Catalyst-free dynamic exchange of aromatic Schiff base bonds and its application to self-healing and remolding of crosslinked polymers

Article abstract of DOI:10.1039/c5ta05788d

The present work reveals that catalyst-free dynamic reversible exchange of aromatic Schiff base bonds is enabled at room temperature. Attachment of aryl groups to both nitrogen and carbon atoms of carbon-nitrogen double bonds is a critical structural factor that contributes to the dynamic characteristics and complete reaction between aromatic aldehyde and aromatic amine. The exchange mechanism is studied by model compounds, and free radical intermediates are found to be involved in the non-equilibrium stage of the exchange reaction. By taking advantage of the dynamic equilibrium of aromatic Schiff base bonds, infusible and insoluble crosslinked polyacrylate is self-healed and reprocessed through rearrangement of macromolecular networks. The reprocessed polymer is still self-healable. Either the self-healing or reprocessing is completed in air without heating. Considering that Schiff base polymers have shown diverse properties and stimulus-response behaviors, activation of the dynamic exchange of the inherent Schiff base bonds would result in a series of novel functionalities. More interestingly, traditional crosslinked polymers might thus be provided with a facile way of being smarter.

Full text of DOI:10.1039/c5ta05788d

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Materials Chemistry A
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Catalyst-free dynamic exchange of aromatic Schiff
base bonds and its application to self-healing and
remolding of crosslinked polymers†
Cite this: DOI: 10.1039/c5ta05788d
Received 27th July 2015
Accepted 3rd September 2015
*
*
Zhou Qiao Lei, Pu Xie, Min Zhi Rong and Ming Qiu Zhang
DOI: 10.1039/c5ta05788d
www.rsc.org/MaterialsA
The present work reveals that catalyst-free dynamic reversible ligand coordination,12 and supramolecular interactions,13 while
exchange of aromatic Schiff base bonds is enabled at room tempera- the latter deals with the Diels–Alder (DA) reaction,14 imine
ture. Attachment of aryl groups to both nitrogen and carbon atoms of bonds,15–22 trithiocarbonate,23 thiuram disulde units,24 disul-
carbon–nitrogen double bonds is a critical structural factor that de-thiol exchange reactions,25 disulde metathesis,26 coumarin
contributes to the dynamic characteristics and complete reaction derivatives,27 alkoxyamine moieties,28 siloxane equilibration,29
between aromatic aldehyde and aromatic amine. The exchange transesterication,30,31 diarylbibenzofuranone links,32 Ru-cata-
mechanism is studied by model compounds, and free radical inter- lyzed olen metathesis,33,34 urea bonds,35 transalkylation
mediates are found to be involved in the non-equilibrium stage of the exchanges of C–N bonds,36 and boronic ester linkages.37,38
exchange reaction. By taking advantage of the dynamic equilibrium of
Comparatively, reversible covalent bonds are more suitable
aromatic Schiff base bonds, infusible and insoluble crosslinked poly- for constructing self-healing polymers towards structural
acrylate is self-healed and reprocessed through rearrangement of application owing to their stronger nature. In addition, the
macromolecular networks. The reprocessed polymer is still self-heal- dynamic reversible reaction, during which the reversible
able. Either the self-healing or reprocessing is completed in air without processes synchronically occur, is preferentially favored for self-
heating. Considering that Schiff base polymers have shown diverse healing, because the healing can be completed in a single-step
properties and stimulus–response behaviors, activation of the dynamic fashion,24–26,28,30–37 rather than the two-step mode required by a
exchange of the inherent Schiff base bonds would result in a series of conventional reversible reaction like the DA reaction.14 In a
novel functionalities. More interestingly, traditional crosslinked poly- polymer built up by dynamic reversible covalent bonds,
mers might thus be provided with a facile way of being smarter.
although a great deal of covalent bonds are disconnected at any
time in the course of crack healing, the rest of the covalent
bonds keep on being connected. Accordingly, the possible
collapse of the material due to ssion of all reversible bonds39 is
prevented and the material can maintain its integrity and load-
bearing capability.
