Content uploaded by Nataša Jović Orsini
Author content
All content in this area was uploaded by Nataša Jović Orsini
Content may be subject to copyright.
Materials and Manufacturing Processes, 24: 1053–1057, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6914 print/1532-2475 online
DOI: 10.1080/10426910903022346
Mechanochemical Exfoliation of Graphite and Its Polyvinyl Alcohol
Nanocomposites with Enhanced Barrier Properties
C. Borriello
1
, A. De Maria
1
, N. Jovic
2
, A. Montone
3
, M. Schwarz
3
, and M. Vittori Antisari
3
1
Department of Physical Methods and Materials, ENEA, Granatello, Napoli, Italy
2
The Vin
ˇ
ca Institute of Nuclear Sciences, Laboratory of Solid State Physics, Belgrade, Serbia
3
Department of Physical Methods and Materials, ENEA, Roma, Italy
Polyvinylalcohol/graphite nanocomposites with graphite nanosheets have been prepared by a mechanical method based on grinding of graphite
powder, under low energy pure shearing milling, using water or KOH as lubricant. The use of different lubricant concurs to obtain graphite sheets
that differently disperse in hydrophilic polymeric matrix. An improvement of water vapor permeability (up to 12%), compared with homopolymer,
has been observed.
Keywords Barrier properties; Graphite; Nanocomposites; Polyvinyl alcohol.
1. Introduction
In recent years, polymer/layered-inorganic and
polymer/clay nanocomposites are being explored
extensively due to their high promise for advanced
technologies, such as electrochemical displays, sensors,
catalysis, etc. In fact nanocomposites show dramatic
changes in properties at very low loadings of such
nanofillers as exfoliated nanoclays [1, 2], graphite
nanoplatelets [3–6], and carbon nanotubes (CNTs) [7–9].
This performance is achieved, not only by using the
inherent properties of the nanofiller, but more importantly
by optimizing the dispersion, interface chemistry, and
nanoscale morphology to take advantage of the enormous
surface area per unit volume that nanofillers have.
Graphite is a layered material consisting of one-atom-
thick sheets of carbon. Its structural analogy to layered
silicates and chemical analogy to carbon nanotubes make
graphite an attractive nanofiller in both scientific study
and technological application. By separating the graphite
layers a potential high aspect ratio graphene sheets can be
obtained satisfying the high-aspect-ratio criterion needed for
nanocomposites.
In particular, graphene-based polymer composites possess
potential applications in radiation and electromagnetic
shielding, antistatic, corrosion-resistant coatings, and other
mechanical and functional attributes such as barrier,
conducting capabilities, light emitting devices, batteries, and
other functional applications [10].
However, the manufacturing of polymer/graphite
nanocomposites requires that graphite sheets be produced
on a sufficient scale, and they also be homogeneously
distributed into various matrices.
Received September 25, 2008; Accepted January 12, 2009
Address correspondence to A. Montone, Department of Physical
Methods and Materials, ENEA, C. R. Casaccia, Via Anguillarese 301,
00123 Roma, Italy; E-mail: amelia.montone@enea.it
Many methods have been developed exclusively
to prepare graphite nanosheets, which include
micromechanical cleavage [11, 12], chemical vapor
deposition (CVD) [13], solvent thermal reaction [14],
thermal desorption of Si from SiC substrates [15], and
chemical routes via graphite intercalation compounds (GIC)
[16] or graphite oxide (GO) [17]. Among these, the latter
route by chemical treatment of natural graphite [17, 18], is
the most used for fabricating graphenes in large quantities
for industrial applications but involve relatively undesirable
solvents and extreme conditions.
In order to avoid impurities due to the chemical treatments
during the process of chemical exfoliation, the mechanical
milling process is an alternative method for the production
of graphite nanopowder and nanosheets.
In this article we present the preparation of two type of
graphite nanosheets by using the prolonged milling under
low intensity pure shear stress previously described [19], but
using two different lubricants, water or potassium hydroxide
(KOH). In this way, it has been possible to investigate the
influence of the lubricant on the surface functionalization
of the graphite sheets and, consequently, the different
capability to disperse in hydrophilic polymeric matrix
during the preparation of nanocomposites. In fact, surface
functionalization of the filler can improve the interfacial
adhesion between the filler and the matrix determining
properties of nanocomposites.
Polyvinylalcohol has been used as polymeric matrix for
the preparation of graphite nanocomposites by solution
method. Barrier properties of the nanocomposites have been
investigated, too.
Polyvinylalcohol is a water-soluble synthetic polymer
with a high hydrophilicity, biocompatibility, and
nontoxicity. It has excellent film forming, emulsifying, and
adhesive properties, so it is used as warp sizing, paper
coating agents, adhesives, a carrier in drug delivery, and a
component of biomedical and packaging material [20, 21].
