A stretchable pillararene-containing supramolecular polymeric material with self-healing property

Article abstract of DOI:10.3390/molecules26082191

Constructing polymeric materials with stretchable and self-healing properties arise increasing interest in the field of tissue engineering, wearable electronics and soft actuators. Herein, a new type of supramolecular cross-linker was constructed through

Full text of DOI:10.3390/molecules26082191

molecules
Article
A Stretchable Pillararene-Containing Supramolecular
Polymeric Material with Self-Healing Property
1
1
1
1
1
1
2,
Meng Zhao , Changjun Li , Xiaotao Shan , Huijing Han , Qiuhua Zhao , Meiran Xie , Jianzhuang Chen *
1
,
and Xiaojuan Liao *
1
School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241, China;
1184300134@stu.ecnu.edu.cn (M.Z.); LCJ0027@163.com (C.L.); sxt1369130195@163.com (X.S.);
5
hjhan@chem.ecnu.edu.cn (H.H.); qhzhao@chem.ecnu.edu.cn (Q.Z.); mrxie@chem.ecnu.edu.cn (M.X.)
School of Materials Science and Engineering, East China University of Science and Technology,
Shanghai 200237, China
2
*
Correspondence: chenjianzhuang@ecust.edu.cn (J.C.); xjliao@chem.ecnu.edu.cn (X.L.)
Abstract: Constructing polymeric materials with stretchable and self-healing properties arise in-
creasing interest in the field of tissue engineering, wearable electronics and soft actuators. Herein, a
new type of supramolecular cross-linker was constructed through host-guest interaction between
pillar[5]arene functionalized acrylate and pyridinium functionalized acrylate, which could form
supramolecular polymeric material via photo-polymerization of n-butyl acrylate (BA). Such material
exhibited excellent tensile properties, with maximum tensile strength of 3.4 MPa and strain of 3000%,
respectively. Moreover, this material can effectively dissipate energy with the energy absorption
efficiency of 93%, which could be applied in the field of energy absorbing materials. In addition, the
material showed self-healing property after cut and responded to competitive guest.
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Zhao, M.; Li, C.; Shan, X.;
Han, H.; Zhao, Q.; Xie, M.; Chen, J.;
Liao, X. A Stretchable
Keywords: pillararene; host-guest chemistry; supramolecular chemistry; supramolecular polymer
Pillararene-Containing
Supramolecular Polymeric Material
with Self-Healing Property. Molecules
1
. Introduction
Polymeric materials with stretchable property have got plenty of attention due to
their widespread applications in the fields of tissue engineering, wearable electronics and
soft actuators [ ]. It is well known that there is always a trade-off between strength and
toughness for the stretchable polymeric materials [ 10]. In general, strong interactions
2
021, 26, 2191. https://doi.org/
1
0.3390/molecules26082191
1–7
Academic Editor: Feihe Huang
8
–
and rigid structures, such as covalent bonded gels, often provide higher tensile strength but
with very low strain at break and slow self-healing, while weaker interactions lead to faster
self-healing, but soft and viscoelastic materials [11–14]. For example, Huang reported a
Received: 25 March 2021
Accepted: 7 April 2021
Published: 10 April 2021
polymer gel based on the formation of acylhydrazone bonds [15]. The tensile breaking
stress reached 1.2–2.7 MPa after solvent exchange from DMSO to water. However, the
gel did not show self-healing behavior and the maximum strain at break was 40%, which
limited their application. Therefore, it is still a challenge to explore a facile method to obtain
polymeric materials with excellent strength and toughness as well as self-healing property.
Scientists have made great efforts to solve this problem by combining covalent
bonded polymer with non-covalent interactions, including H-bond and coordination
bond [16–18]. Among these various non-covalent interactions, host-guest chemistry, the
central of supramolecular chemistry, has attracted tremendous attention, since it endows
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the system with reversible, tunable and dynamic properties [19–26]. For example, Suzuki
and Takata reported the formation of tough films by evaporation of water from dispersions
of elastomer microspheres crosslinked with rotaxane supramolecules, which contained
crown ether [27]. Guo and colleagues prepared ultraductile, notch and stab resistant
supramolecular hydrogels by copolymerization of acrylamide and adamantane monomer
in the presence of cyclodextrin-functionalized polymer [28]. As the pioneer in this field,
Harada and colleagues prepared flexible and tough elastomers with self-healing abili-
4
.0/).
Molecules 2021, 26, 2191. https://doi.org/10.3390/molecules26082191
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2191
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ties by using cyclodextrin derivatives [29]. Hence, host–guest interactions could make
great contributions in preparation of polymeric materials with good mechanical and self-
healing properties.
On the other hand, pillararenes, as a new fascinating family of macrocyclic host
molecules, have attracted increasing attention due to their novel symmetric structures and
extensive applications in the construction of novel supramolecular architectures [30–36].
For instance, Ogoshi succeeded in molecular weight fractionation by the confinement
of single polymer chains of poly(ethylene oxide) in the one-dimensional channels of
crystalline pillar[5]arene [37]. Huang constructed adaptive photosensitizers based on a
host-guest complex containing a water-soluble pillar[5]arene and an AIEgen photosensi-
tizer [38]. However, few supramolecular polymeric materials based on pillararene were
reported [39–41], of which the mechanical performance was not good enough and need to
be improved.
Herein, we successfully prepared a new type of supramolecular polymeric material
with excellent stretchable and self-healing properties. Firstly, a bifunctional cross-linker of
host-guest supramolecule (HGS) was constructed by mixing pillar[5]arene functionalized
acrylate (AAP5A) with pyridinium functionalized acrylate (G) in acetone. Then, n-butyl
acrylate (BA) was added and further transformed into a cross-linked supramolecular
polymeric material (SP) by free radical polymerization under UV light (Scheme 1). Such
material exhibited excellent tensile properties, with maximum tensile strength of 3.4 MPa
and strain of 3000%, respectively. Moreover, this material can effectively dissipate energy
with the energy absorption efficiency of 93%, which could be applied in the field of energy
absorbing materials. In addition, the material showed self-healing property after cut and
responded to competitive guest.
Scheme 1. Schematic illustration of preparation of stretchable supramolecular polymeric material with self-healing property.
2. Results and Discussion
To construct the supramolecular crosslinker, pillar[5]arene functionalized acrylate
(
(
AAP5A) and pyridinium functionalized acrylate (G) were synthesized and characterized.
Scheme 2, Figures S1–S11, Supplementary Materials). The host-guest interaction between
1
AAP5A and G was firstly studied by H-NMR (Figure 1). Curves a and d refer to the
solution of G and AAP5A, respectively. The addition of AAP5A to G solution leads to
remarkable changes (Figure 1, curve b), that is, the signals of protons H from pyridinium
j
group and H from methylene group of G broadened severely and the signal of proton
i
H shifted upfield. According to the literature [42], the complexation induced upfield
k
shift and broadening effect were remarkable when the carbon chain was long. On the
other hand, the signal of proton H shifted downfield, due to the shielding effect of the
m
electron-rich cavities of pillararenes. All of these phenomena revealed that the pyridinium
and methylene groups threaded into the cavity of pillararenes similar to the literatures

