Facile fabrication of fast recyclable and multiple self-healing epoxy materials through diels-alder adduct cross-linker

Article abstract of DOI:10.1002/pola.27655

J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2094-2103 A reversibly cross-linked epoxy resin with efficient reprocessing and intrinsic self-healing has been prepared from a newly synthesized diamine Diels-Alder (DA) adduct cross-linker and a commercial epoxy oligomer. The methodology developed in this work takes smart and environmentally benign epoxy to the next level of simplicity and provides new insights in the modification of traditional engineering plastics as a functional material.

Full text of DOI:10.1002/pola.27655

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Facile Fabrication of Fast Recyclable and Multiple Self-healing Epoxy
Materials through Diels-Alder Adduct Cross-linker
Xiao Kuang,1 Guoming Liu,1 Xia Dong,1 Xianggui Liu,1 Jianjun Xu,2 Dujin Wang1
1Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry,
Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China
2DSM Resolve, P.O. Box 18, 6160 MD, Geleen, the Netherlands
Correspondence to: D. Wang (E-mail: djwang@iccas.ac.cn)
Received 17 February 2015; accepted 26 March 2015; published online 11 April 2015
DOI: 10.1002/pola.27655
ABSTRACT: A reversibly cross-linked epoxy resin with efficient
reprocessing and intrinsic self-healing was prepared from a
diamine Diels-Alder (DA) adduct cross-linker and a commercial
epoxy oligomer. The newly synthesized diamine cross-linker,
comprising a DA adduct of furan and maleimide moieties, can
cure epoxy monomer/oligomer with thermal reversibility. The
reversible transition between cross-linked state and linear
architecture endows the cured epoxy with rapid recyclability
and repeated healability. The reversibly cross-linked epoxy fun-
damentally behaves as typical thermosets at ambient condi-
tions yet can be fast reprocessed at elevated temperature like
thermoplastics. As a potential reversible adhesive, the epoxy
polymer with adhesive strength values about 3 MPa showed
full recovery after repeated fracture-thermal healing processes.
The methodology explored in this contribution provides new
insights in modification of conventional engineering plastics as
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functional materials.
2015 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2015, 53, 2094–2103
KEYWORDS: adduct crosslinker; adhesives; Diels-Alder poly-
mers; epoxy resin; recycling; self-healing; thermosets
desirable attributes of epoxy thermosets and stimulus-
responsive behavior yields remendable or reprocessable
materials. The covalent adaptive epoxy networks based on
dynamically exchange bonds, such as transesterification,8,9
disulfide exchange,10–12 and alkoxyamines moieties13,14 ena-
ble network rearrangement in one-step method, but usually
suffer from long time and the need of various catalysts. To
balance the efficient recycling and self-healing behaviors, the
“two-step” DA reaction seems to be a most possible solution.
In furan/maleimide moieties based DA reaction, DA bond
formation by [412] cycloaddition reaction is predominant at
lower temperatures (50–80ꢀC), while retro DA (rDA) reac-
tion is taking place at 110–130ꢀC.4,15,16 Currently, most stud-
ies are focused on modifying the epoxy monomer/oligomer
to incorporate the DA groups to achieve epoxy healing.17–25
Unlike the above strategy, Bai et al.26 synthesized a diamine
cross-linker incorporating two DA adducts of furan and mal-
eimide moieties to cure common epoxy monomer.
INTRODUCTION Traditionally, thermosets are strong, insolu-
ble, largely inert, and covalently cross-linked polymer net-
works, which are hard to recycle or remold due to the
difficult flow under post thermal-processing compared with
thermoplastics.1,2 Thermosetting materials cannot be
replaced by thermoplastics in a wide range of applications,
especially those requiring high-performance engineering
areas such as the aircraft and automotive.3 As a well-known
thermoset polymer, epoxy resin finds extensive applications
including electronics encapsulation, sealants, coatings, adhe-
sives, and reinforced composites.1 However, conventional
epoxy thermosets suffer from limited functional performance
and have rising environmental concerns by disposal or incin-
eration originating from the permanent and irreversible
cross-linking nature. Additional functional properties of mul-
tiple self-healing would be appealing to extend the service
life and safety for epoxy. Meanwhile, highly efficient recycling
strategy is urgently needed from both environmental and
economic considerations.
