Chapter 10 Graphene-based
Nanocomposites
10.1 Introduction of composites
10.2 Introduction of graphene-polymer nanocomposites
10.3 Processing of graphene-polymer nanocomposites
10.4 Mechanical properties of graphene-polymer
nanocomposites
1
10.1 Introduction of composites
Definition: A composites is a material having two or more distinct
constituents or phases, and have to satisfy the following criteria:
1. Both constituents have to present in reasonable proportions, say
greater than 5%;
2. The constituents phases have different properties, and hence the
composite properties are noticeably different from the properties of
the constituents;
3. A man-made composite is usually produced by intimately mixing and
combining the constituents by various means.
Nanocomposites: Composites contain two or more nano-sized fillers.
(Nano-structural composites)
2
10.1.1 Category of fibers and matrix for composites
Fibers:
Natural fibers
Synthetic organic fibers
Aramid fiber
Polyethylene fiber
Synthetic inorganic fibers
Glass fibers
Alumina fibers
Boron fibers
Carbon fibers
Si C fibers
Whiskers(SiC is available in the form of whiskers, i.e., small
single crystals):A few tens of microns in length and less than
one micro in diameter.
Matrix:
Polymer (polymeric composites)
Metals (metal matrix composites)
Ceramic (ceramic matrix composites)
Carbon (carbon/carbon composites)
3
10.1.2 Fillers and matrix for nanocomposites
Fillers:
Carbon nanotubes
Graphene
Matrix:
Polymer (polymeric composites)
Carbon (carbon/carbon composites)
The major concern of CNT- or graphene-based nanocomposites is
improvement in physical property (such as electrical conductivity)
rather than mechanical properties of the nanocomposites.
4
10.1.3 Reinforcement-matrix interface of composites
Wettability: Interfacial bonding is due to adhesion between the reinforcement
and the matrix and to mechanical keying. Clearly, for adhesion to occur, the
matrix must be brought into intimate contact with reinforcements. Wettability
defines the extent to which a liquid will spread over a solid surface.
Interfacial bonding: Once th ematrix has wet the reinforcement, and is therefore
in intimate contact with the reinforcement, bonding will occur.
(a) Mechanical bonding
A mechanical interlocking or keying of two surfaces can lead to a
reasonable bond.
(b) Electrostatic bonding
Bonding occurs between the matrix and the reinforcement when
one surface is positively charges and the other negatively charged.
(c) Chemical bonding
The bonds formed between chemical groups on the reinforcement
surface and compatible groups in the materix.
(d) Reaction or interdiffusion bonding
The atoms or molecules of the two components of the composite
may interdiffuse at the interface.
5
10.2 Introduction of graphene-polymer nanocomposites
Graphene with the following attracted characteristics:[10-1] @1
1.
2.
3.
4.
5.
High specific surface area (2600 m2/g)
High modulus of ~1 TPa
High fracture strength of ∼130 Gpa,
High failure strain of 25%
High thermal conductivity of 4840−5300 W/mK (estimate
using the shift in the Raman G peak)
6. High electron mobility as high as 15 000 cm2/(V sec).
Challenges:
1. Producing large scale quantities and high quality of isolated pristine
graphene sheets.
2. Uniform dispersion of graphene in polymer and bonding of
graphene with polymer matrix.@2
6
To produce isolated pristine graphene sheets, the direct exfoliation in
highly polar organic solvents such as dimethylformamide (DMF) and Nmethyl-pyrrolidone (NMP) by sonication or in chlorosulfonic acid through
simple dissolution, but these methods are not currently suited for polymers
since the colloidal suspensions cannot support high graphene concentrations
and the stability of the mixture strongly depends on the surface energy of the
solvent. [10-1]
7
10.3 Processing of graphene-polymer nanocomposites [10-2]
Three primary routes for preparing graphene-polymer nanocomposites
1. In situ polymerization[10-1]
In situ polymerization intercalates the graphene oxide with a hydrophilic
polymer or polar monomers, then polymerizes the intercalated to isolate
the single layer sheets.
2. Solution mixing[10-1]
Solution casting or melt processing can produce nanocomposites
from a variety of polymers in a highly scalable manner. While this is a
simple approach, it has not yet been demonstrated to produce single
layer graphene sheets without the use of stabilizers or surfactants that
may negatively affect the composite, specifically the interface.