Molecules 2021, 26, 2191
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reported [42
and H
,
m
43]. Furthermore, 2D NOESY spectrum showed the correlation peaks between
H
j−k
of G and H
of AAP5A (Figure S12), indicating that the guest molecule G
10−11
successfully entered the cavity of AAP5A, which demonstrated the host-guest interaction
between AAP5A and G. On the basis of H-NMR titration experiment (Figure S13) and the
1
nonlinear curve fitting method (Figure S14), the binding constant of AAP5A with G was
−
1
calculated to be 156
±
41 mol
·
L
. According to the literature reported, the driving force
of this supramolecular system can be attributed to the combination of hydrophobic and
cation-π interaction between electron-lacking G and electron-rich P5A cavities [44].
.
Scheme 2. Synthetic routes of AAP5A and G.
Figure 1. 1H-NMR spectra (500 MHz, acetone-d6, 298 K) of 4 mM G (
mM G + 8 mM AAP5A +20 mM G1(c) and 8 mM AAP5A (d).
a), 4 mM G + 8 mM AAP5A (b),
4
Since the cavity of AAP5A can be threaded by the pyridinium and methylene groups
of G, it is possible to construct a supramolecular cross-linker based on the pillararene-
pyridinium group recognition motif. Next, polymerization of the monomer n-butyl acrylate
(BA) was performed in acetone, using 2,4,6–trimethylbenzoyldiphenylphosphine oxide
(TPO) as the photo initiator and the host-guest supramolecule (HGS) as supramolecular
crosslinker and a supramolecular polymeric material with optical transparency (Figure 2
,
inset) was obtained. Since the polymerizable groups were the same in BA, AAP5A and
G, their reaction activities were similar, which made the component proportion in the

