Synergistic Covalent and Supramolecular Polymers for Mechanically Robust but Dynamic Materials

Article abstract of DOI:10.1002/anie.202004152

Nature has engineered delicate synergistic covalent and supramolecular polymers (CSPs) to achieve advanced life functions, such as the thin filaments that assist in muscle contraction. Constructing artificial synergistic CSP materials with bioinspired mec

Full text of DOI:10.1002/anie.202004152

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Accepted Article
Title: Synergistic Covalent and Supramolecular Polymers for
Mechanically Robust yet Dynamic Materials
Authors: Zhaoming Zhang, Lin Cheng, Jun Zhao, Lei Wang, Kai Liu,
Wei Yu, and Xuzhou Yan
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10.1002/anie.202004152
Angewandte Chemie International Edition
RESEARCH ARTICLE
Synergistic Covalent and Supramolecular Polymers for
Mechanically Robust yet Dynamic Materials
Zhaoming Zhang,a Lin Cheng,a Jun Zhao,a Lei Wang,a Kai Liu,a Wei Yu,a and Xuzhou Yan*a
[a]
Dr. Z. Zhang, L. Cheng, J. Zhao, L. Wang, Dr. K. Liu, Prof. W. Yu, and Prof. X. Yan
School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules,
Shanghai Jiao Tong University, Shanghai 200240, P. R. China
E-mail: xzyan@sjtu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Nature has engineered delicate synergistic covalent and
supramolecular polymers (CSPs) to achieve advanced life activities
like the role of thin filament in muscle contraction. Constructing
artificial synergistic CSP materials with bioinspired mechanically
adaptive features, however, represents a challenging goal. Here, we
report an artificial CSP system to illustrate the integration of covalent
polymer (CP) and supramolecular polymer (SP) in a synergistic
fashion, along with the emergence of notable mechanical and
dynamic properties which are unattainable when the two polymers are
formed individually. The synergistic effect relies on the peculiar
network structures consisted of SPs and CPs, and thus makes the
resultant CSPs an overall improvement in the mechanical
performance of stiffness, strength, stretchability, toughness, and
elastic recovery. Moreover, the dynamic properties including self-
healing, stimuli-responsiveness, and reprocessing are retained in
CSPs as well, manifesting themselves as programmable and tunable
materials.
CPs, the traditional sense of polymers, possess good
robustness and strength but lack of dynamic properties. On the
contrary, SPs have extraordinary performances in stimuli-
responsive, self-healing, and self-adapting properties,[5] whereas
the excellent dynamic properties are also accompanied by
relatively poor mechanical properties. Owing to these inherent
characteristics of CPs and SPs, developing polymeric materials
in which good mechanical performance coexists with dynamic
behaviors is always a significant challenge. Taking inspiration
from bio-system, we speculate that construction of synergistic
CSPs is able to achieve the goal, because intelligent integration
of judiciously designed CPs and SPs in CSPs could combine their
merits, and the synergistic effect in CSPs would further improve
these merits or even create evolved properties.
Herein, we report a de novo chemical design of synergistic
CSPs which simultaneously possess robust yet dynamic
characteristics in a programmable manner. The key principle to
the design is that the CSPs adopt network structures formed by
the combination of SPs and CPs, which favors the synergistic
effect to improve the mechanically adaptive properties. In the
networks, the CPs serve as skeleton and maintain the integrity of
the materials, and the SPs are able to undergo energy dissipation
under external forces and thus tune the mechanical behaviors
through the reversible dissociation and reassociation of the
host−guest recognition. When they work synergistically, the
mechanical performances of CSPs are much better than the
simple sum of the corresponding properties of SPs and CPs. The
improvements of the mechanical properties are comprehensive,
including stiffness, strength, toughness, stretchability, and elastic
recovery, which are also tunable by controlling the host−guest
complexation in CSPs. Along with the dynamic properties of self-
healing, stimuli-responsiveness, and reprocessing inherited from
SPs, the CSPs, therefore, possess not only remarkable
mechanical adaptabilities but also multiple dynamic properties.
