Composites: Part A 37 (2006) 1252–1259
www.elsevier.com/locate/compositesa
Multiwall carbon nanotube modified vinylester and
vinylester – based hybrid resins
O. Gryshchuk a, J. Karger-Kocsis
a
b
a,*
, R. Thomann b, Z. Kónya c, I. Kiricsi
c
Institut für Verbundwerkstoffe GmbH, Technische Universität Kaiserslautern, Erwin-Schrödinger-Str. 58, D-67663 Kaiserslautern, Germany
Institut für Makromolekulare Chemie und Freiburger Materialforschungszentrum, Albert-Ludwigs-Universität Freiburg, Stefan-Meier-St. 31,
D-79104 Freiburg, Germany
c
Applied and Environmental Chemistry Department, University of Szeged, Rerrich Béla tér 1, H-6720 Szeged, Hungary
Received 4 June 2005; received in revised form 4 September 2005; accepted 4 September 2005
Abstract
Purified, catalytic chemically vapor deposited multiwall carbon nanotubes (MW-CNT) were incorporated in vinylester (VE), vinylester-urethane (VEUH) and vinylester/epoxy (VE/EP) systems in up to 2 wt% amount. The MW-CNT dispersion was studied by transmission electron microscopy (TEM). The electrical conductivity, thermomechanical, fracture mechanical and failure properties of the
related nanocomposites were investigated. It was found that the MW-CNT of very high aspect ratio and thus prone for nesting can
be only partly disintegrated. The effect of MW-CNT on the electrical conductivity was much larger than on the fracture mechanical performance. As MW-CNT can be treated as a flexible reinforcement its reinforcing efficiency is likely better in tough than in brittle
thermosets.
2005 Elsevier Ltd. All rights reserved.
Keywords: A. Nano-structures; A. Thermosetting resins; B. Electrical properties; B. Fracture toughness; B. Thermomechanical
1. Introduction
Since the discovery of carbon nanotubes (CNTs) vigorous research and development were started to manufacture
CNT – containing composites with unique properties. The
interest was mostly focused on the improvement of the
mechanical and electrical properties of both thermoplastic
and thermoset polymers. The related achievements are already summarized in reviews (e.g. [1–3]). Among the thermoset resins epoxy resins (EP) were the favored matrices
for CNT and carbon nanofiber modifications (e.g. [3–
11]). It was recognized that the major factors affecting
the reinforcing efficiency of CNTs are: strong interfacial
bonding between the CNT and polymer and good dispersion of CNT in polymer matrix. Both aspects, i.e., adhesion
*
Corresponding author. Tel.: +49 631 205 3522; fax: +49 631 205 3521.
E-mail address: karger@ivw.uni-kl.de (J. Karger-Kocsis).
1359-835X/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2005.09.003
[8–10] and dispersion [11], have been already addressed for
EPs. Nevertheless, EP is likely not the best model system
for CNT modification. Note that both single- and multiwall CNTs (SW-CNT and MW-CNT, respectively) can
well be dispersed in organic solvents, like toluene, tetrahydrofurane, dimethylformamide, etc. The related process
is often termed ‘‘solubilization’’. Recall that this strategy
has been used for EPs [11]. On the other hand, unsaturated
polyester and vinylester resins contain as active diluent styrene (up to 40 wt%) which is a suitable solvent for CNTs.
So, such resins are suitable matrices in order to check the
application potential of CNTs when making use of the
‘‘solvent route’’. Interestingly, this aspect was less considered in the works reported until now (e.g. [12]). This paper
was aimed at the incorporation of MW-CNT in vinylester
(VE) and VE-based hybrid resins. The latter contained
vinylester-urethane (VEUH) and interpenetrated VE/EPbased systems. Attention was paid to assess the thermomechanical, fracture and electrical behavior of the related
O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
systems as a function of MW-CNT content and to clarify
the dispersion of the MW-CNT.
2. Experimental
2.1. Preparation of the MW-CNT containing
nanocomposites
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an acceleration voltage of 120 kV. Specimens were sectioned for ca. 50 nm thickness using an Ultracut E device
(Reichert & Jung, Vienna, Austria) and a Diatome diamond knife at room temperature. TEM pictures from the
initial MW-CNT were taken after placing them on a carbon coated copper grid (Fig. 1).
