A novel self-healing power cable insulating material based on host-guest interactions

Article abstract of DOI:10.1039/c8ra04882g

The insulating materials used in power cables are susceptible to damage and cracks during installation and operation. To solve this problem, we have prepared a self-healing material PVP/p(HEMA-co-BA), which is synthesized by radical polymerization using HEMA, BA, PVP and a host-guest assembly. The host-guest assembly is constructed through interactions between host and guest molecules (CD-Al₂O₃ NPs act as the host, and HEMA-Ad acts as the guest). The characterization results of the materials show that there are two kinds of supramolecular interactions, namely, the host-guest interaction and the hydrogen bonding. The material possesses good thermal stability (heat-resisting temperature can reach 200 °C) and good electrical performance. The storage modulus of the material can be increased up to 432 MPa using a cross-linking agent at 20 °C. Furthermore, the material exhibits self-healing property, and it can self-heal several times; its self-healing efficiency is relative to the dosage of the cross-linking agent.

Full text of DOI:10.1039/c8ra04882g

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A novel self-healing power cable insulating material
based on host–guest interactions†
ab
Cite this: RSC Adv., 2018, 8, 25313
Manjun Zhang,a Musong Lina and Qiang Fua
*
Lei Peng,
The insulating materials used in power cables are susceptible to damage and cracks during installation and
operation. To solve this problem, we have prepared a self-healing material PVP/p(HEMA-co-BA), which is
synthesized by radical polymerization using HEMA, BA, PVP and a host–guest assembly. The host–guest
assembly is constructed through interactions between host and guest molecules (CD–Al2O3 NPs act as
the host, and HEMA–Ad acts as the guest). The characterization results of the materials show that there
are two kinds of supramolecular interactions, namely, the host–guest interaction and the hydrogen
bonding. The material possesses good thermal stability (heat-resisting temperature can reach 200 ꢀC)
and good electrical performance. The storage modulus of the material can be increased up to 432 MPa
using a cross-linking agent at 20 ꢀC. Furthermore, the material exhibits self-healing property, and it can
self-heal several times; its self-healing efficiency is relative to the dosage of the cross-linking agent.
Received 7th June 2018
Accepted 2nd July 2018
DOI: 10.1039/c8ra04882g
rsc.li/rsc-advances
At present, one type of self-healing material (the lling type)
exhibits self-healing property due to the repair agent carriers such
as hollow bers,6,7 microcapsules,8–10 or microvasculatures,11,12
1 Introduction
Insulating material of a power cable is the outermost protective
layer of the cable core, and it is exposed to the external environ-
ment directly. During the process of installation and operation,
damages caused by aging and external forces affect the service life
and even cause leakage and blackout accidents, thereby incurring
tremendous economic losses and compromising safety.1,2 For
example, in 2011, across China, 15 of 48 (31%) cable accidents
resulted from external damages.3 At present, methods available for
repairing the damage to the cable insulating layer mainly include
injected liquid method, heat-sealing method, and thermal
contractible tube method; however, these three types of repair
methods have disadvantages such as complex steps and poor
repairing effect, and they also require the fault location technique
to nd out the fault points, due to which they are difficult to
operate.4 Therefore, there is a need to develop new techniques to
repair damaged cable insulation.
Self-healing materials (SHMs) are a type of smart materials that
can automatically identify damages or structural defects (perceive)
and repair them (respond) partially or completely.5 Considering
this feature of automatic identication and repair, we have
attempted to introduce such self-healing supramolecular mono-
mers into the insulating material of power cables for damage
repair.
which are dispersed in the polymer matrix. When the material is
damaged, the carriers break down, and the repair agents release
and diffuse to the crack location. The repair agents can initiate
cross-linking reactions to achieve the purpose of repairing.
Although such materials can self-repair, it is difficult to achieve
repeated repairs due to the limited capacity of the repair agents.
However, another kind of self-healing materials repair the defects
through the intermolecular interactions of their components.