Introduction
Polymers are basic yet crucial components for living things
due to their roles in constructing structural materials and
participating in life process. These polymers are not only the well-
known covalent polymers (CPs) but also a large variety of
sophisticated supramolecular polymers (SPs). In fact, the
implement of some complex functions in organisms relies on the
polymers in a special form, namely, synergistic covalent and
supramolecular polymers (CSPs). For example, in thin filament,
tropomyosin as a CP binds to the SP of actin filament through
noncovalent bonds to construct a synergistic CSP, which plays an
important role in muscle contraction.[1] Mimicking nature is always
an effective way to develop new materials.[2] Although it can be
expected that synergistic CSPs would possess advanced
properties that are unable to achieve by individual CPs or SPs,
materials based on synergistic CSPs are really rare. In 2016, one
ingenious system of simultaneous covalent and noncovalent
hybrid polymerizations was reported by Stupp and coworkers, in
which higher molecular weight covalent polymers were obtained
compared to the polymers generated by the same polymerization
without the synergy of a supramolecular polymer.[3] Similarly,
other reported polymeric systems with coexisted SPs and CPs
were also focused on the study of concurrent covalent and
supramolecular polymerizations.[4] However, synergistic CSPs
designed for specific properties or functions like thin filament have
yet to be demonstrated.
Results and Discussion
Design, synthesis, and structural characterization.
Polynorbornenes (PNBs) have attracted much research attention
due to their great stability and easy preparation by ring opening
metathesis polymerization (ROMP) of norbornene derivatives.[6]
Host−guest molecular recognition between benzo-21-crown-7
(B21C7) and its complementary secondary dialkylammonium salt
has been widely used to construct functional SPs owning to their
1
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Scheme 1. a) Chemical structure and schematic representation of SP. b) Chemical structures and schematic representation of CPs. Boc-protected amino groups
cannot be recognized by the crown ethers. c) Chemical structures and schematic representation of CSPs in which the inherent characteristics of SPs and CPs and
their synergistic effect endow the CSPs with remarkable mechanical and dynamic properties.
moderate association constant (Ka = 250 M−1 in CH3CN).[7]
Therefore, we selected PNB as the covalent backbone and SP
based on B21C7/secondary ammonium salt recognition motif as
the noncovalent counterpart to construct synergistic CSPs. As
shown in Scheme 1c, the PNB and SP chains interweave with
each other through the node of the cyclopentane moiety, forming
the synergistic CSP network structures. In fact, the network
structures of CSPs can also be understood in two simple ways:
on the one hand, CSPs-1−3 can be seen as the CP chains strung
together by SPs; on the other hand, the CSPs can be considered
as the SP chains linked together by CPs as well. Obviously, the
structures of synergistic CSPs are different from those reported
systems in which the supramolecular interactions only serve as
discrete crosslinks of CPs.[8] Notably, there would be some
defects in the CSP networks due to the formation of cyclic
oligomers during the supramolecular polymerization. But the
linear supramolecular polymers are able to be predominant in bulk
according to the ring-chain supramolecular polymerization
mechanism (Scheme S1).[7c] With different amounts of SPs in the
system, synergistic CSPs-1−3 have been constructed. To make
comparison, individual SP and CPs were also prepared as shown
in Scheme 1a and 1b, respectively. The self-assembly of B21C7
and secondary ammonium salt units-decorated norbornene
derivative forms the SP, which is nearly the same as the SP part
extracted from the CSPs. As for the CPs, their structures are very
similar to the CSPs except that the amino group was protected by
the Boc rather than in the form of secondary ammonium salt,
which is unfavorable for the host−guest recognition. It is worth
noting that the CPs are also the starting materials to prepare
CSPs, hence the molecular weight and structural composition of
CSPs are very close to the corresponding CPs.
The formation of SP was supported by the 1H NMR (Figure
1b). Compared with the spectrum of Boc-protected monomer 1
(Figure 1a and Scheme S2), the aromatic protons of
supramolecular monomer showed both the complexed and
uncomplexed peaks which can be attributed to the slow-exchange
nature of the recognition motifs on the proton NMR timescale.
