Self-healing and moldable material with the deformation recovery ability from self-assembled supramolecular metallogels

Article abstract of DOI:10.1039/c4cc06154c

A self-assembled non-covalent metallogel system with self-healing, deformation recoverable, moldable and bottom-up load-bearing properties was prepared using tetrazolyl derivatives and Pd(OAc)₂. This journal is

Full text of DOI:10.1039/c4cc06154c

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Self-healing and moldable material with the
deformation recovery ability from self-assembled
supramolecular metallogels†
Cite this: Chem. Commun., 2014,
0, 12847
5
Received 6th August 2014,
Accepted 4th September 2014
a
ab
ab
a
a
Liwei Yan,* Shaohua Gou, Zhongbin Ye, Shihong Zhang and Lihua Ma
DOI: 10.1039/c4cc06154c
www.rsc.org/chemcomm
A self-assembled non-covalent metallogel system with self-healing,
deformation recoverable, moldable and bottom-up load-bearing
2
properties was prepared using tetrazolyl derivatives and Pd(OAc) .
Materials that are capable of autonomous healing upon damage are
being developed rapidly due to their numerous potential applica-
tions (e.g. wound or fracture healing). Over the past few decades,
Scheme 1 Chemical structures of ligands 1–5.
1
considerable efforts have been made to introduce covalently cross- To the best of our knowledge, 2 has been the lowest molecular
linked polymers or supramolecular polymers into the self-healable mass organic unit reported to date without any appending
2,3
materials.
However, an external trigger (energy or healing moieties that participate in the formation of metallogels. A
reagents) is often required when a self-healing action is performed. dimethylformamide (DMF) solution of 1–4 reacted with a DMF
2
+
2
Although significant progress has been made in polymers described solution of Pd(OAc) at a 2 : 1 molar ratio of tetrazolyl unit : Pd
above, the creation of self-healing materials by non-covalent inter- at room temperature to spontaneously yield complete and homo-
actions without ‘‘polymers’’ continues to be a challenging and geneous gels (named gels 1–4) with critical gel concentrations (CGC)
10
fascinating task. In this regard, self-assembled supramolecular gels of 3, 4.5, 3.6 and 3.9% (w/v), respectively. To explore the noncovalent
are envisioned to be potential candidates for developing self-healing interactions that actually influence the formation of the metallogel,
materials due to their superior reversibility of their formation and the NH group of 1 was blocked by methyl groups to form the N-Me
tunability compared to other polymeric gels. Although numerous derivative 5. Remarkably, 5 was not capable of evolving into any gel
reports have elucidated the details of characterization and respon- with Pd(OAc)
2
under the same conditions. This observation distinctly
4
siveness of supramolecular gels, considerably less is known about demonstrated that the cooperative hydrogen bonding interaction of
5–7
their macroscopic self-healing behavior.
the NH group of tetrazoles played a key role in the formation of gels
Moldable materials that can form free-standing, shape- (Fig. S1, ESI†). In this study, gel 1 was chosen as a model system
persistent objects are currently attracting much attention in because of its relatively low CGC compared to other gels.
the soft materials area. So far, most of the materials displaying
2
The AFM images clearly revealed that 1–4 and Pd(OAc) sponta-
both moldable and self-healing properties are a few hydrogels neously self-assembled into spherical particles with uniform shape
8
,9
derived from polymers or a mixture of a dendrimer and clay,
and a limited number of supramolecular gels.
and size. Taking gel 1 as an example, the average diameter of the
5
–7c
particles estimated from the size distribution after subtracting the
11
Herein we present a readily prepared self-healing and moldable tip-broadening parameters was 35.6 nm, which is in good agree-
supramolecular metallogel system which stemmed from tetrazole- ment with the TEM results. The dynamic light scattering (DLS)
2
based ligands 1–4 (Scheme 1) and Pd(OAc) . The tetrazole-based experiment of gel 1 also confirmed the formation of supermolecular
ligands feature easy preparation, achirality, and structural simplicity. aggregates in solution with an average diameter of about 30.5 nm
(
Fig. S2, ESI†). Interestingly, the morphology evolution of gel 3
a
b
College of Chemistry and Chemical Engineering, Southwest Petroleum University,
Xindu Road, Xindu, Chengdu, 610500, China. E-mail: yanliwei@swpu.edu.cn
observed by TEM images provided considerable insight into the
formation mechanism of a gel. Fig. 1a shows the dispersed spherical
particles with a diameter of ca. 120–160 nm. However, as shown in
Fig. 1b and c, more dense particles with a diameter of ca. 160–
8
State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest
Petroleum University, 8 Xindu Road, Xindu, Chengdu, China
Electronic supplementary information (ESI) available: Synthetic procedures of
–5, AFM, TEM, DLS, rheology and swelling and transmittance studies. See DOI:
0.1039/c4cc06154c
†
1
1
1
80 nm and interconnected bundles were unexpectedly observed on
different matrices of the same copper grid. We speculated that
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 12847--12850 | 12847