Introduction
Herein, for the rst time, we report that catalyst-free dynamic
reversible exchange is able to operate at room temperature
between a single type of aromatic Schiff base bond, i.e. specic
imines containing carbon–nitrogen double bonds with the
nitrogen and carbon atoms connected to aryl groups. On the
basis of this nding, a simple self-healing chemistry is devel-
oped, which takes advantage of both reversible C]N covalent
bonds and their dynamic reaction. Although Zhang et al. have
introduced the Schiff base bond into self-healing gels,17 it does
not carry aryl groups on both nitrogen and carbon atoms, as the
aromatic Schiff base bond dened above. They actually made
use of the reversible reaction between Schiff base bond products
and aldehyde and amine reactants, rather than reversible
exchange between Schiff base bonds themselves. The forward
and reverse reactions took place under different conditions (e.g.,
Self-healing polymers have been substantially studied in recent
years to autonomically recover the cracks created during
manufacturing and/or usage.1–3 Unlike extrinsic self-healing that
requires incorporation of healing agent-loaded microcapsules4
or micropipelines,5,6 intrinsic self-healing takes effect by means
of intramolecular interactions. So far, the available functioning
units of the latter strategy include reversible non-covalent and
covalent bonds.7 The former involves hydrogen bonding,8 p–p
stacking,9 ionic interactions,10 host–guest interactions,11 metal-
Key Laboratory for Polymeric Composite and Functional Materials of Ministry of
Education, GD HPPC Lab, School of Chemistry and Chemical Engineering, Sun
Yat-Sen University, Guangzhou 510275, P. R. China. E-mail: cesrmz@mail.sysu.edu.
cn; ceszmq@mail.sysu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details and
supplementary gures referred in the text. See DOI: 10.1039/c5ta05788d
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pH) in their gels. In contrast, dynamic reversible exchange of
To reveal the dynamic exchange feature between aromatic
Schiff base bonds should allow for efficient shuffling of the Schiff base bonds, a model mixture of small molecules (i.e.,
carbon–nitrogen double bonds under a constant condition (e.g. benzylideneaniline and N-(4-methylbenzy-lidene)-3-chloroani-
room temperature for the present case), so that fractured line) is investigated. As shown in the high-performance liquid
surfaces of the polymer containing Schiff base bonds as inter- chromatography (HPLC) chromatograms in Fig. 1, the exchange
chain linkages can be covalently re-bonded without manual products emerge quickly at room temperature. With a rise in
intervention soon aer damaging. Chow et al. synthesized time, the molar fractions of benzylideneaniline and N-(4-
several dynamic imine polymers displaying constitutional methylbenzy-lidene)-3-chloroaniline continuously decrease,
dynamics by reorganization and exchange of the monomers in while the fractions of the exchange products increase. The
polymer chains through bond recombination between their reaction approaches equilibrium aer 8 h, when the relative
dialdehyde and diamine components under solvent/heating intensities of all peaks nearly no longer change. The exchange
stimuli.40 An aromatic imine was included in two polymers, but products lead to two peaks, which correspond to N-(4-methyl-
reshuffling of the chains was only revealed among the different benzylidene)aniline
and
N-benzylidene-3-chloroaniline,
imine polymers rather than in a single aromatic imine polymer. respectively, as revealed by mass spectroscopy analysis (Fig. S4,
In this work, polyacrylate is modied to bring in an aromatic ESI†). Clearly, the exchange reaction takes place without the aid
aldehyde group (Fig. S1 and S2, ESI†), and then reacts with 4,40- of a catalyst or heating.
diaminodiphenyl methane, forming aromatic Schiff base bond
Owing to the lone pair electrons of its N atom, the Schiff base
crosslinked networks (Scheme 1 and Fig. S3, ESI†). The possesses specic properties and has been used to synthesize a
condensation reaction between aromatic aldehyde and amine is ame retardant.42 Additionally, the condensation product of
completely forward under suitable conditions, producing stable aldehyde and amine is one of the oldest antioxidants, mainly
networks without reverse reactions. Meanwhile, the exchange serving as a rubber stabilizer (e.g., antiager 3-hydro-xybutyr-
reaction between aromatic Schiff base bonds can be activated at aldehydenaphthylamine). Since either the ame retardant or
room temperature. Based on the dynamic reversible exchange antioxidant works by acting as a free radical scavenger in the gas
of aromatic Schiff base bonds, the polymer has acquired the or solid phase, we assume that free radicals might be involved
ability of self-healing and reprocessing. Compared with in the above exchange reaction. Accordingly, (2,2,6,6-tetrame-
conventional covalent bonds, Schiff base bonds are created thylpiperidin-1-yl)oxy (TEMPO) was added to the reaction
under mild reaction conditions without by-products. Molecular system as a radical trap to ascertain the underlying mechanism
assembly based on Schiff base bonds exhibits interesting by means of electron spin resonance (ESR) spectroscopy.43 If
stimulus–response behavior.41 By exploring the mechanism radicals existed in the system, the detected ESR signal intensity
involved in the dynamic reversible exchange of aromatic Schiff would be lower than that of neat TEMPO. It is seen from Fig. 2
base bonds and its application in self-healing and reprocessing that free radicals indeed appear during the exchange reaction
of crosslinked polymers, a new platform for constructing smart
responsive and adaptive materials is hoped to be established.
