Materials Science s a nd Applications, 2011, 2, 1667-1674
doi:10.4236/msa.2011.211222 Published Online November 2011 (http://www.SciRP.org/journal/msa)
1667
Viscoelastic Response of Graphite Platelet and
CTBN Reinforced Vinyl Ester Nanocomposites
Brahmananda Pramanik, P. Raju Mantena*
Department of Mechanical Engineering, University of Mississippi, Oxford, USA.
Received August 20th, 2011; revised October 3rd, 2011; accepted October 17th, 2011.
ABSTRACT
Developing stronger, safer and more cost-effective structural materials for the new generation naval ships is the focus
of ongoing research at University of Mississippi. The light-weight nanoparticle reinforced glass/carbon polymeric
based composites and structural foams for blast, shock and impact mitigation are emphasized in this research. Dera-
kane 510A-40 brominated vinyl ester nanocomposite resin systems are considered to be used in the composite face
sheets of sandwich structures with fire-resistant foam core to reduce flammability along with optimal flexural rigidity,
vibrational damping and enhanced energy absorption. In this work, the viscoelastic performance of 1.25 and 2.5 weight
percent exfoliated graphite nanoplatelet (xGnP) added with 10 weight percent Carboxy Terminated Butadiene Nitrile
(CTBN) reinforced brominated vinyl ester nanocomposites are studied. A Dynamic Mechanical Analyzer (DMA)—TA
Instrume nts Model Q800 was used to obtain the viscoelastic properties, modulus (stiffness), creep/stress relaxation, and
damping (energy dissipation), of the exfoliated graphite platelet and CTBN reinforced brominated vinyl ester. Effects of
frequency (time) on the viscoelastic behavior were investigated by sweeping the frequency over three decades: 0.01, 0.1,
1.0 and 10 Hz, temperature range from 30˚C to 15˚C at 4˚C per minute step rate. Master curves were generated by
time-temperature supe rpositioning (TTS) of the experimental data at 50˚C reference temperature. Addition of CTBN in
xGnP reinforced bro mina ted vin yl ester comp osite s re sulte d in g reater intrin sic materia l d amp ing, indica ting the possi-
bility of higher energy absorption with the new configuratio n.
Keywords: Nanocomposite, CTBN, Viscoelastic, TTS, Material Da mping, Energy Absorption
1. Introduction
The objective of ongoing research at University of Mis-
sissippi is developing stronger, safer and more cost-ef-
fective structural materials for the new generation naval
ships. The emphasis is on the light-weight nanoparticle
reinforced glass/carbon polymeric based composites and
structural foams for blast, shock and impact mitigation.
The optimal flexural rigidity, vibrational damping and
enhanced energy absorption characteristics of composite
structures are extensively investigated in this research.
Thermoset resins, such as vinyl ester, are typically bri-
ttle. To improve their fracture resistance or toughness,
they are usually blended or reacted with different addi-
tives and modifiers, which generally forms a second dis-
persed phase. The most frequently used modifiers are li-
quid rubbers. The morphology of the final modified ther-
moset can significantly affect the toughening mechanism
and consequently its fracture toughness [1-3].
Auad et al. [1] investigated the mechanical behavior of
vinyl ester resin cured with styrene and modified with
liquid rubber CTBN. A sharp drop in density causing de-
trimental fracture toughness was observed in higher CTBN
concentrations (>10 wt%). Balakrishnan et al. [2] exam-
ined the fracture behavior of rubber dispersed epoxy and
inferred cavitations, yielding, plastic deformation of ma-
trix, crack diversion and energy dissipation caused by ru-
bber particles which contribute to the improvement of the
ductility of the epoxy nanocomposite system. Frohlich et
al. [3] suggested the compatibility matching as the key to
novel phase-separated nanocomposites with significantly
improved toughness.
The study of time dependent deformation process is
highly relevant when long-term applications are in con-
sideration. Polymeric composites are viscoelastic in na-
ture and show the time-temperature dependant behavior.
The time-dependent deformation of materials subjected
to a constant stress is defined as creep [4]. In multiple
literatures, the creep behavior of polymer/clay nanocom-

