Enhanced mechanical properties of multiwalled carbon nanotubes/thermoplastic polyurethane nanocomposites

Abstract

Carbon nanotubes are considered to be ideal candidates for improving the mechanical properties of polymer nanocomposite scaffolds due to their higher surface area, mechanical properties of three-dimensional isotropic structure, and physical properties. In this study, we showed the improved mechanical properties prepared by backfilling of preformed hydrogels and aerogels of individually dispersed multiwalled carbon nanotubes (MWCNTs-Baytubes) and thermoplastic polyurethane. Here, we used the solution-based fabrication method to prepare the composite scaffold and observed an improvement in tensile modulus about 200-fold over that of pristine polymer at 19 wt% MWCNT loading. Further, we tested the thermal properties of composite scaffolds and observed that the nanotube networks suppress the mobility of polymer chains, the composite scaffold samples were thermally stable well above their decomposition temperatures that extend the mechanical integrity of a polymer well above its polymer melting point. The improved mechanical properties of the composite scaffold might be useful in smart material industry.

Full text

Article
Enhanced mechanical properties
of multiwalled carbon nanotubes/
thermoplastic polyurethane
nanocomposites
P Kalakonda
1,2
, S Banne
3
, and PB Kalakonda
4
Abstract
Carbon nanotubes are considered to be ideal candidates for improving the mechanical properties of polymer nano-
composite scaffolds due to their higher surface area, mechanical properties of three-dimensional isotropic structure, and
physical properties. In this study, we showed the improved mechanical properties prepared by backfilling of preformed
hydrogels and aerogels of individually dispersed multiwalled carbon nanotubes (MWCNTs-Baytubes) and thermoplastic
polyurethane. Here, we used the solution-based fabrication method to prepare the composite scaffold and observed an
improvement in tensile modulus about 200-fold over that of pristine polymer at 19 wt% MWCNT loading. Further, we
tested the thermal properties of composite scaffolds and observed that the nanotube networks suppress the mobility of
polymer chains, the composite scaffold samples were thermally stable well above their decomposition temperatures that
extend the mechanical integrity of a polymer well above its polymer melting point. The improved mechanical properties of
the composite scaffold might be useful in smart material industry.
Keywords
Polymer nanocomposites, carbon nanotubes, elastic modulus and mechanical integrity
Date received: 9 February 2018; accepted: 21 February 2019
Topic Area: Polymer Nanocomposites and Nanostructured Materials
Topic Editor: Emanuel Ionescu
Associate Editor: Emanuel Ionescu
Introduction
Nanotube polymer composite scaffold has been studied for
many industrial applications such as electronic packing,
shielding, and storage capacitors.
1–13
Recently, a signifi-
cant improvement in the mechanical properties was also
observed in the composites scaffold.
14
However, there is
a big challenge to fabricate the composite scaffold beyond
certain higher loading of nanofiller composition due to
possible aggregation. Consequently, higher aspect ratio
nanofillers are good for larger reinforcement. Carbon nano-
tubes (CNTs) are the best choice in this regard due to their
high aspect ratio and large interfacial area.
15,16
Addition-
ally, the surfaces of nanotubes can be functionalized easily
1
Department of Materials Science and Engineering, Carnegie Mellon
University, Pittsburgh, PA, USA
2
Division of Physical Sciences and Engineering, King Abdullah University of
Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia
3
Department of Materials Science and Metallurgical Engineering, Maulana
Azad National Institute of Technology (MANIT), Bhopal, Madhya
Pradesh, India
4
School of Pharmaceutical Sciences and Innovative Drug Research Centre,
Chongqing University, Chongqing, People’s Republic of China
Corresponding author:
P Kalakonda, Department of Materials Science and Engineering, Carnegie
Mellon University, Pittsburgh, PA 15213, USA..
Email: parvathalu.k@gmail.com
Nanomaterials and Nanotechnology
Volume 9: 1–7
ªThe Author(s) 2019
DOI: 10.1177/1847980419840858
journals.sagepub.com/home/nax
Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License
(http://www.creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without
further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/
open-access-at-sage).