Meanwhile, the characteristic peaks of crown ether (H1−H5)
exhibited obvious downfield shift, which also supported the
formation of the host−guest complexes. The detailed process of
supramolecular polymerization in solution was tracked by
concentration-dependent 1H NMR and 2D diffusion-ordered NMR
spectroscopies (Figures S25 and S26).
The syntheses of CPs-1−3 were based on two monomers: as
shown in Scheme S2, the monomer 1 is a norborene derivative
containing substituents of B21C7 and Boc-protected amino group,
and the monomer 2 is norbornenyl exo-di-n-butylester which acts
as a diluent to tune the content of SPs in the final CSPs. Initiated
by the third-generation Grubbs catalyst, the homopolymerization
of monomer
1
produced the CP-1, and the random
copolymerization of monomers 1 and 2 with the feed ratios of 1:2
and 1:6 afforded copolymers CP-2 and CP-3, respectively. Taking
CP-3 as an example, the H10 signal of monomer 1, the
characteristic peak of double bond in norbornene moiety,
1
disappeared after polymerization in the H NMR spectra (Figure
1d). At the same time, broader signals emerged at 5.54 and 5.34
ppm could be ascribed to the trans and cis double bonds in the
polymers.[9] The integration of the 1H NMR spectra revealed that
the ratios of the two repeating units corresponding to monomers
1 and 2 in the copolymers are roughly consistent with the feed
ratios of the two monomers. Benefitting from controlling the ratios
of the monomers/catalyst, the number-average molecular weights
2
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Figure 1. Partial 1H NMR spectra (400 MHz, acetone-d6, r.t.) of monomer 1 (a), SP (b), CSP-3 (c), and CP-3 (d). e) GPC elution curves of CPs-1−3 with DMF as
the eluent and PS as the standard. f) ATR-FT-IR spectra of CSPs-1−3 and comparison with those of corresponding CPs-1−3.
(Mn) of the CPs-1−3 measured by size exclusion chromatograph
(SEC) were basically close to each other with the values of 68, 75,
and 78 kDa, respectively (Figure 1e).
SP chains interweave with each other to form network structures,
which can restrict the movements of CP chain segments.
Furthermore, the more SPs were involved, the denser networks
would be formed, hence leading to higher Tg.
Based on CPs, the preparations of CSPs were readily
realized by converting the N-Boc group into secondary
ammonium salt: the N-Boc group was firstly deprotected by
CF3COOH, and followed by a treatment of NH4PF6 to exchange
Mechanical properties of SP, CPs, and CSPs. To study the
effect of SP ratios on the mechanical properties, the tensile tests
of CSPs-1−3 were performed. The stress-strain curves and the
calculated Young’s modulus and toughness were shown in Figure
2b and 2c, respectively. The Young’s moduli of CSPs-1−3
decreased gradually from 285 to 216 and then to 92 MPa. On the
contrary, the strain at break values were 3% for CSP-1, 68% for
CSP-2, and 723% for CSP-3, showing an increasing trend.
Meanwhile, the toughness of CSP-3 (47 MJ/m3) was also much
higher than those of CSP-1 (0.56 MJ/m3) and CSP-2 (9.7 MJ/m3).
These results demonstrated that the ratios of SP played a crucial
role in determining the mechanical properties of the synergistic
CSPs. Overall, CSP-3 has a more balanced performance, and
thus was selected as a representative of CSPs in the following
research.