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites
1668
posites have been modeled [4-9], and proposed that na-
noparticles improve the creep resistance of polymer ma-
trices depending on success in level of exfoliation.
The fatigue strength, cyclic deformation and strain in-
compatibility of clay-reinforced nylon have been demon-
strated by Yang et al. [4] Fatigue strength slightly in-
creased by addition of clay reinforcement to nylon 6. The
cyclic deformation has been examined applying stress-
strain hysteresis loops. The strain incompatibility near the
phase boundary caused both relaxation of three dimen-
sional stress field and extraction of clay platelets in the
nano-composites under study.
Tensile compliance of the nano-clay reinforced Poly-
ethylene composites was investigated by Pegoretti et al.
[5]. The viscoelastic component is low corresponding on-
ly to a few percent of the compliance even at relatively
high stresses. The compliance of the composites is only
slightly lower than that of the neat rPET, the Cloisite®
25A reinforcement being somewhat stronger. Both clays
were illustrated beneficial effect on the dimensional sta-
bility of the composites as compared to the neat rPE, the
creep rate did not rise at long time periods.
Galgali et al. [6] presented an experimental investiga-
tion on the creep behavior of molten polypropylene or-
ganically modified clay nanocomposites and concluded
that the solid-like rheological response of this nanocom-
posite develops from large frictional interactions of the
clay crystallites. Compatibilizer showed a significant in-
fluence in modifying the rheological behavior.
Non-linear time dependent creep of polyethylene (PE)
montmorillonite layered silicate (MLS) nanocomposites
was investigated by Ranade et al. [7]. Non-linearity in
the creep response was modeled using the Burger model
and the tensile-creep response was attributed to disper-
sion effects with marginal effects of crystallinity.
Perez et al. [8] studied the clay content and temepera-
ture dependent creep behavior of biodegradable compos-
ites based on starch/polycaprolactone commercial blends
reinforced with organo-modified nanoclay, processed by
melt-intercalation. The experimental response was corre-
lated with Findlay’s power law and Burger’s model. This
investigation showed that the addition of clay to the neat
matrix leads to a significant improvement of creep resis-
tance.
The effects of incorporating various montmorillonite
nanoclays into several starch samples were by rheologi-
cally examined by Chiou et al. [9]. Frequency sweep and
creep results for starch–nanoclay samples at room tem-
perature indicated that the Cloisite Na+ samples formed
more gel-like materials than the other nanoclay samples.
The Cloisite Na+ samples exhibited a large increase in
modulus at higher temperature. In contrast, the more hy-
drophobic nanoclay samples had comparable modulus
values to the pure starch sample. These results suggested
that during gelatinization, the leached amylose interacted
with the Cloisite Na+ interlayer, producing better rein-
forcement and higher modulus values.
These multiple studies showed the significance of cha-
racterizing visoelastic behavior of nanocomposites in
their respective application fields. Hence, investigating
time-temperature depended response of rubber-toughened
nanocomposites is an important consideration. The aim
of this paper is to describe the viscoelastic behavior of
vinyl ester nanoreinforced composites added with an al-
most unreactive liquid carboxy terminated butadiene ni-
trile (CTBN) rubber, a toughening agent for thermoset
resins. The viscoelastic response of Carboxy Terminated
Butadiene Nitrile (CTBN) on the viscoelastic behavior of
Derakane 510A-40 brominated vinyl ester reinforced
with 1.25 and 2.5 wt% exfoliated graphite nano platelets
was studied using dynamic mechanical analyzer (DMA).
Single frequency and frequency sweep across 3 decades:
0.01, 0.1, 1.0 and 10 Hz were applied over 30˚C to 150˚C
temperature range at a 4˚C/min step rate. The time- tem-
perature superposition (TTS) principle was applied to de-
velop master curves of the dynamic storage modulus at a
reference temperature of 50˚C.
2. Experimental Techniques
2.1. Dyna m ic M ec hanic a l Analyzer (DMA)
DMA tests were performed in accordance with ASTM
D4065-01 standard [10]. TA Instruments Model Q 800
DMA (Figure 1) is a stress-controlled Combined Motor
and Transducer (CMT) machine where the motor applies
a force and displacement sensors measure strain, force
and amplitude in the form of raw signals recorded by the
machine [11]. Experiments were performed using the
single-cantilever clamp. Hence the most of the strain oc-
Figure 1. TA Instruments Model Q800 DMA for dynamic
tests [11].

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites1669
curred at the sample surface, while the center experienced
no strain. The stress and strain equations, applied in these
experiments, are based on theory of linear visoelasticity
of the materials (Equations (1) to (4)).