Results and discussion
A schiff base can be synthesized from an aliphatic or aromatic
amine and a carbonyl compound by the nucleophilic addition
reaction. When the carbon or nitrogen atom of C]N double
bonds is attached to a phenyl or a substituted phenyl group, the
Schiff base becomes a stable imine. This is true especially for
the nitrogen atom carrying an aromatic chain. Imine has been
known for its reversible reaction feature: bond formation and
hydrolysis, where an equilibrium is established between imine/
water and aldehyde (or ketone)/amine. However, no reports
show that (i) the dynamic reversible exchange reaction can
proceed among the individual aromatic Schiff bases and (ii) the
polymer containing a single type of aromatic Schiff base unit
behaves like a dynamic material, to the best of our knowledge.
Fig. 1 HLPC chromatograms of the mixture containing an equivalent
amount (0.005 mol Lꢀ1) of benzylideneaniline and N-(4-methylbenzy-
lidene)-3-chloroaniline in methanol as a function of reaction time at
Scheme 1 Crosslinked aldehyde-functionalized polyacrylate (PA-SH). 25 ꢁC.
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between a single type of aromatic Schiff base compound (i.e. N-
(4-methylbenzy-lidene)-3-chloroaniline, refer to Fig. 2c) as well
as between different aromatic Schiff base compounds (i.e.
benzylideneaniline and N-(4-methylbenzy-lidene)-3-chloroani-
line, refer to Fig. 2a and b). It is worth noting that radicals were
not found in benzylideneaniline by the same experimental
method. Because benzylideneaniline is liquid at room temper-
ature, this phenomenon suggests that the exchange equilib-
rium must have been already reached and the exchange
reaction is able to proceed at the equilibrium state in the
absence of free radicals. The decline trend of radical amount
with time as revealed in Fig. 2 offers support to the deduction.
Based on the above discussion, the possible reaction mecha-
nism is plotted in Fig. 3.
Actually, the two aromatic Schiff base compounds used in
this work belong to a conjugated system, in which p electrons
are delocalized across all the adjacent aligned p-orbitals. The
electronic absorption effect of a chlorine atom and the lone pair
electrons on the nitrogen atom of C]N double bonds leads to
uneven distribution of electronic clouds. When aromatic Schiff
base compounds approach one another, the p-orbitals of C]N
double bonds and the delocalized electrons would overlap,
creating a transition state (Fig. 3a) and its resonance form
(Fig. 3b). Aerwards, a four-membered-ring structure (i.e. the
transition state) is built up between aromatic Schiff base
compounds (Fig. 3c), which is then split into double-radical
intermediates (Fig. 3d). Finally, two exchange products are
obtained (Fig. 3e). According to the ESR results, it is known that
the free radicals only appear in the early stage of the exchange
reaction. When the reaction is equilibrated, no free radicals can
be detected. The radical intermediates must be quite unstable
and quickly transformed to the exchange products. It is thus
reasonable to deduce that the high reaction speed at the
beginning of the reaction originates from the concentration-
driven effect. When the equilibrium state is approached, the
reaction slows down. As a result, the high energy barrier radical
intermediates are skipped, while the four-membered-ring
structure is directly converted to the exchange products.
