. In Figure 5(d), graphene–poly(acrylic acid)
(PAA) exhibited the strong peaks of OH stretch at 3305 cmꢁ1
,
CAH, OAH, and CAO stretch at 2971, 2930, and 1180 cmꢁ1
,
respectively. Peaks at 1641, 1537, 1457, and 1370 cmꢁ1 can
be attributed to the C¼¼O, CAH band, and CAC stretch.
Analysis of Graphene and Graphene–Polymer Composites
Using TEM, SEM, and AFM
Conductivity of Composite Papers
The conductivity of the graphene composite papers with dif-
ferent polymers but the same degree of polymerization was
analyzed using a four-probe resistance analyzer. As shown in
Figure 6(a), the pure graphene paper has the highest con-
ductivity while the conductivity values of the graphene com-
posites with different polymers were similar but more than
10 times lower. By designing the polymers with positive-
charged PDMAEA, negative-charged PAA, and neutral PS we
like to explore whether the polymer charge will influence
the electron transfer behavior. The similar conductivity val-
ues for the composites prepared from different charged poly-
mers indicated that the polymer charges may not signifi-
cantly affect the electron transfer through the composite
matrix. It is surprising to observe that conductivity of gra-
phene composite paper prepared from PAA/PDMAEA ran-
dom copolymer is much lower than those of other composite
papers of the homopolymers, which can be attributed to the
phase separation due to the irregular random polymer
chains. However, we found that the graphene content had
significant effect on the composite conductivity. As shown in
Figure 6(b), the increasing polymer percentage resulted from
the longer chain length can significantly decrease the con-
ductivity of composite papers. It can be envisioned that the
longer polymer chains will insulate the graphene sheet bet-
ter, thus greatly reduce the electrical communication
between the graphene sheets. Similar conclusion was also
obtained by He and Gao.22 They studied the effect of poly-
mer composition on the conductivity of the graphene–poly-
mer composites. The composites designed with the same
polymer percentage but different polymer chain length
exhibited different conductivity. The longer the polymer
chain the higher the conductivity of the composites was
The morphology of graphene prepared using Hummers
method was analyzed by atomic force microscope (AFM). As
shown in Figure 4(a), the thickness of graphene sheets was
measured to be 1.1 nm, which is consistent with the values
previously reported.34,57–59 When poly(dimethyl aminoethyl
acrylate (PDMAEA) polymer chains were grafted onto the ba-
sal planes of graphene via p–p stacking interactions the
thickness of the graphene–PDMAEA composite nanosheets
was found to be 4.0 nm [Fig. 4(b)], evidencing the successful
surface polymer grafting. Graphene sheets after modification
with PDMAEA were also analyzed using TEM. As shown in
Figure 4(c) separate graphene–PDMAEA composites were
clearly recognized, evidencing that the as-prepared graphene
composites were well dispersed. These graphene–PDMAEA
composites can be easily prepared into composite papers by
simple filtration, which exhibited layered structure through
the side view imaging by scanning electron microscopy
(SEM) [Fig. 4(d)].
Characterization of the Unmodified Graphene and its
Polymer Composites Using ATR-IR Spectroscopy
Total reflection infrared (ATR-IR) spectroscopy was used to
qualitatively characterize the graphene and its composites.
After reduction trace residual C¼¼O at 1675 cmꢁ1 still
existed and the ¼¼CAH peak is hardly observed [Fig. 5(a)].
When the graphene was modified with PFRA the C¼¼O char-
acteristic peak at 1738 cmꢁ1 evidenced the successful RAFT
attachment [Fig. 5(b)]. The spectrum of graphene–PDMAEA
is shown in Figure 5(c). The peak at 2800 cmꢁ1 should be
attributed to CAH stretch absorption in CH3NA of PDMAEA.
Peaks at 1720, 1230, and 900 cmꢁ1 can be assigned to C¼¼O,
CAO stretch, and ¼¼CAH bend respectively. The 1594 and
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FIGURE 4 I m a g i ng c h a r a ct e r i z a ti o n s o f g r a ph e ne b e fo r e a n d a f te r s u r f a c e g r af t i n g o f P D M A E A v ia p–p s t a c k ing . ( a ) A F M i ma g e o f
u n m o d if i ed g ra p he n e , ( b ) A F M i m a g e o f g r a p h en e – P D M A EA c o m p o si te , ( c ) T E M i m ag e , a n d ( d ) s i d e v ie w o f h i g h r e s o l u t io n S E M
i m a g e o f g ra ph e ne – P D MA E A c o mp o si t e .
