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Effect of morphological state of graphene on mechanical properties of nanocomposites
Osman Bayrak
a
, Mariana Ionita
b
, Emrah Demirci
a
, Vadim V. Silberschmidt
a1
a
Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, LE11 3TU, UK
b
Advanced Polymer Materials Group, University Politehnica of Bucharest, 132 Calea Grivitei, 010737, Bucharest, Romania
Abstract
In the last decade, graphene has emerged as one of the best-performing reinforcement materials
for nanocomposites. Incorporation of graphene into polymer results in a nanocomposite with a
new microstructure responsible for its enhanced features. A morphological state of graphene
flakes is one of the factors that govern formation of microstructure. Studies showed that
graphene-oxide (GO) flakes can be found either as fully exfoliated or intercalated in polymer-
based nanocomposites. While traditional parameters are commonly taken into consideration in
theoretical assessment of properties of composites by means of micromechanical models, the
morphological state is often ignored. This research aims to investigate the effect of morphological
state of GO flakes on stiffness of nanocomposites with widely used micromechanical models,
e.g., rule of mixtures, Hui-Shia and Halpin-Tsai. Pure sodium alginate and nanocomposites on its
basis reinforced with 1.0 and 2.5 wt% GO were used in the study. Parameters required for
modelling were quantified with microstructural characterisation. Micromechanical models were
adapted to account for the morphological state of intercalation observed in the characterisation
study. Tensile experiments were employed to assess the adopted models, and the effect matrix
stiffness, GO thickness, spacing of intercalates as well as the Poisson’s ratio and stiffness of
inter-flake polymer layers was studied.
Keywords: Graphene; Nanocomposites; Morphology; Elastic properties; Modelling
Introduction
Excellent properties of graphene in many areas has been emphasised by specialists [1]: it
outperforms many other nano-reinforcements in terms of boosting mechanical performance of
nanocomposites [2,3]. The main reasons behind this are its strong carbon-carbon covalent bonds
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Corresponding author: E-mail: V.Silberschmidt@lboro.ac.uk
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and an inherently very large surface-to-volume ratio. Compared to other types of nanoparticles,
graphene platelets can have a much larger surface area[4], which plays a key role in load
transfer.
Pristine graphene is formed of a nearly defect-free, one-atom layer lattice of pure carbon atoms,
bonded to each other with sp
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hybridization. Other forms of graphene such as graphene oxide
(GO), functionalized GO and graphene fluoride are also known. Before a synthesis method of
pristine graphene in bulk quantities was realized [5], its other forms, especially GO, have been
intensely used for manufacturing of graphene-based nanocomposites. As a result, many
advances have been made in manufacturing of GO-reinforced nanocomposites. Thanks to its
compatibility with polymers, GO is still widely researched and manufactured for reinforcement of
polymer nanocomposites. A type of polymer is also an important matter to consider for
compatibility with oxidized graphene. Ionita et al. [6] analysed alginate (Alg)-based GO
nanocomposites (GO-Alg) and found good compatibility between functional groups of GO and
alginate. GO-Alg nanocomposites are the main focus of this study.
To optimise performance of nanocomposites, various fractions of graphene nanocomposites are
usually employed. While numerical tools, such as, molecular dynamics [7] and finite-element
simulations [8], can be preferred for design and analysis, some existing theoretical models of
composites can be used for practical calculations. Some of these models preferred in graphene-
nanocomposite research are the rule of mixtures (RoM), Halpin-Tsai (HT) and Mori-Tanaka (MT)
methods[9]. The RoM, which was originally developed for composites reinforced with continuous
constituents, was modified by Padawer and Beecher [10] for discontinuous planar-particle
composites. May et al. [11] investigated reinforcement efficiency of pristine graphene flakes
employing a modified RoM (MRoM), While Yousefi et al. [12] used the RoM to compare the
performance assessment of GO/polyurethane nanocomposites with those in literature. Voigt and
Reuss approximations of the RoM were used to provide upper and lower bounds for the Young’s
modulus of nanoparticle-reinforced nanocomposites. Many studies employed the HT
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micromechanical model that was originally formulated for nanocomposites both with unidirectional
and randomly oriented particles [13] for prediction of the Young’s modulus of graphene
nanocomposites [12,14-16]. In this study, only the version for unidirectional alignment is
considered since the microstructural analysis demonstrated that reinforcements are preferentially
aligned [6]. Another analytical model, by Tandon and Weng (TW), which was derived from the MT
method [17], is also used in the literature for calculation of the Young’s modulus of graphene-
reinforced nanocomposites [9,18,19]. Hui and Shia (HS) obtained simple closed-form expressions
based on the TW method [20]; their method can be used to implement the MT approach in
prediction of the elasticity modulus of graphene-nanocomposites [21]. The HS scheme assumes
flakes as aligned in the matrix.
