Chem. Listy 105, s171s174 (2011) LMV 2008 Regular Lectures
s171
VILMA BURŠÍKOVÁa
, ZUZANA
KUČEROVÁa
, LENKA ZAJÍČKOVÁa
,
ONDŘEJ JAŠEKa
, VÍT KUDRLEa
,
JIŘINA MATĚJKOVÁb
, and PETR
SYNEKa
a
Department of Physical Electronics, Masaryk University,
Kotlářská 2, 611 37 Brno, b
Institute of Scientific Instruments,
Academy of Sciences of the Czech Republic, Královopolská
147/62, 612 64 Brno, Czech Republic
vilmab@physics.muni.cz
Keywords: nanocomposites, carbon nanotubes, polyurethane,
hardness, storage modulus, loss modulus, indentation creep
1. Introduction
Composite materials or composites are multiple-phase
materials that exhibit a significant proportion of the properties
of all constituent phases such that a better combination of
properties is realized. Those phases must be chemically and
physically dissimilar and separated by a distinct interface.
There are two categories of constituent materials: the matrix
and the reinforcement or the dispersed phase. In a composite,
at least one portion of each kind is required. The properties of
composites are a function of the properties of the constituent
phases, their relative amounts and the geometry of the disperse
phase (i.e. the shape of the particles, the particle size
distribution and orientation).
2. Nanocomposites
In contrast to conventional composites, where the reinforcement
is on the order of microns, nanocomposites are
reinforced with filler with at least one dimension that is less
than 100 nm. Although some nanofilled composite have been
used for more than 100 years (e.g. carbon black or fumed
silica filled1,2
), the discovery of carbon nanotubes become
a new great challenge for nanocomposite research and industry.
According to3
, six following characteristics distinguish
the nanocomposite from the classic filled systems: particleparticle
correlation (orientation and position) arising at low
volume fractions (<0.001); large number density of particles
per particle volume (106108 particles/m3
); extensive interfacial
area per volume of particles (103104 m2
ml1
); short
distances between particles (1050 nm at 1.8 vol.%); comparable
size scales among the rigid nanoparticle inclusion, distance
between particles and the relaxation volume of the polymer
chains.
The interfacial region has different properties than the
bulk material because of its proximity to the surface of the
filler. Nominally, the spatial extent of this perturbed matrix is
thought to extend into the bulk one to four times the radius of
gyration of the matrix, Rg, which has a value of around tens
of nanometers. Because of the increased number of particles,
the distance between particles in the nano-filled system is
comparable to the size of the interfacial region (10 nm). Thus,
the relative volume fraction of interfacial material to bulk is
drastically increased. This implies a necessity to optimize the
properties of the interfacial region in nanocomposites. On the
other hand, this region is a source of many potentially novel
properties of nanocomposites derived from the interface. It
can influence the mechanical properties of the nanocomposites
as well as their conductivity and percolation behavior.
The final microstructure and properties of the carbon
nanotubes (CNT)-based polymer composite will depend on
how the CNTs will distribute, disperse and orient. The CNT
orientation state determines the anisotropic functionality.
Properties of composites are significantly better, if CNTs are
oriented in one direction, than if they are oriented randomly4,5
.
Another challenge in fabrication of the CNT-based polymer
composites is the homogeneous dispersion of the CNTs in the
polymer matrix so that it has uniform properties and can efficiently
handle load transfer during structural excitation4,6,7
. It
is a significant challenge in fabrication of the CNT-based
polymer composites because the CNTs, in their manufactured
state, cluster together in any suspension due to strong Van der
Waals forces. Many techniques such as application of ultrasonic
bath or chemical modification of the CNT surface have
been attempted to separate and disperse CNTs in polymer
resins to various degree of success4,8–10
. There are four important
parameters that will describe the suspension characteristics:
CNT dispersion, concentration, aspect ratio and orienta-
tion11
. The importance of dispersion and orientation of CNTs
is obvious from the above paragraph. The CNT concentration
and aspect ratio determine how easily CNTs can move and
interact with each other to build interconnecting network
which can transfer heat and electrons. There are three general
ways of dispersing nanofillers in polymers. The first way is
direct mixing of the polymer and the nanoparticles either as
discrete phases or in solution. The second way is in-situ polymerization
in the presence of nanoparticles and the third is
in-situ formation and in-situ polymerization. These are socalled
hybrid composites12
.
Carbon nanotubes are increasingly being used in nanocomposite
preparation as a filler of many different matrices,
ceramics, metal, polymer and also in clay-polymer compo-
sites1316
. With their high strength, high modulus, light weight
and high aspect ratio they seem to be ideal filler. However,
they are held in bundles a do not disperse easily, they are inert
with low wettability and that hinders their full utilization as a
filler. It has already been reported that functionalization of
CNTs greatly improves their dispersion in a matrix and consequently
the properties of the composite17
. In this work the
preparation of the polyurethane (hereafter PU) composite with
nanotubes is described. Both unmodified and modified multiMEASUREMENT
OF MECHANICAL PROPERTIES OF COMPOSITE MATERIALS
Chem. Listy 105, s171s174 (2011) LMV 2008 Regular Lectures
s172
walled CNTs (hereafter MWCNTs) were used. The structure
and properties of composite with both kinds of filler are compared.
