Healing by the Joule effect of electrically conductive poly(ester-urethane)/carbon nanotube nanocomposites

Article abstract of DOI:10.1039/c5ta09793b

Recent demands for polymers with autonomous self-healing properties are being constantly raised due to the need for high-performance and reliable materials. So far, the advances in this field are limited to the production of self-healing materials requiring a high energy input. Therefore there is an urgent need to develop self-healing polymer systems, in which healing can be easily and specifically induced by external stimuli for economical and viable applications. In the current work we demonstrate, for the first time to our knowledge, the possibility to heal local macroscopic damage by a confined temperature increase arising from the Joule effect. The damage healing is promoted by the resistance to an electrical current at the crack tip. This new concept is studied on thermo-reversible and electrically conductive poly(ester-urethane)/carbon nanotube nanocomposites derived from thermo-reversible Diels-Alder reactions between furfuryl- and maleimide-functionalized poly(ε-caprolactone) (PCL)-based precursors. Electrically conductive materials are then obtained after incorporating multi-walled carbon nanotubes into the thermo-reversible networks using reactive extrusion. Under mild electrical conditions, temperature in the range of the retro-Diels-Alder reaction can be obtained near the damaged site. The obtained results reveal the potential of this new approach for healing materials locally while maintaining the overall material properties.

Full text of DOI:10.1039/c5ta09793b

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Journal Name
ARTICLE
Healing by Joule effect of electrically-conductive poly(ester-
urethane)/carbon nanotubes nanocomposites†
B. Willocq,a R. K. Bose,b F. Khelifa,a S. J. Garcia,*b Ph. Dubois, a and J.-M. Raquez*a
Received 00th January 20xx,
Accepted 00th January 20xx
Recent demands for polymers with autonomous self-healing properties are being constantly raised due to the need of
high-performance and reliable materials. So far, the advances in this field are limited to the production of self-healing
materials requiring a high energy input. Therefore there is an urgent need to develop self-healing polymer systems, in
which healing can be easily and specifically induced by external stimuli for economical and viable applications. In the
current work we demonstrate, for the first time to our knowledge, the possibility to heal locally macroscopic damages by a
confined temperature increase arising from Joule effect. The damage healing is promoted by the resistance to an electrical
current at the crack tip. This new concept is studied on thermo-reversible and electrically conductive poly(ester-
urethane)/carbon nanotube nanocomposites derived from thermo-reversible Diels-Alder reactions between furfuryl- and
maleimide-functionalized poly(ε-caprolactone) (PCL)-based precursors. Electrically conductive materials are then obtained
after incorporating multi-walled carbon nanotubes into the thermo-reversible networks using reactive extrusion. Under
mild electrical conditions, temperature in the range of the retro-Diels-Alder reaction can be obtained near the damaged
site. The obtained results reveal the potential of this new approach for healing materials locally while maintaining the
overall material properties.
DOI: 10.1039/x0xx00000x
www.rsc.org/
linked epoxy resin.4 These self-healing resins were obtained by
thermo-reversible Diels-Alder reactions between tetrafuran (diene)
and trimaleimide (dienophile) monomers. It was shown that when a
Introduction
Inspired by the human skin, self-healing polymers have been widely
developed since the 1980s to overcome some major limitations of
common engineering polymers on failure.1 During their lifetime,
conventional polymers undergo many mechanical and
photo/thermal aggressions leading to microcracks and with time to
major fractures of the material affecting its efficiency, sustainability
and safety. Therefore, many efforts have been made during these
last decades to heal microcracks and restore the initial mechanical
properties of a given material.
crack occurs at the material surface, by simply heating the material
up to 90-120°C (retro-Diels-Alder favored) and by subsequently
cooling it down to 50°C (Diels-Alder favored to restore the covalent
bonds) self-healing of the material was possible. Although many
self-healing polymers of different architectures designed following
reversible bonding such as furan-maleimide Diels-Alder chemistry
5-7
have been developed since this pioneering work,3,
the self-
healing process however requires external heating across the
overall materials and cannot be specifically localized to the
damaged regions to be repaired. Therefore there is a need to
develop new alternatives to locally and specifically repair any
(micro)cracks, and more preferably in the bulk of materials.