The mechanical properties of the three kinds of polymers
were well reflected by the stress-strain curves (Figure 2d). Before
linked by SP, the CP-3 was highly stretchable (> 2000%), but
possessed poor stiffness (Young’s modulus = 0.15 MPa) and
strength (maximum stress = 0.063 MPa). In contrast, SP was a
stiff (Young’s modulus = 68 MPa) but brittle material which can
only be stretched to about 3% of its original length, and thus had
inferior toughness (5.2 × 10−3 MJ/m3). The synergistic CSP-3
exhibited an overall improvement of mechanical properties
including the aspects of stiffness (Young’s modulus = 92 MPa),
strength (maximum stress = 11 MPa), stretchability (723%), and
toughness (47 MJ/m3), which were clearly displayed by the radar
chart (Figure 2f). Notably, the quantified values of these properties
for CSP-3 are much higher than the sum of the corresponding
the counter anion into PF6 . In the 1H NMR spectrum, the H1−H5
−
signals of B21C7 moiety in CSP-3 shifted downfield compared
with those of CP-3, which was caused by the recognition of
B21C7 to secondary ammonium salt to form SP (Figure 1c and
1d). Thus, both the CP and SP were present in the system,
indicating the formation of the CSP. So far, the synergistic CSPs
in solution have been demonstrated, then the CSPs in the
condensed state were investigated by FT-IR spectroscopy with
attenuated total reflection (ATR) mode. As shown in Figure 1f, the
peaks around 1140 cm−1 assigned as the vs (−C−O−C) of the
B21C7 moiety shifted slightly to the lower frequency area when
compared with those of the CPs, which was also observed in the
similar case.[10] Besides, the changes of vas (=C−O−C) in B21C7
moiety at 1255 cm−1 also supported the formation of the
host−guest complex.
Thermal properties of SP, CPs, and CSPs. In the thermal
gravimetric analysis (TGA) curves of CSPs as well as CPs and
SP, the decomposition temperatures of all these polymers were
higher than 200 °C, exhibiting good thermal stabilities (Figure
S27). Differential scanning calorimetry (DSC) curves can reveal
some structure information of the synergistic CSPs. As shown in
Figure 2a, the glass transition temperature (Tg) values of SP and
CP-3 were 14 and −22 °C, respectively. When the SPs were
attached on the CPs to form CSPs, the Tg enhanced significantly.
The values showed a trend of CP-3 < CSP-3 < CSP-2 < CSP-1.
This can be explained that in the synergistic CSPs, the CP and
3
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Figure 2. a) DSC curves of SP, CP-3, and CSPs-1−3 recorded by the second heating scan from −50 to 80 °C with a heating rate of 20 °C/min. b) Stress-strain
curves of CSPs-1−3 recorded at room temperature with a deformation rate of 100 mm/min. c) Young’s moduli and toughness of CSPs-1−3 calculated based on
their stress-strain curves. d) Stress-strain curves of SP, CP-3, and CSP-3 recorded at room temperature with a deformation rate of 100 mm/min. e) Normalized
stress-relaxation curves of SP, CP-3, and CSP-3 measured at 60 °C. f) Comparison of mechanical properties and stress-relaxation time of CSP-3 with those of SP
and CP-3.
data for CP-3 and SP, implying that synergistic effect is involved
in the mechanical behaviors of CSP-3. As mentioned in the
introduction part, SPs suffer from poor mechanical robustness,
which is the fatal defect for their practical applications. The results
in this work indicated that the combination of CPs and SPs to
construct synergistic CSPs is an efficient means to improve the
mechanical properties of SPs. In addition, the stress-relaxation
behaviors of the three polymers were also investigated. As shown
in Figure 2e, the CSP-3 released stress much slower than
individual CP-3 or SP, and the corresponding characteristic
relaxation time (1/e) was also included in the radar chart (Figure
2f). The slower stress relaxation of CSP-3 can be attributed to that
the network structure formed by CP-3 and SP could restrict the
movements of polymer chains, and the dissipation of strain
energy may mainly rely on the dissociation/reassociation of
host−guest interactions.
Insights into the mechanical performance of CSP-3. What
are the roles of CP and SP in the mechanical behaviors of CSP-
3? And how they realize their synergies? To clarify these issues,
a series of rheology and cyclic tensile loading measurements
were carried out. We firstly performed the cyclic temperature ramp
tests of CSP-3 (Figure 3a). When temperature ramped down from
120 to 40 °C, both G′ and G″ increased substantially. During this
process, a crossover temperature (Tc) appeared at around 79 °C.
At high temperature (T > Tc), G″ was larger than G′, indicative of
the break of the network structures owing to the dissociation of
host−guest interactions in SP. Upon reducing temperature, the
network structures restored with G′ > G″ at the region of T < Tc.
