Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
ISSN 2050-7526
PAPER
Kun Dai, Chuntai Liu, Zhanhu Guo et al.
Lightweight conductive graphene/thermoplastic polyurethane foams with
ultrahigh compressibility for piezoresistive sensing
Volume 5 Number 1 7 January 2017 Pages 1–240
This journal is
©
The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 73--83 | 73
Cite this: J. Mater. Chem. C, 2017,
5,73
Lightweight conductive graphene/thermoplastic
polyurethane foams with ultrahigh compressibility
for piezoresistive sensing †
Hu Liu,
ab
Mengyao Dong,
a
Wenju Huang,
a
Jiachen Gao,
a
Kun Dai,*
a
Jiang Guo,
b
Guoqiang Zheng,
a
Chuntai Liu,*
a
Changyu Shen
a
and Zhanhu Guo*
b
Lightweight conductive porous graphene/thermoplastic polyurethane (TPU) foams with ultrahigh
compressibility were successfully fabricated by using the thermal induced phase separation (TISP)
technique. The density and porosity of the foams were calculated to be about 0.11 g cm
3
and 90%
owing to the porous structure. Compared with pure TPU foams, the addition of graphene could
effectively increase the thickness of the cell wall and hinder the formation of small holes, leading to a
robust porous structure with excellent compression property. Meanwhile, the cell walls with small holes
and a dendritic structure were observed due to the flexibility of graphene, endowing the foam with
special positive piezoresistive behaviors and peculiar response patterns with a deflection point during
the cyclic compression. This could effectively enhance the identifiability of external compression strain
when used as piezoresistive sensors. In addition, larger compression sensitivity was achieved at a higher
compression rate. Due to high porosity and good elasticity of TPU, the conductive foams demonstrated
good compressibility and stable piezoresistive sensing signals at a strain of up to 90%. During the cyclic
piezoresistive sensing test under different compression strains, the conductive foam exhibited good
recoverability and reproducibility after the stabilization of cyclic loading. All these suggest that the
fabricated conductive foam possesses great potential to be used as lightweight, flexible, highly sensitive,
and stable piezoresistive sensors.
1 Introduction
Conductive polymer composites (CPCs), achieved through the
addition of conductive fillers into the normal insulating polymer
matrix, have shown great potential applications in the fields of
smart sensors. The sensing mechanism is mainly based on the
change in conductive networks, i.e., the variation of electrical
resistance arising from the exposure to external stimuli (stress,
organic vapor, temperature, etc.).
1–17
Piezoresistive sensors,
which convert the external applied compression stress or strain
into an obvious electrical resistance signal, can be effectively
used in many industrial fields.
18–20
However, the rigidity and
small strain of conventional metal or semiconductor based
sensors limit their applications for the fabrication of flexible
devices.
21
Herein, CPCs with good flexibility have been considered
as an ideal substitute and extensively researched. For example,
Chen et al. fabricated a finger-sensing conductive graphite
nanosheets/silicone rubber composite with remarkable and
reversible piezoresistivity.
22
Dang et al. reported flexible carbon
nanotubes (CNTs)/methylvinyl silicone rubber composites with
markedly sensitive linear piezoresistive behavior under low
pressure.
23
Meanwhile, lightweight and high compressibility are of great
importance for piezoresistive sensors to satisfy different practical
demands. Due to the merits of lightweight, large specific surface
area and high porosity, porous polymer foams have been widely
used in many fields, including electromagnetic interference (EMI)
shielding,
24–27
biological scaffolds
28–31
and super adsorbents.
32–34
As
for the CPC based piezoresistive sensors, the introduction of a
three-dimensional porous structure must be an effective strategy
for achieving promising piezoresistive performance for widespread
applications. Recently, several newly developed conductive
sponges have also been prepared and used as pressure sensors.
The porous structure not only helps to reduce the density of
CPCs effectively but also enables the CPCs with excellent
compressibility in a large strain region. For example, Si et al.
prepared carbonized three-dimensional nanofibrous aerogels
a
College of Materials Science and Engineering, The Key Laboratory of Material
Processing and Mold of Ministry of Education, Zhengzhou Univers ity, Zhengzhou,
Henan 450001, P. R. China. E-mail: kundai@zzu.edu.cn, ctliu@zzu.edu.cn
b
Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular
Engineering, University of Tennessee, Knoxville, TN 37996, USA.