Intrinsic self-healing polymers thereby represent a promising way to
heal microcracks due to their autonomous healing properties. This
intrinsic healing process is based on reversible bonding allowing to
heal a material in a repetitive way after a sufficient energy input.2
Such reversibility can be triggered by light illumination, supra-
molecular interaction or thermal activation.3 In 2002, Chen et al.
reported for the first time on self-healing polymers based on [4+2]
Diels-Alder reactions by designing a thermo-remendable and cross-
Among the approaches investigated to overcome these limitations,
the concept of externally-activated heating via the incorporation of
magnetic, photo-thermal and electrical nanoparticles into self-
healing polymeric matrices has recently been developed but it
remains scarcely addressed to few examples such as temperature-
based self-healing of networks filled with magnetic chromium oxide
particles, gold nanoparticles, carbon fibers or graphite.8-15 In the
present study, we propose for the first time to the best of our
knowledge the design of thermo-reversible and electrically
conductive poly(ester-urethane) nanocomposites containing multi-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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walled carbon nanotubes (MWCNTs) that are able to be locally
Experimental
healed using Joule effect (Fig. 1). Due to their unique mechanical,
electrical and thermal properties,16 carbon nanotubes are widely
used as nanofillers for the mechanical reinforcement of
polymeric materials17 and for the synthesis of composites
electrically conductive at low MWCNTs amount (less than 1-2%
wt.).18
Materials
α,ω-Dihydroxyl poly(ε-caprolactone) (PCL-diol) (CAPA2402, Mn =
4000 g.mol-1; PDI = 1.48) and α,α’,ω,ω’-tetrahydroxyl poly(ε-
caprolactone) (PCL-tetraol) (CAPA4801, Mn = 8000 g.mol-1; PDI =
1.48) were kindly supplied by Perstorp and used as received. 1-
(3-hydroxylpropyl)-1H-pyrrole-2,5-dione
prepared from 3-aminopropan-1-ol (Acros, used as received),
exo-3,6-epoxy-1,2,3,6-tetrahydrophtalic anhydride (Sigma
(MAL(OH))
was
Although the implementation of MWCNTs in the field of electro-
active shape memory polymers was already reported,19 no
studies deal with self-healing of polymers based on Joule effect
using MWCNTs. As far as self-healing is concerned, we have
herein considered thermo-reversible poly(ester-urethane)
networks obtained by reactive extrusion, a solvent-free process,
involving linear and branched poly(ε-caprolactone) (PCL)
terminated with furfuryl moieties, noted PCL(FUR)2 and
PCL(FUR)4 respectively, and a PCL-based dimaleimide coupling
agent, noted PCL(MAL)2.20, 21 The main advantages of using PCL,
an aliphatic polyester, lies on its good chain mobility at ambient
temperature (Tg = -60°C), low melting temperature (Tm = 60°C -
below the Diels-Alder activation temperature) and a good
processability. To obtain the electro-conductive networks,
MWCNTs were directly dispersed into the polymeric matrix using
extrusion method. The thermo-reversibility of the networks in
presence or not of MWCNTs, important for the healing process,
was attested after five heating/cooling cycles using a rheometer
coupled with FTIR spectrometer. Finally, the local healing of the
resulting nanocomposites was demonstrated under electrical
current and monitored by an IR-camera on scratched films.
Aldrich, used as received) and ethanol 96%vol (VWR) (Supporting
Information 1). Furfuryl isocyanate (Sigma Aldrich), methylene
diphenyl diisocyanate (Sigma Aldrich), dichloromethane (Alfa
Aesar), chloroform (VWR), methanol (VWR) and multi-walled
carbon nanotubes NC7000 (Nanocyl, 1.5 µm average length and
9.5 nm diameter) were used as received.
Synthesis of PCL end-functionalized with furfuryl moieties
(PCL(FUR)2 and PCL(FUR)4)
Reactions were carried out in bulk, under inert atmosphere, in a
250 ml previously conditioned flasks. PCL-diol and PCL-tetraol
were end-functionalized in presence of an excess of furfuryl
isocyanate (1.5 eq.) overnight at 110°C. Oligomers were
dissolved in CH2Cl2 and poured in an excess of methanol in order
to remove out the excess of furfuryl isocyanate. Chemical
characterizations were performed using 1H NMR spectroscopy
(500 MHz, CDCl3). Yield 99% (Supporting Information 2).