Besides, the temperature ramped up process exhibited opposite
viscoelastic transition. The ramp curves roughly overlapped with
each other, showing relatively good thermo-reversible
performance.
(TTS) principle, which can be divided into four different regions
(Figure 3b). The region above fg was assigned as the glassy
regime where the network was frozen and chain movements were
restricted. Below the frequency of fg, the frequency region (fd < f <
fg) is called the dissipative regime where G′ and G″ have similar
magnitude. The dissipative relaxation was caused by the
relaxation of network strands between the connections of CP and
SP, and was governed by the Tg.[11] In the intermediate frequency
range (fs < f < fd), a pronounced plateau where the material
exhibited predominately elastic property was observed. Two
possible elements could be responsible for the elastic plateau,
which are crosslinks and entanglements. Based on the theory, the
plateau modulus (GN) can be applied to calculate the molecular
weight between two effective interactions (Me) using the equation
of GN = ρRT/Me.[12] If we assume that the modulus fully results
from the chain entanglements of CSP-3, the calculated value of
Me, namely, the molecular weight between two entanglement
points, is about 64 kDa. This value is close to the molecular weight
of the CSP-3, indicating that the chain entanglements are
negligible in CSP-3. Thus, the elastic properties of CSP-3 are
mainly dominated by the network formed by CP and SP. Finally,
the terminal regime of the master curve (f < fs) is the viscous flow
regime. In general, polymers possessing true terminal relaxation
show the slope of G′ and G″ with values of 2 and 1, respectively,
which fits the Maxwell model. These values of CSP-3 are lower
than 2 and 1, which can be interpreted that the chain dynamics
are affected by the secondary interactions, that is, the recurring
dissociation/reassociation of the host−guest interactions.
Then, we proceeded to study the relationships between the
synergistic effect and mechanical behaviors of the CSP-3. In the
stretching-speed-dependent tensile tests, both Young’s modulus
and strength of CSP-3 increased along with the deformation rates
from 1 to 1000 mm/min (Figure S34). It is well known that the
mechanical strength of polymer network with dynamic crosslinks
is highly dependent on deformation rates.[13] Therefore, these
To discriminate the dynamic behaviors of CSP-3 at different
timescales, master curves were obtained at the reference
temperature of 40 °C using the time-temperature superposition
4
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Figure 3. a) Cyclic temperature ramp curves range from 45 to 120 °C with a heating/cooling rate of 2 °C/min. b) Master curves of CSP-3 at a reference temperature
of 40 °C. c) Cyclic tensile test curves of CSP-3 recorded at room temperature with increased maximum strains. d) Hysteresis area for each circle of cyclic tensile
test curves. e) Cyclic tensile test curves of CSP-3 loaded at a strain of 150% with rest intervals ranging from 0 to 20 min for recovery property study. The tests were
performed at room temperature with a deformation rate of 100 mm/min. f) Hysteresis ratio and residual strain for each circle of cyclic tensile test curves.
results revealed that the dynamic characteristics of CSP-3 may
largely originate from the SP part. The role of SP was further
disclosed by cyclic tensile tests in a broad range of maximum
applied strains from 25 to 300% (Figure 3c). After reloading, CSP-
3 exhibited pronounced hysteresis and notable residual strain
which increased substantially with enlarged strains. The
hysteresis areas of each loading and unloading cycle were
summarized in Figure 3d. These results further verified the
important role of SP in energy dissipation in which the host−guest
recognition acted as the sacrificial bond and dissipated the energy
through its rupture.
recovery. Moreover, the fast recovery of deformation is beneficial
to the recovery of the whole mechanical properties.