and prepared to interact with suitable polymers through the
use of suitable coupling agents.
17
However, there is a big challenge in handling of CNTs,
such as aggregation that leads to a nonuniform dispersion
and poor interfacial interaction between the nanotubes and
the polymer matrix. The nanotubes aggregate easily with a
polymer in simple mixing process due to van der Waals
attraction between the nanotubes and the polymer matrix.
Recently, loading of 1–20 wt%of CNTs into polymers,
the most widely used fabrication method, leads to 40–
800%strengthening in modulus.
18–20
This poor reinforce-
ment has been attributed due to lower aspect ratio,
slippage, and bundling of nanotubes at their atomically
smooth surfaces. The fictionalization method is one of the
methods through which we can overcome the aggregation,
but their intrinsic properties such as electrical and
mechanical properties are reduced. To achieve homoge-
neous dispersion, it is necessary to overcome the aggrega-
tion issue in polymers without covalent functionalization
and fabricate porous networks of nanotubes backfilled
with the polymer matrix. These porous nanotube net-
works are fabricated via chemical vapor deposition
method (CVD), and they contain impurities from synth-
esis that cannot be removed without damaging the scaf-
fold. They also offer well-controlled size, shape, and
pore size as well as reduced control over properties of
the constituent nanotubes. Fabrication of backfilling net-
works has yielded only 100–600%and 60–100%
enhancement in Eand ultimate tensile strength (UTS),
at the loading of 1.5–60 vol%CNTs. This would be
possibly due to nanotubes bundle during polymer infil-
tration fabrication process and have poor interfacial
adhesion with polymers.
21,22
Experimental methods
Materials and sample preparation
A schematic representation of our composite fabrication
method is shown in Figure 1. The polymer used in this
work is a commercial-grade elastomeric random of thermo-
plastic polyurethane family (TPU; Texin Sun-3006HF,
Bayer Materials, Pittsburg, Pennsylvania, USA) and is
composed of hard and soft segments (HS and SS, respec-
tively). This class of polymers is often utilized as a polymer
matrix in nanocomposite studies because it has many
industrial applications and simple processing. The aliphatic
hard segments of TPU are incompatible with soft segments
and phase segregate into amorphous or crystalline domains
within a network of soft segments. The composition and the
hierarchical structures formed by both soft and hard seg-
ments indicate the thermomechanical responses of TPU:
the hard segments typically influence the modulus and
strength, while the soft segments provide stretchability.
The TPU used here has approximately 62 wt%of soft
segments with the rest being hard segments. The hard
segments either do not crystalize or form small crystallized
domains that have a melting temperature (T
mHS
) of approx-
imately 60C. Further, the glass-transition temperature of
the hard segments (T
gHS
) is nearly 30C and the soft seg-
ments are rubbery at room temperature with a T
gSS
of
nearly 45C. The MWCNTs (Baytubes C150P, purity
*95%, mean diameter of 13–16 nm, and the average
length of 1–4 mm) used in this work were purchased from
Bayer Material Science.
We have used a solution fabrication method with
MWCNTs in hydrogel and aerogel-forms concentra-
tion.
23–32
The porous network is stood together primarily
via van der Waals interactions at discrete nanotube cross-
over linking points.
33–38
The nanotubes are dispersed indi-
vidually within scaffolds and randomly oriented for
effective load transfer from polymers to nanofillers. Pore
diameters of nanotube network 10–20 nm allow for easy
infiltration of polymers. We have partially backfilled the
scaffolds with the polymer by soaking them in a polymer
solution of concentration 1–6 wt%for 5–10 h at 50C. By
adjusting the polymer solution concentration and soaking
time, we have tuned the final nanotube wt%in the compo-
sites. At the backfilling temperature, the polymer is in a
rubbery state, less viscous and facilitated easy polymer
infiltration into the nanotube network. We then evaporated
the solvent, annealed the composites under vacuum at
150C for 12 h and removed all voids by hot-pressing
method at 130C for 10–15 min. The composite scaffolds
were used for all measurements in a rectangular shape with
an average thickness of 150–200 mm. These composites
Figure 1. A schematic of the fabrication steps of nanotube
polymer composites.
2Nanomaterials and Nanotechnology