Synthesis of PCL end-functionalized with maleimide moieties
(PCL(MAL)2)
Typically, PCL-diol (1eq.), 1-(3-hydroxylpropyl)-1H-pyrrole-2,5-
dione (2eq.) and methylene diphenyl diisocyanate (2.1eq.) were
introduced into a 15 cm³ twin-screw DSM micro-compounder at
55°C, 30 rpm and under nitrogen flow. Then both temperature
and speed were raised to 140°C and 70 rpm respectively. The
reaction was considered to be complete when the torque was
constant (generally after 40 min). The protected maleimide end-
groups were deprotected by placing the obtained polymer in an
oven under vacuum at 110°C overnight. Chemical
characterizations were performed using 1H NMR spectroscopy
(500 MHz, CDCl3) (Supporting Information 3).
Synthesis of thermo-mendable nanocomposite networks
The desire amount of MWCNTs (between 0.1 and 5% wt.) was
added to a solution of PCL(FUR)x (where x = 2 or 4) in CH2Cl2. The
dichloromethane was evaporated overnight under a fumed hood
and one hour under vacuum at room temperature to get a
master-batch of PCL(FUR)x/MWCNTs with a perfect control of
MWCNTs amount. This premix was then introduced in a micro-
compounder (Supporting Information 4) in presence of
2
Fig. 1. Development of thermo-responsive and electrically
conductive networks based on Diels-Alder moieties by reactive
extrusion and thermal activation at crack tip by Joule effect

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ARTICLE
PCL(MAL)2 (stoichiometric ratio between furfuryl and maleimide
functions) at 55°C, 30 rpm and under nitrogen flow. Then both
temperature and speed were increased up to 80°C and 70 rpm
where Pd is effective penetration depth (Pd = Ps – Pe).
For each scratch a hundred of depth recoveries were calculated
at different points of the scratch and an average was taken.
respectively. After
a mixing of 30 min, the obtained
nanocomposites were placed in an oven at 65°C to ensure the
completion of Diels-Alder coupling reactions.
Further qualitative scratch recovery after electrical activation
was imaged with
a field emission gun scanning electron
microscope (FEG-SEM Hitachi SU8020).
Films preparations
Cross-linking density evaluation
0.4-mm thick films for rheological measurement, evaluation of
Joule heating and volume conductivity measurements were
obtained by compression molding. Typically, 7 g of the samples
were placed in circular molds and compressed at 80°C, for 5 min
in contact, 3 min at 10 bar and then 2 min during cooling.
Between 50 and 100 mg of the thermo-reversible networks were
swollen in chloroform for 24 h at 25°C and polymer volume
fractions (VR) were determined using Equation 2.22
ꢅꢆꢁꢇ
ꢃꢄ =
(2)
Electrical properties determination
(
)
ꢅꢆ ꢁꢇꢈꢁꢉ ꢊ ꢅꢋꢁꢉ
Volume resistivity (ρ, Ω.cm) of the nanocomposites was
evaluated with the four-probe method on rectangular samples
with length, width and thickness of respectively 6 cm, 1 cm and 2
mm for low resistivity samples (1 to 107 Ω.cm) and with the
resistivity test chamber Keithley 8009 method on circular films
with a diameter of 7 cm and a thickness of 0.4 mm for high
resistivity samples (> 107 Ω.cm). The volume conductivity σ (S.cm-
1) was calculated with σ = 1/ρ.
Where m1 and m2 are the weights before and after swelling tests;
ds and dr are the density of the solvent and the polymer
respectively. Then VR values were used in the Flory-Rehner
equation to obtain cross-linking densities of the networks (Mc/2)
following Equation 3:
ꢎꢏ (ꢏꢑꢆ/ꢒꢈ ꢏꢑ
)
ꢐ
ꢓ
ꢋ
ꢌꢍ = −
(3)
ꢔꢕꢖ ꢗꢈꢏ ꢊ ꢏꢑꢊ ꢘꢏꢑꢋꢙ
(
)
ꢑ
Where Mc (g.mol-1) is the average molecular weight between the
crosslinks, ρ (g.cm-3) is the density of the polymer, χ is the
solvent-polymer interaction parameter (0.343 for the
PCL/chloroform system), Vs (cm3.mol-1) is the molar volume of
the solvent and VR is volume fraction of the polymer.