E-mail: zguo10@utk.edu
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6tc03713e
Received 28th August 2016,
Accepted 14th October 2016
DOI: 10.1039/c6tc03713e
www.rsc.org/MaterialsC
Journal of
Materials Chemistry C
PAPER
Open Access Article. Published on 17 October 2016. Downloaded on 8/26/2022 5:38:04 AM.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
View Journal
| View Issue
74 | J. Mater. Chem. C, 2017, 5, 73--83 This journal is
©
The Royal Society of Chemistry 2017
with high compressibility and conductivity. During the cyclic
compression strain of up to 50%, a 70% decrease of normalized
resistance (termed negative piezoresistive behavior) was observed
together with good recoverability and reversibility.
35
In our
previous research, porous CNTs/thermoplastic polyurethane
(TPU) nanocomposites with a density of approximately 0.1 g cm
3
have been prepared by the thermal induced phase separation (TIPS)
technique. The porous nanocomposites also exhibited negative
piezoresistive behavior with good recoverability and reproducibility
over a wide strain range of up to 90%.
6
Meanwhile, it has also
been demonstrated that the conductive network distributed in
the polymer matrix could be tuned by changing the loading
or the type of conductive fillers, causing different sensing
behaviors.
11,36
Carbon based conductive materials such as
carbon black, CNTs and graphene are the most frequently used
fillers to fabricate CPCs. In particular, the single layer two
dimensional graphene has been considered as a good candidate.
Due to the high conductivity of graphene, it could reduce the
percolation threshold of CPCs significantly, avoiding the sacrifice
of mechanical properties of CPCs.
37,38
On the other hand, the
large specific surface area and good flexibility of graphene will
also lead to an extraordinary conductive network and dispersion
morphology in CPCs. To explore its application, the three-
dimensional architectures of graphene or its derivatives (such
as graphene aerogel/polydimethylsiloxane (PDMS),
39
graphene
foam (GF),
40
GF/PDMS,
41,42
etc.) have been investigated for piezo-
resistive sensors. But, their complex preparation process and
high-cost hindered their practical applications. Based on the
aforementioned discussions, it is both scientifically meaningful
and necessary to fabricate porous graphene based CPC foams,
aiming to acquire new types of lightweight piezoresistive sensors
with ultrahigh compressibility and interesting sensing behaviors.
In the present work, TPU was chosen as the host polymer
matrix owing to its good elasticity and good affinity for carbon
fillers.
2,16
The porous graphene/TPU foams were fabricated by
using the simple TIPS technique, which was verified to be
a good manufacturing route to three-dimensional porous
CPCs.
6,30
The effects of graphene loading levels on the electrical
conductivity, mechanical properties, thermal properties and cell
morphology of the porous CPCs were systematically studied.
Their structures were also characterized. Stepwise compression
and cyclic compression under different compression strains
were conducted separately to investigate the piezoresistive sensing
behaviors of the porous CPC foams.
2 Experimental
2.1 Materials and chemicals
Polyester based thermoplastic polyurethane (TPU, Elastollan
1185A) purchased from BASF Co. Ltd, China was used as a
polymer matrix. Graphene was purchased from Chengdu
Organic Chemicals Co. Ltd, China. According to the supplier,
the characteristics of graphene were: thickness 0.55–3.74 nm,
diameter 0.5–3 mm, specific surface area 500–1000 m
2
g
1
,
and purity >90 wt%. Dioxane was purchased from Zhiyuan
Reagent Co., Ltd Tianjin, China, and used as-received without
further treatment.
2.2 Fabrication of porous graphene/TPU foams
Porous graphene/TPU foams were prepared using the TIPS
technique. First, graphene (in ratios of 0.5, 1, 1.5, 2, 2.5 and
3 wt% to TPU, correspondin g to volume concentrati ons of 0.024,
0.05, 0.76, 0.1, 0.13 and 0.16 vol%. The conversion method is
detailed in the ESI†) was dispersed in dioxane using an ultrasonica-
tion instrument (SCIENTZ-II, 285W, Ningbo Scientz Biotechnology
Co. Ltd, China) to achieve a homogenous graphene/dioxane
mixture. Second, TPU pellets (5 g per 100 mL dioxane) were
dissolved in a graphene/dioxane mixture with rapid stirring
below 40 1C for 30 min. Subsequently, the obtained mixture
was added into glass tubes with a diameter of 20 mm and placed
in a 25 1C freezer for 12 h to ensure complete phase separation.
The tubes were then transferred to a freeze-drying vessel at 80 1C
for 72 h at 8 Pa, forming the porous structure after the sublimation
oficecrystals.ThepureTPUfoamwasalsofabricatedwithoutthe
addition of graphene.
