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Mechanochemical Exfoliation of Graphite and Its Polyvinyl Alcohol Nanocomposites with Enhanced Barrier Properties

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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.
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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.
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The substitution of conventional plastic materials with biodegradable and bio-based materials is an interesting alternative for dealing with environmental problems related to their wide use. The aim of this thesis is to develop new nanocomposite materials, dedicated to food packaging, safe for the consumer and the environment. The bio-nanocomposites are produced by extrusion using two biopolyesters as matrix (PBSA or PHBV), chosen for their differing transfer properties, and lamellar nanoclays as reinforcement. The investigations were focused on two major scientific issues: (i) to understand the impact of the introduction of nanoclays on the material properties at all stages of its life cycle: after production, during its use in food contact and during its end-of-life; (ii) to study the benefits-risks balance related to the addition of nanoclays in biodegradable materials dedicated to food packaging. The first part of this work aims to evaluate the impact of the addition of nanoclays on the properties of transfer and inertia of these materials and consequently of their suitability for food contact. In this way, the conditions defined in the European regulation No 10/2011 dedicated to non-degradable materials were first used, then methods more adapted to this type of materials have been developed. Subsequently, the benefits in term of barrier properties and the risks related to consumer exposure to additives present in the materials were quantified using different case studies. Finally, a study of the impact of the nanoclays on the end-of-life of the materials was conducted by biodegradation tests in soil and ecotoxicity tests on plants and soil.
Article
In this work, the physicochemical and rheological properties of a nanocomposite of graphene oxide (GO) and thermoplastic polyvinyl alcohol (PVA) are reported. After adding deionized water to PVA, the sample was heated and mixed. The polymer solution was divided in two portions, each of them containing PVA in water. GO nanoribbons were added to one portion of the PVA solution in water to obtain the nanocomposite suspension (1% w/w). The similarity of the Fourier-transform infrared spectra of PVA and its related nanocomposite was assessed (both having similar peaks, an important observation as it indicated the water absorption on the samples). The scanning electron microscopy images showed that PVA lost its spherical shape in composites due to the GO sheets coating. The characteristic peaks O ⁻ ( m/ e 16) and OH ⁻ ( m/ e 17) were observed in the time-of-flight secondary ion mass spectrometry negative mode spectra of PVA and nanocomposite of GO nanoribbons with PVA. The analysis of the OH ⁻ , O ⁻ , C 2 H ⁻ , and C 2⁻ peaks of PVA and the nanocomposite of GO nanoribbons revealed that a reduction occurred for GO nanoribbons when coated with PVA. The rheological behaviors of samples in solution were studied and their shear rate versus shear stress were assessed. A non-Newtonian rheological behavior of the nanocomposite of GO nanoribbons with PVA was observed. The dielectric response of the nanocomposite of GO-PVA featured a simultaneous increase of both real and imaginary part of the complex permittivity toward low frequency due to the contribution of charge carriers. These results provide more understanding about the physical, mechanical, and dielectric properties of GO-PVA nanocomposite to make it appropriate in the construction of embedded capacitors. The novelty of our work lies in the rheological and physical properties of this nanocomposite in comparison with those of PVA that have not been previously reported.
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Graphite nano-sheets, having a thickness of the order of 10 nm, high shape asymmetry, and a well preserved crystalline order, have been prepared by a mechanical method based on the grinding of graphite powder under low energy pure shear milling using water as a lubricant. The small applied stress activates the plastic deformation process mainly on the easy gliding (00 1) hexagonal planes so that planar flakes bounded by the basal planes are obtained by a purely mechanical process. (c) 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
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A review is given of the academic and industrial aspects of the preparation, characterization, materials properties, crystallization behavior, melt rheology, and processing of polymer/layered silicate nanocomposites. These materials are attracting considerable interest in polymer science research. Hectorite and montmorillonite are among the most commonly used smectite-type layered silicates for the preparation of nanocomposites. Smectites are a valuable mineral class for industrial applications because of their high cation exchange capacities, surface area, surface reactivity, adsorptive properties, and, in the case of hectorite, high viscosity and transparency in solution. In their pristine form they are hydrophilic in nature, and this property makes them very difficult to disperse into a polymer matrix. The most common way to remove this difficulty is to replace interlayer cations with quarternized ammonium or phosphonium cations, preferably with long alkyl chains.
Article
Poly(vinyl alcohol) (PVA) is a polymer of great interest because of its many desirable characteristics specifically for various pharmaceutical and biomedical applications. The crystalline nature of PVA has been of specific interest particularly for physically crosslinked hydrogels prepared by repeated cycles of freezing and thawing. This review includes details on the structure and properties of PVA, the synthesis of its hydrogels, the crystallization of PVA, as well as its applications. An analysis of previous work in the development of freezing and thawing processes is presented focusing on the implications of such materials for a variety of applications. PVA blends that have been developed with enhanced properties for specific applications will also be discussed briefly. Finally, the future directions involving the further development of freeze/thawed PVA hydrogels are addressed.