Tensile properties determination
Tensile tests were performed based on ASTM D638 using a Zwick
universal tensile testing machine (speed = 10 mm.min-1 and
preload = 1 N).
Thermal reversibility of the PCL-based networks
Thermogravimetric characterization
Thermal reversibility was evaluated by a rheometer coupled with
FTIR spectrometer (Haake Mars III, Thermofisher coupled with
Nicolet iS10). This device allows the simultaneous measurement
of the rheological properties and the related changes in the
chemical composition of the sample with temperature.23 The
diamond attenuated total reflectance (ATR) element that collects
spectroscopic data simultaneously with rheological data is
located at the bottom part of the rheometer. Oscillatory shear
mode (at a shear strain of 0.1% and a frequency of 1 Hz) with
parallel 20-mm-diameter stainless steel plates was used for all
experiments. The sample thickness was fixed at 0.4 mm. In order
to follow the reversibility of the Diels-Alder reactions five
heating/cooling cycles were performed (20 min at 140°C and 1 h
at 50°C per cycle) but only two heating/cooling cycles are
presented in the manuscript for a better visualization. The data
were recorded every 30 s at 140°C and every minute at 50°C. The
consumption of the Diels-Alder moieties was quantified by
following the intensity of the furan ring breathing band centered
at 1024 cm-1 and the maleimide double band centered at 695 cm-
Thermogravimetric analyses (TGA) of the samples were carried
out on a TA Instrument Q500 model, under nitrogen flow, with a
heating rate of 20 °C.min-1 from 25°C to 800°C, in an open
platinum pan.
Scratch recovery evaluation
A Rockwell 100-µm-diameter diamond tip apparatus was used to
scratch the samples and for the evaluation of associated
recoveries. Superficial surface scratches (Ps) were performed at
25°C by applying
a load of 0.5 N and post-scans were
subsequently carried out to estimate recoveries. For this, post-
scans after 5 min and after a heating treatment at 140°C for 20
min were used to quantify respectively the elastic recovery of the
polymer (Pe) and the residual depth Rd. Finally in order to
quantify the scratch recovery (η) after the heating treatment and
get rid of the elastic recovery (Ps – Pe) Equation 1 was used:
η = (1 – ꢀꢁ/ꢂꢁ ) ∗ 100 (1)
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1, normalized with respect to the carbonyl stretching band
centered at 1726 cm-1, which is not affected by the temperature
cycles.
some coupling reactions between carbon nanotubes and
PCL(MAL)2 or PCL(FUR)2 chains are still possible, there are less
pronounced as compared to the desirable reactions between
PCL(MAL)2 and PCL(FUR)2 that enables the synthesis of thermo-
mendable Diels-Alder networks.
Evaluation of the thermal activation by Joule effect
Copper stickers connected to electrodes connected in turn to a
voltage generator (Agilent U8002A, max 30V-5A) were placed on
samples (with a distance of 1 cm) to ensure good electrical
contact. Temperature increase was monitored by an infrared
camera (FLIR A655sc) at one image/30s frequency.
Besides, in order to obtain an efficient healing of the polymer
arising from Joule effect a good electrical conductivity is
required. For this end, MWCNTs were incorporated into the
matrix and electrical properties of the obtained nanocomposites
as function of MWCNTs amount were addressed (Fig. 2).
As expected, the electrical conductivity of the resulting
composites derived from PCL(FUR)2 increases with the MWCNT
content, leading to conductive materials after ca. 1.5% wt. of
MWCNTs (Fig. 2, top). The electrical percolation threshold,
corresponding to the inflection point of the curve, was also
estimated at ca. 1% wt. of MWCNTs. To have a good compromise
between MWCNTs amount (relatively low) and good electrical
properties, an amount of 2% wt. of MWCNTs was selected for
the subsequent part of the study. It is worth noting that TEM
images performed on this film confirm the good dispersion of
MWCNTs within the matrix as shown in Fig. 2 (bottom). Tensile
test performed on a nanocomposite filled with 2% wt. of MWCNT
and a neat matrix also revealed an increase of the Young
modulus with the addition of the fillers (425 ± 5 MPa vs. 559 ± 12
MPa), without clearly compromising the elongation at break of
the polymeric matrix (745 ± 43% vs. 636 ± 19%). Moreover the
presence of 2% wt. of MWCNTs slightly increased the thermal
degradation temperature of the resulting material with a main
degradation peak of 422°C for the neat matrix and 438°C for the
composite (Supporting Information 5).