1053
1054 C. BORRIELLO ET AL.
However these properties are dependent on humidity so
its exploitation in many packaging applications is reduced.
For this reason, the interest in mixing of PVOH with fillers
is rapidly growing [22].
2. Experimental
2.1. Materials
The materials used in this investigation were Graphite
(Aldrich, 99.999% pure) and Polyvinylalcohol (M
w
124000-
186000) Aldrich; methanol and KOH Carlo Erba Reagenti.
2.2. Preparation of Graphite Nanosheet
Graphite powders have been pulverized in an agate mortar
and later processed in a Retsch Ultra-Fine mortar grinder
(Type KM1) [4], using water or KOH as lubricant to obtain
G
W
and G
KOH
, respectively. G
W
has been milled for 20
hours in Pulverisette 2 mill, in the presence of water and
subsequently dried and G
KOH
has been obtained by dry
milling 3 g of graphite powder with 10 g of potassium
hydroxide (KOH), for 10 hours in the same kind of mill,
after which washing in deionized water and in methanol
were applied till the pH value of 7 of washing water was
reached, followed by drying.
2.3. Synthesis of Nanocomposites
Polyvinylalcohol/Graphite
Nanocomposites PVOH/G
W
and PVOH/G
KOH
containing 5% wt of G
W
and G
KOH
, respectively, have been
prepared. A suspension of graphite in water has been added
to a hot aqueous solution of PVOH, and the mixture has
been refluxed for 4 hours, concentrated, and deposited on
a glass surface to obtain films by casting.
2.4. Analysis
X-ray diffraction (XRD) peak profiles of the (002)
and (004) reflections of starting graphite powder and G
W
were collected on Bruker D8 Advance diffractometer in
glaze angle (2
) incident geometry using CuK radiation
(1.5406 nm), voltage of 38 kV, and a current of 30 mA.
Water vapor permeability has been measured by
Extrasolution PermeH
2
O instrument at 23
C, 85% RH,
using a testing range of 0.002–500g/m
2
/d.
2.5. Morphological Characterization
The morphology of the filler G
W
and G
KOH
and cross-
section of composites were examined by scanning electron
microscopy (SEM, Cambridge 250 MKIII) equipped with
EDS microanalysis and backscattered electron detector.
3. Results and discussion
Graphite G
W
and G
KOH
have been obtained as described
above. The XRD analysis (Fig. 1) of starting graphite
powder and G
W
indicate the broadening of (002) and (004)
reflections after performing milling due to exfoliation of the
graphite powder along [001] direction under shear effect.
On the other hand, it is not observed a notable angular shift
of reflections indicating that a d-spacing between graphite
Figure 1.—XRD profile of (002) and (004) reflections of G
W
before and after
the milling.
sheets stay preserved in the individual graphite nanoplatelets
(in graphite d = 034 nm). The same has been observed
for G
KOH
.
In Figs. 2(a) and (b) we can see the difference in the
microstructure of these two types of graphite sheets. While
the presence of water during mechanical milling process
led to the production of platelet graphite sheets with rather
smooth surface, dry milling with KOH led to increase of
surface roughness and by-product of graphite dust [particles
with notable smaller dimensions, seen in Fig. 2(b)].
The state of dispersion in the polymeric matrix has been
analyzed by XRD and SEM analysis of PVOH/G
W
and
PVOH/G
KOH
nanocomposites.
Figure 3 shows the XRD patterns of 5 wt%
polyvinylalcohol/graphite nanocomposites.
While pure polymer shows only a broad peak at 2 of
19.5
, the nanocomposites show sharp peaks at 2 values
corresponding to the characteristic peaks of pure graphites
(26.5
). Therefore, the occurrence of peaks confirms not
only the presence of pure graphite into the nanocomposites
but also the fact that the individual graphite nanosheets
MECHANOCHEMICAL EXFOLIATION OF GRAPHITE AND ITS POLYVINYL ALCOHOL 1055
Figure 2.—SEM images of graphite sheets (a) milled with water and (b)
milled with KOH.
consist of multilayer graphite sheets, and no intercalation
has occurred in the space between carbon layers.
SEM images clearly show, in Fig. 4, that the distribution
and dispersion of the graphite G
KOH
in the polymer matrix
is better than graphite G
W
. In fact, Figs. 4(a) and 4(b) show
the presence of wide size aggregates. Inferior distribution
of G
W
particles inside polymer matrix probably is due to
the larger size of G
W
particles and their pursuit to settle to
the bottom of the holder during preparation procedure. The
same has been observed in the G
W
/epoxy composites.
Figure 3.—XRD characterization of PVOH/graphite nanocomposites.
