Amphiphilic poly(N-vinyl pyrrolidone) grafted graphene by reversible addition and fragmentation polymerization and the reinforcement of poly(vinyl acetate) films

Article abstract of DOI:10.1039/c3ta11853c

The reversible addition and fragmentation (RAFT) polymerization of vinyl pyrrolidone (VP) from graphene oxide (GO) is used to produce GO-g-PVP (GP) and the grafting is confirmed from Fourier transformed infrared (FTIR) and nuclear magnetic resonance spectra. The average thickness of GP (8.2 nm) obtained from atomic force microscopy is higher than that of GO (1.2 nm), indicating the wrapping of grafted PVP on the GO sheets. Transmission electron microscopy of GP exhibits swollen domains (white spots) characterizing the grafted PVP chains from the GO surface. The dispersibility of the GP sheets becomes greatly improved over that of GO and they are dispersible in the solvents of Hansen solubility parameter (δ_p + δ_H) range 6.3-58. Three nanocomposites GP1, GP3 and GP5, produced by mixing with 1, 3 and 5 (w/w)% GP with poly(vinyl acetate) (PVAc), produce a stable dispersion in dimethyl formamide, although mixtures of GO and PVAc do not. The field emission scanning electron microscopy of the GP5 sample indicates a good homogeneous dispersion of GP sheets within the PVAc matrix, although both GO and PVP are individually immiscible with PVAc. The FTIR data indicates a specific interaction between GP and PVAc. The glass transition temperature (T_g) of the pure PVAc increases in the GP composites, but in the GO composite it remains unchanged. In the GPP5 hybrid containing the GO, PVP and PVAc mixture produced at the same composition as in GP5, an increase of T_g is seen to a lesser degree than that of GP, indicating that GO acts as a compatibilizer of a PVP and PVAc immiscible blend. The mechanical properties of PVAc exhibit a strong reinforcement and the Young's modulus & tensile strength data show a 190% and 169% increase over PVAc in the GP5 sample due to the homogenous dispersion and unidirectional (parallel) orientation of GP sheets in the composite film.

Full text of DOI:10.1039/c3ta11853c

Journal of
Materials Chemistry A
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PAPER
Amphiphilic poly(N-vinyl pyrrolidone) grafted
graphene by reversible addition and fragmentation
polymerization and the reinforcement of
poly(vinyl acetate) films†
Cite this: J. Mater. Chem. A, 2013, 1,
0863
1
Rama K. Layek, Atanu Kuila, Dhruba P. Chatterjee and Arun K. Nandi*
The reversible addition and fragmentation (RAFT) polymerization of vinyl pyrrolidone (VP) from graphene
oxide (GO) is used to produce GO-g-PVP (GP) and the grafting is confirmed from Fourier transformed
infrared (FTIR) and nuclear magnetic resonance spectra. The average thickness of GP (8.2 nm) obtained
from atomic force microscopy is higher than that of GO (1.2 nm), indicating the wrapping of grafted
PVP on the GO sheets. Transmission electron microscopy of GP exhibits swollen domains (white spots)
characterizing the grafted PVP chains from the GO surface. The dispersibility of the GP sheets becomes
greatly improved over that of GO and they are dispersible in the solvents of Hansen solubility parameter
(d
p
+ d
H
) range 6.3–58. Three nanocomposites GP1, GP3 and GP5, produced by mixing with 1, 3 and
5
(w/w)% GP with poly(vinyl acetate) (PVAc), produce a stable dispersion in dimethyl formamide,
although mixtures of GO and PVAc do not. The field emission scanning electron microscopy of the GP5
sample indicates a good homogeneous dispersion of GP sheets within the PVAc matrix, although both
GO and PVP are individually immiscible with PVAc. The FTIR data indicates a specific interaction
between GP and PVAc. The glass transition temperature (T
composites, but in the GO composite it remains unchanged. In the GPP5 hybrid containing the GO, PVP
and PVAc mixture produced at the same composition as in GP5, an increase of T is seen to a lesser
g
) of the pure PVAc increases in the GP
g
Received 10th May 2013
Accepted 8th July 2013
degree than that of GP, indicating that GO acts as a compatibilizer of a PVP and PVAc immiscible blend.
The mechanical properties of PVAc exhibit a strong reinforcement and the Young's modulus & tensile
strength data show a 190% and 169% increase over PVAc in the GP5 sample due to the homogenous
dispersion and unidirectional (parallel) orientation of GP sheets in the composite film.
DOI: 10.1039/c3ta11853c
www.rsc.org/MaterialsA
mechanical property reinforcement. For example, the Young's
modulus of single-layer graphene can reach up to 1 TPa, while
Introduction
One of the major challenges of this century is the gradual
replacement of traditional materials with the new synthetic
its strength can be extended to 130 GPa, making graphene the
9
strongest material ever acheived. To exploit these properties
ones having a light weight and good reinforcing properties. In the incorporation of exfoliated graphene sheets into the poly-
this respect, the formation of ‘polymer nanocomposites’ is one mer matrix with uniform dispersion and ne interfacial inter-
of the most economical routes to modify the bulk properties of actions may lead to the formation of high performance
polymers appreciably and a great deal of research has been composites.1 However, it is very difficult to process graphene
focused on this area in the past few years. Amongst the various as a nanoller for its difficulty in exfoliation due to strong
5,16
1
–8
nanollers (such as silicate, clay, carbon nanotube, graphene)
cohesive force between its layers and a lack of dispersion in
common solvents. Its oxidized form, graphene oxide (GO), is
recently graphene has generated major interest for its possible
technological applications in various elds, due to its exciting
homogeneously dispersed in aqueous medium due to its
9
–14
physical, mechanical, optical and electronic properties
and hydrophilic nature, but it becomes precipitated out from
17
its most promising application is associated with the common organic media. As a result GO is found to be
incompatible with hydrophobic polymers, hence the use of GO
nanosheets is limited within a few polymer matrices due to
the fragile interfacial interaction between the GO and the
Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata – 700032, India. E-mail: psuakn@iacs.res.in
†
Electronic supplementary information (ESI) available: WAXS, Derivatogram of surrounding matrix.