In this work we aim to address several challenges for the
preparation of self-healing and recyclable epoxy, including
complex synthesis, low versatility, and low reprocessing effi-
ciency. Herein, a diamine DA adduct cross-linker was newly
synthesized to cure common epoxy monomers. The rDA and
Recently, a variety of dynamic covalent bonds, such as Diels-
Alder (DA) reaction,4 disulfide chemistry,5,6 alkoxyamine
moieties,7 and transesterification reaction8 are incorporated
into covalent adaptive epoxy network. The convergence of
Additional Supporting Information may be found in the online version of this article.
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DA reactions could proceed effectively in the cross-linkages
of one DA adduct, which were characterized by NMR, FTIR,
and differential scanning calorimetry (DSC) in thermal cyclic
tests. The structure–property relationship of the fast recycla-
ble and repeated self-healing epoxy was analyzed.
EXPERIMENTAL
Materials
N-Boc-ethylenediamine was obtained from Accela (>95%).
Maleic anhydride (99.5%), furfurylamine (99.0%), di-tert-
butyl dicarbonate (98%), triethylamine (Et3N, ꢁ99%), and
nickel (II) acetate tetrahydrate (99.9%) were supplied by
Aladdin. Trifluoroacetic acid (TFA, >99.0%) was provided by
Xing Jin Chemical Reagents Corp. (China). Ion exchange resin
Ambersep 900 (OH) was obtained from Alfa Aesar. Ion
exchange resin was first washed with water and then with
methanol to replace water phase. DGEBA (EPON 828) was
obtained from Momentive. All the other reagents and sol-
vents, analytical grade, were purchased from Beijing Chemi-
cal Reagents Corp. (China) and used without purification.
SCHEME 1 Synthesis of diamine DA adduct cross-linker (Com-
pound 4).
Measurements
1H NMR measurements were carried out on a Bruker DRX
400 and the chemical shifts were calibrated by CDCl3
(d 5 7.26), MeOD (d 5 4.80), or DMSO-d6 (d 5 2.50). 13C
NMR spectra were recorded on a Bruker DRX 300 and the
chemical shifts were calibrated using CDCl3 (d 5 77.16) or
MeOD (d 5 49.00). Mass spectra (ESI) were recorded on a
Bruker Solarix 9.4T. Attenuated total reflection infrared
(ATR-IR) spectra were obtained with a Thermo Scientific
Nicolet 6700 spectrometer. In situ FTIR measurement was
carried out using a Linkam FTIR 600-vertical hot stage
under nitrogen atmosphere in transmission mode. IR spectra
were collected with a resolution of 4 cm21 by averaging 16
scans. DSC experiments were performed on a TA Q2000
using aluminum pans under nitrogen purge. The thermal sta-
bility was tested on a Perkin Elmer Pyris 1 TGA instrument
at a heating rate of 20ꢀC min21 under nitrogen atmosphere.
Rheology tests were performed on a TA Discovery HR-2 rhe-
ometer, using parallel plate geometry (25 mm diameter). The
dynamic temperature sweep tests were conducted at a cool-
ing rate of 5ꢀC min21. Shear frequency and strain were 1
rad s21 and 0.05%, respectively. Single-lap shear tests
according to ASTM D 1002 were carried out using steel
plates at room temperature on an Instron 3365 tensile tes-
ter. For the thermal healing tests, the sample ends of two
fracture parts were kept in contact using a paper clip for
thermal treatment. The as-cured and healed samples
were tested with a crosshead speed of 1 mm min21. Four
experiments were performed for one sample for averaged
values.
Then the above two compounds proceeded DA reaction to
form adducts (Compound 3). Compound 3 (10.0 g, 22.9
mmol) was first dissolved in CH2Cl2 (100 mL) at 0ꢀC, and
then TFA (40 mL) was added and stirred for 3 h, during
which the solution was allowed to reach the room tempera-
ture. The mixture was concentrated and washed with cold
Et2O four times (4 3 30 mL). After evaporating the remain-
ing solvent, the obtained white powder was dissolved in
30 mL MeOH. The yellow brown solution turned light yellow
after flashing through ion exchange resin for several times.