Young’s moduli, volume fractions, orientations and aspect ratios of nano-reinforcement are the
main parameters used in the analytical models to predict the effective stiffness of the
nanocomposite. In these analytical models, linear elasticity of the phases is assumed and stress
transfer is considered to be continuous throughout the interphases. Also, the models assume that
the reinforcement flakes are homogeneously dispersed and fully exfoliated. However, depending
on a manufacturing method and a type of materials constituting the nanocomposite, morphology
of graphene flakes can differ. Barrett et al. [22] studied an effect of molecular structure of three
different polymer matrices on mechanical performance and physical changes of their
nanocomposites reinforced with reduced GO (rGO). They applied the same preparation method
for all the nanocomposites, but while rGO dispersed as exfoliated in a polyhydroxyoctanoate
matrix, it exhibited an intercalated morphology in polyhydroxyoctenoate. According to [23],
nanocomposites produced with two different methods, namely, solvent mixing and melt blending,
yield different morphologies of graphene flakes. While the solvent-mixing method gave a
microstructure with homogeneously exfoliated and preferentially aligned graphene distribution in
the matrix, the melt-blending method produced an intercalated and random distribution of flakes.
Mechanical tests in these studies indicated from the effect of the resulting microstructures with
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different morphologies of the reinforcements: nanocomposites with intercalated graphene flakes
showed poorer mechanical performance. Hence, the theoretical prediction of mechanical
properties of nanocomposites can be faulty if the morphology of the reinforcement is not taken
into account.
Generally, graphene dispersion in polymer nanocomposites is classified into three main states:
stacked, intercalated and exfoliated [18]. Graphene or GO flakes with a stacked morphology are
characterized by presence of more than several sheets. A flake with more than 10 atomic layers
is not accepted as graphene [24,25]. Therefore, the main morphological states of graphene (and
GO) are intercalated and exfoliated. As experiments showed [22,23], morphological differences
caused a major effect on mechanical performance of composites. Lower stiffness reported for
composites with intercalated graphene is mainly due to the fact that reinforcement in the matrix
phase has a form of intercalated clusters – sandwiches of graphene and polymer. Obviously,
such clusters are weaker than graphene and their aspect ratios are smaller.
In this study, a micromechanical analysis of GO nanocomposites with intercalated morphology of
reinforcement is performed. Transmission electron microscopy (TEM) images were used to
assess microstructure and morphology of the studied materials. Thickness of GO flakes was
found with atomic force microscopy, while effective mechanical properties were obtained with
quasi-static tensile tests. For the sake of better prediction of the Young’s modulus of such
materials, the micromechanical models were adapted to account the intercalated morphology.
The effect of matrix stiffness, GO thickness, layer spacing in intercalates as well as the Poisson’s
ratio and stiffness of inter-flake polymers were researched with the adapted models and a
developed finite-element scheme.
Experimental and modelling details
Characterization
A previous X-ray diffractometry (XRD) study, reported in [6], showed that a characteristic peak of
a GO spectrum shifted towards lower angle values for GO-Alg nanocomposites. This observation
revealed that morphology of GO flakes was intercalated. For a further understanding of
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morphology and microstructure of this composite, a TEM study was conducted. To implement it,
small pieces of composites were first embedded in epoxy. To obtain ~100 nm-thin sections of the
samples, the materials were sliced using an ultra-microtome machine with a diamond knife. The
sections were placed on copper grids and monitored using a JEOL JEM-2000FX TEM system
with 100 kV acceleration voltage of electron beams. Micrographs with magnifications at 8000x to
300000x were obtained. In the AFM study, a scanning probe microscope NTEGRA Aura - NT-
MDT (NT-MDT Co., Russia) was used to evaluate the thickness of GO. For this study, GO was
dispersed in water and deposited on fresh cleaved mica substrate.
Quasi-static tensile tests were conducted on the samples that were prepared according to
“Standard Practice for Cutting Film and Sheeting Test Specimen, ASTM D6287-09”; their
dimensions were 50 mm (length) × 3 mm (width). Instron 3345 uniaxial testing system with its
Bluehill
®
software was used to perform the tensile tests. After maintaining the gauge length of 30
mm, parameters of the test were set according to ASTM D3039. The tests were performed with
three samples for each type of material.
Modelling
The traditional micromechanical models used to calculate the Young’s modulus of graphene-
nanocomposites consider reinforcements as fully exfoliated and homogeneously distributed. For
composites with intercalated clusters of graphene, the use of such models can result in errors, as
can be found in literature. For this reason, appropriate adjustments of models employed should
be implemented before micromechanical analysis of such nanocomposites. Luo and Daniel [26]
treated the intercalated structure of polymer-clay nanocomposites as a parallel system of
platelets to calculate its Young’s modulus with the RoM and use it as input parameter in the MT
model. GO-polymer intercalations were treated in similar way in that study, i.e. assuming them as
a system of continuous and parallel platelets. The Young’s modulus of the intercalated structures
was calculated using the Voigt’s RoM as they are composed of parallel layers of GO and
polymer. Then,
was implemented as the Young’s modulus of inclusions in such widely used