These have the advantages of repeated repair times, which are not
restricted by the capacity of the repair agents. The mechanism of
such kind of self-healing materials is based on dynamic covalent
bonds. Under the stimulation of external factors such as thermal
energy,13 light14 and pH,15 reversible cleavage and generation of
intermolecular covalent bonds occur. In the process of covalent
bond regeneration, the cracks can be repaired. However, in the
actual environment in which the power cable insulation sheath
exists, the above-mentioned external stimulating factors are not
present, which hinders the self-healing efficiency of these
materials.
Based on the research of this kind of covalent bond-based
self-healing model, scientists have widened the research eld
to non-covalent bond models. The self-healing property is
gained mainly due to the presence of non-covalent bonds such
as hydrogen bonds,16 p–p bonds,17,18 and metal coordination
bonds19 as well as topology structure.20 The most important
advantage of such materials is that they do not require external
stimulatory factors to activate the self-repair property; among
these, a particular kind of supramolecular material based on
a host–guest interaction model is of great interest.
aElectric Science and Research Institute of Guangdong Power Grid Co. Ltd, China.
E-mail: penglei1025@whu.edu.cn
bSchool of Electrical Engineering, Chongqing University, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04882g
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Currently, basic research on the host–guest interaction model
has been carried out, and researchers have also developed
materials with excellent repairing effects. For example, the group
led by Professor Huang at Zhejiang University has developed two
kinds of supramolecular gels based on host–guest reactions;
these gels exhibit excellent self-healing performance, and they
can rapidly self-heal even under 10 000% strain.21 Professor
Mynar's group from Tokyo University has prepared a gel that can
self-heal completely, and it is synthesized using clay and tele-
chelic dendritic macromolecules with multiple adhesive
termini.22 The group of Zhang in Sichuan University has devel-
oped an elastomer that can restore 90% of its mechanical
strength rapidly.23 However, the mechanical properties of these
self-healing materials have been proven to be unsatisfactory
(some materials are even “jelly-like”).24 The Young's modulus of
self-healing materials based on non-covalent bonds is generally
in the range of 1–10 MPa.16,25 Therefore, to meet the requirements
for use as a cable material, enhancing the mechanical strength of
the self-healing material is of great signicance.
Fig. 1 Schematic of the process used in this study for the synthesis of
self-healing material.
(98%) were obtained from Chengdu Kelong Chemical Reagent
Factory. All the above-mentioned materials were analytically pure.
3.2 Instruments and characterization
Therefore, in this paper, we proposed a new PVP/p(HEMA-co-
BA) self-healing material with suitable mechanical strength.
The material adopted a host–guest interaction topology model,
and the guests were graed by special reinforcements. The
host–guest assembly was then prepared by free-radical copoly-
merization with HEMA, BA, and PVP to obtain the self-healing
material having suitable mechanical strength.
1H and 13C NMR spectra were recorded using a Bruker AVANCE AV
II-600 NMR spectrometer. Mass spectrometry measurements were
conducted on LCQ Deca LC-MS. One-dimensional Fourier infrared
(FT-IR) spectra were recorded with a Nicolet 560 Fourier transform
infrared spectrometer, and two-dimensional Fourier infrared (2D-
IR) characterization was conducted by a Nicolet 710 FT-IR spec-
trometer (coupled with a control temperature instrument) and
a DTGS detector. The storage modulus and glass transition
temperature of samples were determined using a Q800 Dynamic
mechanical thermal analyzer. Tensile tests were performed on an
AG-10TA tensile testing machine. Thermogravimetry analysis of
Al2O3 NPs, NH2–Al2O3 NPs, and CD–Al2O3 NPs was carried out
using a Q-500 thermal analyzer of TA. The self-healing property of
the damage was observed under VHX-1000C Digital Microscopes,
and the tensile stresses of the sample before and aer self-healing
were tested by an AG-10TA tensile testing machine.