The resulting solution was filtrated with alkali alumina and
dried over anhydrous Na2SO4. The solvent was evaporated
under reduced pressure at 35ꢀC to get the Compound 4
(4.79 g, 88%) as a viscous liquid with brown color. And it is
a mixture of exo and endo isomers (exo/endo53.4/1). Exo
1
isomer: NMR [400 MHz, MeOD, d (ppm)]: 6.65 (s, 2H), 5.20
(s, 1H), 3.61 (t, J 5 6.2 Hz, 2H), 3.28 (d, J 5 14.0 Hz, 1H),
3.19 - 3.11 (m, 2H), 3.03 (d, J 5 6.3 Hz, 1H), 2.84 (t, J 5 6.2
Hz, 2H). 13C NMR [300MHz, MeOD, d (ppm)]: 178.6, 177.4,
138.9, 138.6, 93.4, 82.2, 51.9, 51.4, 47.1, 42.0, 40.3. ATR
(KBr/cm21): 3361, 3289, 2969, 2931, 2874, 1769, 1698,
1584, 1457, 1375, 1107, 916, 863, 715. ESI-MS (m/z):
[M 1 H]1 calcd. C16H17N3O3, 238.1; found 238.1. For more
details about the synthesis and properties of Compounds 1,
2, and 3, refer to the Supporting Information (Figs. S1–S3).
Synthesis of Reversibly Cross-linked Epoxy Polymers
Diamine cross-linker (Compound 4; 1.96 g, 8.27 mmol) and
DGEBA (6.75 g, 17.23 mmol) were mixed at room tempera-
ture directly and degassed under vacuum at 30ꢀC for 20
min. After further degassing at 60ꢀC for another 10 min, the
mixture was transferred into the mold and heated at 60ꢀC
for 12 h to get the cross-linked epoxy polymer. To obtain a
control material (denoted as “control”), polyetheramine
Synthesis of Diamine DA Adduct Cross-linker
The diamine DA adduct cross-linker with one DA adduct was
synthesized by a four-step method (Scheme 1) similar to lit-
erature.26 First, the Compounds
1 and 2 with tert-
butoxycarbonyl (Boc) protected amines were synthesized.
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SCHEME 2 DA / rDA reactions in Compound 3.
D-230 as a curing agent, which has similar molecular weight
to the synthesized cross-linker, was used with the same for-
mulation and curing condition.
Thermo-reversibility of DA Adduct Cross-linker by DA
and rDA Reactions
In the cross-linker component, the DA adduct can undergo
DA and rDA reactions providing the molecular foundation for
the reversibility. The DA and rDA reactions of the cross-
linker were evaluated by the 1H NMR cyclic tests. The pri-
mary amine can react with C@C bond of maleimide group
due to Michael addition.28 Thus, Boc protected diamine
cross-linker (Compound 3) was tested as a model to investi-
gate the feasibility of the DA and rDA reactions in the cross-
linker component itself.
RESULTS AND DISCUSSION
Synthesis of Diamine DA Adduct Cross-linker
Scheme 1 shows the synthesis procedure of the diamine DA
adduct cross-linker. First, the N-Boc-ethylenediamine reacted
with maleic anhydride in CH2Cl2, then under the catalysis of
acetic anhydride (dehydrating agent), nickel (II) acetate (cat-
alyst), and triethylamine (co-catalyst) in refluxed acetone to
obtain Compound 1 with much higher yield than that of pre-
vious research.27 Furfurylamine was protected by Boc group
to get Compound 2. Then, Compounds 1 and 2 reacted in
ethyl acetate to obtain Compound 3 as pure exo isomer.