3. Melt blending
Melt compounding utilizes both high-shear forces and high-temperature melting
to blend the filler and matrix material.
8
10.3.1 In situ reduction and polymerization
Dispersed GO in N,N-dimethylformamide (DMF) through vortex mixing and
mild sonication to form a colloidal suspension of single layer GO sheets;
Dissolved PVDF into the DMF/GO solution, then the mixture was casted
onto a Telfon substrate to dry overnight at 60 °C under atmospheric.
Once the solvent is entirely removed from the polymer matrix, the graphene
oxide sheets become immobilized in the rigid polymer matrix (Figure 1a2).
The in situ reduction process is then carried out by hot pressing the PVDF/GO
film at 200 °C for 2 h, yielding nanocomposites with isolated single layers of
reduced-graphene oxide (Figure 1a3).
9
The color of the film changes during heat treatment from light brown to black as
shown in Figure 1b. Color change indicates removal of the oxygen functionalities
and the partial restoration of the graphitic structure during thermal treatment.
These results demonstrate a high level of exfoliation of the GO with a typical
sheet thickness less than 1 nm.
10
FE-SEM
AFM)
Figure 1e,f shows the fracture surface of virgin PVDF and PVDF with reduced
graphene oxide, respectively. The nanocomposites with reduced-graphene
oxide become rough compared to the cross section of PVDF. The wrinkled
surface indicates the reduced graphene oxide in the polymer as shown in arrow
of Figure 1f.
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
are used to characterize the reaction progress during thermal reduction of the
graphene oxide. Figure 2 shows the mass loss of the graphene oxide filler during
reduction together with the heat flow and the temperature profile used.
11
There is a mass loss below 100 °C that can be attributed to the removal of desorbed
water. At 200 °C, there is a dramatic mass loss accompanied by an exothermic peak,
which is ascribed to the decomposition of oxygen functional groups present on the GO
surface. Samples were prepared by reducing the graphene oxide in the
nanocomposite at 200 °C for two hours and thus the temperature profile used in the
TGA analysis held the materials at 200 °C to capture the reduction process and
determine if longer holding times could benefit the material.
There is a sharp exothermic spike around 500 °C, indicating that oxidation of
graphene occurs and formation of CO2. Thermal analysis of the material demonstrates
that 200°C is sufficient to remove a large portion of the oxygen containing functional
groups and shows that higher temperatures are effective in further reducing the 12
materials but not mandatory to achieving filler conductivity.
Two samples of the filler were produced on copper substrates, one of GO drop
cast from DMF at room temperature and another drop cast and reduced at 200 °C
for two hours in air. High-resolution C1s scans provide a good assessment of the
state of carbon atoms in graphene oxide and reduced graphene oxide because the
data can distinguish between different numbers of carbon−oxygen bonds (zero, one,
two, or three carbon−oxygen bonds).[10-1]@1
I
(I)
II
(II)
I
III
IV
(III)
(IV)
The process is suitable for PAni and epoxy resin.[10-2]
13
The thermal reduction removes many of the hydroxyl groups (∼3400, 1380,
1052 cm−1) through desorption. Thermal reduction at 200 °C is incapable of
removing all carbonyl moieties, thus a significant peak at 1720 cm−1, attributed to
C=O stretching, remains after reduction of the graphene oxide. Two closely
overlapping peaks are observed at ∼1650 and ∼1620 cm−1, attributed to COOH
and aromatic C=C stretching, respectively.
C=O
stretching
COOH
C=C
stretching
C=C
stretching
cyclic
ether
After reduction, the unstable COOH
groups are removed from the material while
the conjugated network of aromatic groups
Increases substantially. The reduced
graphene oxide shows no peak at 1650
cm−1 and the increased sp2 hybridization
pushes the C=C peak from 1620 to 1580
cm−1.The peak at 1225 cm−1 corresponds
to cyclic ether moieties, such as furan or
pyran. Graphene oxide has relatively few
of these groups and thus a relatively
small peak; however, after reduction
some of the hydroxyl groups convert
into ethers and appear as defects in the
14
graphene structure.