3.1. Barrier Properties
The high aspect ratio of graphitic additives, which is
defined as half the flake width divided by its thickness,
suggests their potential use for reducing gas permeability of
polymer films. Gas permeability through a polymer filled
with high aspect ratio, impermeable flakes can be decreased
substantially via a reduced cross-section for gas diffusion
and a tortuous path mechanism [22].
In this article, we have studied diffusion of water vapor
across the PVOH nanocomposites. In the case of the
PVOH/G
KOH
sample, the permeability is reduced for 11.5%
passing by 9.68 g/m
2
d value for PVOH to 8.57 g/m
2
d for
PVOH/G
KOH
. For PVOH/G
W
, the permeability measured
is 9.10g/m
2
d.
State of clay nanodispersion has been examined also
by evaluation of aspect ratios of clay nanoplatelets from
permeability measurements and using a permeability model.
In fact, the effective permeability of a nanocomposites P
c
is reduced from that in the pure polymer film P
m
by the
following equation [23, 24]:
P
m
/P
c
= 1 +
2
2
/1 − (1)
where is the volume fraction of the filler of aspect ratio .
Using Eq. (1), the effective aspect ratio of the organoclay
platelets in nanocomposites has been calculated.
The volume fraction of organoclay () is obtained from
weight percent (wt%) using Eqs. (2) and (3) below:
= wt%
c
/100
G
W
or
G
KOH
(2)
100/
c
= wt%/
G
W
+ 100 − wt%/
m
(3)
where
c
,
G
W
,
G
KOH
, and
m
are densities of nano-
composite, G
W
, G
KOH
, and matrix, respectively. Density of
G
W
and G
KOH
are 1.56 g/cm
3
and 1.44 g/cm
3
, density of
PVOH is about 1.18 g/cm
3
. Therefore, 5 wt% of filler inside
composites correspond to 3.81 and 4.13 vol% of G
W
and
G
KOH
, respectively.
The resulting aspect ratio of organoclay is higher for
G
KOH
(73) than for G
W
(42) indicating a better dispersion of
the clay in PVOH/G
KOH
. These results are consistent with
SEM analysis.
However, platelets fully dispersed can have aspect ratios
between 100 and 300, even at lower mineral loading of the
order of 1.5 vol%. PVOH nanocomposites performance is
not realized to that expectation. It is necessary to improve
intercalation of polymeric matrix into carbon layers of
graphite nanosheets, probably using a different lubricant
during the milling process.
4. Summary
Two different types of graphite nanosheets, G
W
and
G
KOH
, have been obtained by low energy pure shear
milling in the presence of water and KOH as lubricants,
respectively. SEM analysis has shown difference in platelet
dimensions and surface roughness of G
W
and G
KOH
.
Inevitably, different surface functionalization of these
nanosheets has been obtained according to untreated
graphite powder. It can improve the dispersion into a
1056 C. BORRIELLO ET AL.
Figure 4.—SEM images of (a, b) PVOH/G
W
and (a
,b
) PVOH/G
KOH
nanocomposites; a, a
are secondary electron images while b, b
are backscattered
electron images.
hydrophilic polymeric matrix as is polyvinylalcohol matrix
(PVOH).
Five wt% of G
W
and G
KOH
were mixed with PVOH
to produce nanocomposites PVOH/G
W
and PVOH/G
KOH
.
Water vapor permeabilities of PVOH dropped after
incorporation of graphite due to its aspect ratio and
impermeability [23]. PVOH/G
KOH
has shown better
performance. We suggested that this is consequence
of a different functionalization of these two types
of graphite. G
KOH
is more homogeneously dispersed
into the polyvinylalcohol matrix and probably causes a
better interfacial bonding at the graphite nanosheet/matrix
interface. No reports, to our best knowledge, have
studied the barrier properties of polyvynilalcohol/graphite
nanocomposites.
References
1. Ray, S.S.; Okamoto, M. Polymer/layered silicate
nanocomposites: A review from preparation to processing.
Progress in Polymer Science 2003, 28, 1539–1641.
2. Usuki, A.; Hasegawa, N.; Kato, M. Polymer–clay
nanocomposites. Advances in Polymer Science 2005, 179,
135–195.
3. Zheng, W.; Wong, S.C. Electrical conductivity and dielectric
properties of PMMA/expanded graphite composites. Composites
Science and Technology 2003, 63, 225–235.
4. Zheng, W.; Lu, X.H.; Wong, S.C. Electrical and mechanical
properties of expanded graphite reinforced high-density
polyethylene. Journal of Applied Polymer Science 2004, 91,
2781–2788.
5. Cho, D.; Lee, S.; Yang, G.M.; Fukushima, H.; Drzal, L.T.
Dynamic mechanical and thermal properties of phenylethynyl-
terminated polyimide composites reinforced with expanded
graphite nanoplatelets. Macromolecular Materials and
Engineering 2005, 290, 179–187.