obtained. It is obvious that when the polymer percentage is
fixed the longer the polymer chain the less chains will exist
on the graphene surface, therefore, the graphene will be less
insulated.
phene composites of PDMAEA, PS, PAA, and PDMAEA/PAA
have different polymer weight percentages of 48, 44, 30,
and 41%, respectively. This result is consistent with their
monomer MW. It can be seen from the TGA analysis the
polymer percentage was low which is probably due to the
free PFRA existed in the polymerization mixture which con-
sumed the most monomers in a very short time. To increase
the polymer ratio in the composites the in situ polymeriza-
tion was performed in the absence of free PFRA in the poly-
merization mixture. The polymerization was performed fol-
lowing the procedure as described in the experimental. The
as-prepared graphene–polymer composites were analyzed
by TGA. As shown in Figure 7(b), the polymer percentage
was significantly increased up to 99%. This protocol sup-
plied a convenient tool to easily manipulate the polymer
composition.
Thermal and Mechanical Analysis
The graphene–polymer composites were also analyzed
using thermogravimetric analysis (TGA) under nitrogen
atmosphere to 800ꢀC at a rate of 10ꢀC/min. As shown in
Figure 7(a), the TGA curve of graphene revealed 20% mass
loss before the furnace temperature reached 500ꢀC, which
is due to the evaporation of absorbed water and decompo-
sition of some residual oxygen-containing groups.5 The TGA
curves of the graphene–polymer composites are similar in
trend. It can be seen that they all contain about 10% vola-
tiles like water, which can be removed below 200ꢀC. The
graphene–polymer composites are not thermally stable, and
mostly lost below 400ꢀC, which is similar to the results of
Fang et al.60 The TGA analysis also revealed that the gra-
The TS of graphene and its polymer composites were also
investigated. As shown in Figure 8(a), TS of the graphene
6
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FIGURE 5 A T R - I R s p e c tr a o f ( a ) g r a ph e n e , ( b ) g r ap h e ne – P F R A , ( c ) g r ap h e ne – P D M A E A , a n d ( d ) G r a p h en e–P A A .
composites were significantly enhanced for all the composites,
among which TS of graphene–PDMAEA reached 24.8 Mpa.
The TS enhancement might be attributed to Van der Waals
force and the entanglement effect among polymer chains.61
The longer side groups of the PDMAEA polymer chains should
accounts for the highest TS of graphene–PDMAEA composite
FIGURE 6 ( a ) C o nd u c t i vi t y o f p a p e r s o f g r a p h e ne a n d g r ap h e ne c o m p o si te s w i t h d i f f er en t p o ly m e r s a n d ( b ) C o n d u c t i vi t y o f g r a -
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FIGURE 7 T GA t h e r m o gr a ms o f g r a ph en e a n d g r a p h ene – p o ly m er c o m p o s i t e p r ep ar e d b y in situ p o ly m e ri za t i o n i n t h e p r e s e n c e
(
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as the longer side groups will certainly enhance the chain
entanglement and interfacial adhesion.62,63 We also observed
TS is more sensitive to the percentage of polymer. As shown
in Figure 8(b), TS of graphene–PS composite papers
increased with the increasing content of PS, which is due to
the increased interactions between the polymer chains.
that the PFRA agent was attached on the graphene basal
planes by p–p stacking interactions. The p–p stacking inter-
actions were found strong enough to stand 80ꢀC polymeriza-
tion condition without obvious de-association. It was also
observed that the longer the polymer chain grafted the lower
the conductivity. Our study revealed that the polymer
charges might not play an important role in manipulating
the electron transfer through the composite paper. Graphene
composites of different polymers with the same polymeriza-
tion degree exhibited similar conductivity; however, when
the polymer chain was designed as random copolymer the
conductivity was significantly decreased. This grahene-poly-
mer composite preparation method is facile, efficient and
can be used to tailor the graphene–polymer composites with
versatile properties.
CONCLUSIONS
In summary, we have successfully demonstrated a facile
‘‘graft from’’ method to prepare graphene composites of dif-
ferent polymers using RAFT polymerization via just one
RAFT agent functonalized graphene precursor. This is a facile
method that can maximally avoid the graphene aggregation,
therefore can be used to prepare graphene–polymer compo-
sites at molecular level. Fluorescence spectroscopy evidenced
FIGURE 8 T S o f g r a p h e ne c o mp o si t e s w i t h d i f f er e n t p o ly m e rs ( a ) a n d w i t h d i f f er en t g r ap h e ne c o n t e n t s.
8
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ACKNOWLEDGMENTS
JL thanks the NSF of China (51173087), NSF of Shandong
(ZR2011EMM001) and Taishan Scholars Program for financial
support.
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