Finally, two Boc groups were deprotected by TFA and the
salt was neutralized by ion exchange resin in MeOH, and
Compound 4 as brown color viscous compound was success-
fully obtained after vacuum evaporation. It is noteworthy
that the diamine cross-linker can be prepared using cost
effective resources in a large scale without chromatography
purification. Whereas synthesis of similar reported com-
pounds require tedious purification such as column
chromatography.26
1
The H NMR spectrum of Compound 3 was first measured at
room temperature in DMSO-d6. Then the sample was kept at
120ꢀC for 30 min and subsequently quenched quickly to
1
room temperature for the next H NMR run. Afterwards, the
above sample was treated at 75ꢀC for 12 h and then cooled
down to room temperature and retested. Peaks 1, 2, and 3
are assigned to DA adduct, and peaks 10, 20, and 30 are
assigned to the furan ring in rDA product (Scheme 2). As
clearly shown in Figure 1(a), only peaks 1, 2, and 3 were
observed before heating. After heating at 120ꢀC for 30 min,
peaks 1, 2, and 3 disappeared and peaks 10, 20, and 30
appeared [Fig. 1(b)]. It means that most of the DA adducts
cleaved at 120ꢀC by rDA reaction. After treating the solution
1
FIGURE 1 H NMR spectra of DA and rDA reaction cycles for Compound 3: (a) before heating, (b) after heating at 120ꢀC for 30 min
and quenching to room temperature, and (c) annealing (b) at 75ꢀC for 12 h and quenching to room temperature.
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SCHEME 3 Curing reaction between diamine DA adduct cross-linker and DGEBA.
at 75ꢀC for 12 h, peaks 1, 2, and 3 appeared once again [Fig.
1(c)] indicating the reformation DA adduct by furan and mal-
eimide moieties. Therefore, the 1H NMR results verify the
reversibility of the cross-linker component by rDA and DA
reactions. It should be noted that the 1H NMR experiment
was tested in low concentration solution (10 mg/mL) with-
out stirring in an NMR tube. And the estimated DA reaction
conversion under that condition is calculated to be about
polymer. The molar ratio of NH groups and epoxide groups
was 0.96:1. After being degassed in a vacuum oven, the mix-
ture of DA adduct cross-linker and DGEBA was poured into
the mold and kept at 60ꢀC for 12 h to get the cross-linked
epoxy polymer, which was found to be insoluble.
FTIR spectroscopy was used to characterize the curing pro-
cess (Fig. 2). The peaks at 1609 cm21 assigned to the car-
bon–carbon stretching vibration of phenyl group were used
as an internal standard. After curing at 60ꢀC for 12 h, the
cured mixture of DGEBA and diamine cross-linker shows an
obvious decrease for the band intensity of 916 cm21, which
is attributed to the oxirane ring breathing of epoxide group.
By comparing with the normalized intensity of epoxide
1
51% by H NMR spectrum.
The reversibility of the cross-linker was further confirmed
by DSC tests at bulk state (Supporting Information Fig. S4).
During the first heating process, only one endothermic peak
at 155ꢀC was observed which is attributed to the pure exo-
adduct melting or rDA reaction.15,29 After annealing the melt
at 75ꢀC for 2 h, pure exo-adduct was reformed as indicated
by the single melting peak at 155ꢀC. The exothermal
enthalpy is calculated to be 196.2 J g21, corresponding to
85.7 kJ mol21 per C-C r bond in the DA adduct. This result
demonstrates that DA bond is 3–4 times weaker than the
conventional C-C r bond.30 The above 1H NMR and DSC
results suggest that the cross-linker bearing one DA adduct
can undergo efficient DA and rDA reactions with good
reversibility both in solution and at bulk state.
Preparation of the Reversibly Cross-linked Epoxy
Polymers
The cross-linked epoxy polymer was synthesized by step
growth polymerization of epoxide and amine groups. The
diamine cross-linker is able to react with at most four
epoxide groups yielding
(Scheme 3).1
a cross-linked epoxy polymer
FIGURE 2 FTIR spectra of the mixture of DGEBA and diamine
DA adduct cross-linker: (a) before curing, (b) after curing at
60ꢀC for 12 h. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
A common, widely used diglycidyl ether of bisphenol A
(DGEBA) epoxy monomer was utilized to illustrate the prep-
aration and performance of the reversibly cross-linked epoxy
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Figure 3(a) shows the curing behaviors using reversible and
irreversible cross-linkers by DSC. With consideration of the
1H NMR results on occurrence of the DA and rDA reactions,
we could assign exo- and endo-thermic peaks to different
reaction processes. The epoxy-amine curing reaction started
from room temperature with an exothermic peak locating at
about 89ꢀC. The endothermic peak at about 142ꢀC can be
attributed to the rDA reaction which releases maleimide moi-
ety. The second exothermic peak at 169ꢀC can be assigned to
the Michael addition of residual amine to maleimide moieties
and further curing.26 The curing reaction temperature of this
system should be carefully selected in avoid of rDA reaction
during the main curing process, thus a moderate tempera-
ture of 60ꢀC was used here.