2 Principle
In this study, the host–guest inclusion model is used for the
synthesis of a self-healing material with b-cyclodextrin as the host
and adamantane as the guest. The b-cyclodextrin molecule has
a lipophilic cavity inside, whereas adamantane shows excellent
lipophilicity. Moreover, the size of the cavity in b-cyclodextrin is
0.7–0.8 nm, and the diameter of adamantane molecule is about
0.7 nm.26 Thus, the host and guest molecules match perfectly in
size, which means that adamantane can t in the cavity of
cyclodextrin rmly. When the material is damaged, based on the
lipophilic cavity of b-cyclodextrin and the oleophilic structure of
the adamantyl group, the adamantane group is stuck inside the
cavity of the cyclodextrin molecule by the same polar attractive
force, resulting in a stable supramolecular network. Thus, the
damage is healed. The process of preparation of the PVP/
p(HEMA-co-BA) self-healing material is shown in Fig. 1.
3.3 Fabrication method
(a) Synthesis of guest and host molecules. The details of the
synthesis of guest molecule HEMA–Ad and host molecule CD–
Al2O3 NPs are presented in ESI 2.†
(b) Synthesis of self-healing material PVP/p(HEMA-co-BA).
First, CD–Al2O3 NPs were ultrasonically dispersed in 3 mL of DMF
for 30 min and then, HEMA–Ad was added. Aer stirring for 24 h,
PVP, HEMA, BA, EGDMA, and AIBN were added in certain
proportions. Aer reaction at 75 ꢀC for some time with stirring, the
solution was allowed to react for 10 hours without stirring. Finally,
the solvent was removed in a vacuum drying oven to obtain PVP/
3 Materials and methods
3.1 Materials
2-Hydroxyethyl-methacrylate (HEMA), b-cyclodextrin (b-CD), butyl
acrylate (BA), azobisisobutyronitrile (AIBN, 99%, recrystallization),
polyvinylpyrrolidone (PVP-K30, Mn ¼ 40 000), Al2O3 nanoparticles
(NH2–Al2O3 NPs, 30 nm, a-phase), ethylene glycol dimethacrylate
(EGDMA), (3-aminopropyl) triethoxysilane (APTES), and ada-
mantanecarboxylic acid (Ad-COOH) were obtained from Aladdin
(Shanghai) Reagent Co., Ltd. Hydrochloric acid, dichloromethane
(CH2Cl2), dimethyl sulfoxide (DMSO), p-toluenesulfonyl chloride
(TOS-Cl), thionyl chloride (SOCl2), and concentrated sulfuric acid Fig. 2 Chemical reaction diagram of the self-healing material.
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Table 1 Composition of samples
Sample
HEMA/mL
BA/mL
PVP/g
HEMA–Ad/g
CD–Al2O3 NPs/g
EGDMA/mL
1
2
3
4
5
6
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2.37
2.37
2.37
2.37
2.37
2.37
2.37
2.37
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.3
0.3
0.3
0.3
0.2
0.1
0
0
0.5
1
2
1
1
1
1
Ref. sample 1
Ref. sample 2
0.3
p(HEMA-co-BA) self-healing material. The reaction diagram is
shown in Fig. 2, and the amounts of reagents added are shown in
Table 1.
4 Results and discussion
4.1 Graing effects of host molecule and guest molecule
The graing effects of host molecule CD–Al2O3 NPs and guest
molecule HEMA–Ad were analyzed by 1H NMR and 13C NMR
spectra, and the results are shown in S1a and b and S2a and b.†
4.2 Host molecule CD–Al2O3 NPs
The FT-IR spectra of Al2O3 NPs, CD–Al2O3 NPs, and TOS–CDs
were examined; the results are shown in Fig. 3. Compared to the
peaks in the FT-IR spectra of Al2O3 NPs, the three new charac-
teristic peaks of CD–Al2O3 NPs at 1038, 1149, and 1490 cmꢁ1
corresponded to nC–O–C and dC–O–C on cyclodextrin and dNH on
the amide groups. In addition, compared to the peaks in the FT-
IR spectra of TOS–CD, nC]C (at 1369 cmꢁ1 and 1419 cmꢁ1) on
the benzene ring was not observed in the infrared spectrum of
CD–Al2O3 NPs, conrming that Al2O3 NPs were successfully
modied by cyclodextrin.
On the other hand, thermogravimetric analyses (TGA) of Al2O3
NPs, NH2–Al2O3 NPs, and CD–Al2O3 NPs are shown in Fig. 4.