Article
Polymethylmethacrylate (PMMA)/expanded graphite (EG) and PMMA/untreated graphite (UG) composites were prepared by direct solution blending of PMMA with EG and UG fillers. A four-point resistivity probe system was used to measure the electrical conductivity of the composites. With the increase of filler content, the electrical conductivity of the composites showed the transition from an insulator to a semiconductor. The transition can be described by classic percolation theory with a critical exponent of 2.1±0.1 for PMMA/EG and 1.8±0.1 for PMMA/UG composites. Interestingly, only 0.6vol% filler content was required to reach the percolation threshold of transition in electrical conductivity using PMMA/EG. The thickness of the EG sheet was found to be at the nanometer scale. The filler content necessary to reach the percolation threshold in PMMA/EG was found to be much lower than those required for PMMA/UG (2.0vol% graphite) and conventional PMMA/carbon black (4.5vol% CB) composites. Evidence was presented in this to demonstrate the improvement in electrical conductivity which was effected by the increase in filler form factor and their enhanced dispersion.
Article
This article reports on the dynamic mechanical and thermal properties of thermosetting phenylethynyl-terminated polyimide (PETI-5) composites reinforced with expanded graphite (EG) nanoplatelets having various average particle sizes and content. The EG nanoplatelets with varying particle sizes were prepared by different pulverization techniques through intercalation and exfoliation of natural graphite flakes. The effect of the EG particle size and concentration of the thermal behavior of PETI-5/EG composites was investigated with several thermal analysis methods (DMA, TMA, and DSC). The storage modulus dynamic mechanical properties and glass transition temperature significantly increased with increasing concentration of EG nanoreinforcements regardless of size. The coefficient of thermal expansion significantly decreased, especially in the glass transition region.
Article
High-density polyethylene (HDPE) was reinforced with expanded and untreated graphite in a melt-compounding process. Viscosity increased upon addition of graphite phase, with the expanded graphite (EG) showing more dramatic rise than the untreated graphite (UG) in viscosity. The increase in viscosity was attributed to the increased surface-to-volume ratio for the EG filler after acid treatment. Electrical conductivity also increased from that pertaining to an insulator to one characteristic of a semiconductor. The EG system showed a lower percolation threshold for transition in conductivity compared to that in the UG system. DSC results indicated that the fillers acted as a nucleating agent in inducing the crystallization of HDPE in the composites. However, the overall degree of crystallinity and melting temperature of HDPE decreased with the addition of EG and UG. Mechanical properties improved as a function of filler content but the overall enhancement was not impressive. It was conjectured that the filler–matrix interface was not optimized in the melt-mixing process. However, the role of EG as a reinforcement phase for both electrical and mechanical properties was unambiguously established. The EG composites demonstrated potentially useful attributes for antistatic, barrier, mechanical, electrical, and cost-effective applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:2781–2788, 2004
Article
The local elasticity of individual single-walled carbon nanotube (SWNT) bundles and the load transfer in epoxy composites containing SWNTs and carbonaceous soot material formed during nanotube synthesis were studied. The composites were loaded to failure, axially in tension and compression, after which the fracture surface was examined. Micro-Raman spectra and scanning electron micrographs revealed that it is the low-modulus features of the bundles, and not the axial modulus of individual tubes, that control the mechanical stability and strength of the composite. Nanotube reinforcement increases the toughness of the composite by absorbing energy because of their highly flexible elastic behavior during loading.
Article
Polymer films with aligned impermeable flakes can have permeabilities 10–100 times smaller than films of the same dimensions but without flakes. However, if the flakes have a distribution of sizes, this reduced permeability could be compromised by the shorter diffusion paths in any region containing smaller flakes. This work shows theoretically and experimentally that this is not the case. For example, a film with 3 vol.% of 5 μm and 3 vol.% of 50 μm flakes has a diffusional resistance 20% higher than a film with 6 vol.% of only 5 μm flakes and only 1% less than a film with 6 vol.% 50 μm flakes.
Article
Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ∼10 nm. Solution-based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007
Article
In this paper, we present results for polymer nanocomposites of poly- (methyl methacrylate) (PMMA) and amide-functionalized SWNTs. The results demonstrate that even at very low loadings, 1 wt % (0.5 vol %), the mechanical and electrical properties are significantly improved. The improvement over PMMA properties exceeds the theoretical bounds for composites with the same volume fraction loading of randomly oriented, straight, individually dispersed nanotubes. The modeling and experimental results thus suggest that the nanotube bundles are well dispersed in the polymer matrix, that the functionalization significantly improves interaction with polymer, and that the interphase formed has improved mechanical properties over that of the matrix material. Loss modulus results indicate a significant difference between functionalized and nonfunctionalized tubes in the composite. Functionalized tubes result in a composite in which relaxation mechanisms are shifted by 30 °C from that of the matrix material, indicating extensive interphase regions and absence of PMMA with bulk properties. Unfunctionalized composites demonstrate a broadening of relaxation modes, but still retain the signature of bulk PMMA properties. These data suggest a morphological difference with a discrete interphase layer in unfunctionalized composites and a fully transformed matrix in the case of functionalization. This difference is consistent with electrical and mechanical property data. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2269–2279, 2005