Now, the roles of SP and CP and their synergistic effect in the
mechanical behaviors of CSP-3 have been elucidated. Separately,
SP could reversibly dissociate and reassociate to dissipate the
energy, which is crucial to the mechanical properties such as
stiffness and toughness. CP serves as the skeleton of the CSP-3
network, and maintains the integrity of the material at a large
deformation. Synergistically, these merits of SP and CP work with
each other in CSP-3 to endow it not only outstanding mechanical
performance in stiffness, strength, stretchability, toughness, etc.,
but also notable recovery property. The rheological analysis has
The role of CP and its interplay with SP were demonstrated
by the recovery properties of CSP-3. During the cyclic tensile tests, revealed that chain entanglements are negligible in CSP-3, hence
CSP-3 was repeatedly loaded to a 150% strain and unloaded with
different rest intervals ranging from 0 to 20 min. As shown in
Figure 3e, the virgin circle of CSP-3 had a plump curve due to the
energy dissipation of SP. The second circle without relaxing
exhibited a very small hysteresis loop because most of the
dissociated host−guest interactions were not recovered.
Meanwhile, the unrecovered deformation of the polymer network
also led to a large residual strain of about 53%. Along with
extending rest time, the host−guest recognition and the polymer
network gradually recovered, and basically finished in 20 min.
Thus, CSP-3 displayed good recovery property. It is well known
that during the recovery process, there is a competition between
the elasticity of the polymeric system and the strength of the
temporarily reformed host−guest interactions. The reformed
host−guest interactions could slow down the recovery of the
primary chain to its equilibrium state.[14] In CSP-3, the elasticity
was mainly contributed by the covalent chains and the unbroken
part of the network. As shown in Figure 3f, the recovery of residual
strain for the sample was much faster than the recovery of
hysteresis area. Because of the relatively slow reassociation of
host−guest recognition, it is reasonable to deduce that compared
with the hindrance from the reformed host−guest recognition, the
elasticity offered by the CP and unbroken network is dominant
during the recovery process, thus leading to the fast deformation
the mechanical properties mainly originate from the synergistic
interactions of SP and CP in the network. Therefore, we can
conclude that the rationally designed CSPs are able to utilize the
advantages of SP and CP synergistically to produce a particular
property, thereby achieving a result that one plus one is greater
than two. Next, we wonder whether the innate dynamic properties
of SP still retain in CSP-3.
Dynamic properties of CSP-3. Self-healing is highly
dependent on the dynamic properties of materials and also an
important indicator of dynamic properties. Here, the bulk self-
healing of CSP-3 was studied at 50 °C, and was detected by
tensile tests. With the extension of healing time, the overall
mechanical properties of the rejoined films including Young’s
modulus, tensile strength and strain, and toughness gradually
restored (Figures 4a and S35). The healing efficiency evaluated
by the strain at break of the restored films compared with that of
the uncut one, was up to 93% after 24 h (Figure S35a), showing
notable healing property.
As we know, the host−guest recognition between B21C7 and
secondary ammonium salt is sensitive to pH changes through the
reversible deprotonation and protonation of secondary
ammonium salt. Here, triethylamine (TEA) and trifluoroacetic acid
(TFA) were employed as the acid/base pair to study the pH-
responsiveness of CSP-3. As shown in Figure 4b, When treated
5
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temperature should follow Arrhenius law. As shown in Figure 4d,
the strict linear correlation of ln(τ) and 1000/T indicates that the
host−guest interactions are responsible for the stress relaxation.
The activation energy of the relaxation process was calculated to
be 43.3 kJ/mol, which is associated with the dissociation barrier
of the B21C7/secondary ammonium salt recognition. Compared
with the materials cross-linked by ureidopyrimidinone (UPy)
groups or coordination bonds,[15] the activation energy of the
host−guest recognition is lower. It is therefore that the host−guest
complexation can work more sensitively both in the mechanical
behaviors and dynamic responsivenesses.