From Fig. 4, it can be seen that the loss mass of Al2O3 NPs in the
region between 50 ꢀC and 800 ꢀC is 0.21% and that of NH2–Al2O3
NPs is 5.71%, thus indicating that organics are modied on the
nanoparticles. The difference between the loss mass of CD–Al2O3
NPs (24.35%) and that of NH2–Al2O3 NPs is 18.64%, which proves
that cyclodextrin is successfully modied on Al2O3 NPs.
Fig. 4 Thermogravimetric analyses of Al2O3 NPs, NH2–Al2O3 NPs, and
CD–Al2O3 NPs.
According to previous reports,27 an estimated total of 3710
cyclodextrins can be modied on each nanoparticle.
4.3 PVP/p(HEMA-co-BA) self-healing material
To establish that the crosslinked network in PVP/p(HEMA-co-
BA) material is formed by PVP and p(HEMA-co-BA) through
hydrogen bonding between the carbonyl groups on poly-
vinylpyrrolidone and the hydroxyl groups, we used infrared
spectroscopy to characterize the hydrogen bonds of PVP and
PVP/p(HEMA-co-BA) material (sample 3 and 1 in Table 1). The
results are shown in Fig. 5.
Table
2 dynamic thermo-mechanical analysis of PVP/
p(HEMA-co-BA)The peaks of PVP at 2923, 1655, 1458, and
1285 cmꢁ1 correspond to the stretching vibration of O–H, the
stretching vibration of C]O, the bending vibration of CH2, and
the vibration of C–N. In the spectrum of PVP/p(HEMA-co-BA)
Fig. 3 Infrared spectra of Al2O3 NPs, CD–Al2O3 NPs and TOS–CD.
Fig. 5 Infrared spectrum of PVP and PVP/p(HEMA-co-BA) material.
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Table 2 Dynamic thermo-mechanical analysis of PVP/p(HEMA-co-
BA) materials
Storage modulus/
MPa
0
0
rE 1
rE 1
0
0
0
0
Sample 20 ꢀ
C
50 ꢀ
C
90 ꢀ
C
(E 20 C/E 50 C
)
(E 50 C/E 90 C
)
Tg/ꢀC
ꢀ
ꢀ
ꢀ
ꢀ
2
3
4
300.8 48.8
399.0 79.1
432.3 64.8
1.1
2.3
1.0
6.16
5.04
6.67
43.32
34.21
62.08
78.1
81.3
95.8
material, it can be seen that the stretching vibration peak of C]O
shis from 1655 cmꢁ1 to 1663 cmꢁ1, indicating that the carbonyl
and hydroxyl groups in the material form hydrogen bonds.
To verify the host–guest interaction between cyclodextrin
and adamantane in the PVP/p(HEMA-co-BA) material, it (sample
3 in Table 1) was tested by two-dimensional infrared spectros-
copy, which is an effective tool to study molecular interactions
in many systems.28 The results are shown in Fig. 6a and b (a and
b are the synchronous and asynchronous 2D infrared signals of
the material, respectively). According to the theory of 2D IR
spectroscopy developed by Noda,29 a correlation peak is formed
when two dipole transition moments associated with the
molecular vibrations of different functional groups are reor-
ienting simultaneously. The cooperative motion of local struc-
tures is expected when strong interactions exist among different
functional groups. In Fig. 6a, the peaks at 1161 cmꢁ1 and
1082 cmꢁ1 correspond to the C–O–C bending vibration and the
C–C–C–O bending vibration of cyclodextrin. Moreover, the peak
at 1103 cmꢁ1 is due to the bending vibration of CH2 on ada-
mantane. It is clear that the characteristic peaks of b-CD (at
1161 cmꢁ1 and 1082 cmꢁ1) and Ad (at 1103 cmꢁ1) show clear
correlations (shown in the red box in Fig. 6a). The results
indicate that b-CD and Ad in the material interact with each
other strongly due to the host–guest interactions.
Fig. 7 Glass transition temperature curve of PVP/p(HEMA-co-BA)
material (samples 2, 3, and 4).