Results and discussion
Synthesis of electro-conductive network
Carbon nanotubes, characterized by sp2 bonds, can react as
diene or dienophile agents in the frame of [4+2] Diels-Alder
reactions. It has recently been reported that, in presence of
maleimide or furfuryl compounds, MWCNTs can be efficiently
surface-functionalized through Diels-Alder reactions.24, 25 In the
present study such competitive reactions can be detrimental to
the synthesis of electrically responsive materials and, in the
meantime, to the healing process. Preliminary experiments
showed that the length of dimaleimide is of great importance to
reduce this competitive reaction as much as possible. For
instance, when a short dimaleimide coupling agent, i.e., the N,N’-
(1,3-phenylene)dimaleimide, was initially employed in the
preparation of these thermo-mendable PCL(FUR)2 networks in
presence of MWCNTs (from 0.25 to 1% wt.), the extrusion force,
remaining constant (between 50 to 70 N) during the process,
indicated that the coupling reactions between the PCL(FUR)2 and
the dimaleimide coupling agent were relatively low (i.e., no
increase in the molecular weight and the viscosity) and only
brittle materials, mainly oligomers, were obtained. However,
FTIR performed on the resulting samples showed the
disappearance of the maleimide band at 830 and 695 cm-1 (See
Supporting Information 4), confirming that the maleimide
moieties most likely reacted at the MWCNTs side walls as
reported elsewhere.25
To reduce the coupling reactions between MWCNTs and
maleimide moieties as much as possible, PCL oligomers end-
functionalized with maleimide groups (PCL(MAL)2) were
preferred in this work. Interestingly and in contrast to the short
maleimide coupling agent investigated, coupling reactions
between PCL(FUR)2 and PCL(MAL)2 did occur in presence of
MWCNTs (from 0.1 to 5% wt.) as attested by an increase of the
extrusion force (observed at 80°C). Such a behavior could be
explained by the steric hindrance of the long PCL(MAL)2 chains
compared to the short N,N’-(1,3-phenylene)dimaleimide, being
in favor of the reaction towards PCL(FUR)2. At this point, even if
Fig. 2. Electrical properties of the linear PCL-functionalized
network with MWCNTs amount (top) and TEM micrographs of a
sample filled with 2% wt. of MWCNTs (bottom).
4

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ARTICLE
Synthesis of thermo-reversible conductive cross-linked
networks
7
6
5
4
3
2
These results, indicating the achievement of a well-defined
electro-conductive network by reactive extrusion, allowed us
building up the thermo-reversible cross-linked materials in
presence of MWCNTs for healing investigations. Since the cross-
linking density of a network may be of great importance for the
healing processes,3, 26 the linear PCL(FUR)2 was partially replaced
by a four-arm PCL end-functionalized by furan groups, PCL(FUR)4.
For this, the network cross-linking density in terms of number of
thermo-responsive moieties and availability towards Diels-Alder
reactions was investigated through the synthesis of MWCNT-
nanofilled networks containing 0/100, 25/75, 50/50, 75/25 and
100/0% wt. of PCL(FUR)4/PCL(FUR)2 and 2% wt. of MWCNTs by
reactive extrusion. It is worthwhile to note that independently of
the PCL(FUR)4/PCL(FUR)2 ratio, the proportion of end-
functionalized furan and maleimide (PCL(FUR)/PCL(MAL)2) was
maintained at 50/50% (molar) for all the compositions (See
Supporting Information 6). For each composition, coupling
reactions were monitored by following the extrusion force
evolution with time (See Supporting Information 7) and as
expected, the force and consequently the viscosity increased
with the PCL(FUR)4 fraction. This indicated that the higher the
PCL(FUR)4 amount in the PCL(FUR)2/PCL(FUR)4 ratio, the higher
the cross-linking degree of the network.