The functionalization of MWCNTs in inductively coupled
plasma ICP is also reported.
3. Mechanical properties of the composites
There are several basic problems associated with the
determination of the mechanical properties of composite film
consisting of hard particles and viscoelastic-plastic matrix.
The evaluation of material parameters such as elastic modulus
and plastic hardness of the viscoelastic-plastic film is problematic
because these materials exhibit also a significant time
dependent plastic deformation (creep). If we load such system
to a maximum load and then keep penetration depth constant,
the load will relax under some plastic or viscoelastic response
(anelastic – time dependent plastic deformation). Two different
indentation methods were used to study the mechanical
properties of the composites.
A Fischercope H100 depth sensing indentation (DSI)
tester was used to study the indentation response of composites
on glass substrates. Several different testing conditions
were used in order to find the optimum procedure allowing
the suppression of the influence of the time dependent indentation
response of CNT-filled PU composite on a glass substrate.
The loading period of 20 s was followed by a hold time
of 20 s or 60 s, an unloading period of 20, 5 or 1 s and finished
after holding the minimum load for 20 or 60 s. The tests
were made for several different indentation loads in order to
study the composite mechanical properties of the film/
substrate system from near surface up to film-substrate interface.
The applied load varied from 1 to 1000 mN. An example
of the time dependence of the load is shown in Fig. 1.
In the case of the second, dynamic method, the CSM
ultrananoindenter (UNHT) was used. The indentation measurement
was based on the application of an oscillating force
superimposed to the increasing indentation load in which the
transfer function between the load and indentation depth provides
a method of calculating storage and loss modulus of the
material.
The PU-CNT nanocomposite preparation was as follows.
Appropriate amounts of CNTs, polyol, diluter and antistatic
agent were weighted out, stirred well manually, glass
balls ensuring the mechanical perturbance were added and the
whole mixture was ultrasonicated for 60 minutes. Than isocyanate
was added and the mixture was stirred once more. After
that it was poured onto glass substrate surrounded by the
frame. Liquid in the frame created a flat level and let dried at
the laboratory temperature. After 48 hours the composite was
dry enough to be removed from the frame. The chemicals
weight rate was 7 : 2 : 1 (polyol : isokyanate : diluter) and the
CNT concentration in composites was 0.1 wt%.
4. Results
In Fig. 1 the testing conditions (dependence of the applied
load on the time) used and the corresponding time dependences
of the indentation depth are illustrated for several
studied samples.
Fig. 1. Comparison of the time dependence of the load with the
time dependence of the indentation depth for 3 different types of
polyurethane matrices PU1-PU3 and four composites marked
A,B,C,D, reinforced by CNTs
0 10 20 30 40 50 60 70
0
5
10
15
20
25
30
35
PU1
PU2
PU3
A
B
C
D
Indentationdepth[m]
Time [s]
0
20
40
60
80
100
Load[mN]
Load
Fig. 2. Experimental indentation creep and relaxation data fitted
with Maxwell-Voigth (M-V) reological model obtained for pure
PU samples and PU-CNT nanocomposites
10
15
20
25
30
35 fits according to M-V modelPU1
PU2
PU3
A
B
C
D
Creep[m]
0 2 4 6 8 10 12 14 16 18 20
4
6
8
10
12
14
16
18
Time [s]
Anelasticresponse[m]
fits according to M-V model PU3
A
B
C
D
Chem. Listy 105, s171s174 (2011) LMV 2008 Regular Lectures
s173
The mechanical properties of PU depend substantially
on its preparation conditions, it is well-known that the composition
of PU can be varied to produce hard and stiff to soft and
rubbery materials. In Fig. 1, the indentation responses of three
different PU coatings to the applied load change in time are
shown. The substrate material was glass. Using the indentation
study, we have found that mechanical properties of PU
coatings depend not only on the preparation mixture, but also
on the amount of chemicals used for the preparation, i.e. they
are dependent on the coating thickness. Probably it is caused
by different solidification process at the surface, which is in
contact with air and in the bulk of the material. In case of the
sample PU1, which was thicker than 300 m, there was
a harder skin at the surface and the hardness in the bulk decreased
of one order of magnitude. In case of samples PU2
and PU3 with thicknesses lower than 300 m the solidification
process was more homogeneous, and the differences in
surface and bulk properties are less. The hardness of the most
thin and stiff PU matrix P3 was 22 MPa, its indentation
modulus EIT was 1.14 GPa, the loss modulus EL was
0.15 GPa.
The PU matrix was in each case softer than the multiwall
carbon nanotube (MWCNT) containing PU (samples
A,B,C,D) and showed more significant indentation creep. The
unloading cycle showed for PU coatings sudden rush decrease
in indentation depth. The composite film A consisting of nonmodified
nanotubes in polyurethane matrix exhibited slightly
higher resistance against indentation compared to P3. However,
the COOH activated carbon nanotubes in case of sample
B (0.05 % of MWCNT) and sample C (0.1 % of MWCNT)
substantially improved the PU matrix properties. The COOH
functionalisation of MWCNTs increased the composite resistance
against indentation and creep and resulted in better relaxation
(anelastic response) ability (see Fig. 1 and Fig. 2).