DSC thermograms, HR TEM images of GO and GP, tables of solubility
parameters and mechanical property data are available free of cost. See DOI:
In recent times, several methods have been reported to
produce graphene polymer composites, such as melt
1
0.1039/c3ta11853c
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1
8,19
20,21
casting,
to aid dispersion.
used functionalized GO sheets to improve the properties of
solution blending
and the addition of surfactant distilled water was made in the laboratory and was used
22,23
A number of research groups have also throughout the whole work.
24–26
graphene polymer nanocomposites.
of GO can be achieved by: (i) covalent
The functionalization
and (ii) non covalent
Synthesis of graphene oxide (GO)
24,25
26
Graphene oxide was prepared according to the Hummers
approaches. The major disadvantage of the non-covalent
approaches over the covalent functionalization of GO sheet is
that the efficiency of the load transfer is low since the force
between the wrapping molecules and the GO surface is weak.
Very recently, organic molecules, such as phenyl isocyanate and
porphyrin, have also been anchored on GO surfaces to enhance
their dispersion in nonpolar polymers.
stable dispersion of GO sheets into the polymer matrix func-
tionalization of GO sheets with another polymer and having
30
method. In this typical procedure, 46 ml concentrated sulfuric
acid was added into a conical ask in an ice bath and was stirred
with a bar magnet. Then 0.1 gm sodium nitrate was added into
the ask with constant stirring. 2 gm graphite powder was
added into the conical ask very slowly with vigorous stirring
and aer the complete addition of the graphite powder, 6 gm
potassium permanganate was added into the conical ask very
25b,27
But for better and
ꢀ
slowly with the temperature at 0–5 C. Then the mixture was
allowed to stir at room temperature forming a thick paste for
28
good interaction with the matrix polymer is necessary. But
there is a possibility of well dispersed composite formation even
if there is no specic interaction of the anchored polymer with
6
hours. Aer that 92 ml double distilled water was poured into
the ask with constant stirring. Then the temperature of the
ꢀ
solution was raised to about 90 C and the mixture was allowed
28c
the host matrix, due to the compatibilizing property of GO.
to stir for 30 minutes. Finally 280 ml of water was added fol-
lowed by the slow addition of 3 ml H O (30%). The color of the
2 2
PVAc is a thermoplastic polymer that belongs to the
poly(vinyl ester) family and is an archetypal exible polymer
that has been widely used as a stable expanded lm, as an
emulsion in water and has been used as an adhesive for porous
materials, wood, paper, clothes and as a component for porous
building stone, in particular sandstone. But the mechanical
properties of PVAc are very low and to improve it graphene may
be a promising nanoller due to its remarkable properties and
no such report yet exists in the literature. Herein, we report a
novel method to prepare poly(vinyl pyrolidone) (PVP) func-
solution changed from dark brown to yellow. The solution was
centrifuged to collect the product at the bottom and the
supernatant liquid was thrown out. The product was re-
dispersed with water and was centrifuged until the pH of the
solution became neutral. Finally the brown mass collected was
29a
ꢀ
dried at 60 C in vacuum to obtain the GO.
Synthesis of GO – 2-bromopropionyl bromide (GO-BPB)
tionalized amphiphilic GO [Go-g-PVP, (GP)] by growing PVP 500 mg GO was placed in 30 ml of anhydrous pyridine and it was
chains from the GO surface via the RAFT polymerization tech- dispersed by ultrasonication using an ultrasonic bath cleaner.
nique to yield dispersible GO sheets in solvents of varying Then 5 ml of 2-bromopropionyl bromide was added drop wise at
ꢀ
polarity e.g. water (highly polar) in one hand to very nonpolar 0 C into this solution. Aer that the mixture was allowed to stir
ꢀ
solvents, such as chlorobenzene, chloroform etc. on the other. for 3 h at 0 C followed by stirring at room temperature for 48 h.
The GP chains aim to increase the physical and mechanical All the operations were carried out under a nitrogen atmo-
properties of PVAc, though PVP and PVAc are reported to be sphere. Then the raw product was dispersed in CHCl and was
3
incompatible due to the presence of similar functional centrifuged from this solution. This procedure was repeated
29b
groups. But there are reports where the presence of a third three times to remove any adsorbed 2-bromopropionyl
substance (a compatibilizer) e.g. bisphenol A, enhances the bromide. The obtained black mass obtained by centrifugation
29b
compatibility of the above two polymers. Here we report that was collected and was dried under vacuum.
GP makes a well dispersed nanocomposite with PVAc, also GO
mixed with PVP (non covalent functionalization) is found to
produce a composite with PVAc, thus GO acts as a compatibil-
izer of a PVP and PVAc immiscible blend. The Young's modulus,
tensile strength and toughness of the GP/PVAc composite lms
exhibit signicant increases compare to those of pure PVAc.