25
50
75
100
Weight content in PCL(FUR)4 (%)
Fig. 3. Cross-linking density quantification after swelling test in
chloroform for networks containing different amounts of
PCL(FUR)4 with (ꢀ) and without (ꢁ) MWCNTs (2% wt.).
formation of cross-linked structure related to high amount of
Diels-Alder adducts.26 One should point out that the cross-linking
degree does not seem to affect the electrical conductivity of
these highly cross-linked composites since, for conversions of
more than 99%, volume conductivities of about 1.10-2 S.cm-1
were measured (Supporting Information 8). In addition, good
dispersions evaluated by TEM images were obtained for the fully
cross-linked
nanocomposite
(0/100%
wt.
of
PCL(FUR)2/PCL(FUR)4) (Supporting Information 9) .
To better appraise the cross-linking density of the obtained
networks, swelling tests were performed in chloroform and
correlated with Flory–Rehner equation for the determination of
the cross-linking density of the networks (see Experimental part).
In the last sections, two systems containing 100/0% and 0/100%
of PCL(FUR)4/PCL(FUR)2 (extreme compositions) were selected
for the thermo-reversibility study in presence of MWCNT. These
systems give rise to different healing behaviors, since the highly
cross-linked structure may be at the origin of intense Diels-Alder
reactions with restricted chain mobility, whereas the linear
system should have less Diels-Alder reactions but higher chain
mobility.27
Fig.
3 shows cross-linking densities of the PCL networks
containing different amount of PCL(FUR)4 with and without
MWCNT. One can notice a continuous increase of cross-linking
density with the amount of PCL(FUR)4, confirming the qualitative
results obtained by the extrusion force monitoring during the
material processing. Interestingly, as shown in Fig. 3, the network
cross-linking density can be easily tuned by simply adjusting the
PCL(FUR)4 content. It should be mentioned that, except for the
linear composition (0% wt. of PCL(FUR)4) being fully soluble, the
remaining systems were insoluble regardless of the MWCNTs
content. This supports our aforementioned statement that the
formation of the cross-linked networks through Diels-Alder
reactions is not strongly perturbed by the presence of MWCNTs.
However, higher cross-linking density values obtained for the
systems containing MWCNTs in comparison to the MWCNTs-free
systems likely indicating the formation of a joint-network
between nanotubes and the polymeric network. This noteworthy
effect is especially interesting when considering the fully cross-
linked system (100% PCL(FUR)4, 2% wt. MWCNTs), which
presents a good potential for healing applications due to the
Thermo-reversibility of the selected systems
The influence of MWCNTs on the thermo-reversibility of the
selected PCL-based networks was monitored by rheological
analysis simultaneously recorded with in situ FTIR measurements
(Fig. 4). Multiple temperature cycles (between 140 to 50°C
corresponding to the retro-Diels-Alder and Diels-Alder reactions
respectively) were performed and the maleimide band intensity
at 695 cm-1 was more particularly followed during the
temperature cycles.
In absence of MWCNTs, storage modulus (G’) and loss modulus
(G”) curves intersect after each heating/cooling cycle showing
the reversibility of the cross-linked system. With temperature,
the network changes from a cross-linked system at 50°C, where
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G’ is higher than G”, to an uncross-linked network at 140°C,
where G” becomes higher than G’ (Fig. 4a top). Even after
multiple temperature cycles this behavior remains well defined
and perfectly reversible. For the linear system, G” is always
higher than G’ due to the lower viscosity of the linear chains
compared to the cross-linked network. For both systems in
absence of MWCNTs, in situ FTIR spectra of the PCL-based films
reveals modifications with temperature and more particularly in
the maleimide band region at 695 cm-1. A closer look at
maleimide band intensity shows an increase of the band intensity
at higher temperatures and a decrease of the band intensity at
50°C, in accordance to the retro- and Diels-Alder reactions
respectively. Those perfectly reversible band modifications
indicate a good response of the networks to temperature cycles
(Fig. 4a bottom).
By adding 2% wt. of MWCNTs to the linear and cross-linked
systems, surprisingly the intersection of G’ and G” with
temperature is no longer observed, likely due to the formation of
a physical MWCNT network incorporated into the cross-linked
network, thus limiting the chain mobility. Despite this lack of
reversibility evidenced with rheological test, in situ FTIR
confirmed the occurrence of Diels-Alder and retro-Diels-Alder
reactions, as attested by an increase and decrease of maleimide
band with temperature, similar to that obtained for the MWCNT-
free network (Fig. 4b).