The thermal properties of as cured epoxy polymer was also
evaluated by DSC. Figure 3(b) shows a glass transition tem-
perature (Tg) about 52ꢀC for the as cured sample. An endo-
thermic peak appears at 111ꢀC representing the rDA
reaction. The exothermal process at 137ꢀC can be attributed
to post curing, which was also found in the control sample.
The exothermal process at 234ꢀC may originate from the
homo-polymerization of the released maleimide moiety,20
which was absent in the control sample. The rDA reaction
occurred at a lower temperature in the polymer than the
melting temperature of exo-adduct (155ꢀC). It indicates good
miscibility of the cross-linker and the epoxy monomer.29 To
demonstrate the wider application of this methodology,
another commercial novolac epoxy oligomer (F-44) was also
cured by the diamine DA adduct cross-linker (Supporting
Information Fig. S5).
Thermo-reversibility of Reversibly Cross-linked Epoxy
Polymer
FIGURE 3 (a) DSC heating curves of unreacted DGEBA and dia-
mine cross-linker mixture. The heating rate was 2ꢀC/min. (b)
DSC heating curves of as-cured epoxy polymers. The heating
rate was 5ꢀC/min. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
The reversibility of the cross-linked epoxy was first exam-
ined by the thermal behavior of the cross-linked epoxy, as
measured by the repeated DSC cycles. The first cycle run
ended at 120ꢀC and the following cycles ran to 140ꢀC. DSC
thermograms show obvious endothermic peaks during the
successive six thermal cycles on the heating curves [Fig.
4(a)]. For the first cycle, only one peak ranging from 70 to
120ꢀC was observed, corresponding to the rDA reaction of
the DA adduct cross-linkages. Two shoulder-peaks in the
next five heating cycles are attributed to the coexisting of
endo- and exo-adduct in the thermal cycles.15,29,33,34 Slight
deviations of the repeated DSC traces were observed. On one
hand, the conversion of DA reaction is incomplete within the
relatively short time scale of the DSC experiments. On the
other hand, increasing the heating cycles leads to increasing
amount of thermodynamically stable exo-product during the
DA reaction.30 Both of the exo- and endo-adduct isomers
could undergo DA and rDA reactions, and thus serve as
reversible cross-linkages purpose.
group peak before and after curing, the curing percentage is
semi-quantitatively evaluated to be about 70%.
Meanwhile, two splitting peaks at 3200–3400 cm21 assigned
to primary amine stretching vibration disappeared and the
strong absorbance at 3200–3600 cm21 for the hydroxyl
groups emerged. The amount of DA adduct was kept during
the curing process as indicated by comparable characteristic
band intensity at 1770 cm21 (–C@O stretching in malei-
mide). These results clearly show that the effective curing
reaction between the epoxy monomer and the diamine
cross-linker has occurred.
DSC measurement was performed to better understand the
curing process of the reactive mixture. The DSC scan of the
mixture of DGEBA and diamine cross-linker is shown in Fig-
ure 3. Complex reactions proceed in the DSC heating process,
including curing reaction by epoxy and amine, rDA reaction,
Michael addition by maleimide and amine, and homo-
polymerization by maleimide moieties.31, 32
To understand the reversibility in molecular structure, in situ
FTIR of the cured epoxy was examined in thermal cyclic
tests. The evolution of the maleimide band (696 cm21
)
intensity reflects the DA/rDA reactions progress.35,36 Figure
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rDA reaction. The rDA reaction started from about 70ꢀC and
proceeded steadily from 90ꢀC to 120ꢀC. The intensity kept
constant for prolonged time at 120ꢀC, indicating the rDA
reaction equilibrium. In subsequent cooling process, steady
drop followed by a plateau in intensity near 80ꢀC is corre-
sponded to the evolution of DA cross-linking reaction. Lim-
ited maleimide conversion about 55% evaluated from
normalized maleimide band intensity was attained within
the relatively short time scale of the cooling experiments.