Molecules 2021, 26, 2191
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network facile to be adjusted by varying the feed ratio. As shown in Figure 2, the trans-
mittance reached 96% in the wavelength range of 500–800 nm. Concerned the 3D network
of supramolecular materials, FT-IR characterization was facile to take insight into this
polymerization process. As shown in Figure S15 (Supplementary Materials), curves a
and b referred to the signals of AAP5A and G, respectively. Mixing AAP5A with G lead
to some changes. As shown in Figure S15, curve c, the stretching vibration peak of C-H
− −1
1
(
3037 cm ) and the aromatic group (1581 cm ) as well as the out-of-plane bending vi-
−
1
bration absorption peak of trisubstituted C-H (894 cm ) from the pyridinium group of
G disappeared, which was caused by the host-guest interaction between AAP5A and G.
After polymerization, as shown in Figure S15, curve d, the out-of-plane bending vibration
absorption peaks of benzene group (880, 853 cm ) on the pillararene did not move sig-
nificantly, while the out-of-plane bending vibration absorption peak of C-H (810 cm
−
1
−
1
)
from C=C disappeared, indicating that the raw material basically reacted completely in
the process of photo polymerization. In addition, the stretching vibration absorption peak
−
1
of the carbonyl group upshifted from 1718 to 1728 cm , due to the disappearance of the
double bond, which destroyed the conjugated system of the double bond with the carbonyl
group. All the phenomena revealed the successful polymerization of double bond and the
formation of polymeric material.
Figure 2. UV-vis transmittance and the picture (inset) of the supramolecular material.
The density of crosslinker may have influence on the mechanical properties [45–47]. In
order to investigate it further, three supramolecular polymeric materials (named SP-1, SP-2
and SP-3, respectively) with different content of host-guest supramolecule HGS (9, 7 and
5
%) were prepared. The feed ratio was shown in Table S1 in detail. In the rheological test
0
(Figure 3), the storage modulus G of the two samples SP-1 and SP-2 were both greater than
0
0
the corresponding loss modulus G and there was no crossover within the experimental test
range, indicating that these two samples were stable. As compared, the storage modulus
G and loss modulus G of sample SP-3 crossed at the frequency of 1.00 Hz, showing
0
00
0
its gelation ability becomes weak. In addition, the rheological test showed that the G
0
0
0
00
and G of SP-1 hardly changed as the frequency varied and G >> G compared with
the other two samples, revealing SP-1 is most stable among these samples. Therefore,
with increasing of the content of HGS, the crosslinking density increased, leading to the
enhancement of strength of these supramolecular polymeric materials. Previously, we
0
reported a supramolecular gel with G of only 2 KPa by mixing pillararene-containing
copolymers with a bis(pyridinium) dication guest [40]. Interestingly, as compared, the
0
5
G of SP-1 was higher (1.5
×
10 Pa) by two orders of magnitude, indicating that mixing
host-guest first and then doing the polymerization can effectively improve the mechanical
stability of the supramolecular polymeric materials.