showed that the nanotube networks were well preserved
even after the fabrication process with no voids.
Characterization
The tensile stress (s) was measured as a function of tensile
strain (e00) at the rate of 0.2 mm s
1
at room temperature
with a 50-N load cell using an Instron 5940 series tabletop
testing system (TA Instruments). For the tensile measure-
ments, we followed the ASTM D 882 standard including
the testing of plastic sheets with the thickness <0.25 mm.
For thermal analysis, differential scanning calorimetry
(DSC) measurements were carried out with a Q20 DSC
(TA Instruments) at a heating rate of 3Cmin
1
. Most of
the measurements were collected over a temperature range
of 30Cto230
C. Thermogravimetric analysis (TGA)
was carried out in atmospheric air over a temperature range
of 25–800C using a Q50 TGA (TA Instruments). The
specimens were heated at a rate of 5Cmin
1
.
For electrical conductivity measurements of the com-
posite, copper wire leads were attached to the short ends
of the rectangular composites with silver paste (DuPont
4929 N), and resistance was measured using two-probe
contact direct current method with EC-Lab V10 and
Fluke Ohmmeter.
Results and discussion
To study the mechanical reinforcement of the polymer
(TPU) using the Baytube networks, we compare the
mechanical characteristics of the composite scaffolds with
up to 19 wt%nanotubes loading from measurements of
tensile stress versus tensile strain at room temperature
(Figure 2). The polymer (TPU) shows modulus (E)of
6.7 MPa, then plastically yielded to a more gradual defor-
mation, followed by a steep rise in up to a stretchability of
about 345%at which point the specimen broke. Modulus
(E) of these composites increases dramatically with Bay-
tube loading and reaches 1590 MPa at 19 wt%nanotube
(Figure 3(a)), which corresponds to more than 20,000%
improvements over that of the pristine polymer. However,
stretchability decreases, and the yield point begins at a
smaller strain with increasing Baytube loading. These com-
posites did not show any sudden drop in yield. The point
that is commonly observed for TPU composites
27
is due to
the disintegration of the nanotube network,
which demonstrates the robustness nanotube scaffold
within the polymer.
Further, these composites show enhancement in UTS
from 11 MPa to 30 MPa at 19 wt%nanotube loading
(Figure 3(b)), which corresponds to approximately 300%
improvements over that of the pristine polymer. The elon-
gation at break of these composite scaffold decreases with
CNTs loading and reaches 2.5%strain at 19 wt%nanotubes
(Figure 2). The elongation at break of these composite
decreases with CNT loading and reaches 2.5%strain at
19 wt%nanotubes (Figure 2).
The UTS and tensile modulus of the composites show
significantly more brittle due to the stronger interfacial
interactions between CNTs and polymer. These composite
scaffolds with 19 wt%CNT loading showed higher mod-
ulus and UTS, higher stiffness compared to the pristine
polymer due to highly packed morphology with no voids
between the CNTs and the polymer. It could also be specu-
lated that the reduction of pore size leads to more friction
between the CNTs and the polymer, which leads to higher
tensile strength. From the cyclic fatigue test (Figure 4), the
composites scaffold aerogels show significantly higher
strain hardening at 80C than the pristine polymer. The
CNTs network suppress the mobility of polymer chains
and therefore the thermal motion at the interface of CNT
and TPU, which lead to have a strain hardening in the
elastic region (Figure 4). As the CNTs have higher sur-
face area and mechanical properties of isotropic 3-D
structure, their composite scaffold aerogels show higher
mechanical characteristics.
The improved mechanical properties of these composite
scaffolds might also originate from an improved structure,
which can be measured using thermal stability measure-
ments by the DSC and TGA. For the polymer, the DSC
curve from the first heating cycle shows peak at T
mHS
¼
58.8C with an absorbed heat dH
m
of 3.3 J g
1
, calculated
from the area under the peak. This indicates that the hard
segments are weakly crystalline, while the soft segments
show no crystallinity (Figure 5(a)). The melting of the
polymer’s hard segments is reversible and shows a much
weaker recrystallization on cooling at approximately 30C.
The height of the endotherm peak decreases with a loading
of CNTs and becomes broader possibly due to the restricted
thermal motion of the hard segments within the nanoporous
nanotube networks. The restrained polymer thermal
motion, which is believed to be due to an increase in the
effective polymer stiffness, is likely to have contributed to
the observed higher mechanical reinforcement.
Figure 2. The tensile stress versus strain curves of TPU polymer
and composites with various Baytube concentrations. TPU:
thermoplastic polyurethane.
Kalakonda et al. 3