Synthesis of potassium O-ethyl xanthate
.63 g (0.1 mol) KOH was stirred in 40 ml (0.645 mol) ethanol
until a clear solution was obtained. Then, 20 ml (0.332 mol) CS
was added slowly to the above solution with stirring and the
5
2
31
stirring was continued overnight. The resultant mixture was
suspended in 200 ml ether and was ltered. The precipitate was
washed thrice with ether and was dried under vacuum at room
temperature overnight. The observed mp of the light yellow
Experimental section
Samples
ꢀ
31
color product was 210 C.
Graphite (Aldrich, USA), 2-bromopropinyl bromide (BPB,
Aldrich USA), sodium nitrate potassium permanganate carbon
disulphide and ethanol (synthetic grade, Merck, Mumbai) were
Synthesis of graphene oxide O-ethyl xanthate (GO-EX)
used as received. Pyridine, N,N-dimethylformamide (DMF) and In a dried and nitrogen purged round-bottom ask 100 mg GO-
dichlorobenzene (Renkem, New Delhi, analytical grade) were BPB, 500 mg of potassium O-ethyl xanthate was taken and then
distilled before use. Vinyl pyrrolidone (VP, Aldrich, USA) was the ask was evacuated and relled with nitrogen gas and
distilled in vacuum and azobis isobutyro nitrile (AIBN) (Aldrich, sealed with a rubber plug. In another dried and nitrogen purged
2 2
USA) was puried by recrystallization from methanol. Doubly round-bottom ask, 2 ml pyridine was dissolved in 5 ml CH Cl
1
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while stirring under nitrogen and was sealed with a rubber plug. tension and were gold-sputtered prior to observation through a
This CH Cl –pyridine mixture was injected into the former eld emission scanning electron microscope (FESEM) (Jeol
2
2
vessel using a syringe, sealed and was sonicated for 5 minutes GSM-5800). The transmission electron microscopy (TEM, JEOL,
using an ultrasonic bath. The reaction mixture was stirred at 2010EX) was conducted via dispersing the GO and GP in water
room temperature for 2 days and then it was diluted with 10 ml and casting a solution drop on a carbon coated copper grid. The
CH
2
Cl
2
. The solution was washed with CH
2
Cl
2
by repeated re- lm was dried by slow evaporation of the solvent at room
ꢀ
dispersion and centrifugation. Finally the product was dried temperature (30 C) and nally in vacuum.
under vacuum for 48 hours.
FTIR spectroscopy. FTIR study of the nanocomposite lms
was performed using a Perkin-Elmer FT-IR instrument (spec-
trum 100). The nanocomposite lms were cast from DMF
solution by spreading over the silicon wafer surface. The lms
were dried via slow evaporation of solvent in air and nally in
Synthesis of GO-g-PVP (GP) by RAFT polymerization
In a typical procedure 100 mg graphene oxide O-ethyl xanthate
and 5 mg of AIBN were dissolved in 2 ml of DMF in a 10 ml dry
reaction tube and then the tube was sealed with a rubber plug.
Then it was purged with nitrogen gas three times. In another
similar reaction tube vinyl pyrrolidone was purged with
nitrogen and it was sealed with a rubber plug. Aer that 1 ml of
vinyl pyrrolidone from the later vessel was injected into the
former vessel using a syringe and was sealed. It was then poly-
ꢀ
vacuum at 60 C for three days.
Raman spectroscopy. Raman spectra of the above GO and GP
solutions in water were recorded using a LabSpec Raman
spectroscope (JY T6400). The sample solutions were dropped on
a microscopic glass slide and were excited at 514 nm with an Ar
ion laser with a scanning duration of 50 s.
ꢀ
X-ray scattering. XRD patterns of the nanocomposite lms
were obtained by xing the nanocomposite lm on an
aluminum holder and the experiment was carried out using a
Bruker AXS diffractometer (model D8) using a Lynx Eye
detector. The instrument was operated at 40 kV and 40 mA
merized at 80 C for 2 days and then it was diluted with meth-
anol and was centrifuged. The residue was re-dispersed with
methanol and was centrifuged repeatedly (5 times) to wash out
the impurities. The residue was dried in vacuum at 60 C for
ꢀ
three days to obtain the GP.
ꢀ
current. The samples were scanned from 2q ¼ 5 to 35 at the
scan rate 0.5 s per step.
Preparation of GP/PVAc lms
Thermal study. A Perkin-Elmer differential scanning calo-
rimeter (Diamond DSC7) working under a nitrogen atmosphere
was used to measure the thermal properties of the nano-
composite lms and it was calibrated with indium before use.
The samples were taken in aluminum pans and were crimped
using a universal crimper. The thermal stabilities of the
composites were measured using a Perkin-Elmer TGA instru-
ment (Pyris Diamond TG/DTA) under a nitrogen atmosphere at
The PVAc reinforced GP lms were prepared by a solvent cast
method. In this procedure, to prepare the GP/PVAc composites,
250 mg PVAc was dissolved in 5 ml DMF in four culture tubes.
In another three culture tubes different amounts of GP were
dispersed very well in 5 ml DMF and were mixed with the
PVAc solution with stirring. The solution was sonicated for
30 minutes to make the homogeneous solution. The solutions
were then poured in Petri discs and were heated slowly for
evaporation of the solvent in open air to obtain lms of the
nanocomposites. Pure PVAc lm was also prepared in the same
ꢀ
ꢂ1
a heating rate of 10 C min
.