2.3. Testing
MW-CNT has been produced by catalytic chemical
vapor deposition technique using acetylene as feedstock
and Co–Fe/Al2O3 catalyst. It has been purified by washing
first with NaOH, followed by HCl treatment to dissolve
the alumina support and the metal content of the raw product of synthesis. Prior to final rinsing with distillated water,
the amorphous carbon has been removed by oxidation in
strongly acidic K2MnO4 solution. The specific BET surface
of MW-CNT proved to be 276 m2/g. The diameter of MWCNT was ca. 40 nm. A transmission electron microscopic
(TEM) picture taken from MW-CNT is given in Fig. 1. Further information to the MW-CNT can be taken from [13,14].
As thermoset matrices VE, VEUH and interpenetrating
VE/EP resins served. Their characteristics and curing
are described elsewhere, viz. VE [15], VEUH [15–17] and
VE/EP with interpenetrating network (IPN) structure
[15,18,19]. MW-CNT, introduced usually in 0.5, and exceptionally in 1 and 2 wt%, respectively, was always dispersed
in the VE first by mechanical stirring prior to sonication
(15 min at ambient temperature using Sonorex Super RK
103H, Bandelin Electronic, Berlin, Germany). Note that
sonication may lead to both rupture and grafting of
MW-CNT which were, however, not studied in this work.
The cure cycle ended for all formulations at T = 200 C.
2.2. Assessment of the MW-CNT dispersion
TEM measurements were carried out with a Zeiss Leo
Cem 912 microscope (Oberkochen, Germany) applying
The thermo-mechanical performance of the nanocomposites was studied by dynamic-mechanical thermal analysis (DMTA). DMTA traces (storage modulus, E 0 ; and the
mechanical loss factor, tan d vs. temperature) were determined by an Eplexor 25N device (Gabo Qualimeter, Ahlden, Germany) in flexural mode at 10 Hz frequency. The
scan rate set for a broad temperature range (T = 100 to
>200 C) was 1 C/min.
The fracture toughness (Kc) and fracture energy (Gc)
were determined on the compact tension (CT) specimens
(dimension: 35 · 35 · 3 mm). The CT specimens were produced by pouring the resins to open PTFE-molds and
cured in a thermostatic oven (regime: ambient temperature
for 2 h, 60 C for 15 min, 80 C for 30 min, 140 C for
30 min, and finally 200 C for 1 h). The sawn notch of
the CT specimens was sharpened by razor blade tapping
prior to testing on a Zwick 1445 machine (Zwick, Ulm,
Germany) at room temperature with a v = 1 mm/min
crosshead speed. Kc and Gc were computed in accordance
with the ESIS testing protocol [20] except that the maximum load and related energy value were always taken into
account (which affected only the fracture mechanical data
of the IPN-structured VE/EP system).
The fracture surface of the CT specimens was inspected
in a scanning electron microscope (SEM; JSM-5400 of
Jeol, Tokyo, Japan). Prior to SEM the fracture surfaces
were sputtered with a Pt/Pd alloy to create a conductive
cover layer.
Owing to the very high aspect ratio of the MW-CNT it
was expected that the nanocomposites become electrically
conductive even at low nanotube content. Thus, the volume resistivity of the nanocomposites was determined by
a Hiresta UP high resistivity meter (Mitsubishi Chemicals,
Tokyo, Japan). A dc voltage of 10 V was applied across the
sample thickness when the resistance of the sample was less
than 108 X, while for higher resistances voltages up to
1000 V were applied. The samples were dried over night
in an oven at 40 C under vacuum and then kept in dried
environment, in order to eliminate any moisture effects.
3. Results and discussion
3.1. MW-CNT dispersion
Fig. 1. TEM pictures taken from the MW-CNT. Note the very high
aspect ratio and related nesting of the nanotubes.
TEM pictures taken at various magnifications from the
VE + 0.5 wt% MW-CNT are displaced in Fig. 2. One can
resolve regions with both poor (Fig. 2(a)) and well dispersed MW-CNT (Fig. 2(b) and (c)). Poor dispersion is
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O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
Fig. 2. TEM pictures taken from the VE-based nanocomposites containing 0.5 wt% MW-CNT.
due to the very large aspect ratio of the nanotubes causing
pronounced ‘‘nesting’’. Recall that this is an inherent feature of the MW-CNT used (Fig. 1). Accordingly, the nested
areas could be broken up only partially during the sample
preparation. It is worth of calling the attention on how
flexible the nanotubes are (Fig. 2(c)).
Introducing additional crosslinks between the secondary
hydroxyl groups of the VE and isocyanate groups of the
polyisocyanate compound in VEUH ([15–17]) does not improve the dispersion of the nanotubes (Fig. 3).
The TEM pictures illustrate again that some MW-CNT
are individually dispersed, whereas others remained fully
entangled, agglomerated (Fig. 3).
The scenario is very similar also for the IPN-structured
VE/EP hybrids (Fig. 4).