2.3 Characterization
Fourier-transform infrared (FT-IR) spectra were recorded on a
Nicolet Nexus 870 instrument using the attenuated total reflectance
(ATR) technique. All the spectra were scanned at a resolution of
4.0 cm
1
in the range from 500 to 4000 cm
1
.
X-ray diffraction (XRD) analysis was carried out on a Rigaku
Ultima IV X-ray diffractometer with Cu Ka radiation. The scanning
was performed from 5 to 801 with a speed of 0.021 min
1
.
Differential scanning calorimetry (DSC) analysis was carried
out on a DISCOVERY DSC Q2920 instrument. Approximately
8 mg of sample was encapsulated in an aluminum pan and
heated from 30 to 200 1C at a heating rate of 10 1C min
1
. Then
it was maintained for 5 min to erase the thermal history. After
that, the sample was cooled down to 60 1C and reheated to
220 1C at the same rate. All the tests were performed in a
nitrogen atmosphere at a flow rate of 20 mL min
1
.
The thermal stability was investigated using thermogravimetric
analysis (TGA/STDA851e, Mettler Toledo, Switzerland). A sample of
about 8 mg was heated from ambient temperature to 700 1Cata
constant heating rate of 10 1Cmin
1
in a nitrogen atmosphere.
The nitrogen flow rate was 40 mL min
1
.
The Raman spectra were measured using a Renishaw inVia
Raman confocal microscope with 532 nm laser excitation at
1cm
1
resolution in the range from 80 to 4000 cm
1
.
Field emission scanning electron microscopy (FE-SEM)
(JEOL JSM-7500F instrument) was adopted to observe the
morphology of the fabricated foam. The specimens were cryo-
genically fractured in liquid nitrogen. The fracture surfaces were
then coated with a thin layer of platinum for better imaging.
The mechanical properties were characterized via compression
tests using a universal test ing machine with a 100 N load cell
(UTM2203, Shenzhen Suns Technology Stock Co. Ltd, China).
Cylindrical samples with a diameter of 15 mm and a height of
10 mm were compressed to a compression strain of 50% at a
Paper Journal of Materials Chemistry C
Open Access Article. Published on 17 October 2016. Downloaded on 8/26/2022 5:38:04 AM.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
This journal is
©
The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 73--83 | 75
rate of 5 mm min
1
. The results were averaged over at least five
different specimens for each sample.
The volume resistance was measured using a precision
digital resistor (Model TH2683, Changzhou Tonghui Electronics
Co. Ltd, China) under a constant voltage of 10 V. As shown in Fig. 1,
cylindrical samples were sandwiched between two aluminum
electrodes. Silver paste was used to ensure good contact between
the electrode and the sample. The corresponding volume
conductivity was calculated by using the formula: s = L/RS,
where s represents the volume conductivity, R represents the
volume resistance, S and L represent the cross-sectional area
and the height of the cylinder, respectively. The precision digital
resistor and the universal testing machine were coupled with a
computer to record the piezoresistive behavior online. In order
to ensure the reliability and reproducibility of the piezoresistive
behavior, at least five specimens were used for each test in the paper.
3 Results and discussion
3.1 Thermogravimetric analysis
Fig. 2 shows the TGA curve and the corresponding differential
thermogravimetric (DTG) curve of the TPU foam and its CPC foams
with different graphene loadings in a nitrogen atmosphere. By
comparison with the TPU foam, the onset degradation temperature
of CPC foams increased with increasing graphene content,
and an improvement of about 6.6 1C was observed for the CPCs
with about 3 wt% graphene loading. In addition, a two-step
degradation pattern was observed from the DTG curves of all
foams. The first step between 260 and 330 1C was mainly
related to the cleavage of urethane bonds of TPU,
10
and the
decomposition peak weakened slightly with increasing graphene
content and almost disappeared for the CPC foam with 2 wt%
graphene, but it obviously appeared again for the CPCs containing
3 wt% graphene. As for the second step between 330 and 500 1C
related to the decomposition of soft segments of TPU, the
temperature of the maximum decomposition rate increased
firstly and reduced subsequently with increasing graphene
content. The CPC foam with 2 wt% graphene possessed the
highest temperature of the maximum decomposition rate at
about 387.48 1C, 22 1C higher than that of pure TPU. Such trend
was also observed in linear low density polyethylene (LLDPE)
based composites incorporated with functionalized graphene.