Since the dynamic exchange habit of aromatic Schiff base
Fig. 2 Time dependences of ESR spectra of (a) the mixture of TEMPO
solution (0.02 wt%) and equimolar solutions of benzylideneaniline and
N-(4-methylbenzy-lidene)-3-chloroaniline in methanol (0.2 mol Lꢀ1);
(b and c) mixture of TEMPO solution (0.01 wt%) and N-(4-methyl-
benzy-lidene)-3-chloroaniline solution in methanol (0.2 mol Lꢀ1). For
(a), TEMPO solution and solutions of benzylideneaniline and N-(4-
bonds in the absence of a catalyst has been conrmed by the methylbenzy-lidene)-3-chloroaniline were mixed at the same time.
Afterwards, ESR spectra were acquired at certain time intervals. The
plots show that the signal intensity of residual TEMPO decreases with
time. It means that when the reaction approaches equilibrium, the
total amount of free radicals generated during the exchange reaction
small molecule model study, we proceed to the study of the
aromatic Schiff base bond crosslinked polyacrylate, PA-SH
(Scheme 1). Fig. S5, ESI,† plots dynamic mechanical perfor-
mance of the polymer in comparison with that of the control,
PA-ref (Scheme 2 and Fig. S6, ESI†). Clearly, both the temper-
ature dependences of storage modulus and tan d keep
unchanged during the cyclic tests. It means that (i) the polymer
has been fully cured and the yield of imine bonds is almost
100%, and (ii) all ruptured aromatic Schiff base bonds can be
rapidly recovered due to the dynamic reversible equilibrium
between random disconnections and reconnections. As a
result, the polymer would not be liquidized, and PA-SH behaves
like PA-ref without reversible bonds. On the other hand, the
molecular weight between the crosslinks of PA-SH, Mc is esti-
are gradually reduced. In the case of (b), the scenario resembles (a)
except that only a single type of aromatic Schiff base compound is
included, so that a similar trend is perceived because of the exchange
reaction between the same aromatic Schiff base compounds. With
respect to (c), the solution of the aromatic Schiff base compound in
methanol was prepared in advance, allowing the exchange reaction to
occur. After a certain reaction time, a methanol solution of TEMPO was
added and the ESR spectrum was immediately recorded. That is, for
each data point in (c), TEMPO solution was freshly incorporated. The
detected signal intensity of the residual TEMPO is thus inversely
proportional to the real-time radical concentration in the system.
Therefore, the increase of signal intensity of TEMPO with a rise in
reaction time also reflects the decline trend of the radicals produced
mated to be 2.1 ꢂ 103, which is slightly lower than the value of by the exchange reaction and confirms the conclusion drawn from (a
and b). Temperature: 25 ꢁC.
PA-ref, 2.7 ꢂ 103. The higher crosslinking density of PA-SH
than PA-ref coincides with the higher peak position of
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Fig. 3 Radical mediated dynamic exchange of aromatic Schiff base
bonds of benzylideneaniline and N-(4-methylbenzy-lidene)-3-chlor-
oaniline. Prior to the equilibrium, the reaction pathway consists of the
following steps: (a–e). After the equilibrium is reached, the reaction
pathway becomes (a–c) and (e).
Scheme 2 Structure of the control polymer PA-ref.
tan d (i.e. Tg) of the former (ꢀ21.2 ꢁC) than that of the latter
(ꢀ29.6 ꢁC), but is inconsistent with their tan d peak heights. As
shown in Fig. S5, ESI,† the tan d peak of PA-SH is obviously
higher than that of PA-ref. This discrepancy should originate
from the transient response of the temporary dangling chains
carrying disconnected aromatic Schiff base bonds in the course
of synchronous dissociation/recombination of aromatic Schiff
base bonds.
One thing needs to be mentioned is that PA-SH is a so
material especially because of the introduction of butyl acrylate.
This would help to improve molecular mobility for crack heal-
ing with high efficiency. Besides, the lower Tg of PA-ref than PA-
SH is appropriate, as it means that PA-ref has higher mobility
than PA-SH. Accordingly, when comparing the room tempera-
ture self-healability of the two materials hereinaer, one should
not correlate the lower healing efficiency of PA-ref with its chain
mobility.
To have deeper understanding of the material's response to
the dynamic exchange of Schiff base bonds, rheological
measurements were carried out as a function of angular
frequency. Fig. 4a indicates that storage shear modulus, G0, of
PA-SH is nearly independent of frequency at higher frequency
regime like conventional thermosetting polymers. The insig-
nicant decline at lower frequency regime implies that (i) the
rates of disconnection and reconnection of aromatic Schiff base
Fig. 4 Storage shear modulus, G0, and loss shear modulus, G00, as a
function of angular frequency, u, for (a) PA-SH and (b) PA-ref. (c) Stress
relaxation and (d) creep behaviors of PA-SH and PA-ref.
bonds start to synchronize with the testing frequency and (ii)
the polymer partly loses elasticity due to the transient dissoci-
ation of a certain amount of aromatic Schiff base bonds.