The degradation temperature of the pristine polymer is
300C, and it shifts to a higher temperature by 40Cat19
wt%CNTs loading. The mass loss in composite scaffolds
associated with burning of polymer reduces in the presence
of nanotube network because of nano-confinement.
Furthermore, it shows that the degradation temperature of
composite scaffold aerogels shifts to a higher temperature
with increasing CNT loading. The rate of mass loss shows
that degradation temperature of hard segments significantly
shifts to higher temperature with an addition of nanotubes
and is consistent with DSC measurements. The composites
show extended thermal stability and a very slow mass loss
rate when burned in the presence of atmospheric air in TGA
(Figure 5(b)), and it is likely to the shift the dissociation
temperature of the hard segments to a higher temperature.
The nanotube network remained intact even after the com-
posites are heated to 800C but had a thin coating of resi-
dues, probably decomposed polymer. This higher
decomposition temperature extends mechanical integrity
and thermal stability of the polymer.
The intrinsic properties of individually dispersed
MWCNTs may also add several beneficial features to the
Figure 3. Mechanical characteristics of composites. (a) The values of E(black solid circles) of the composites increase by >20,000%
with loading of 19 wt% of nanotubes. &&&The enhancement in Eis well-predicted by the Halpin–Tsai model (block solid line). (b) The
UTS of the composites (black solid square) also increases with the loading of nanotubes. The error bars are obtained from mea-
surements on multiple samples. E: tensile modulus; UTS: ultimate tensile strength.
Figure 4. Cyclic fatigue test of pristine polymer TPU and composite scaffolds aerogel. TPU: thermoplastic polyurethane.
4Nanomaterials and Nanotechnology

nanocomposite scaffolds. The electrical conductivity of
nanocomposite scaffold aerogel increases as the function
of nanotubes loading. The conductivity of the aerogel
composites is higher compared to the hydrogel compo-
sites (Figure 6). It may be due to the penetration of the
polymer into the nodes and degradation of the electrical
contacts between the nanotubes. At lower CNT concen-
tration, the conductivity is lower, and it might be due to
less electrical contacts between the nanotube junctions.
We also observe that the electrical conductivity is low
at higher loading of CNTs, which might be due to higher
contact resistance between the CNT junctions (Figure 6;
SEM image). In these composite scaffold aerogels, larger
internal surface area and stronger interfacial interaction
with 3-D random network may lead to higher electrical
conductivity.
Figure 5. (a) DSC measurements of polymer and composites show reduction in polymer crystallization in the composite with
increasing nanotube concentration. (b) TGA of nanotubes, polymer and composites under atmospheric air. DSC: differential scanning
calorimetry; TGA: thermogravimetric analysis.
Figure 6. Electrical conductivity of aerogel-based composite scaffolds as a function of nanotubes loading (wt%) and the SEM image of
CNT, TPU-coated CNTs. TPU: thermoplastic polyurethane; CNT: carbon nanotube; SEM: scanning electron microscope.
Kalakonda et al. 5

Conclusion
In conclusion, we performed solution-based fabrication
method to overcome the aggregation of nanotubes and
improve tensile modulus about 200-fold. The nanopores
of the nanotube network reduce the polymer thermal
motion, resulting in the suppression of polymer glass tran-
sition and with an extension of mechanical integrity. Also,
the nanotubes and polymer are thermally stable well above
their degradation temperatures. Our fabrication method
allows creating nanocomposites from polymers that are
incompatible with nanotubes and the ones produced in this
study have superior mechanical properties as well as pro-
mising smart material industrial applications.
Acknowledgments
The authors would like to acknowledge Bayer Materials for
providing thermoplastic polyurethane and Baytubes.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) received no financial support for the research,
authorship, and/or publication of this article.
ORCID iD
P Kalakonda https://orcid.org/0000-0003-1793-2069
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