Thus, the network was not fully restored after directly cool-
ing. However, additional annealing process would contribute
to the complete recovery which would be discussed later on.
The reversibility of the epoxy polymer cured by the diamine
DA adduct cross-linker was visualized by solubility test. The
as-cured epoxy swelled in DMF at room temperature (Sup-
porting Information Fig. S6). On heating in DMF at 130ꢀC,
the swollen cross-linked polymer was fully soluble within 30
min and yielded a light yellow solution. The full dissolving of
as-cured epoxy in DMF at 130ꢀC suggests that the high tem-
perature promotes decross-linking by rDA reaction.
It can be deduced from the above results that the cross-
linked epoxy could turn into branched or even linear mole-
cules by rDA reaction at elevated temperature then recovers
to network state by DA reaction at lower temperature
(Scheme 4). Though the rDA reaction temperature increases
after exposing the mild-temperature cured epoxy to a higher
temperature, the reversibility of the formed epoxy polymer
is maintained. This highlights the capability of the current
system as a fast reversibly cross-linked material for efficient
recycling and repeated self-healing.
FIGURE 4 (a) DSC heating curves of as-cured epoxy polymer
in six repeated cycles; (b) IR band evolution of maleimide
(696 cm21) peak during thermal cycles for as-cured epoxy. The
scan rate was 5ꢀC/min. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Fast Recycling of Reversibly Cross-linked Epoxy Polymer
We anticipated that the cross-linked epoxy developed in this
work should be capable of reprocessing efficiently without
catalyst. To gain insight into the recycling behavior of the
epoxy polymer, dynamic temperature sweep rheological mea-
surement was performed.
4(b) shows the band intensity of 696 cm21 in the first heat-
ing and cooling cycles between 20ꢀC and 120ꢀC. The rise in
band intensity during heating process indicates the genera-
tion of maleimide moiety, demonstrating the occurrence of
Figure 5(a) shows the rheological test of as cured epoxy
polymer. At 120ꢀC, a liquid state with the crossover of
SCHEME 4 Schematic illustration of thermally reversible transition between network and branched molecules in cross-linked epoxy
polymer by DA and rDA reactions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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The first and the second recycled epoxy samples were visual-
ized with color darkening. It was also found that the color
change depended on the temperature and the time of thermal
treatments. During the heating progress, the color changed
from light yellow to dark yellow [Fig. 5(d)] then light red to
dark red [Fig. 5(e)] for as-cured epoxy. Similar color changes
in DA system has been reported in other polymer matrix.37
The recycled epoxy polymer with red color just swells in
DMF at room temperature, but reverts into yellow solution at
150ꢀC after 30 min. Maleimide homo-polymerization and
Michael addition between malemide and amine could occur
at 140–150ꢀC especially in the presence of epoxy function,31,
32
which would lead to irreversible network. It is inferred
that the inconvenient side reactions are negligible at the recy-
cling temperature range (120–140ꢀC) in a short time.
Additionally, the mixture of epoxy monomer and furfuryl-
amine (without maleimide component) did not turn red at
both 60ꢀC for 15 h and additional 120ꢀC for several hours,
FIGURE 5 Dynamic temperature sweep rheology tests with
cooling rate of 5ꢀC/min: (a) as-cured, (b) the first recycled, the
second recycled and control epoxy polymers. The pictures of
epoxy polymers: (c) as cured, (d) pulverized to grains, and (e)
the first (left) and the second (right) recycled epoxy polymers
by hot compressing at 120ꢀC and 140ꢀC, respectively. The scale
bar is 1 cm. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
storage modulus (G0) and loss modulus (G00) was observed.
During the cooling process, G0 became the dominant modulus
leading to elastic rubber state, which is ascribed to network
formation by the DA reaction.