Meanwhile, loss shear modulus, G00, of PA-SH rstly decreases
with decreasing frequency and then increases. At frequency
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lower than the crossing point of the G0 ꢃ T and G00 ꢃ T curves, G0 polymer, which is also supposed to function in the damaged
< G00, meaning the crosslinked networks of PA-SH are changed material for re-bonding cracks. The simple qualitative exami-
from elastic-like to viscous-like solid. It means that it has to take nation in Fig. S7, ESI† shows that this is the case. Here, the PA-
longer time for the disconnected networks to be reconstructed SH specimen was cut, recombined and healed. Eventually, it
through dynamic exchange of aromatic Schiff base bonds under
low angular frequency. Consequently, viscous-like behavior
plays the leading role.
A careful survey of Fig. 4a reveals that the crossing frequency
of G0 ¼ G00 increases with an increase in temperature. High
temperature favors appearance of the viscous characteristics.
This is reasonable as the crossing frequency of G0 ¼ G00 is related
to the dissociation rate constant of reversible networks.44 The
exchange reaction among the aromatic Schiff base bonds is
always accelerated at elevated temperature, so that the cross-
linked polymer becomes viscous-like at higher frequency under
high temperature conditions.
The rheological performance of the control material, PA-ref,
which does not contain any reversible bond, forms sharp
contrast to that of PA-SH. As exhibited in Fig. 4b, G0 of PA-ref is
almost constant within the entire frequency range of interest.
No matter how G00 changes with frequency, it is always lower
than G0. These habits are peculiar to polymers with permanent
covalent bonds, and further reect the importance of dynamic
exchange of aromatic Schiff base bonds in the time and
temperature dependent mechanical properties of PA-SH.
The results of Fig. 4a demonstrate the variation in mechan-
ical performance of PA-SH as a result of network rearrangement
under dynamic shear loading. If such a network rearrangement
induced by exchange of aromatic Schiff base bonds does exist,
the polymer should also respond to long term static loading.
The stress relaxation and creep behaviors illustrated in Fig. 4c
and d prove that it is true. As shown in Fig. 4c, there is complete
stress relaxation aer about 800 s (i.e. the stress relaxes to 0) in
the case of PA-SH. The crosslinked networks in the polymer
must have been rearranged to reach a less stretched state during
stress relaxation.26 Comparatively, PA-ref lacks this mechanism
and can only conduct limited stress relaxation through
conformation change. The material is allowed to bear certain
equilibrium stress for long time.
Similarly, the result of creep test of PA-SH (Fig. 4d) exhibits
that the creep rate gradually decreases with time at the begin-
ning and then increases rapidly, entering an unstable devel-
opment phase. With respect to PA-ref, however, its creep strain
remains unchanged within the most time range of the experi-
ment, except for the initial deformation arising from movement
of crosslinks and chain segments. It is evident that the cross-
linked networks of PA-SH keep on managing to accommodate
the applied force in terms of rearrangement through exchange
of aromatic Schiff base bonds. When a few disconnected
aromatic Schiff base bonds fail to be reconnected due to
(a) effect of healing time, (b) effect of repeated repair (because tensile
reduced collision probability at larger deformation, the stress
Fig. 5 Typical tensile stress–strain curves of virgin and healed PA-SH:
strength of the virgin specimen was not fully restored, the recon-
concentration would inevitably occur and lead to irreversible nected interface became the weakest part, and had to break again
during the subsequent tensile tests. This ensured that the second and
third tensile failures happened to the same healed portion), and (c)
effect of waiting time taken for intentionally separating the broken
specimens before recombination. (d) Typical tensile stress–strain
curves of virgin and healed PA-ref. Healing conditions: 25 ꢁC in air.
failure of the polymer. As for PA-ref, the permanent crosslinked
networks prohibit plastic ow at room temperature. Therefore,
its creep rate and creep strain are far lower than those of PA-SH.