6. Ramanathan, T.; Stankovich, S.; Dikin, D.A.; Liu, H.; Shen,
H.; Nguyen, S.T.; Brinson, L.C. Graphitic nanofillers in PMMA
nanocomposites. An investigation of particle size and dispersion
and their influence on nanocomposite properties. Journal of
Polymer Science B: Polym. Phys. 2007, 45, 2097–2112.
7. Ajayan, P.M.; Schadler, L.S.; Giannaris, C.; Rubio, A. Single-
walled carbon nanotube-polymer composites: Strength and
weakness. Advanced Materials 2000, 12, 750–753.
8. Thostenson, E.T.; Ren, Z.F.; Chou, T.W. Advances in the
science and technology of carbon nanotubes and their composites:
A review. Composites Science and Technology 2001, 61,
1899–1912.
9. Ramanathan, T.; Liu, H.; Brinson, L.C. Functionalized
SWNT/polymer nanocomposites for dramatic property
improvement. Journal of Polymer Science B: Polym. Phys. 2005,
43, 2269–2279.
10. Zheng, W.; Wong, S.C.; Sue, H.J. Transport behavior of
PMMA/expanded graphite nanocomposites. Polymer 2002, 43,
6767–6773.
11. Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.;
Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric
field effect in atomically thin carbon films. Science 2004, 306,
666–670.
12. Bunch, J.S.; van der Zande, A.M.; Verbridge, S.S.; Frank, I.W.;
Tanenbaum, D.M.; Parpia, J.M.; Craighead, H.G.; McEuen, P.L.
MECHANOCHEMICAL EXFOLIATION OF GRAPHITE AND ITS POLYVINYL ALCOHOL 1057
Electromechanical resonators from graphene sheets. Science
2007, 315–490.
13. Wang, J.J.; Zhu, M.Y.; Outlaw, R.A.; Zhao, X.; Manos, D.M.;
Holloway, B.C. Synthesis of carbon nanosheets by inductively
coupled radio-frequency plasma enhanced chemical vapor
deposition. Carbon 2004, 42, 2867–2872.
14. Kuang, Q.; Xie, S.-Y.; Jiang, Z.-Y.; Zhang, X.-H.; Xie, Z.-X.;
Huang, R.-B.; Zheng, L.-S. Low temperature solvothermal
synthesis of crumpled carbon nanosheets. Carbon 2004, 42,
1737–1741.
15. Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A.Y.; Feng, R.;
Dai, Z.; Marchenkov, A.N.; Conrad, E.H.; First, P.N.; de Heer,
W.A. Ultrathin epitaxial graphite: 2D electron gas properties
and a route toward graphene-based nanoelectronics. Journal of
Physical Chemistry B 2004, 108, 19912–19916.
16. Viculis, L.M.; Mack, J.J.; Kaner, R.B. A chemical route to carbon
nanoscrolls. Science 2003, 299, 1361–1361.
17. Stankovich, S.; Dikin, D.A.; Dommett, G.H.B.; Kohlhaas, K.M.;
Zimney, E.J.; Stach, E.A.; Piner, R.D.; Nguyen, S.T.; Ruoff,
R.S. Graphene-based composite materials. Nature 2006, 442,
282–285.
18. Gogotsi, Y.; Libera, J.A.; Kalashnikov, N.; Yoshimura, M.
Graphite polyhedral crystals. Science 2000, 290, 317–320.
19. Vittori Antisari, M.; Montone, A.; Jovic, N.; Piscopiello, E.;
Alvani, C.; Pilloni, L. Low energy pure shear milling: A method
for the preparation of graphite nano-sheets. Scripta Materialia
2006, 55, 1047–1050.
20. Hassan, C.M.; Peppas, N.A. Structure and application of
poly(vinyl alcohol) hydrogels produced by conventional
crosslinking or by freezing thawing methods. Advances in
Polymer Science 2000, 153, 37–65.
21. Lai, S.; Casu, M.; Saba, G.; Lai, A.; Husu, I.; Masci, G. Solid-sate
13C NMR study of poly(vinyl alcohol) gels. Solid State Nuclear
Magnetic Resonance 2002, 21, 187–196.
22. Kalaitzidou, K.; Fukushima, H.; Drzal, L.T. Multifunctional
polypropylene composites produced by incorporation of
exfoliated graphite nanoplatelets. Carbon 2007, 45, 1446.
23. DeRocher, J.P.; Gettelfinger, B.T.; Wang, J.; Nuxoll, E.E.;
Cussler, E.L. Barrier membranes with different sizes of aligned
flakes. Journal of Membrane Science 2005, 254, 21–30.
24. Lape, N.K.; Nuxoll, E.E.; Cussler, E.L. Polydisperse flakes in
barrier films. Journal of Membrane Science 2004, 236, 29–37.