Typically, 5 min of hot compressing could reprocess the
epoxy polymer. As a demonstration, the as-cured epoxy was
first pulverized and then recycled by hot-compressing at
120ꢀC. After cooling down and annealing at 75ꢀC for 1 h in
the air, an integrated transparent red plate (first recycled)
was obtained. The first recycled sample was further recycled
and remolded at 140ꢀC (second recycled). Similar to the
result of as-cured epoxy, both the first and the second
recycled epoxy polymers show steady rise in G0 at the start
of cooling in temperature sweep rheological tests [Fig. 5(b)].
In contrast, the storage modulus of control sample, which
does not contain reversible cross-linkages, keep constant and
shows a long plateau at elevated temperature.
FIGURE 6 (a) IR band evolution of maleimide (696 cm21) peak
during thermal cycles for first recycled epoxy. The scan rate
was 5ꢀC/min. (b) FTIR spectra of epoxy polymer during three
successive thermal cycles: heating to elevated temperature,
cooling down, and annealing at 75ꢀC for 30 min in each cycle.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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TABLE 1 Lap Shear Strength of the Reversibly Cross-linked
Epoxy
Sample
As-Cured
First Healed
Second Healed
Strength (MPa)
2.0 6 0.2
3.3 6 0.9
3.4 6 0.4
Self-healing Properties of Reversibly Cross-linked Epoxy
Polymer at High and Mild Temperatures
In principle, a two-step process with reestablishment (DA
reaction) of intramolecular interactions proceeding after
bond disconnection (rDA reaction) is necessary to achieve
self-healing in DA polymer self-healing. On one hand, high
temperature treatment for decross-linking (rDA reaction)
improves chain inter-diffusion at micro-crack interface. On
the other hand, the free furan and maleimide moieties at
opposite sides of the crack tend to reconnect (DA reaction)
resulting in better crack adhesion.
FIGURE 7 Adhesion strength of epoxy samples by single-lap
shear test: as cured, first healed (healing 120ꢀC for 20 min),
and second healed (healing 150ꢀC for 20 min).
The healing behavior of the reversibly cross-linked epoxy
was first investigated by lap shear test in fracture-thermal
healing cycles. The lap shear strength, expressed in MPa, is
calculated by dividing the load required to break the adhe-
sive bond by contact overlapping area (12.5 3 25 mm2). The
first and second healing process was first treating the sam-
ple at 120ꢀC and 150ꢀC for 20 min, respectively, and then
annealing at 75ꢀC for 1 h before tests.
but rapidly turned dark red after the addition of bismalei-
mide at 120ꢀC (Supporting Information Fig. S7). A detailed
study of color darkening is beyond the focus of this work. It
is tentatively proposed that the possible formation of p-
complexes of maleimide instead of side reactions is contrib-
uted to the color changes.
Figure 6(a) shows the IR band evolution of 696 cm21 during
heating and cooling process for the first recycled sample.
The absorbance increases sharply by rDA reaction above
80ꢀC and slows down approaching 140ꢀC. Some drop during
the subsequent annealing process at 140ꢀC would be attrib-
uted to exo-adduct isomer formation or side reaction of mal-
eimide. Then the DA reaction occurs immediately at cooling
and slows down near 80ꢀC.
The shear strength of epoxy samples before and after heal-
ing are plotted in Figure 7 and summarized in Table 1. The
lap sheer strength of as-cured epoxy is about 2 MPa.
Remarkably, shear strength of the first healed samples is
around 3.4 MPa which shows 170% cohesive recovery to
the as-cured samples. The reversible DA/rDA reactions are
critical for the cohesive healing. Also, it is believed that the
postcuring during the thermal healing process contributes
to the improved adhesion strength. In addition, the compa-
rable shear strength between the first and second healed
samples further demonstrates excellent cohesive healing
provided by the reversible DA adduct cross-linkages. It is
noted that the reversibly cross-linked epoxy shows moder-
ate thermal resistance throughout the temperature range of
the recycling and healing process by TGA (Supporting
The multiple reversibility is shown in Figure 6(b). Repeated
cycles of thermal treatment at elevated temperatures lead
to rDA reaction for decross-linking as indicated by a distin-
guishing maleimide characteristic band (696 cm21). After
cooling down and subsequent annealing at 75ꢀC for 30 min,
the disappearance of the characteristic band shows a com-
plete cross-linkage recovery. Therefore, this reversibly
cross-linked epoxy fundamentally behaves like a typical
chemically cross-linked thermosets at ambient conditions,
yet can be reprocessed or remolded at elevated tempera-
ture like thermoplastics and then recovers cross-linked
state after cooling or annealing. The excellent reprocessabil-
ity is explained by the feature that the cross-linked epoxy
would turn into branched or linear states by rDA reaction
at elevated temperature, thus providing necessary process-
ing fluidity.