Mechanical properties. Tensile tests were carried out from
the water cast lms of uniform thickness using a universal
testing machine (Zwick Roell, model Z005) at a strain rate of
ꢀ
manner. The composite lms were kept at 60 C in vacuum for
three days to remove any trapped solvent and they were desig-
nated as GP1, GP3, and GP5 respectively; the numbers indicate
the percent of GP (w/w) in the composites. Two other
composites containing 95% PVAc were prepared by mixing 5%
GO (w/w) (GO5) and a total of 5% mixture of GO and PVP
ꢂ1
ꢀ
1
mm min at room temperature (30 C). Each experiment
was repeated for three times to observe reproducibility.
Results and discussion
(
having the composition of GP) (GPP5) in DMF medium fol-
In Scheme 1 the graing of PVP on the GO surface by the RAFT
polymerization technique is illustrated. GO is produced by the
oxidation of graphite powder by the Hummer's method (step-1)
and it was then coupled with 2-bromo-propynyl bromide (BPB)
to produce the GO-BPB (step-2). In step 3 the GO-BPB was
reacted with potassium O-ethyl xanthate (EX) producing the GO-
EX and in step-4 the RAFT polymerization of vinyl pyrrolidone
lowed by drying as above. Also a blend of PVAc and poly-
pyrrolidone containing 5% (w/w) of the latter was made by the
solvent (DMF) cast method followed by drying as above and is
designated as PP.
Characterization
Microscopy. The atomic force microscopy (AFM) was con- (VP) from the GO-EX surface occurs by reversible addition and
ducted in the non contact mode at a resonance frequency of the fragmentation of the C–S bond producing the GO-g-PVP (GP).
tip end of ꢁ250 KHz. GO and GP were dispersed in water
AFM topographic images and the corresponding height
0.01%, w/v) by sonication and then lms were cast on the mica proles are very helpful to characterize the polymer brushes
(
ꢀ
surface. The samples were dried at 60 C on a hot plate and graed on the GO surface and are presented in Fig. 1. It is
ꢀ

nally in vacuum at 60 C for three days. The morphology of the apparent from Fig. 1a that GO has an exfoliated sheet like
dried lms were studied using atomic force microscope (Veeco, structure and from the height prole the thickness of the GO
32
model AP 0100). The composite lms were fractured under sheets is found to be 1.1 nm. It is to be noted here that the
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Scheme 1 Synthesis of GO-g-PVP by RAFT polymerization.
surface of the GO sheet is very smooth, but the GP sheets
To substantiate the covalent linking of the monomer with
(Fig. 1b) exhibit roughness of the surface and the swollen the GO surface the FTIR spectra of GO, GO-BPB, GO-EX and GP
domains (white spots) characterize the graed PVP chains from are compared in Fig. 3. GO shows characteristic peaks for the
ꢂ1
the sheet surface. The average thickness of the GP obtained O–H stretching vibration at 3421 cm , C]O stretching vibra-
ꢂ1
ꢂ1
from the height prole of the AFM image is 8.5 nm, which is tion at 1730 cm , C–O stretching at 1047 cm , and at 1626
ꢂ1
35
much higher than that of GO. The higher thickness of the GP cm for the unoxidized graphite domain. The FTIR spectrum
compared to that of GO certainly indicates the wrapping of the of GO-BPB exhibits the >C]O stretching peak shis to a lower
ꢂ1
graed polymer on the surface of the graphene sheets. The energy vibration at 1720 cm indicating a decrease of bond
33
TEM images of the GO and the GP nanosheets, presented in order due to the ester formation with BPB. The IR spectrum of
ꢂ1
Fig. 2 indicate the exfoliated sheet of GO (Fig. 2a), but in GP the GO-EX shows strong absorption bands at 1050 cm , arising
fully exfoliated GO sheets become coated with molecular from the asymmetric stretching mode of the CS
2
group and at
ꢂ1
brushes of PVP (Fig. 2b). These results, therefore, signify that 1138 cm , arising from the asymmetric stretching mode of the
36a
the PVP chains become successfully graed on the GO surface C–O–C group. The two sets of absorption bands between 1367
ꢂ1
via the RAFT polymerization technique and this type of and 1405 cm are due to the C–H bending modes of GO-EX. It
morphology is also very similar to the other polymer brushes is to be noted here that GO also exhibits small peaks at 2921
33,34
ꢂ1
ꢂ1
anchored on the graphene surface.
The HRTEM images cm
and 2848 cm
resulting from the >CH2 stretching
of GO and GP are presented in the ESI Fig. 1† and one vibration and the >CH group may be present in trans-linked
2
important observation is that in GO the fringe patterns are cyclohexane chairs of graphitic domains in GO, if it is not fully
36b
2
clearly observed, but it is not prominent in GP due to the oxidized But in reduced GO the intensity of the >CH peaks
36c
coating of GO by PVP.
decreases and sometimes become negligible. These changes
1
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Fig. 1 AFM images and corresponding height profiles of (a) GO and (b) GP.
Fig. 2 TEM images of (a) GO and (b) GP.
can be attributed to the alleviation of the groups considered as
defects (epoxy, hydroxyl and carboxylic) in the graphene lattice
and re-establishment, at least to some extent, of the aromatic
3
6d
ꢂ1
structure. In the present system the peaks at 2921 cm and
2
ꢂ1
848 cm become weaker in GO-BPB and GO-EX than that of
GO and it may therefore be attributed to this that GO becomes
partially reduced during the initiator attachment. In the GP, the
FTIR spectrum shows the characteristic vibration peak of GO at
Fig. 3 FTIR spectra of GO, GOPBr, GO-EX and GP.