81 ꢀC and 78 ꢀC, respectively, indicating that the glass transition
temperature of the sample increases with the increasing
amount of the crosslinking agent as the chemical crosslinking
points limit the segmental motion of the material. At the same
time, it can be observed that all the three samples have a very
wide glass transition temperature, e.g., sample 3 with Tg of 20–
ꢀ
140 C. This is because of the presence of two kinds of supra-
molecular forces in the material, namely, host–guest interaction
of adamantane and cyclodextrin and hydrogen bonding of
hydroxyl and carbonyl groups, which make the material micro-
phase separated, and the different segmental motions of the
micro-phases give rise to different Tg temperatures, ultimately
resulting in very wide Tg peak overall. In addition, at 20 ꢀC, the
storage moduli of the three samples increase (308, 399 and
432 MPa, respectively) with the increasing amount of cross-
linking agents. The resistance temperature of the material
was obtained through thermogravimetric analysis (TGA), as
shown in Fig. 8.
ꢀ
There is no clear mass loss of the material till 257 C, indi-
cating that the PVP/p(HEMA-co-BA) material can withstand
temperatures up to 257 ꢀC. At the same time, it can also be seen
that the mass percentage of Al2O3 NPs in the material is 16.7%,
which is consistent with the ratio present in sample 3.
4.4 Thermal properties of PVP/p(HEMA-co-BA)
To study the glass transition temperature (Tg) of PVP/p(HEMA-
co-BA) material, dynamic thermo-mechanical analysis (DMA)
was performed, and the results were used to obtain the tan d
curve, as shown in Fig. 7 and Table 2.
The glass transition temperature of sample 4 is the highest
(Tg ¼ 95 ꢀC), whereas the Tg values of sample 3 and sample 2 are
Fig.
6 Two-dimensional infrared spectra of the material: (a)
synchronous infrared signal, (b) asynchronous infrared signal.
Fig. 8 Thermogravimetric curves of sample 3.
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Table 3 Insulation properties of PVP/p(HEMA-co-BA)
Table 4 Mechanical properties and self-healing efficiency of PVP/
p(HEMA-co-BA)
No. Properties
Unit
U m
U
Results
3/%
s/MPa
3.2 ꢂ 107
4.1 ꢂ 108
7.2 ꢂ 1010
3.5 ꢂ 1010
21.7
Self-healing
1
2
3
4
5
Volume resistivity
Surface resistivity
Normal
50 ꢀC, vacuum 2 h
Normal
Sample
Er/MPa
Before
Aer
Before
Aer
efficiency (h)
50 ꢀC, vacuum 2 h
1
2
3
4
5
6
0.36
7.64
9.42
15.0
9.30
9.22
9.14
337
173
161
98
165
159
158
225
24
70
29
43
24
0
0.61
0.81
1.45
2.04
1.48
1.40
1.43
0.52
0.59
0.99
1.34
0.77
0.46
0
85%
73%
68%
66%
52%
33%
0
Relative permittivity Normal
—
50 ꢀC, vacuum 2 h
18.7
AC electrical strength
(in oil)
DC electrical strength
(in oil)
MV mꢁ1 12.5
MV mꢁ1 14.7
Ref. 1
material in Fig. 9b has a damage 28.5 mm in width and 16.54 mm
in depth. Aer self-healing, only a trace of the damage is
retained on the surface, indicating that the PVP/p(HEMA-co-BA)
material indeed exhibits self-healing properties.
4.5 Insulation properties of PVP/p(HEMA-co-BA)
The insulation properties of PVP/p(HEMA-co-BA) were tested.
The results are shown in Table 3. It can be seen from Table 3
that the volume resistivity of the material reached 107–108, and
the surface resistivity reached 1010, which can meet the insu-
lation requirements for general engineering use (usually greater
than 106). The relative permittivity of the material under normal
state was 21.7, which was higher than that of polyvinyl chloride
(4.8). The material also exhibited good AC and DC electrical
strength values (its values were as high as 12.5 MV mꢁ1 and 14.7
MV mꢁ1, respectively), which were greater than the limit of
general cable sheath insulation materials (the requirement is
greater than 1 MV mꢁ1).