106
106
a)
140
140
105
105
120
104
100
103
80
120
104
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103
80
102
102
60
40
160
60
40
160
101
101
100
100
0
80
Time (min)
0
80
Time (min)
rDA
rDA
rDA
rDA
DA
DA
DA
DA
0
80
160
0
80
160
Time (min)
Time (min)
106
105
104
103
106
105
104
103
b)
140
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100
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60
60
40
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0
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Time (min)
160
Time (min)
rDA
rDA
rDA
rDA
DA
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160
0
80
160
Time (min)
Time (min)
c)
DA, 50°C
rDA, 140°C
Fig. 4. Rheological and maleimide FTIR band intensity results of the linear (left) and cross-linked (right) network (a) without MWCNTs and
(b) with 2% wt. of MWCNTs. (c) Diels-Alder reaction between furfuryl and maleimide moieties.
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The impact of MWCNTs in the chain-mobility of the films based on
PCL(FUR)x (X=2 or 4) in terms of scratch recovery efficiency was
further investigated by microscratch tests using a Rockwell 100-µm-
diameter diamond tip (see Experimental part). While the scratch
recovery η of the fully cross-linked system was of ca. (77.3±16.3)%
(the high value of the error bar is due to the quite high surface
roughness of the cross-linked surface), the recovery of the scratch
was complete in the case of the linear system (100.0±0.4)%.
This can be attributed to the higher mobility of the linear chains as
compared to the branched ones in presence of MWCNTs. In their
work, AbdolahZadeh et al. already reported a systematic study of
the relationship between the cross-linking density, the amount of
reversible groups and healing efficiency in a disulfide-based intrinsic
self-healing hybrid sol-gel.27 In their work, the authors introduced a
dual network containing permanent crosslinks and disulfide
reversible groups and by modifying the cross-linking density,
authors showed that, for short healing time (10 min at 70°C), the
healing efficiency (500-µm wide crack closure) could be doubled for
networks containing a lower cross-linking degree. On the other
hand, a minimal amount of reactive groups is necessary for a
sufficient healing efficiency. Based on these results, the linear
composition was selected for the electrically induced healing
process by Joule heating.
Thermal activation of a scratched film by Joule heating
In order to appraise the thermal activation of a scratched film by
Joule effect, a PCL(FUR)2/PCL(MAL)2 with a ratio of 50/50% (molar)
film containing 2% wt. of MWCNTs was connected to two
electrodes and the heat increase was followed with an IR camera
(Fig. 5). In the absence of a crack and with an applied voltage of 25
V, the temperature between the electrodes after 3 min reached ca.
70°C (Fig. 5a). At this temperature, the material is not thermally
remendable since the temperature is not high enough to induce
retro-Diels-Alder reaction.
Fig. 5. Evaluation of the thermal activation by Joule heating at 25V
of non-cut (a) and cut (b) samples. (c) Thermo-reversibility of
PCL/MWCNTs nanocomposite at crack tip attested by ex situ FTIR
measurements before electrical activation (red), after electrical
activation (black) and after cooling after electrical activation (blue).
After a cut between the two electrodes and by applying the same
voltage (25 V) on the scratched sample different temperatures at
the film surface are visualized by IR camera (Fig. 5b). After only 90 s
a local increase of the temperature up to ca. 110°C nearby to the
scratched region is observed. In this case, electrons are not
transferred directly from one electrode to the other because of the
physical barrier created by the scratch. Instead, electron flow
remains concentrated at the crack tip and a local increase of heat is
noticed (in the scratched region) even after the polymeric interface
recovery. At this point, the temperature reached near the damaged
area is high enough to promote retro-Diels-Alder reactions and
enables the self-healing of the system through an
‘unzipping/zipping effect’. It is worth mentioning that the current
value increased slightly (from 10 to 70 mA) during the whole
healing process due to a facilitated flow of electrons after the
damage repair.