2 312
11
12 5
KL
s
EFI L
c




 

 

t
(1)
0.7616 0.027130.1038ln
c
L
L
Ftt

 

(2)
2
6
xPL
wt
  (3)

2
2
3δ
12
11
5
c
x
tF
t
LL










(4)
where, E = elastic modulus, Ks = measured stiffnes, Fc =
clamping correction factor, L = clamp span length, I =
sample moment of inertia,  = Poisson’s ratio, t = sample
thickness, 
x = stress, P = applied load, w = width of the
specimen, 
x = strain,
 = amplitude of deformation.
The viscoelastic properties, such as, modulus (stiffness)
and damping (energy dissipation), of the exfoliated gra-
phite platelet added with CTBN reinforced brominated
vinyl ester were studied over a range of temperature and
frequency. Creep and stress relaxation experiments were
also conducted using DMA.
2.2. Single Frequency Dynamic Test
Dynamic mechanical analysis was carried out using the
TA Instrument model DMA Q800 V7.5 on rectangular
cross-sectioned specimens under single-cantilever clamp-
ing mode, with a span length of 17.5 mm. The 1.25 and
2.5 weight percent nanoclay, graphite platelet and graph-
ite platelet added with 10 weight percent Carboxy Ter-
minated Butadiene Nitrile (CTBN) reinforced bromi-
nated vinyl ester nanocomposites were characterized and
compared with the base pure brominated vinyl ester ther-
moset composite under single frequency—temperature ramp
method. Samples of 35 mm  10 mm  1.6 mm thick
were clamped with 30 gm clamp mass and subjected to 1
Hz single frequency with 25 µm displacement amplitude
assuming linear visco-elastic characterization. Test tem-
perature was equilibrated at 30˚C and maintained iso-
thermal for 5 minutes, and then elevated with 3˚C/min
steps up to 150˚C in test duration. Three specimens were
tested from each configuration of the nanocomposites.
The output data were processed by Rheology data analy-
sis software to produce characteristic graphs [11].
2.3. Multi-Frequency Dynamic Test
DMA measurements included frequency sweep with time
temperature steps, to which time-temperature super-po-
sition (TTS) was applied to predict the long-term time
dependent properties of the material. The dynamic stor-
age modulus (E’) and damping of nano-reinforced bro-
minated 510A-40 vinyl ester specimens were character-
ized as a function of temperature and frequencies. Dy-
namic mechanical testing was used to perform multi-
frequency measurements with accelerated temperature
and theoretical time-temperature superposition post-pro-
cessing of the data. Effects of CTBN inclusion in exfoli-
ated graphite platelet reinforcement were investigated.
Vinyl ester nanocomposites were characterized by per-
forming a multi-frequency isothermal mode test, in which
the sample is equilibrated at different temperatures and
subjected to a series of frequencies. Specimens with di-
mensions of 35 mm  10 mm  1.6 mm were subjected
to frequencies of 0.01, 0.1, 1.0 and 10 Hz with a tem-
perature step rate of 4˚C per minute starting from 30˚C
(RT) to 150˚C. Only 25 µm displacement amplitude was
applied for the test since the analysis assumes linear do-
main for viscoelastic characterization, and two specimens
were tested from each configuration. The raw data was
then processed using the Rheology data analysis software
to generate the master curves.
2.4. Creep and Stress Relaxation
The creep and stress-relaxation response of brominated
vinyl ester and its nanocomposites was investigated using
the DMA. Nano-reinforced brominated 510A-40 vinyl
ester specimens were tested in a TA Instruments Model
Q800 DMA using single-cantilever clamp with a span of
17.5 mm and pre-load stress of 3 MPa. The preload stress
of 3 MPa was chosen due to apply the linear viscoelastic
theory at low stresses. Two samples were tested from
each configuration.
Short term creep tests were carried out by subjecting
the samples to a constant load over 30 minutes duration
at isothermal temperatures in the DMA. The room tem-
perature (RT) varied between 30˚C and 32˚C. A tem-
perature range of 30˚C through 100˚C was chosen, as this
covered the glass transition temperature for all the nano-
composites considered in this research. The sample was
initially equilibrated at 30˚C for about 4 minutes to make
sure that the sample temperature settles down. After
equilibrium, the sample was subjected to a fixed stress of
3 MPa for about 30 minutes. The temperature was then
incremented by 4˚C and the above procedure repeated till
the final temperature of 100˚C.
In the stress relaxation mode, the sample was held at a
constant strain and the stress level measured as a function