Molecules 2021, 26, 2191
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◦
Figure 3. Rheology results of the supramolecular material at 4 C. (SP-1 in black line, SP-2 in red line,
0
00
SP-3 in blue line; storage modulus G (ꢀ), loss modulus G (ꢁ), complex viscosity η (F)).
To compare this supramolecular polymeric material with traditional network crossslinked
by covalent crosslinker, some polymeric materials (samples 1–5) with different content of
crosslinker diethylene glycol diacrylate (DEGDA) were prepared (Table S1). To our surprise,
the covalent cross-linked polymeric materials were very brittle and could not form a fixed
shape after photo polymerization (Figure S16, Supplementary Materials). Therefore, we failed
to do the tensile tests. On the other hand, samples prepared with the ratio of HGS to BA with
1:10, 1:15, 1:20 were tough enough to form supramolecular gels, but samples with the ratio of
HGS to BA with 1:3, 1:5 could not (Table S1). According to the literature reported [39], this
may be caused by the high content of pillararenes, which brought about steric hindrance and
prevented the polymerization.
With the increasing of HGS content, the tensile strength of supramolecular polymeric
materials increased significantly from 0.4 MPa to 3.4 MPa (Figure 4, Table S2), which
was in good agreement with the rheological results. This was mainly caused by the
rigid structure of pillararenes which formed the “hard segment” in the network and
provided strong mechanical property. On the other hand, while the non-covalent cross-
linking density inside the polymeric materials increased, the modulus of the materials
increased accordingly; hence, the tensile strength of the materials increased significantly.
Learning from the literatures [48], we speculated that small clusters of supramolecular
crosslinkers with local high concentration may exist after polymerization, due to the
π-
π
conjugation effect arising from pillararenes. Therefore, wide-angle X-ray scattering
(
XRD) measurements of three samples were carried out. As shown in Figure S17, all these
◦
samples showed a characteristic peak at 2
accumulation of pillararenes arising from the
strength of the material. According to Bragg’s Law (
θ
= 22.5 , which was attributed to the ordered
π
-π
interaction and lead to the high tensile
λ
= 2dsin ), the corresponding spacing
θ
distance d was 0.395 nm. Compared with the regular structure formed by poly(ethylene
◦
glycol) (PEG) and
α
-cyclodextrin (2
θ
= 19.8 ), the spacing distance of this pillararene-
based regular structure was smaller, revealing that the supramolecular polymeric material
was more transparent than those prepared from PEG and cyclodextrin, which is usually
opaque [49].

Molecules 2021, 26, 2191
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Figure 4. The stress−strain curves of SP-1 (black curve), SP-2 (red curve) and SP-3 (blue curve).
The polymeric materials prepared in this way were very flexible as the elongations at
−
3
break varied from 400 to 3000% and the toughness of the SP-2 could reach 16.11 MJ m
·
−
3).
m
(
Figure 5), which was much greater than the polybutyl acrylate (pBA) (1.71 MJ
Moreover, necking appeared in the latter stage of stretching, which was a typical duc-
tile fracture behavior [50 51]. It was speculated that the butyl acrylate flexible group
·
,
constituted the “soft segment” of the network, which effectively relieved the rigidity of
pillararenes, acting as the “hard segment”. To demonstrate this speculation, differential
scanning calorimetry (DSC) characterizations were conducted. As shown in Figure S18,
the glass transition temperature (T
increasement of supramolecular crosslinker and the highest T
g
) of the supramolecular material increased with the
was below the room tem-
g
perature, indicating that the supramolecular crosslinked materials were flexible. Therefore,
the rigid pillararenes and soft polybutyl acrylate chains, as well as the dynamic nonco-
valent bonds played important roles in enhancing the toughness of this supramolecular
polymeric materials.
Figure 5. The toughness of SP-1 (black), SP-2 (red) and SP-3 (blue) and pBA (grey).
It is well known that the mechanical properties of materials usually involve a trade-off
between modulus and elongation. In general, strong interactions often result in higher
tensile strength with the loss of toughness, which makes the material brittle and hard.
The reversible dynamic non-covalent bond can achieve higher toughness and self-healing
properties of the material, as well as dissipate energy effectively, so it can be used as a
candidate for energy-absorbing materials [52]. As shown in Figure 6, there was an obvious