43
All these indicate that the addition of graphene is beneficial
for the improvement of thermal stability of composites, but
higher loading also brings a negative effect. There are two main
aspects in this phenomenon. First, the graphene nanosheets
with a large surface area act as a so-called ‘tortuous path’ to
retard the decomposition of the polymer matrix and the release
of volatile products.
44,45
Second, when a higher amount of graphene
is added in the CPC foam, the high thermal conductivity of
graphene enables it to act as the heat source to accelerate the
Fig. 1 Schematic diagram of sample preparation, resistance test and piezoresistance behavior test.
Fig. 2 (a) TGA and (b) DTG thermograms of the TPU foam and its CPC foams with graphene loadings of 1, 2 and 3 wt%; the inset in (a) shows the onset
degradation temperature (the temperature for 5% weight loss).
Journal of Materials Chemistry C Paper
Open Access Article. Published on 17 October 2016. Downloaded on 8/26/2022 5:38:04 AM.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
76 | J. Mater. Chem. C, 2017, 5, 73--83 This journal is
©
The Royal Society of Chemistry 2017
decomposition of TPU. Herein, an appropriate graphene content is
therefore important for the properties of the composites.
43
3.2 Differential scanning calorimetry analysis
Fig. 3 displays the melt temperature (T
g
) and glass transition
temperature (T
m
) curves of the TPU foam and its CPC foams
with different graphene loadings obtained from the DSC
thermograms. From Fig. 3(a), the T
m
of the TPU foam was
found to be about 192 1C, which was referred to the melting
point of hard segments crystalline structures of the TPU matrix.
No variation of T
m
was found for graphene/TPU foams, indicating
that the original crystal structure of the host matrix remained
unchanged in spite of the incorporation of graphene.
46
In
addition, the value of melting enthalpy is about 5.097 J g
1
,
showing a small fraction of hard segments in the TPU used in
the research. However, the increase of graphene loading led to
wider and shallower endothermic curves and the melting
enthalpy of the conductive foam with 3 wt% loading dropped
to only 3.383 J g
1
, which might be due to the fact that the
addition of graphene inhibited the crystallization of the TPU
hard segments, leading to the decrease in the crystalline size of
TPU molecules.
28,47–49
As for the influence of graphene on the
T
g
of CPC foams, it can be seen that the T
g
decreases with
increasing graphene content, Fig. 3(b). Compared with the T
g
of
TPU foams at about 25.76 1C, the T
g
values of the CPC foams
with 1, 2 and 3 wt% graphene are 26.9, 27.93 and 28.11 1C,
respectively. The main reason for this phenomenon is that the
entanglement degree of the soft segment molecules of TPU may
be reduced in the presence of graphene, causing the higher
mobility of soft segments. On the other hand, the destruction of
the crystalline structure of hard segments, which act as physical
crosslinking points of the TPU molecules, will release the
constrained soft segments and improve its mobility, so a
reduction of T
g
occurred.
3.3 X-ray diffraction analysis
Fig. 4 shows the XRD patterns of graphene, the TPU foam and
its CPC foams. A small broad diffraction peak at a 2y value of
25.41 appears in the pattern of graphene, which is assigned to
the (002) planes of a graphitic structure with short-range order in
some stacked graphene sheets. The TPU foams display a strong
diffraction peak at a 2y value of 20.51, which is relevant to the
existence of a short range regularly ordered structure of both hard
and soft domains along with a disordered structure of the
amorphous phase of the TPU matrix.
47,50
The XRD patterns of
all CPC foams display obviously the peak assigned to the TPU,
indicating that the crystal structure of the TPU matrix remains
unchanged after the addition of graphene. But the intensity of
the peak is attenuated with increasing graphene loading. The
reason may be due to the interfacial interaction between the
graphene and TPU, which causes the decrease in the crystalline
size of TPU molecules.
50
This phenomenon is consistent with
the DSC results. Besides, the diffraction peak corresponding to
graphene disappears completely, which is ascribed to the full
exfoliation during the sonication process, leading to the destruction
of the short-range ordered graphitic structure of graphene.
3.4 Fourier transform infrared spectroscopy analysis
The FT-IR spectra of the TPU foam and its CPC foams with
different graphene loadings were recorded to investigate the
Fig. 3 (a) T
m
and (b) T
g
curves of the TPU foam and its CPC foams with graphene loadings of 1, 2 and 3 wt%.
Fig. 4 XRD patterns of graphene, the TPU foam and its CPC foams with
graphene loadings of 1, 2 and 3 wt%.
Paper Journal of Materials Chemistry C
Open Access Article. Published on 17 October 2016. Downloaded on 8/26/2022 5:38:04 AM.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online