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites
1670
of time over the same temperature range. The method
segments executed during the relaxation test was the
same as that used in creep. The sample is initially equili-
brated at RT for about 4 minutes, and then displaced 0.01
strain for 30 minutes. The temperature was then incre-
mented by 4˚C and the process repeated until the final
temperature of 100˚C.
3. Analysis and Computation
3.1. Linear Viscoelastic Theory
A viscoelastic material is characterized by possessing
both viscous and elastic behavior. Elastic material is one
which returns all the energy stored during loading after
the load is removed [12]. As a result, the stress and strain
response for elastic materials moves totally in phase. For
elastic materials, Hook’s law applies, where the stress is
proportional to the strain, and the modulus is defined at
the ratio of stress and strain. A purely viscous material
returns none of the energy stored during loading. All the
energy dissipated as “pure bending” once the load is re-
moved. In this situation, the stress is proportional to the
strain rate rather than strain. These materials, known as
inelastic materials, have only damping, instead of stiff-
ness. Both of these two types are ideal in existence. The
real-life materials fall into neither of the above categories.
These are called viscoelastic materials. Some of the en-
ergy stored in a viscoelastic system is recovered upon
removal of the load, and the remainder is dissipated as
heat [12]. Figure 2 [12] describes the cyclic stress at a
loading frequency of  is out-of-phase with the strain by
certain angle  , where 0 <  <π2. The angle  is a
measure of the materials damping level; the larger angle
denotes greater damping. The viscoelastic modulus is
represented by a complex quantity. The real part of this
complex parameter, known as storage modulus (E1), re-
lates the elastic behavior of the material, and defines the
Figure 2. Cyclic stress and strain curves vs. time for a vis-
coelastic material [12].
stiffness. The imaginary component, known as loss modu-
lus (E2), explains the material’s viscous behavior, and
defines the ability of energy dissipation of the material.
The complex viscoelastic modulus (E*) is defined as:
*0
12
0
i
E
EiE e



  (5)
3.2. Time-Temperature Superposition (TTS)
Molecular motion in materials occurs at larger rates un-
der elevated temperatures. The time-temperature super-
position principle is based on this temperature dependent
response of the materials. The change in property which
occurs relatively quickly at higher temperatures can be
made to appear as if they occurred at longer times or
lower frequencies simply by shifting the data with re-
spect to time (1/frequency) [11]. By shifting the data
with respect to frequency to a reference curve, a master
curve is generated, which covers time (frequencies) out-
side the accessible range. The shifting mechanism used
to shift a set of data upon a reference curve follows WLF
[11] model. This model assumes that the fractional free
volume increases linearly with respect to temperature in
the transition region, and when the free volume increases,
its viscosity decreases. In this model, the degree of shift-
ing is calculated according to Equation (6):



10
10
log T
CT T
aCTT

 (6)
4. Results and Discussion
4.1. Modulus
Figures 3 shows the storage modulus variations with
temperature for brominated vinyl ester nanocomposites
with the single-frequency test. Addition of CTBN to
graphite reinforcement shifted the drop of storage modu-
lus to higher temperature.
Figure 4 shows the detrimental effect of CTBN rein-
forcement in storage modulus with respect to xGnP pre-
sented at 30˚C. Figures 5 and 6 show the storage mo-
dulus variations with temperature for brominated vinyl
ester nanocomposites with the multi-frequency tests. It
can be observed that the modulus remains higher at high-
er frequency up to the glass transition temperature.
4.2. Glass Transition
Figures 7 shows the loss factor variation with tempera-
ture for brominated vinyl ester nanocomposites at single
frequency. Figure 8 shows a marginal increase of glass
transition temperature in nanocomposites with CTBN ad-
dition.

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites1671
Figure 3. Storage modulus for bromi nated vinyl ester na no-
composites at single-frequency .
Figure 4. Storage modulus at initial temperature for bro-
minated vinyl ester nanocomposite.
Figure 5. Storage modulus for 1.25 wt% reinforced bromi-
nated vinyl este r nanocomposites at multi-frequency.
Figure 6. Storage modulus for 2.5 wt% reinforced bromi-
nated vinyl este r nanocomposites at multi-frequency.
Figure 7. Loss factor for brominated vinyl ester nanocom-
posites at s ingl e-frequency.
Figure 8. Glass tra nsition temperature f or bro mi nated vi nyl
ester nanoco mposites.