Molecules 2021, 26, 2191
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hysteresis appeared in the cyclic stress–strain test with a fixed applied strain of 100%
and the energy absorption efficiency
ω was as high as 93%, indicating that the reversible
host–guest interaction led toeffective dissipation of energy. When subjected to external
forces, the flexible host–guest supramolecules with ability to move can effectively dissipate
the external force, which could be used in the field of energy-absorbing materials.
Figure 6. The energy absorbing properties of SP-1.
The host-guest complexation is a reversible dynamic non-covalent bond, which may
enable the polymeric materials with self-healing property. In order to study the self-healing
behavior of this material, two samples of different colors (one sample was stained with
rhodamine B for better observation) were cut into two pieces with the same size and then the
different samples were put together to make the sections contacted. After 30 min, we found
that the two samples were connected as a whole (Figure 7). In contrast, the covalent bond
cross-linked samples could not form an integrity after being cut. Moreover, this reformed
material could sustain a load of 333 g and still endure a large strain (Figures S19–S20
Supplementary Materials). These results indicated that we have successfully constructed a
supramolecular polymeric material with self-healing property.
,
Figure 7. The photos of the two cut samples (left) and re-formed one (right).
This supramolecular polymeric material may be responsive to chemical stimulus
owing to the dynamic non-covalent bond. According to the literature [53], the strong
interaction between butanedinitrile (G1) and pillararene can push the pyridinium group
out of the pillararene cavity. Therefore, we choose G1 to investigate the competitive process.
1
First, the complexation behavior of AAP5A, G and G1 was studied by H-NMR (Figure 1
,

Molecules 2021, 26, 2191
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curve c). After five times equivalent of G1 was added to the mixed solution of AAP5A
and G, the signals of the protons H and H of G appeared again, the protons signal of
j
i
H and H
m
returned to the initial values, which proved that G1 and AAP5A formed a
k
stronger complex in acetone and could destroy the structure of the supramolecular cross-
linker formed by AAP5A and G. Therefore, with the addition of excessive amount of G1,
the polymeric material turned to be a flowing sol state (Figure 8), showing the stimulus
responsiveness to competitive guest molecules.
Figure 8. Reversible gel−sol transition of the supramolecular polymeric material.
3. Materials and Methods
1,4-Dibromobutane (99%), 1,6-Dibromohexane (98%), 4-methoxyphenol, acrylic acid,
paraformaldehyde (99%) and boron trifluoride etherate (48%) were purchased from Shang-
hai Titan Scientific Co. (Shanghai, China) and used as received. 1,4-Dimethoxybenzene
was purchased from Aladdin Industrial Corporation (Shanghai, China). Pyridine was
purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). TPO was purchased
from Shanghai Excellent Chemical Co. Ltd. All other solvents were obtained from commer-
cial suppliers and were used directly without further purification. Ultrapure water was
used in all relevant experiments.
1H-NMR (500 MHz) and 13C-NMR (126 MHz) spectra were recorded on a Bruker
DPX500 spectrometer (Bruker BioSpin, Switzerland) using tetramethylsilane as an inter-
nal standard in CDCl or acetone-d .MALDI-TOF was recorded on an UltrafleXtreme
3
6
III MALDI-TOF mass spectrometer (Bruker Daltonics, Germany). UV-vis spectra were
recorded on a Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan). FT-IR spec-
troscopy was obtained on a NEXUS 670 FTIR spectrometer (Thermo, USA). The rheological
properties were studied by a MARS-3 (Thermo Hakke Instruments, USA). Frequency
◦
sweeps from 0.01 to 100 Hz were performed at 0.1% strain under 4 C on circular samples
(20 mm diameter). Uniaxial tensile measurements were performed on a tensile testing
machine HY-0508 (Shanghai Hengyi Test Instrument Co., Ltd., China) equipped with a
−
1
5
0 N load cell with a crosshead speed of 30 mm min at room temperature. The testing
rectangular samples were prepared with the length of 5 mm, width of 5 mm and thickness
of 0.8~1 mm. As for the loading-unloading test and the successive loading-unloading tests,
−
1
the crosshead velocity were both kept at 30 mm min . The toughness was calculated from
the area of stress-strain curves [54]. The energy absorbing efficiency were calculated
ω
according to the calculate equation in the literature [52]. Small angle and wide angle X-ray
scattering were performed on a Smartlab SE (Bruker, Germany). All samples were prepared
with thickness of <1 mm. DSC was performed on a Q2000 DSC (TA, USA) in nitrogen
◦
◦
−1
atmosphere. All the samples were first heated from 25 to 180 C at a rate of 50 C min
and held at this temperature for 3 min to eliminate the thermal history; then, they were
◦
◦
90 to 180 C at a heating or cooling rate of
cooled to
−
90 C and heated again from
.
−
◦
−1
10 C min
4
. Conclusions
In conclusion, we have successfully constructed a new type of host–guest supramolec-
ular crosslinker based on pillar[5]arene functionalized acrylate and pyridinium functional-