This is somewhat surprising as the IPN organization of
this VE/EP might have affected the MW-CNT dispersion.
It was found in atomic force microscopic (AFM) studies
that the mean width of the interpenetrating, intermingling
bands is at about 80 nm [15,19]. So, provided that the
MW-CNT is better ‘‘soluble’’ in one of the resin components (namely in VE), the IPN structuring should have directed the nanotube dispersion. This is, however, not
obvious for the VE/EP hybrid.
3.2. Electrical conductivity
Based on the above TEM study the electrical conductivity of the VE-based systems should be similar. The data in
Table 1 demonstrate that this prediction is fairly met. The
conductivity of the nanocomposites is strongly improved
compared to the parent resins by adding 0.5 wt% MWCNT. On the other hand, the type of the VE system, except
VE/EP, influenced the conductivity marginally. For the lat-
O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
Fig. 3. TEM pictures taken from the VEUH-based nanocomposites containing 0.5 wt% MW-CNT.
Fig. 4. TEM pictures taken from the interpenetrated VE/EP nanocomposites with 0.5 wt% MW-CNT.
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O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
Resin
composition
Electrical
conductivity
(S/cm)
MW-CNT
content
(wt%)
Kc
(MPa m1/2)
VE
–
1.94 · 1006
2.49 · 1004
4.05 · 1004
0
0.5
1
2
0.56
0.58
0.71
0.48
0.14
0.17
0.19
0.13
VEUH
–
1.30 · 1006
0
0.5
0.63
0.45
0.17
0.11
VE/EP
–
3.84 · 1004
0
0.5
1.85
1.88
9.91
11.60
Gc
(kJ/m2)
Note. Systems marked by ‘‘–’’ were electrically insulators (<1012 S/cm).
ter system a higher conductivity than that of VE and
VEUH was found. This may be related with solubilization
(hybrid resin) and/or segregation effects (IPN structuring).
Increasing the MW-CNT amount was accompanied with
further improvement of the conductivity as checked on
the example of VE (Table 1).
3.3. DMTA behavior
Fig. 5 depicts the E 0 vs. T and tand vs. T traces for the
VE nanocomposites. Note that incorporation of 0.5 wt%
MW-CNT has a negligible effect on the stiffness. By contrast, 1 wt% MW-CNT causes pronounced stiffness
enhancement. It is the right place to mention that the reinforcing efficiency of CNTs decrease with their increasing
wall number. This is due to the fact that only the outer
layer of MW-CNT is coupled with the matrix via the
interphase.
The glass transition temperature (Tg) or a-relaxation
shifts toward lower temperatures. The related difference
(20 C) should be linked with the interphase which is
likely less crosslinked than the bulk. Note that adding
MW-CNT in thermosets often results in Tg shift toward
higher temperatures (e.g. [10]). On the other hand, organophilic layered silicates caused a negative shift in Tg for VE/
10000
EP [21] and EP ([22] and references therein), similar to the
present case with MW-CNT. The slight decrease of the Tg
peak owing to MW-CNT incorporation is a further hint for
the reinforcing action of the latter.
Interestingly, an adverse effect for stiffness was registered
for the VEUH/MW-CNT nanocomposites (Fig. 6). The observed stiffness reduction suggests that the additional crosslinking via polyurethane chemistry is affected by the MWCNT. On the other hand, the Tg when read from the peak
of the a-transition, did not change. This can be attributed
to the formation of a rather well crosslinked interphase.
The findings listed in connection with Figs. 5 and 6 indicate
that MW-CNT may affect the chemical reactions in the
interphase. Accordingly, the recipe (peroxide, catalyst, activator. . .) should be adjusted. Unfortunately for that purpose only a trial and error approach can be used.
The DMTA response was less influenced by MW-CNT
for the IPN structured VE/EP (Fig. 7). Note that the
change in the Tg relaxation is similar to that found for
VE (Fig. 5). This suggests that the MW-CNT is probably
embedded in the VE phase with preference. The preferred
embedment may be the cause for the improved electrical conductivity of the VE/EP-based nanocomposites
compared to the corresponding VE- and VEUH-based
ones.
10000
0.3
0.25
1000
VEUH
0.2
+ 0,5% MWCNT
0.15
100
tan δ
Table 1
Electrical conductivity and fracture mechanical data of the thermoset
systems studied
E' [MPa]
1256
0.1
0.05
10
-100
-50
0
50
100
150
200
250
0
300
Temperature [˚C]
0
Fig. 6. E vs. T and tand vs. T traces for the VEUH with and without
MW-CNT.