To investigate the self-healing efficiency of the PVP/p(HEMA-
co-BA) material and the effect of the cross-linker dosage on self-
healing properties, a tensile testing machine was used to study
the mechanical properties of the samples before and aer self-
healing at the drawing speed of 5 mm minꢁ1. The results
(including Young modulus EY, breaking elongation 3, breaking
strength s, and self-healing efficiency h) of the samples before
and aer self-healing are listed in Table 4; as the cross-linker
dosage in the material increased (the irreversible cross-linking
point increased), the Young modulus EY and the breaking
strength s increased signicantly, whereas the breaking elon-
gation 3 decreased rapidly; this indicated that within a certain
range, the less the cross-linker, the higher the self-healing
efficiency (up to 85%).
To prove that the self-healing property of PVP/p(HEMA-co-
BA) material is derived from the inclusion effect of the host (b-
CD group) and the guest groups (Ad group), three groups of
control experiments were designed: (1) samples without CD–
Al2O3 NPs were prepared. The results showed that the material
had no self-healing property (Fig. 10a); (2) samples without PVP
were prepared. The results showed that the material had self-
healing property (Fig. 10b); (3) the PVP/p(HEMA-co-BA) mate-
rial was cut, and cyclodextrin solution was smeared on the cut
4.6 Self-healing property of PVP/p(HEMA-co-BA)
To investigate the self-healing property of PVP/p(HEMA-co-BA)
material, the sample was cut into two pieces with a blade, and
the broken ends were brought together immediately. Aer 2
hours at room temperature, the pieces were found to have
rejoined by self-healing (shown in Fig. 9a). Moreover, the
sample was cut off at the original fracture to investigate if it
could self-heal again; the results showed that the material had
achieved the ability to self-heal multiple times.
To observe the healing more clearly, a 3D digital microscope
was used for detection. The result is shown in Fig. 9b. The
Fig. 10 Self-healing tests using different materials: (a) samples without
Fig. 9 Self-healing of PVP/p(HEMA-co-BA) material: (a) self-healing CD–Al2O3 NPs; (b) samples without PVP; (c) self-healing test of PVP/
process of the material; (b) 3D ultra-depth of focus images of the p(HEMA-co-BA) after smearing cyclodextrin solution at cutting
material before and after self-healing.
surface.
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surface; then, the self-healing properties were measured. The
results showed that the material could not self-heal (Fig. 10c).
The two sets of experiments shown in Fig. 10a and
b demonstrate that the self-healing property does not rely on
the interaction between the carbonyl group on PVP and the
hydroxyl group on p(HEMA-co-BA) but rather on the host–guest
interaction between cyclodextrin and adamantane. The sample
shown in Fig. 10c cannot self-heal because the cyclodextrin
molecule in the cyclodextrin solution rather than cyclodextrin
molecule in material complex with adamantane molecule on
the cutting surface, which means that the cyclodextrin molecule
in the solution acts as a competitive molecule. Consequently,
the host and guest molecules at the cutting surface cannot form
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this paper presented a novel self-healing material synthesized
1
by free radical copolymerization. The H and 13C NMR spectra
veried that HEMA was successfully graed on Ad, and Al2O3
NPs were successfully graed on CD. One-dimensional and two-
dimensional infrared spectra showed the existence of two kinds
of supramolecular interactions in the material: the host–guest
interaction and the hydrogen bonding interaction. The PVP/
p(HEMA-co-BA) self-healing material showed good thermal
stability with the heat resisting temperature of 200 ꢀC. The
thermogravimetric analysis revealed that the storage modulus
of the sample at 20 ꢀC increased with the increase in cross-
linker dosage, the highest being 432 MPa. The results from
3D ultra-depth of focus imaging and tensile tests showed that
the material possessed self-healing property and it can self-heal
multiple times; the self-healing efficiency increased with
a decrease in cross-linker dosage. Moreover, the tensile test
showed that the mechanical strength (Young modulus EY) of the
material was up to 15 MPa, and the mechanical strength was
satisfactory.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This research is supported by China Postdoctoral Science
Foundation Grant (2018M630920) and China Southern Power
Grid project (number GDKJQQ20152049).
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25318 | RSC Adv., 2018, 8, 25313–25318
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