ꢂ1
ꢂ1
3
424 cm and those of PVP at 1647 cm for >C]O stretching,
ꢂ1
at 1020 and 1102 cm for the C–N vibration, conrming the
graing of PVP from the GO surface. Also a broad peak
37
1
The H NMR spectrum of the GP dispersion in CDCl (Fig. 4)
3
ꢂ1
comprising of two peaks at 2875 and 2962 cm with increased exhibits the methine proton (b) peak at d 3.70 ppm and the peak
intensity is observed due to the presence of the >CH
the PVP chain.
2
group in of methylene protons (c) adjacent to the nitrogen atom in the
pyrrolidonyl ring occurs at d 3.19 ppm. The backbone
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1
Fig. 4
3
H NMR spectra of GP in CDCl .
methylene proton (a) occurs at d 1.30–1.85 ppm, thus over- signicantly, which is also substantiated by the Raman
lapping with the methylene protons (d and e) of the pyrrolidonyl spectra (Fig. 6).
ring, (d 1.85–2.70 ppm). In the GO-g-PVP, essentially all the PVP
In the Raman spectra of GO and GP (Fig. 6) the disorder-
ꢂ1
characteristic resonances are clearly present in the spectrum induced phonon mode (D band) of GO appears at 1367 cm
38a
indicating that the PVP is graed from the GO surface. It and the Raman allowed phonon mode (G band) of the
ꢂ1
should be noted here that like the –COOH functionalized vibrations appears at 1604 cm . The D band arises due to the
3
carbon nanotube GO also does not exhibit any peak in the NMR vibrations of sp carbon atoms (defects), while the G band
38b
2
spectrum.
appears as a result of the vibrations of sp carbon atoms of
The amount of graing of PVP from the GO surface the graphene rings. In the case of GP the D band appears at
ꢂ1
ꢂ1
calculated from the gravimetric analysis before and aer the 1357 cm and the G band appears at 1633 cm . The rela-
polymerization is ꢁ70% (w/w) in the GP sample. However, to tively lower intensity ratio of the D and G bands (ID/IG ¼ 0.2)
estimate the amount of PVP more accurately we have analyzed of GP than that of GO (I
the TGA curves for GO-EX and GP (Fig. 5). It is apparent from graphitic domains in GP than that of GO, probably due to its
Fig. 5 that GO-EX is not thermally stable and begins to reduction during polymerization to produce GP
degrade upon heating below 100 C and it shows a 30% weight of the G band of GP to a higher energy may be attributed to
D
/I
G
¼ 1.14) implies the increased
3
8e,f
The shi
ꢀ
ꢀ
loss up to 250 C. This weight loss is due to the loss of the increase of the graphitic domain size over that of GO
adsorbed water and due to the decomposition of labile oxygen owing to the reduction experienced during graing and
3
8c
containing functional groups as CO and CO2. Aer that, a also due to the difficulty of vibration due to the attachment
very slow weight loss occurs due to the decomposition of the with PVP.
carbon structure. In GP the weight loss in the labile oxygen
The UV-vis spectra of the GO dispersion in water (brown
functional group region is only 10 wt% and then it degrades color) shows absorption peaks at 240 and 305 nm due to the
ꢀ
39a,b
very rapidly in the temperature range 340–400 C exhibiting p–p* and n–p* transitions, respectively (Fig. 7).
But in the
another 74% weight loss due to the decomposition of func- aqueous dispersion of GP (black colour) the above p–p*
tionalized PVP. So it may be inferred that the graing of PVP is absorption peak shows a red shi to 267 nm, characterizing the
7
4 wt% in GP and it is important to note that compared to reduction of GO to rGO during the initiator attachment and
ꢀ
GO-EX the weight loss in GP is 20% less at 250 C and the TGA subsequent polymerization process. A similar phenomenon is
thermogram of GP is very similar to that of RGO. This also reported in the case of ATRP polymerization on the GO
indicates that during polymerization GO-EX gets reduced
38f
Fig. 5 TGA thermograms of GO-EX and GP under a nitrogen atmosphere.
Fig. 6 Raman spectra of GO and GP in aqueous dispersion.
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reason for the broad range solubility of GP may be delineated
by using the Hansen solubility parameters, e.g. d : dispersion
d
cohesion parameter, d : polarity cohesion parameter; and d :
p
H
26,40
hydrogen bonding cohesion parameter.
The dp and dH
values of the different solvents, listed in ESI Table 1,† are the
important factors from the solubility point of view. Very
41
recently, Parades et al. have reported that GO can be
dispersed in almost all of the solvents, except dichloro-
methane, n-hexane, and, to a lesser extent, methanol and
o-xylene. Many of these dispersions (acetone, ethanol, 1-prop-
anol, dimethyl sulphoxide, and pyridine) exhibit only short-
term stability and precipitate out completely from the solution
in few hours to a few days. The GO sheets exhibit a long-term
stability in ethylene glycol, DMF, NMP, and THF but produce
an inhomogeneous dispersion. Similarly Park et al. tried to
disperse GO in DMF at different concentrations (0.1–1 mg
Fig. 7 UV-vis spectra of GO and GP from aqueous solution.