To further confirm the thermo-reversibility of these electro-
conductive thermo-mendable nanocomposites at crack tip under
electrical activation ex situ FTIR measurements (Fig. 5c) were
performed. As expected, before electrical application, FTIR
performed on the sample does not show any maleimide bands at
830 and 695 cm-1 due to their complete consumption with furfuryl
moieties. After the electrical application, an increase of intensity for
the characteristic maleimide bands attests for the occurrence of
retro-Diels-Alder reaction. This thermal reversibility leads to lower
molecular weight oligomers with higher mobility at the scratched
region, and inter-exchange chemical Diels-Alder reactions enabling
the healing of the damaged zone. After cooling down the sample to
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room temperature, the disappearance of maleimide bands at 830
and 695 cm-1 indicates the reformation of Diels-Alder adducts.
A visual evaluation of crack recovery efficiency taken from images
during the electrical experiment and the corresponding SEM images
before and after scratch reparation also confirmed the good
recovery of the material in the scratched region (Fig. 6). For the
thermo-mendable film, the disappearance of the crack was
observed after 180 s under electrical activation (Fig. 6a). For sake of
comparison same experiments performed on
a linear non-
mendable (i.e. non-functionalized, CAPA FB540 Mn = 100 000 g.mol-
1) commercial PCL filled with 2% wt. of MWCNTs revealed the total
absence of material healing after the electrical application. In this
particular case, the absence of furfuryl and maleimide moieties
precludes any Diels-Alder reactions necessary for an efficient
healing of the material. After 180 s at 25 V, only a deformation due
to the melting of the PCL could be observed and no recovery was
noticed in the scratched region (Fig. 6b).
Fig. 7. Influence of the scratch orientation with respect to
To further demonstrate the importance of scratch orientation with
respect to electrodes, an investigation on this orientation relatively
to the electrical current localization was performed. This
orientation should affect the Joule resistivity and consequently the
response of the scratched material to the thermal fluctuation
induced by an electrical current. For this, a sample was scratched
vertically to the orientation of the electrodes and electrically
activated by applying the aforementioned conditions (i.e. 25 V for 3
min). Interestingly, one can notice from Fig. 7 that, in the present
case, the increase of temperature, limited to only 60-80°C, is not
located at the scratch position but rather in the whole sample,
similarly to what was obtained for the non-scratched sample (Fig.
5a). The absence of any physical obstacle able to limit the electrons
diffusion from an electrode to the other prevents from getting
temperatures high enough in the scratched area, necessary to
induce retro-Diels-Alder reactions for healing the material.
electrodes on Joule effect
Conclusions
In this study, we synthesized thermo-mendable and electrically
conductive poly(ester-urethane) nanocomposites by reactive
extrusion, a solvent-free process. It was shown that the cross-
linking density can be easily tuned by simply changing the
PCL(FUR)2/PCL(FUR)4 ratio in the presence of the dimaleimide
coupling agent PCL(MAL)2. Excellent thermo-reversibility, important
for the healing process, was confirmed with a rheometer coupled
with in situ FTIR measurements for the linear system. Incorporation
of multi-walled carbon nanotubes (MWCNTs) in the thermo-
reversible systems was also achieved by reactive extrusion process.
The obtained nanocomposites, with a good MWCNTs dispersion,
presented good electrical conductivity at 2% wt. and showed good
thermal reversibility. Finally, the activation of the Joule effect at a
low voltage (25 V) was followed by infrared camera. In the presence
of macroscale damages (i.e. cut), local changes in electrical
resistivity at the crack tip led to a local temperature increase, within
the range of the retro-Diels-Alder reaction temperatures. Such
temperature increase led to sufficient chain mobility to close and
seal the cut by an ‘unzipping/zipping effect’ within 3 min. The
promising obtained results introduce a new concept for polymer
healing using Joule effect and set the bases for the development of
self-healing electrically conductive polymers.
Acknowledgements
LPCM would like to thank the Belgian Federal Government Office of
Science Policy (SSTC- PAI 6/27) for general support and would also
like to thank both Wallonia and the European Commission ‘FSE and
FEDER’ for financial support in the frame of Phasing-out Hainaut.
The authors thank also Dr Y. Paint from Materia Nova for SEM and
Fig. 6. Joule heating experiment on scratched nanocomposites
based on thermo-mendable PCL sample and corresponding SEM
images (a) and on a commercial PCL sample (b) after 3 min at 25 V.
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