The above results and discussion reveal the effect of
aromatic Schiff base bond exchange in the intact crosslinked Healing times for (b–d): 24 h.
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can still bear load like the original version. It is thus known that
PA-SH can be self-healed at room temperature in air.
The curves in Fig. 5 further quantify the self-healability of PA-
SH. With a rise in healing time, the tensile strength of the
healed specimen approaches to that of the virgin one (Fig. 5a).
Aer 2 h, the strength of the healed version almost reaches the
equilibrium value. According to the results of Fig. 5a, 24 h is set
as the constant healing time for the subsequent tests to ensure
complete healing. Fig. 5b veries the multiple healability of PA-
SH. The specimen can be repeatedly self-healed as other
intrinsic self-healing polymers. The decline of healing efficiency
for the 3rd healing event is due to the fact that the distorted
fracture surfaces fail to be perfectly aligned and recombined.
The smaller effective contact area would certainly reduce the
number of reconnected bonds at the interface, and hence the
healing efficiency decreases.
To identify the predominant healing mechanism, two
control healing tests were made. First, the specimen of PA-SH
was cut and recombined for healing aer waiting times of 24
and 48 h, respectively. If hydrogen bonding governed the crack
healing, the measured healing efficiency would be rather low in
this case, as the neighboring free hydrogen bonds on the same
fracture surface should have been preferentially combined with
each other.8 Fig. 5c shows that it is not true. The healing effi-
ciencies are not affected by the waiting time and are close to the
values reported in Fig. 5b, where the broken specimens were
immediately rejoined for healing. Secondly, the specimen of PA-
ref was also cut and healed like PA-SH under the same
circumstances. The healing efficiency is only 13% (Fig. 5d),
which should be primarily contributed by both dangling chains
and hydrogen bonding. Based on these ndings, we know that
the dynamic exchange of aromatic Schiff base bonds is mainly
responsible for the self-healing capability of PA-SH.
Fig. 6 (a) Fragments of PA-SH. (b) PA-SH sheet remolded from (a) at
room temperature. (c) Typical tensile stress–strain curves of recycled
PA-SH subjected to successive breakage and healing events. Healing
temperature ¼ 25 ꢁC; healing time ¼ 24 h.
Traditionally, crosslinked polymer is reclaimed by breaking
down the irreversible covalent bonds. The decomposed prod-
ucts are hard to be converted into their original version. Here,
the solid state reprocessing by means of reshuffling of the
dynamic crosslinks provides a facile and energy-saving solution
with attractive application potential.45 Therefore, thermosetting
polymers might thus be born with both self-healability and
recyclability.
In fact, Fig. 5c demonstrates an important feature of
dynamic exchange of aromatic Schiff base bonds. That is, the
exchange reaction is always in the dynamic equilibrium so that
the number of aromatic Schiff base bonds available for healing
does not change whether the cut surfaces are in contact with
Conclusions
each other or not. In this context, rearrangement of the Dynamic exchange of aromatic Schiff base bonds is allowed to
aromatic Schiff base bond crosslinked networks is carried out proceed in air at room temperature in the absence of a catalyst.
throughout PA-SH all the time at room temperature. As long as Free radicals prove to be involved in the exchange reaction. By
the surfaces of different PA-SH specimens are in contact, the incorporating aromatic Schiff base bonds into polyacrylate as
exchange reaction of aromatic Schiff base bonds would take intermolecular linkages, dynamically reversible crosslinked
place not only in the bulk but also across the interface forming networks are built up. Accordingly, the polymer acquires cata-
new interactions. This habit would thus enable reprocessing of lyst-free self-healability and reprocessability without the need of
the crosslinked polymer without the need of depolymerization. manual intervention through rearrangement of the crosslinked
Under the guidance of the analysis, PA-SH specimens were networks as a result of dynamic exchange of the Schiff base
cut into small pieces (Fig. 6a), which were then compression bonds. Although the synthesis and structure of the aromatic
molded in air at room temperature under 4.5 MPa for 12 h. Schiff base bond crosslinked polymer have not yet been opti-
Eventually, an intact sheet was recovered (Fig. 6b). The recov- mized, the results of the present work evidence the feasibility of
ered PA-SH possesses almost the same mechanical properties as the concept.