ꢂ1
graphite oxide (GO) ml ) by sonication, but they were unable
to produce a homogeneous GO suspension even on lengthy
sonication (over 24 h) in an ultrasound bath establishing that
3
4
surface where it gets reduced showing a black dispersion. The
red shi in the p–p* absorption peak at 240 nm to 267 nm may,
therefore, be attributed to the high degree of reduction during
the GO sheets are dispersed in organic solvents with the value
42
p H
of d + d lying between 13–53. But GP remains dispersed in
38g
polymer graing from GO as evident from TGA and Raman
spectra.
chlorobenzene, chloroform, DCM, DMF, DMSO, ethanol
methanol and water with the d + d range 6.3–58. From these
p
H
The dispersibility of GP was tested in a series of solvents of
varying polarity (Fig. 8). The color of the aqueous dispersion of
GO was brown and that of GP was black, which might be
results it is evident that the dispersibility of the GO sheets is
greatly improved when it is graed with PVP. This is because
PVP has both a hydrophilic ring part containing a C]O, and
C–N group and hydrophobic vinyl chain so that PVP remains
soluble in a wide variety of solvents. The PVP graed on the GO
surface may therefore facilitate its dispersion ability in the
solvents having a wide range of polarities i.e. a wide range of
34
attributed to its reduction during its preparation. Again GP
dispersion in (i) hexane, (ii) petroleum ether (iii) carbon
tetrachloride and (iv) toluene shows a totally transparent
supernatant part, indicating that the GP is not dispersible in
these solvents. The GP was found to form a stable dispersion in
chlorobenzene, chloroform, 1,2 dichlorobenzene, dimethyl
formamide (DMF), N-methyl pyrrolidone (NMP), dichloro-
methane (DCM), dimethyl sulphoxide (DMSO), ethanol,
p H
solubility parameter (d + d ).
Nanocomposites with PVAc
methanol, and water. Thus it is not dispersible in less polar The three nanocomposites GP1, GP3 and GP5, produced by
solvents than chlorobenzene e.g. in toluene, xylene etc. By mixing PVAc with 1, 3 and 5% (w/w) GP in a common solvent
checking the solubility of GP in different solvents it is noted DMF, were homogenized by sonication and the dispersion was
that on graing PVP polymer GO becomes amphiphilic. The found to be stable for a month. To produce the composites the
ꢂ1
Fig. 8 Dispersibility of GP in different solvents (2 mg ml ) (a) chlorobenzene (b) chloroform (c) 1,2 dichlorobenzene (d) dichloromethane (e) NMP (f) DMF (g) DMSO
(
h) ethanol (i) methanol (j) water and dispersion of pure (k) GO5 and (l) GP5 (m) GPP5 composite in DMF.
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ꢀ
above dispersion was dried at 60 C in air and nally in vacuum. suggesting that GO is fully exfoliated into individual sheets in
43–45
Here, we note that GO does not produce a homogenous all the GP composites.
suspension, even with the long time (2 h) sonication (Fig. 8k)
42
preventing us forming the composite with pure GO. Also, as
mentioned above PVP and PVAc are incompatible, making the
system very interesting with the formation of a stable homog-
enous dispersion of GP with PVAc in DMF (Fig. 8l). In Fig. 8m
GO is noncovalently functionalized with PVP by mixing the
components in DMF and following this by sonication for half an
hour. The above solution was then added to the PVAc solution
to make the composition the same with the GP5 solution and
was sonicated for half an hour and a dispersion of lesser degree
than that of GP5 is observed. These results indicate that PVP
might have some inuence in making the stable dispersion of
GO in the PVAc solution. In other words, it might be possible
that GO acts as a compatibilizer for PVP and PVAc. The GP/PVAc
composites are characterized by morphology, thermal study
and the mechanical properties are presented in the following
sections.
FTIR
To establish the nature of interaction between the PVAc and GP
sheets the FTIR spectra of PVAc and composite lms are pre-
sented in Fig. 10. Pure PVAc displays the characteristic bands at
ꢂ1
1242 and 1736 cm for the C–O and >C]O vibrations of the
46a
ester carboxylate group.
In the composites the vibration
bands of GP (cf. Fig. 4) are not observed at all due to the low
concentration of GP (1–5 wt%). But it is interesting to note that
the stretching frequencies of the ester carboxylate group of
PVAc are shied gradually to a lower frequency with increasing
ꢂ1
GP concentration, e.g. for GP3 1733 cm and for GP5 1731.5
ꢂ1
cm . This result suggests that GP sheets interact with the PVAc
matrix resulting in a molecular level dispersion of GP sheets.
Hence the PVAc composite lms may yield a high performance
nanocomposite due to easy load transfer through its compact
ꢂ1
interface. It is to be noted here that a new peak at 1587 cm
appears at the different GP composites and the peak intensity
increases with increasing GP concentration. The PVP doesn’t
have any characteristic peak in this region and therefore this
FESEM
The cross-sectional morphologies of the pure PVAc and GP/
PVAc (GP5) composite obtained from FESEM images (Fig. 9a
and b) indicate a continuous polymer matrix for pure PVAc and
a good homogeneous dispersion of GP sheets within the PVAc
matrix in the GP5 composite. The GP sheets are identied by
more black contrast than that of PVAc in the composite. A good
dispersion of GP sheets throughout the PVAc matrix in the
solution and especially in the solid state is interesting and may
arise from the synergetic effect of GO and PVP of GP in the
PVAc matrix. It is apparent from the gure that the cross
sectional image of PVAc is very smooth, but that of GP5 is
uneven, which may be attributed to the graed PVP chain on
the GO surface.