To date, there are no literatures of cross-linked epoxy that
exhibits so efficient reprocessability using common, widely
used epoxy monomer/oligomer. Comparing with one-step
manner in bonds exchange systems, our strategy allows
epoxy to be recycled or remolded more efficiently.8,10
FIGURE 8 Pictures before (left) and after (right) healing the first
recycled epoxy polymer at 75ꢀC for 30 min. Inserted was the
optical microscopic image of the healed crack with the scale
bar of 50 mm. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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2 D. Hanna, S. H. Goodman, Handbook of Thermoset Plastics,
William Andrew, San Diego, 2013.
Information Fig. S8). Moreover, the thermal resistance of
the recycled sample can be further improved. All these
results highlight that the reversibly cross-linked epoxy poly-
mer is a promising candidate for high strength reversible
adhesives.
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Mild condition healing was also conducted in this system.
Since the DA bond is weaker than ordinary covalent bonds,
the crack propagates preferentially along the weak DA
adducts to release the stress.24,30,38 The temperature of 75ꢀC
was selected, as this temperature is beneficial for DA reac-
tion and also provides necessary chain mobility (above Tg).
As a demonstration shown in Figure 8, a recycled disk-
shaped plate was first broken into two parts and then the
crack faces were put together at 75ꢀC. After 30 min, the frag-
ments rejoined into an integrated plate. Indeed, the crack
was healed with only tiny scar by optical microscopy. A bet-
ter healing is expected for a higher healing temperature or a
prolonged healing time.
6 T. F. Scott, A. D. Schneider, W. D. Cook, C. N. Bowman, Sci-
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11 J. Canadell, H. Goossens, B. Klumperman, Macromolecules
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CONCLUSIONS
13 Z. P. Zhang, M. Z. Rong, M. Q. Zhang, Polymer 2014, 55,
3936–3943.
We have demonstrated a facile methodology to prepare rever-
sibly cross-linked epoxy with efficient recycling and multiple
self-healing, based on a diamine Diels–Alder adduct cross-
linker and a commercial epoxy oligomer. The newly synthe-
sized diamine cross-linker with one Diels–Alder adduct of
furan and maleimide moieties endows cured epoxy polymer
with reversible cross-linkages. The reversibly cross-linked
epoxy combines thermoplastic and thermoset features. It
behaves as a classic thermoset at ambient conditions, yet can
be fast reprocessed or remolded at high temperature like ther-
moplastics and then recovers to cross-linked state after cool-
ing or annealing. The adhesive strength values about 3 MPa
showed full recovery after repeated fracture-thermal healing
process. These results indicate that the thermally reversible
epoxy will find potential applications in reversible adhesive,
self-healing coating, and so forth. The methodology developed
in this work takes smart and environmental benign epoxy to
the next level of simplicity, affordability, and practicality, and
provides new insights in the modification of traditional engi-
neering plastics as functional materials.
14 Z. P. Zhang, M. Z. Rong, M. Q. Zhang, C. e. Yuan, Polym.
Chem. 2013, 4, 4648–4654.
15 J. Canadell, H. Fischer, G. De With, R. A. T. M. van Benthem,
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ACKNOWLEDGMENTS
25 Q. Tian, M. Z. Rong, M. Q. Zhang, Y. C. Yuan, Polym. Int.
2010, 59, 1339–1345.
The authors wish to thank the financial support of Hong Kong,
Macao, and Taiwan Science and Technology Cooperation
Project (2012DFH50110), Ministry of Science and Technology
of the PeopleÇs Republic of China. Prof. Wei Huang, Prof. Ke
Zhang, and Dr. Xiaojuan Zhao are acknowledged for helpful
discussion on synthesis.
26 N. Bai, K. Saito, G. P. Simon, Polym. Chem. 2013, 4, 724–730.
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