10.3.2. Solution mixing[10-1]
Solution casting or melt processing can produce nanocomposites from a
variety of polymers in a highly scalable manner. While this is a simple approach,
it has not yet been demonstrated to produce single layer graphene sheets
without the use of stabilizers or surfactants that may negatively affect the
composite, specifically the interface.[10-1]
Solution mixing is the most straightforward method for preparation of polymer
composites. The solvent compatibility of the polymer and the filler is
critical in achieving good dispersity. Due to the residual oxygen-containing
functional groups, GO can be directly mixed with water soluble polymers, such
as poly(vinyl alcohol) (PVA), at various concentrations.
For the fillers which are not dissolve in non-polar solvents or limited solubility
in both organic and inorganic solvents,@1 high-speed shearing combined with
ice-cooling has also been applied to mix graphene-based fillers and the polymer
matrices. However, in the two approaches mentioned above, re-stacking,
aggregation and folding of the graphene based nanosheets are unavoidable
during the process, which significantly reduce the specific surface area of the 2D
fillers. [10-2] During the subsequent in situ reduction of GO, the polymer matrix
prevents the re-aggregation of rGO sheets to retain a homogeneous
suspension@2.
15
10.3.3. Melt blending
Blend: A polymer material containing two or more constituents within a given phase.
[10-9]
Melt compounding utilizes both high-shear forces and high-temperature melting
to blend the filler and matrix material. Hence, it does not require a common
solvent for the graphene filler and the polymer matrix. Polylactide (PLA)-exfoliated
graphite (EG) composite and PET–rGO graphene composite were successfully
prepared by using melt compounding. However,
the high shear forces employed in melt
compounding can sometimes result in
the breakage of the filler materials, such as
CNTs and graphenenanosheets.[10-2]
PET/graphene nanocomposites were
prepared by melt compounding at 285 C using
a Brabender mixer. Compounding was
performed with an initial screw speed of 50
rpm/min for 4 min; then the screw speed was
raised to 100 rpm/min within 1 min and the
compounding was conducted at this speed for
5 min. The specimens for microscopy and
electrical conductivity measurement were
prepared by compression molding at 275 C
16
under a pressure of 15 MPa.[10-10]
10.4 Mechanical properties of graphene-polymer
nanocomposites [10-2]
Monolayer graphene is one of the strongest materials with a Young’s modulus
of 1.0 TPa and a breaking strength of 42 N m-1.
By using solution mixing and vacuum filtration, the strong and ductile poly(vinyl
alcohol)(PVA)–GO composite paper has been prepared, which shows a Young’s
modulus of 4.8 GPa and a tensile yield strength of B110 MPa with 3 wt% of the
GO loading.
The mechanical properties of graphene-based nanocomposites are
apparently lower than the prediction of the pristine graphene. To alleviate or
solve this problem, it is necessary to chemically tailor the structure at the
filler/matrix interface to facilitate the efficient load transfer. For example, GO
filler was covalently bonded to isocyanated-PU matrix via the reaction between
the oxygenated groups of GO and the isocyanate groups at the end of PU
chains. This chemical bonding has led to the increase in the Young’s modulus
and hardness by ~900% and ~327%, respectively.@1
Isocyanate is the functional group with the formula R–N=C=O.
17
Graphite oxide nanoplatelets/PU nanocomposites [10-11]
Expandable graphite (EG), known as a graphite intercalation compound, is
produced by intercalating sulfuric acid into natural flaked graphite via chemical or
electrochemical processes. It can expand up to a hundred times in volume at
high temperature. The GO prepared by the oxidation of EG can be exfoliated into
graphite oxide nanoplatelets (GONPs) in DMF by ultrasonication directly. This
type of GO was specifically called expandable graphite oxide (EGO)
Oxygenated groups attached to the GONPs can not only facilitate the dispersion
but also provide active sites to form chemical bonding that is an ideal interface
between the GONPs and appropriate polymers. Polyurethane (PU) is one of
today’s most versatile industrial materials which have been widely used as surface
coatings for various substrates. PU is an appropriate polymer that can form the
chemical bonding with the GONPs via the reaction between isocyanate groups in
the end of PU chains and oxygenated groups on the GONPs., PU materials.