Conclusion
In summary, we have designed and prepared three
synergistic CSPs through successive ROMP of norbornene
moieties and supramolecular polymerization driven by the
host−guest recognition between B21C7 and secondary
ammonium salt. Due to the synergistic effect, CSP-3 exhibited
notable mechanical properties in terms of stiffness (Young’s
modulus = 92 MPa), strength (maximum stress = 11 MPa),
stretchability (723%), toughness (47 MJ/m3), and elastic recovery
(20 min), which achieve a result that one plus one is greater than
two. In addition, these mechanical properties of the synergistic
CSPs can be tuned by controlling the ratios of SPs and CPs. The
structural basis to realize the synergistic effect and improve the
mechanical properties, is the unique 3D network formed by the
combination of SPs and CPs. Particularly, the rheological results
have suggested that the CSP-3 has almostly pure cross-linked
structure with negligible chain entanglements. Notably, in addition
to originating from the synergistic interactions, the properties of
CSP-3 also inherit from the innate properties of SP and CP, such
as the well preserved self-healing, stimuli-responsive, and
reprocessing properties of SP in CSP-3. Therefore, the CSP-3
could possess not only good mechanical performance but also
multiple dynamic properties. As far as we know, this work
represents the first artificial elastomeric materials of synergistic
CSPs. To further develop the concept, constructing synergistic
CSPs with various well-designed structures, enriching and
enhancing their properties and functions, and exploiting their
potentials in specific applications are being actively pursued in our
lab.
by 2 equiv. of TEA, both the stiffness (Young’s modulus = 0.52
Figure 4. a) Stress-strain curves of the virgin and healed CSP-3 specimens
after three different healing times from 4 to 8 and then to 24 h at 50 °C. b) Stress-
strain curves of the virgin CSP-3 sample and those after being treated by
acid/base pair of TFA and TEA. c) Normalized stress-relaxation curves of CSP-
3 at different temperatures. d) Fitting of the relaxation times to the Arrhenius
equation.
MPa) and strength (maximum stress = 0.21 MPa) of the sample
were poor but its stretchability was good (> 2000%), which is
highly similar to the case of CP-3. The reason for this observation
is that the TEA broke the host−guest recognition of SP, converting
CSP-3 into linear covalent polymer like CP-3. The broken SP
could be recovered by protonation of TFA, and the degree of the
recovered mechanical properties can be tuned through gradual
addition of TFA. As indicated by the stress-strain curves, addition
of 0.4 equiv. of TFA partially recovered the mechanical properties
of the sample. Further, increasing the dosage of TFA to 2 equiv.,
the mechanical properties recovered mostly (Figure S37). The
difference between the virgin curve and acid/base-treated one is
reasonable on account of the changed microenvironment of the
sample. The pH-responsiveness of CSP-3 can also be proved by
1H NMR spectra (Figure S38). Besides, CSP-3 also exhibited
obvious ion-responsiveness (Figure S39). When the PF6− counter
anion was changed into Cl− which doesn’t favor the recognition of
B21C7 to secondary ammonium salt, the mechanical properties
of CSP-3 decreased largely. Meanwhile, the mechanical
properties basically recovered by exchanging the counter anion
−
back to PF6 . These results suggested that the dynamic properties
Acknowledgements
of SP are well retained in CSPs, and the mechanical properties of
CSPs are tunable by the changes of external environments.
Furthermore, we envisioned that the dynamic properties
arising from SP could also allow for reprocessing of the
synergistic CSPs. To clarify this, the CSP-3 sample was cut into
small pieces and then hot pressed at 60 °C, which reformed
coherent and smooth samples with different shapes (Figure S40).
Impressively, the stress-strain curves of the remolded samples
only changed slightly after three circles (Figure S41). These
results indicated that the synergistic CSP-3 has good malleability.
To get a deep insight into the dynamic properties of CSP-3,
stress relaxation experiments were performed at different
temperatures. As expected, the material relaxed stress faster at
higher temperatures (Figure 4c). It is well known that if the stress
relaxation occurs via the dissociation/reassociation of the
host−guest complex, the characteristic relaxation time versus
X.Y. acknowledges the financial support of the NSFC/China
(21901161), Program for Eastern Scholar of Shanghai, and start-
up funds from Shanghai Jiao Tong University. W.Y.
acknowledges the financial support of the NSFC/China
(51625303).
Keywords: supramolecular polymers • host−guest chemistry •
synergistic effect • mechanically adaptive properties • dynamic
materials
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Entry for the Table of Contents
Inspired by bio-system, synergistic covalent and supramolecular polymers (CSPs) are developed for the construction of mechanically
robust yet dynamic materials. They are able to integrate and even enhance the merits of covalent polymers and supramolecular
polymers, and achieve a result that one plus one is greater than two.
8
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