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites
1672
Figures 9 and 10 show the loss factor variations in
multi-frequency tests. CTBN inclusion in graphite rein-
forcement contributed in maintaining the peak loss factor
within a higher temperature range during glass transition
(114˚C to 116˚C).
4.3. Damping
Tan-delta, defined as the ratio of loss modulus to storage
modulus, is a measure of inherent material damping.
Peak of Tan-delta is the region over which material ex-
periences a transition from glassy to a leathery behavior,
associated with the onset of short range molecular seg-
ments motion, of which all are initially frozen [12,13].
CTBN addition resulted in greater value of Tan  peak
showing more inherent material damping as shown in
Figure 11, indicating the possibility of higher energy ab-
sorption.
Figure 9. Loss factor for 1.25 wt% reinforced brominated
vinyl ester nanoco mposites at multi -frequen cy.
Figure 10. Loss factor for 2.5 wt% reinforced brominated
vinyl ester nanocompposites at si ngle-fre quency.
Fig ure 11 . T an-de lta pe a ks fo r br o mi nate d vinyl es te r na no-
composites.
4.4. Long-Term Dynamic Properties
Since the glass transition temperature for nanocompo-
sites is observed to be variable for different compositions,
a reference temperature of 50˚C was chosen to generate
master curves for storage modulus. To perform this, data
from higher temperature experiments in the lower portion
of the plot are shifted to the left (lower frequencies) and
curves corresponding to the temperatures lower than 50˚C
are shifted to the right [11].
Figure 12 shows the generated master curves of stor-
age modulus over an extended period of time. From
Figure 12, all vinyl ester nanocomposites were observed
to maintain their rigidity (at 50˚C) with an average dy-
namic storage modulus of (2.5 GPa) over a period of 1010
sec (321.5 years), from where the vinyl ester nanocom-
posites impregnated with CTBN starts to show impro-
vement. This is significant on the long term behavior of
reinforced vinyl ester with CTBN.
4.5. Creep and Stress-Relaxation
Shape of the creep and relaxation curves for brominated
vinyl ester is strongly dependent on temperature ob-
served in this experiment (Figures 13 and 14). The effect
of temperature on creep and relaxation behavior of bro-
minated vinyl ester, 1.25 wt percent × GnP, 2.5 × GnP
reinforced samples with 10 wt percent CTBN additives
over 30 minutes duration were also showed similar trend.
The creep deformations in the initial portion, which are
pure elastic, are relatively small and associated with bend-
ing and stretching of intermolecular bonds. However, the
deformation beyond the proportional limit is same as a
stretching of the wrinkled molecular chains which is not
recoverable instantly. Hence, the mechanical response
exists in the viscoelastic linear region, where no intermo-
lecular slippage causes permanent deformation [12,13].

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites1673
Figure 12. Master curves for brominated vinyl ester nano-
compposites.
Figure 13. Creep-strain for pure brominated vinyl ester.
Figure 14. Relaxation modulus for pure brominated vinyl
ester.
It has been observed that at 100˚C, both increment of
creep-strain and decrement of modulus in stress-relaxa-
tion are less with the addition of toughening agent (Fig-
ures 15 and 16). Accordingly, addition of CTBN to ×
GnP reinforcement improves the creep resistance.
5. Conclusions
The effect of carboxy terminated butadiene nitrile (CTBN)
on the viscoelastic behavior of Derakane 510A-40 bro-
minated vinyl ester reinforced with 1.25 and 2.5 wt per-
cent exfoliated graphite platelets was investigated using
dynamic mechanical analyzer (DMA). Single frequency
and frequency sweep across 3 decades: 0.01, 0.1, 1.0 and
10 Hz were applied over 30˚C to 150˚C temperature
range at a 4˚C/min step rate. The time-temperature su-
perposition principle was used to develop master curves
of dynamic storage modulus at a reference temperature
of 50˚C.
The elastic modulus was observed to decrease by a
small amount with addition of CTBN to the reinforce-
ment; along with marginal increase of the glass transition
Figure 15. Creep-strain for na nocomposites at 100˚C.
Figure 16. R e laxation modulus for nanoc om pos it es at 100˚C.

Viscoelastic Response of Graphite Platelet and CTBN Reinforced Vinyl Ester Nanocomposites
1674
temperature. CTBN addition also resulted in greater va-
lue of Tan  peak showing more inherent material damping
with the possibility of higher energy absorption. Creep
and relaxation curves showed the improvement of creep
resistance due to the CTBN inclusion in the nano-rein-
forcement.
6. Acknowledgements
The support from ONR Grant N00014-07-1-1010, Office
of Naval Research, Solid Mechanics Program (Dr. Yapa
D.S. Rajapakse, Program Manager) has been acknowl-
edged. The nanoclay and graphite platelet reinforced vinyl
ester composite panels were manufactured by Dr. Larry
Drzal’s group at Michigan State University.
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