46b
peak is arising from the graphene. The graphene shows a
characteristic peak for the aromatic C]C stretching in the
ꢂ1
38d
broad range 1570–1622 cm reported in literature. But this
aromatic peak is not observed in the spectrum of GP (Fig. 3) and
it might be due to the overlapping with the >C]O peak of PVP
Wide angle X-ray diffraction (WAXS)
To get an idea of the structure of the composite lms the WAXS
patterns of GO, PVAc and the different PVAc composite lms are
presented in ESI Fig. 2.† The PVAc shows a very broad amor-
phous diffraction peak due to its amorphous nature. It is
apparent that the GO shows its characteristic diffraction peaks
ꢀ
at 2q ¼ 10.55 , but in the composites these peaks are absent
Fig. 9 FESEM cross-sectional images of the tension fractured surface of (a) pure
PVAc and (b) the GP5 composite (arrows indicate the GP sheets).
Fig. 10 FTIR spectra of the PVAc, GP, GO and GPP composites.
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Journal of Materials Chemistry A
ꢂ1 46b
at 1647 cm
.
Also this aromatic C]C stretching peak is Compatibilizer action of GO
absent in GO5 and GPP5 and this may be due to the smaller
amount of C]C aromatic skeleton present in the GO compared
to GP, where an increase of the aromatic part occurs due to the
substantial reduction during polymerization from GO (cf. TGA
and Raman spectra).
A possible mechanism of the compatibilizing action of GO in
the PVP/PVAc system is highlighted here. The essential
requirement to act as a blend compatibilizer is to reduce the
interfacial tension between the incompatible blends and
diblock copolymers have been proven to be the best blend
52a–c
compatibilizers.
producing the polymer nanocomposite, a good dispersion of
) of the pure PVAc and the the nanoller in a common solvent with the polymer is a very
g
In solution the blending method of
Differential scanning calorimetry
The glass-transition temperature (T
GP composite lms obtained from DSC curves in the same important and a necessary requirement. Here, we prepared
range of temperatures are shown in Fig. 11. Analysis of the Tg PVAc solution in DMF and also we prepared the GO dispersion
data from the maximum of the derivatogramm (ESI Fig. 3†) in DMF separately, but the latter is not homogeneously
indicates that the segmental motion of the PVAc chain is dispersed. When the latter solution was poured into the former
hindered due to the incorporation of GP sheets into the PVAc PVAc solution it also exhibited bad dispersion, even aer long
ꢀ
of pure PVAc is 38.5 C and in the case of the GP5 sonication (60 min). In DMF solution the GO sheets have high
g
matrix. The T
sample it increases to 44 C. The increase of T
lms compared to that of pure PVAc may be attributed to the fragile interfacial interaction between the GO and PVAc,
ꢀ
of the composite cohesive energy between the sheets, so there is a chance of
g

more compact nature of the PVAc chains arising from the strong although PVAc has an ester functional group which has the
cohesive force between the GP platelets, restricting the capability to interact with the –OH and –COOH groups of GO.
segmental motion of PVAc chains due to a decrease in free Therefore, GO does not mix well with PVAc, when the GO/PVAc
47
volume fraction and hence increase of Tg. This type of increase composite is fabricated from DMF solution, but in the presence
of T
is also demonstrated in other reports of polymer–graphene of PVP the situation improved signicantly although PVP and
composite lms.
composite containing 5% (w/w) GO (GO5) there is no change of acting as a compatibilizer.
g
48,49
Here we also note that in the GO/PVAc PVAc are reported to be immiscible and it might be that GO is
T
g
, but in the GPP5 composite containing mixture of GO, PVP
PVP is a polar polymer and easily dissolves in water. When
and PVAc at the same composition as in GP5 an increase of T
4
g
to we add PVP into the inhomogeneous GO solution and sonicate
ꢀ
1.3 C was seen. Though the increase of T of PVAc is not as the solution, the cohesive energy between the GO sheets get
g
good as in the covalently functionalized GP5 system, the present reduced due to the interaction between the pendent epoxy,
increase of Tg certainly indicates GO became non-covalently hydroxyl and carboxylic groups of GO and the >C]O group of
functionalized with PVP causing nanocomposite formation polar PVP. Therefore, the dispersibility of GO is greatly
with PVAc. This behavior of GO is very much similar to the increased. When this DMF dispersion of GO/PVP is mixed with
effect of compatibilizer (bisphenol A) in the compatibility of PVAc solution in DMF, GO remains homogenously dispersed.
29b
PVP and PVAc.
As GO sheets remain well dispersed here, PVAc gets sufficient
opportunity to interact through the ester group with the
carboxylic and hydroxyl groups of GO. This facilitates the
formation of a homogenous dispersion of GO in the PVAc
matrix. So PVP decreases the interfacial tension between GO
sheets facilitating the GO sheet to interact with PVAc; thus GO
acts as a compatibilizer between PVP and PVAc.