10000
0.7
VE/EP (IPN)
0.6
+ 0,5% MWCNT
0.6
+ 0,5% MWCNT
0.4
+1% MWCNT
0.3
100
0.4
0.3
100
0.2
0.2
0.1
0.1
10
-100
-50
0
50
100
150
200
0
250
tan δ
VE
E' [MPa]
1000
1000
tan δ
E' [MPa]
0.5
0.5
10
-100
-50
0
50
100
150
0
200
Temperature [˚C]
Temperature [˚C]
Fig. 5. E 0 vs. T and tand vs. T traces for the VE nanocomposites
containing various amounts of MW-CNT.
Fig. 7. E 0 vs. T and tand vs. T traces for the VE/EP (IPN) with and
without MW-CNT.
O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
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3.4. Fracture mechanics and fracture behavior
Table 1 lists the Kc and Gc values determined. The Kc
and Gc data go through a maximum as a function of
MW-CNT content for VE. Nevertheless, the related
changes are modest and at 2 wt% MW-CNT both fracture
toughness and energy fall below those of the parent VE.
Incorporating MW-CNT in VEUH reduces the fracture
mechanical values. On the other hand, Kc marginally
whereas Gc markedly increase in VE/EP owing to adding
MW-CNT. As far as the fracture mechanical values of
VE/EP concern, they were out of limit of the linear elastic
fracture mechanics because of pronounced ductile deformation. Recall that here the maximum load and corresponding energy were taken into account when
computing Kc and Gc.
Fig. 8 shows the fracture surfaces of VE/MW-CNT
nanocomposites. The VE + MW-CNT nanocomposites
failed brittlely. At high magnifications one can see the effect
of the nanotube concentration (Fig. 8(b)–(d)). At 0.5 wt%
MW-CNT small, regular shear steps are resolved. This reflects higher toughness compared to a smooth mirror-like
fracture surface. The regularity is lost owing to large
MW-CNT aggregates when added in 2 wt% (Fig. 8(d)).
Comparing Fig. 8(b) with Fig. 9, one can see that the additional crosslinking via the polyurethane route caused a pro-
Fig. 9. SEM pictures taken from the fracture surface of VEUH containing
0.5 wt% MW-CNT.
nounced matrix embrittlement. The surface is smoother,
the shear steps are smaller for VEUH- than for the VEbased nanocomposites. It is worth of noting that the benefit
of this kind of hybridization was a substantial increase in
the Tg (Figs. 5 and 6). The IPN-structured VE/EP of low
Tg (Fig. 7) failed in a very ductile manner. This was not
affected by the MW-CNT added (Fig. 10(a)). One can even
claim that the MW-CNT and their aggregates even favored
Fig. 8. SEM pictures taken from the fracture surface of VE/MW-CNT nanocomposites Designations: (a)–(c) VE + 0.5 wt% MW-CNT; (d) VE + 2 wt%
MW-CNT.
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O. Gryshchuk et al. / Composites: Part A 37 (2006) 1252–1259
Fig. 10. SEM pictures taken from the fracture surface of VE/EP containing 0.5 wt% MW-CNT.
the shear deformation (Fig. 10(b)). This is well reflected in
the Gc data published in Table 1.
The fracture and failure behavior can be summarized as
follows. The MW-CNT used is prone for nesting due to the
very high aspect ratio of the nanotubes involved. Therefore
MW-CNT can be treated as some kind of ‘‘flexible’’ reinforcement. A flexible reinforcement works well in a very
ductile system as our IPN structured VE/EP. On the other
hand, it is the less efficient the more crosslinked the resin
matrix is (VEUH > VE). If this note is correct then MWCNT of lower aspect ratio (achieved for example by ball
milling) may be a better reinforcement for tigthly crosslinked resins. This speculation disregards eventual changes
in the interphase which, however, have to be taken into account based on our above results.
4. Conclusions
Based on this work devoted to check the reinforcing effects of MW-CNT in VE and VE-based hybrid resins by
exploiting the solubilization effect of styrene on MWCNT the following conclusions can drawn:
Disentangling of the nested MW-CNT can only partially
be achieved by the preparation method used. Rough
information on the dispersion state of MW-CNT can
be received from electrical conductivity measurements.
MW-CNT affects the formation of the interphase which
has to be considered.
The more tightly crosslinked the resin, the less efficient
MW-CNT as reinforcement is. For highly crosslinked
resins the use of MW-CNT of low aspect ratio is
recommended.
Acknowledgments
The authors thank Mr. P. Alvarez Dacosta for his help
in the experimental work. This work was partly supported
by the German Science Foundation (DFG Ka 1202/15)
and Fonds der Chemischen Industrie (JKK).
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