Molecules 2021, 26, 2191
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ized acrylate, which can form a supramolecular polymeric material through photo poly-
merization of n-butyl acrylate. Such supramolecular polymeric material exhibited tunable
stretchable property by varying the content of the supramolecular cross-linker. In addition,
this material showed effective dissipation of energy, which could be applied in the field
of energy absorbing materials. Moreover, this material exhibited self-healing property
and stimulus response to competitive guest molecules, which were obviously different
from covalent bond crosslinked network. This work provides an effective method for
preparation of supramolecular polymeric materials with tunable stretchable, self-healing,
chemical responsive and energy dissipative properties and offers a new paradigm for
preparing multi-functional materials.
Supplementary Materials: The following are available online, Experimental details, Figures S1–S7
:
1
H-NMR, 13C-NMR spectra and MALDI-TOF of AAP5A; Figures S8–S11: H-NMR, C-NMR spectra
1
13
1
of
experiment and the nonlinear curve fitting method of AAP5A and
AAP5A HGS, and SP-1; Figure S16: The picture of the covalent cross-linked polymeric material;
G; Figure S12: NOESY spectrum of a mixture of AAP5A and
G; Figures S13–S14: H-NMR titration
G; Figure S15: FT-IR spectra of
,
G,
Table S1: Feeding ratio of polymeric materials; Table S2. Summary of mechanical properties, Table S2:
Summary of mechanical properties; Figure S17: Wide angle X-ray scattering of SP; Figure S18: DSC
curves of SP; Figures S19–S20: Self-healing property of the supramolecular polymeric materials.
Author Contributions: Conceptualization, M.Z., M.X., X.L.; methodology, M.Z., C.L.; validation,
H.H., Q.Z. and J.C.; investigation, M.Z., C.L., X.S.; writing—original draft preparation, M.Z.; writing—
review and editing, J.C. and X.L.; supervision, X.L.; funding acquisition, X.L. All authors have read
and agreed to the published version of the manuscript.
Funding: National Natural Science Foundation of China (No. 21774033) and National innovation
and entrepreneurship training program for college students (2020PY-336) are greatly appreciated for
the financial supports.
Institutional Review Board Statement: Not applicable.
Acknowledgments: We thank Mingyu Guo for kind suggestion.
Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role
in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the
manuscript, and in the decision to publish the results.
Sample Availability: Samples of the compounds AAP5A and G are available from the authors.
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