Mechanical properties
The mechanical behavior relating to the tensile strength,
toughness, Young's modulus, of PVAc and GP/PVAc composite

lms were studied and the stress–strain curves are presented in
Fig. 12a. It is evident from the gure that the tensile stress
gradually increases and the tensile strain gradually decreases
with an increase in GP concentration in the composites (ESI
Table 2†). To obtain a quantitative and a comparative picture of
the reinforcement of all the above mechanical parameters the
data is presented in the bar diagrams (Fig. 12(b–e)). It is evident
from the Fig. 12(b) and (c) that PVAc gradually becomes harder
as the strain at break gradually decreases and the stress at break
gradually increases with increasing GP concentration. Pure
PVAc lm has the Young's modulus value of 1.47 GPa and it also
increases gradually with an increase of GP concentration
exhibiting a 190% increase for the GP5 sample. The tensile
Fig. 11 The glass transition temperature of PVAc and its composites from DSC
heating thermograms.
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mobility of the PVAc polymer chains causing an increase of
the glass-transition temperature of the GP/PVAc nanocomposite
in comparison to the pristine PVAc, causing enhanced adhesion
51,53a
at the GP– PVAc matrix interface.
A comparison of the mechanical properties of the GP/PVAc
composite with that of the carbon nanotube (CNT)/PVAc
composites would be benecial to understand the reinforce-
ment property of GP. For 1 wt% of ller, the multiwalled carbon
nanotube (MWNT)/PVAc composite stabilized by sodium
dodecyl sulfate exhibits a 56% increase in Young's modulus and
53b
6% increase of tensile strength,
whereas in the GP/PVAc
composite containing 1 wt% of GP the increase of Young's
modulus is 63% and that of tensile strength is 62%. The
dynamic Young's modulus of the pristine MWNT/PVAc
composite increases by 25% and that of the PVAc graed
MWCNT PVAc composites increased by 45% at 1 wt% of the
53c
MWNT. Hence it may be stated that GP is a superior rein-
forcing agent compared to that of MWNT or functionalized
MWNTs.
Generally, the load transfer efficiency depends strongly on
the interfacial interaction between the polymer and the nano-

ller and also to its orientation in the composite. The orienta-
tion of the GP sheets within the PVAc matrix can be judged from
54
the Halpin and Tsai equation in conjunction with the Young's
modulus data. For the random distribution of GP in the PVAc
matrix, the modied form of the Halpin and Tsai equation may
55
be written as
ꢀ
ꢁ
3
8
1 þ ð2L
G
=3T
G
Þh V
G
5 1 þ 2h V
G
L
T
E
Y
¼
þ
(1)
1 ꢂ h V
G
8 1 ꢂ h V
G
L
T
where
Fig. 12 (a) Stress–strain curves of PVAc and its nanocomposites with GP at the
indicated compositions. Bar diagrams showing the variation of mechanical
properties with GP wt percent (b) tensile strain, (c) tensile stress, (d) Young's
modulus, and (e) toughness.
ðE
G
=E
P
Þ ꢂ 1
=3T
hL ¼
ðE
G
=E
P
Þ þ ðL
G
G
Þ
ðE
ðE
G
G
=E
=E
P
P
Þ ꢂ 1
Þ þ 2
hT ¼
strength of PVAc increases gradually from 7 ꢃ 0.3 to 18.8 ꢃ
and E , E and E are the Young's modulus of the composite, GP
and PVAc respectively. L , TG and VG represent the length,
thickness and volume fraction of the GP units present in the
composites. The modied Halpin–Tsai equation for the unidi-
rectional (parallel) orientation of GP nanosheets in the polymer
Y
G
P
1.2 MPa in the GP5 sample showing a 169% increase over that of
G
pure PVAc lms. The toughness was calculated from the area
under the stress–strain curve (Fig. 12a) and is presented in
Fig. 12f. It is interesting to note that it shows a dramatic
increase with GP concentration, showing a maximum for the
GP3 sample (112% increase over PVAc) and a better dispersion
of GP sheets throughout the whole matrix is a probable cause.
Therefore, the addition of GP into the PVAc matrix causes an
overall enhancement of mechanical properties, probably due to
the good reinforcement property of the nanoller (GP) with the

lms is written as:
ꢀ
ꢁ
1
þ ð2L
G
=3T
G
Þh V
G
0
0
L
E
¼
E
P
(2)
Y
1
ꢂ h V
G
L
G
Approximating the E (250 GPa) of GP to be equal to those
host PVAc matrix due to easier load transfer through the GP of graphene and taking the values of E
54
¼ 1.47 GPa, density of
p
ꢂ3
interfaces. The improvement of all the mechanical properties of PVAc ¼ 1.19 g cm , T
¼ 8.2 nm and L
¼ 800 nm the plots of
G
G
PVAc is governed by the good dispersion of GP sheets into the
E are made according to both the random distribution (line 1)
Y
PVAc matrix (cf. Fig. 9) and the strong interfacial interaction and unidirectional orientation (line 2) and are compared with
between GP and PVAc, as evident from the shi of vibration the experimental plot (line 3) in Fig. 13. It is evident that the
frequency of the carboxylate ester group (cf. Fig. 10). Such a experimental E values lie near the line calculated using unidi-
y
molecular level dispersion and strong interfacial interactions rectional (parallel) distribution of the ller in the PVAc matrix.
may play an efficient role in efficient load transfer between the Thus the GP sheets favor aligning parallel to the surface of the
50
GP sheets to the PVAc polymer matrix. These also affect the PVAc lm within the GP nanocomposites. So, the interaction
1
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Acknowledgements
We gratefully acknowledge CSIR, New Delhi (Grant no.
2(0051)/12/EMR-II) and DST Unit of Nanoscience at IACS for
nancial support. RKL and AK acknowledge CSIR, New Delhi
0

for the fellowship.
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0874 | J. Mater. Chem. A, 2013, 1, 10863–10874
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