the original one (cf. Fig. 6c and 5a). In addition, the recovered
Schiff base polymers (or polyazomethines) have been known
PA-SH also has repeated room temperature remendability for their ability to form metal chelates and supramolecular
(Fig. 6c). The results well agree with the prediction based on the interactions with electrophiles, ber-forming ability, good
principle of dynamic reversible bonding. A similar room thermal stability, semiconductivity, photoelectric properties,
temperature reprocessability is also available for cross-linked liquid crystal performance, etc.46–49 Moreover, Schiff base bonds
epoxidized polysulde via disulde metathesis.26
have successfully acted as stimuli-responsive groups in various
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polymers, and the Schiff base compound is an epoxy curing 19 Y. L. Zhang, B. Yang, X. Y. Zhang, L. X. Xu, L. Tao, S. X. Li and
agent used for the underwater and damp environment. Y. Wei, Chem. Commun., 2012, 48, 9305.
Exchange of aromatic Schiff base bonds would be inevitably 20 B. Yang, Y. L. Zhang, X. Y. Zhang, L. Tao, S. X. Li and Y. Wei,
accompanied by changes in the conjugated molecular chains Polym. Chem., 2012, 3, 3235.
and energy state. When the present free radical mediated 21 G. Deng, F. Li, H. Yu, F. Liu, C. Liu, W. Sun, H. Jiang and
dynamic exchange reaction is activated in these polymers, a wide Y. Chen, ACS Macro Lett., 2012, 1, 275.
variety of novel functionality would be aroused due to the hybrid 22 D. E. Apostolides, C. S. Patrickios, E. Leontidis, M. Kushnir
effect, in addition to the self-healability and reprocessability. In and C. Wesdemiotis, Polym. Int., 2014, 63, 1558.
this context, the outcomes of the present work offer a general 23 Y. Amamoto, J. Kamada, H. Otsuka, A. Takahara and
platform for developing a family of new smart materials.
K. Matyjaszewski, Angew. Chem., Int. Ed., 2011, 50, 1660.
24 Y. Amamoto, H. Otsuka, A. Takahara and K. Matyjaszewski,
Adv. Mater., 2012, 24, 3975.
25 M. Pepels, I. Filot, B. Klumperman and H. Goossens, Polym.
Chem., 2013, 4, 4955.
Acknowledgements
The authors thank the support of the Natural Science Founda-
tion of China (Grants: 51273214 and 51333008), the Natural 26 Z. Q. Lei, H. P. Xiang, Y. J. Yuan, M. Z. Rong and M. Q. Zhang,
Science Foundation of Guangdong (Grants: 2010B010800021 Chem. Mater., 2014, 26, 2038.
and S2013020013029), the Science and Technology Program of 27 J. Ling, M. Z. Rong and M. Q. Zhang, Polymer, 2012, 53, 2691.
Guangzhou (Grant: 2014J4100121), and the Basic Scientic 28 C. E. Yuan, M. Z. Rong, M. Q. Zhang, Z. P. Zhang and
Research Foundation in Colleges and Universities of Ministry of
Education of China (Grant: 12lgjc08).
Y. C. Yuan, Chem. Mater., 2011, 23, 5076.
29 P. W. Zheng and T. J. McCarthy, J. Am. Chem. Soc., 2012, 134,
2024.
30 C. Mathieu, M. Damien, T. Francois and L. Ludwik, J. Am.
Chem. Soc., 2012, 134, 7664.
Notes and references
1 M. Q. Zhang and M. Z. Rong, Self-healing Polymers and 31 D. Montarnal, M. Capelot, F. Tournilhac and L. Leibler,
Polymer Composites, John Wiley & Sons, Inc., Hoboken, 2011.
2 R. P. Wool, So Matter, 2008, 4, 400.
3 Y. C. Yuan, T. Yin, M. Z. Rong and M. Q. Zhang, eXPRESS
Polym. Lett., 2008, 2, 238.
Science, 2011, 334, 965.
32 K. Imato, M. Nishihara, T. Kanehara, Y. Amamoto,
A. Takahara and H. Otsuka, Angew. Chem., Int. Ed., 2012,
51, 1138.
4 D. Y. Zhu, M. Z. Rong and M. Q. Zhang, Prog. Polym. Sci., DOI: 33 Y. X. Lu and Z. B. Guan, J. Am. Chem. Soc., 2012, 134, 14226.