The homogeneity of the composite was improved from the
point of view of the indentation test and the scatter in measured
values decreased. The activation of the carbon nanotubes
improved the hardness and the elastic modulus of the prepared
samples. These results were proven also by dynamic
mechanical analysis. Results on the depth dependence of the
hardness and elastic modulus are shown in Fig. 3, it is obvious,
that the functionalised CNTs offer much better mechanical
properties.
In case of sample D (0.1 % of MWCNT), the capacitively
coupled low pressure glow discharge was used to
activate the MWCNT fillers. The MWCNTs were treated with
r.f. power 15 W of at the frequency of 13.56 MHz in mixture
of oxygen and argon. From Fig. 1 and Fig. 2 it is evident, that
the effect of this treatment on the properties of the PU/CNT
composite is of the same quality as in case of commercial
COOH treated CNTs.
In Fig. 4 the viscoelastic properties of composite filled
with plasma modified CNTs (sample D) are characterised
using dynamic mechanical analysis. The hardness of sample
determined using this analysis was 65 MPa, the storage
modulus EST was 1.95 GPa, the loss modulus EL was
0.12 GPa and the loss factor  was 2.8°. An increase in the
hardness approaching the surface region was observed in each
case of prepared coatings.
Moreover, we analysed the time dependent mechanical
properties of the prepared coatings using several mechanical
models (Kelvin-Voigt, Voigt, Maxwell-Voigt) The fourelement
Maxwell-Voigt model was found to be the most suitable
approach to describe the time-dependent behaviour of the
prepared coatings as it is shown in Fig. 2. In this model, the
material response is described by serial combination of Maxwell
model and Kelvin-Voigt model.
5. Conclusion
Complex mechanical characterization of MWCNT/PU
composites was done. Four types of coatings were compared.
The coating with commercial pure MWCNT filler exhibited
viscoelastic properties as the PU matrix, but its mechanical
Fig. 3. Dependence of the hardness and the elastic modulus on the
indentation depth for nanocomposites A, B and C;
A  nanocomposite consisting of nonmodified nanotubes in PU matrix,
B  COOH activated carbon nanotubes  0.05 % of MWCNT,
C  COOH activated carbon nanotubes  0.1 % of MWCNT
20
40
60
Results on dynamic mechanical analysis
A B C
HIT
[MPa]
0 2 4 6 8 10 12 14
1,0
1,5
2,0
2,5
3,0
EIT
[GPa]
Displacement [m]
A B C
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
50
100
150
200
250
Sample D
Hardness
Hardness[MPa]
Indentation depth [m]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Loss modulus EL
EST
,EL
[GPa]
Storage modulus EST
Fig. 4. Results on dynamic mechanical analysis of nanocomposite
sample D. Dependence of the hardness, storage and loss modulus,
EST and EL on the indentation depth for nanocomposite D. D –
nanocomposite consisting of plasma modified carbon nanotubes in
PU matrix  0.1 % of MWCNT
Chem. Listy 105, s171s174 (2011) LMV 2008 Regular Lectures
s174
properties such as hardness, elastic modulus and creep resistance
were slightly improved. Two different concentrations of
MWCNTs commercially functionalized with COOH group
were studied. These composites showed improved mechanical
properties, and the modified nanotubes proved to be much
better fillers than the unmodified MWCNTs due to stronger
filler-to-matrix attachment. Because the modified nanotubes
seem to be much more convenient composite filler, the first
experiments with nanotube modification have been carried
out. Modification using inductively coupled discharge in argon
and oxygen mixture was successful and the mechanical
properties of the composite were increased at the same level
as in case of the commercially COOH functionalized
MWCNT fillers.
This research has been supported by Ministry of Education,
Youths and Sports of the Czech Republic under project
MSM0021622411, by the grant of Czech Science Foundation
No. 202/07/1669 and by Academy of Science of the Czech
Republic by KAN311610701.
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V. Buršíkováa
, Z. Kučerováa
, L. Zajíčkováa
,
O. Jašeka
, V. Kudrlea
, J. Matějkováb
, and P. Synek
(a
Department of Physical Electronics, Masaryk University,
Brno, Czech Republic, b
Institute of Scientific Instruments,
Academy of Sciences of the Czech Republic, Brno, Czech
Republic): Measurement of mechanical properties of composite
materials
The aim of the present work was the study of mechanical
properties of MWCNT/PU. Four types of coatings were
compared. Two different concentrations of MWCNTs commercially
functionalized with COOH group were prepared
and studied. These composites showed improved mechanical
properties compared to PU, and the modified nanotubes
proved to be much better fillers than the unmodified
MWCNTs due to stronger filler-to-matrix attachment. Because
the modified nanotubes seem to be much more convenient
composite filler, the first experiments with nanotube
modification have been carried out. Modification using inductively
coupled discharge in argon and oxygen mixture was
successful and the mechanical properties of the composite
were increased at the same level as in case of the commercially
COOH functionalized MWCNT fillers.