JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 42

graphene

Different mechanical mixing techniques such as high-shear mixing,
calendering and ultrasonication are also being investigated to ensure
an even distribution of graphene in the polymer at nanoscale resolution.
Another key challenge is making graphene affordable. A two inch2
sample of graphene grown on a copper foil costs $300 dollars [5]. It
may not be justifiable for companies to spend this money, especially
when there is no clear demonstration that graphene will give them a
competitive advantage. A cheaper form of graphene called graphene
nanoplatelets (GNPs) is available for around £1 per gram. GNPs consist of 10-20 layers of graphene stacked on top of one another bonded
in the same way as graphite. But it is not clear whether GNPs possess
the same properties as graphene in its one-layer form, so experiments
will be needed to verify this.
The quality, purity and reproducibility in large quantities (rather than
a few grams) of graphene also need to be improved. There are currently no standards for graphene production and there are also health
and safety uncertainties regarding its inhalation [6].

Results so far
Fig.1: Ants are strong for their size a bit like graphene. But can we extrapolate
that strength from a small scale to a scale that structural engineers can use?
Graphene SEM image shown in bottom right

gym. The most it can carry is a leaf. In the same way graphene is very
strong considering it is only one atom thick. But it will never be able
to carry the same loads as a structural steel I-beam because graphene
only exists on the nanoscale. The reason you cannot build a plane or
car entirely out of graphene is because it would break given the forces
exerted in service.
So how can you transfer graphene's strength from the nanoscale to
the macroscale? Suppose you could use a pheromone to control the
movements of a billion ants. If you could get them to all lift a van in
unison, then pound for pound the ants would be better lifters than
a human. Likewise, billions of nano-flakes of graphene can work
together to make an ultra-strong macroscopic material. This is the
idea behind graphene composites. Graphene flakes can be added
to polymer materials to create a composite with enhanced strength,
toughness, electrical and thermal conductivity. However, it is insufficient to just mix in graphene and expect to see improvements. The
material must be carefully dispersed and distributed amongst the
polymer chains for it to take effect. In addition, an optimum bond
must be formed at the polymer/graphene interface so that stress can
be effectively transferred from the polymer to the graphene during
loading. The materials science community has identified graphene
dispersion as a primary challenge area for the advancement of polymer nanocomposite technology. Graphene has a tendency to clump
together to form graphite agglomerates in the polymer, rendering it
ineffective and sometimes behaving like a defect triggering premature
failures. Through chemical functionalisation of the surface of graphene, agglomeration can be resisted and the bond at the polymer/
graphene interface can be enhanced, leading to better stress transfer.

42 jec composites magazine / No99 September 2015

In the world of nanocomposites, graphene has produced mixed
results. There have been improvements which can outperform other
nanomaterials such as carbon nanotubes. For example, [7] have
shown that 0.1 wt% of graphene added to epoxy improved the tensile
strength by 40% compared to a 14% improvement using 0.1 wt%
multi-wall carbon nanotubes (MWCNT), see Figure 2.
Similarly, a 1 wt% loading of graphene sheets functionalised with
oxygen atoms improved the Young's modulus of PMMA by 80% [8].
The same weight percent of single-wall carbon nanotubes (SWCNT)
produces a 50% improvement. These results are encouraging but
the problem is that the trend does not continue at higher loadings.
Adding more graphene does not improve the Young's modulus more
and even at these relatively low amounts, other mechanical properties like strength and toughness may be reduced (see Figure 3). This
is thought to be because at higher loadings, it is harder to prevent the
graphene from agglomerating [9].

Fig. 2: Comparison of Tensile Strength of different nanoparticles added to
epoxy. GLP = Graphene. SWNT = single walled carbon nanotube. MWNT =
Multiwalled carbon nanotube [7]

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015