10.1016/j.progpolymsci.2015.07.002.
5 K. S. Toohey, N. R. Sottos, J. A. Lewis, J. S. Moore and
S. R. White, Nat. Mater., 2007, 6, 581.
6 W. C. Pang and I. P. Bond, Compos. Sci. Technol., 2005, 65,
1791.
7 M. Q. Zhang and M. Z. Rong, Polym. Chem., 2013, 4, 4878.
34 Y. X. Lu, F. Tournilhac, L. Leibler and Z. B. Guan, J. Am.
Chem. Soc., 2012, 134, 8424.
35 H. Z. Ying, Y. F. Zhang and J. J. Cheng, Nat. Commun., 2014,
5, 3218.
36 M. M. Obadia, B. P. Mudraboyina, A. Serghei, D. Montarnal
and E. Drockenmuller, J. Am. Chem. Soc., 2015, 137, 6078.
8 P. Cordier, F. Tournilhac, C. Soulie-Ziakovic and L. Leibler, 37 O. R. Cromwell, J. Chung and Z. B. Guan, J. Am. Chem. Soc.,
Nature, 2008, 451, 977. 2015, 137, 6492.
9 S. Burattini, H. M. Colquhoun, J. D. Fox, D. Friedmann, 38 A. P. Bapat, D. Roy, J. G. Ray, D. A. Savin and B. S. Sumerlin, J.
B. W. Greenland, P. J. F. Harris, W. Hayes, M. E. Mackay
and S. Rowan, Chem. Commun., 2009, 6717.
10 R. J. Varley and S. van der Zwaag, Polym. Int., 2010, 59, 1031.
Am. Chem. Soc., 2011, 133, 19832.
39 B. J. Adzima, H. A. Aguirre, C. J. Kloxin, T. F. Scott and
C. N. Bowman, Macromolecules, 2008, 41, 9112.
11 M. Nakahata, Y. Takashima, H. Yamaguchi and A. Harada, 40 C.-F. Chow, S. Fujii and J.-M. Lehn, Chem. Commun., 2007,
Nat. Commun., 2011, 2, 511.
4363.
12 S. Bode, L. Zedler, F. H. Schacher, B. Dietzek, M. Schmitt, 41 Y. Jia and J. Li, Chem. Rev., 2015, 115, 1597.
J. Popp, M. D. Hager and U. S. Schubert, Adv. Mater., 2013, 42 Y. Liu, Y. Zhang, Z. Cao and Z. Fang, Ind. Eng. Chem. Res.,
25, 1634.
13 F. Herbst, D. Dohler, P. Michael and W. H. Binder, 43 E. G. Janzen, Acc. Chem. Res., 1971, 4, 31.
2012, 51, 11059.
Macromol. Rapid Commun., 2013, 34, 203.
44 M. C. Roberts, M. C. Hanson, A. P. Massey, E. A. Karren and
14 X. Chen, M. A. Dam, K. Ono, A. Mal, H. Shen, S. R. Nutt,
K. Sheran and F. Wudl, Science, 2002, 295, 1698.
15 G. Deng, C. Tang, F. Li, H. Jiang and Y. Chen,
Macromolecules, 2010, 43, 1191.
P. F. Kiser, Adv. Mater., 2007, 19, 2503.
45 H. P. Xiang, H. J. Qian, Z. Y. Lu, M. Z. Rong and M. Q. Zhang,
Green Chem., 2015, 17, 4315.
46 C. J. Yang and S. A. Jenekhe, Macromolecules, 1995, 28, 1180.
16 N. Roy, E. Buhler and J.-M. Lehn, Polym. Int., 2014, 63, 1400. 47 A. Iwan and D. Sek, Prog. Polym. Sci., 2008, 33, 289.
17 Y. Zhang, L. Tao, S. Li and Y. Wei, Biomacromolecules, 2011, 48 M. Grigoras and C. O. Catanescu, J. Macromol. Sci., Polym.
12, 2894.
Rev., 2004, 44, 131.
18 P. Taynton, K. Yu, R. K. Shoemaker, Y. Jin, H. Qi and 49 M. A. Hussein, M. A. Abdel-Rahman, M. A. Asiri, K. A. Alamry
W. Zhang, Adv. Mater., 2014, 26, 3938.
and K. I. Aly, Des. Monomers Polym., 2012, 15, 431.
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