18
Tensile strength
[10-7]
increasing
in loading
Stress-strain plots of poly(vinyl alcohol)/RG-O composites as a function of
filler loading, showing the pronounced reinforcing effect of RG-O. Tensile strength
and elongation at break show opposing trends with increasing volume fraction of
RG-O (adapted from Ref. [10-7]).
19
[10-11]
@2
The storage modulus of the PU composite
containing 4.4 wt% GONPs is about 30%
higher than that of the PU at −60 ◦C.
No shift in the transition peak of damping
factor (tan δ) associated with the glass
transition temperature (Tg) of the soft segment@1
20
21
The properties of polymer nanocomposites depend strongly on how well they
are dispersed.[@1] The synthesis of graphene from graphene oxide leaves some
epoxide and hydroxyl groups; these greatly facilitate functionalization. Since
graphene oxide and CRG are flat sheets, entangled bundles are not an issue.
However, restacking of the flat sheets, especially after chemical reduction, can
significantly reduce their effectiveness. Restacking can be prevented by either
use of surfactants that can stabilize the reduced particle suspensions or blending
with polymers prior to the chemical reduction.
GO readily exfoliates in water or other protic solvents via hydrogen-bonding
interaction.[@2] Nanocomposites have been created with GO and water-soluble
polymers such as poly-(ethylene oxide) (PEO) or poly(vinyl alcohol) (PVA). Using
GO after chemical modification with isocyanate or amine, composites have also
been produced in aprotic solvents with hydrophobic polymers such as polystyrene
(PS), polyurethane (PU), or poly(methyl methacrylate) (PMMA).
R-OH
or
R-COOH
O
∥
C
CH3 CH3
Example of polar
protic solvents
Example of polar
aprotic solvents
O
R
hydrogenbonding
O
H
R
H
22
Isocyanate: A compound has the
functional group with the formula
R-N=C=O
Amines are organic compounds that
contain nitrogen and are basic. The
general form of an amine is shown in
the following. Where R represents
an alkyl group (烷基).
H
R
Electrical conductivity can be restored
via chemical reduction of the graphene
oxide. This can also be done in situ in
the presence of a polymer. For example
added sulfonated polystyrene and then
reduced graphene oxide with hydrazine
hydrate. Without the sulfonated polystyrene,
the reduced sheets rapidly aggregated.
However, depending on polymer type
and the reducing agent, this in situ
reduction technique may result in polymer Polystyrene sulfonate
degradation.
N
H
Polystyrene
23
Graphene composites can be produced via in situ intercalative polymerization
of monomers. Successful polymerizations of PVA, PMMA, epoxy, and
poly(arylene disulfide) with graphene oxide or silicone foams and PU with TRG
have been reported. Especially for poly(arylene disulfide), graphene oxide was
used as an oxidation agent which converts thiol salts to disulfide. However, so far
monomers have only been polymerized in solvents. The high viscosity of even
dilute dispersion of graphene makes bulk-phase polymerization difficult. If
functional groups on the chemically modified graphene are reactive with the
monomer, grafting of polymer chains onto graphene surfaces can occur. Chain
grafting has been demonstrated with the polymerization of poly(2(dimethylamino)ethylmethacrylate) and PVA and with PU formation.
24
[10-11]
Morphologies of graphene-based composites
(a)TEM of GONP/DMF, (b) SEM of PU, (c) 、(d) 、(e) SEM images of different magnification
25
Morphologies of graphene-based composites
[10-9]
c
(a) 、(b) TEM of GONP/DMF,
(c) SEM of PET nanocomposites
26
References
10-1. Nano Letters, Tang 2012 12 84-90
10-2. Chem. Soc. Rev., Huang 2012 41 666-686
10-3. Polymer Int. Trang 2012 61 93-98
10-4. Energy Environ. Sci., Pumera 2011 4 668-674
10-5. Macromolecules, Kim 2010 43 6515-6530
10-6. Macromol. Chem. Phys. Du 2012 213 1060-1077
10-7. Polymer, Potts 2011 5-25
10-8. Nanotechnology, Du 2012 23 475704
10-9. Composite Materials Mel m. Schwartz, Vol. 1, PrinticeHall PTR
10-10 Polymer Zhang 51 (2010) 1191–1196
10-11 Nanotechnology Cai 20 (2009) 085712
27