View Article Online
Communication
ChemComm
Fig. 1 (a–c) TEM images (unstained) of gel 3 on different matrices on a
carbon-coated copper grid from DMF (0.02%).
Fig. 3 (a) The stress sweep data of gel 1 (4%) at a fixed frequency of
1
.0 Hz, (b) the bottom-up load-bearing and sticky properties of gel 1 (4%).
(
1) the different density of particles might be attributed to the
being liquefied to a solution that led to a sudden plummeting
of the moduli. This type of response reveals the robust and
brittle feature of this supramolecular gel. The gel also showed
special bottom-up load-bearing properties, one to three
spatulas (ca. 26 g) were inserted into the gel body and
suspended just by holding the test tube (Fig. 3b). The gel body
was able to support up to 30-fold its own weight without
causing any visible change. To our knowledge, this is the first
example that described such unique bottom-up load-bearing
strength through the insertion of the weights into the gel body,
which is probably associated with the solid-like and viscoelastic
feature of the gel, although several supramolecular gels with
concentration difference as a result of the uneven dispersion of
the sample and (2) the strain release of spherical particles and
surface defects likely accounts for the fusion of particles into
12
thermodynamically more stable bundles. Thus, it is reasonable
to presume that the initially formed aggregates interconnected
and further evolved into more dense and extended bundles, which
created an entangled network that rigidified the solvent DMF.
The transparent gel 1 could be stable for more than one year at
room temperature, and it could survive under a broad range of basic
or acidic conditions (pH = 1–14). Moreover, the gel can tolerate
heating (ca. 100 1C), shaking or sonication without any visible change
or turning into a solution, which indicated the strong intermolecular
interactions (e.g. coordination bonds, solvent-mediated hydrogen-
bonds, etc.), stability and the coordination polymeric nature of this
supramolecular metallogel. The gel could be moulded into any self-
supporting geometrical shape blocks, which demonstrated its
remarkable moldable and self-healing properties (Fig. 2). The block
showed sticky instinct by adhering to a spatula or the bottom of a
beaker (Fig. 2c and d). Interestingly, a gel sculpture (Fig. 2j) was taken
out with care from a template after aging a solution of gel 1 for
several minutes, which depicts the head portrait of a girl. Thus,
the gel could be fabricated into various geometric shapes using
appropriate molds.
6
,7c
load-bearing abilities in up-down mode have been reported.
A stir bar could also be adhered to the surface of the inverted
gel without falling down which confirmed the viscous feature
of the gel.
13
Very surprisingly, the fragments of gel 1 could be restored to the
gel phase within seconds by adding DMF or H O (Fig. 4). This result
2
gave an indication of the solvent-mediated cooperative hydrogen-
bond interactions that stabilized the gel. We rationalized that this
particular phenomenon might be related to the swelling behavior of
the gel fragments. Subsequently, a swelling experiment was con-
ducted to test this hypothesis. A gel block with original size, 9 mm
(length) ꢀ 8 mm (width), was found to expand to 12 mm ꢀ 10 mm
Frequency sweep measurements showed that the storage modulus
accordingly after immersing into DMF for several minutes (less
obvious for height). Also, the same volume solvent could be absorbed
by an integral gel upon resting overnight, which is consistent with
the swelling and restoration process (Fig. S4, ESI†). Unexpectedly, the
solvent-healed gel displayed extraordinary deformation recovery
ability by shaking and re-shaking (Fig. 4a and Movie 1, ESI†). This
process could be repeated at will, without observing any loss of
recovery ability. It is reasonable to assume that this observation
0
G approaches a plateau and is always greater than the loss modulus
00
G over the entire range of frequencies, which implies the substantial
elastic response of the sample (Fig. S3a, ESI†). In addition, stress
sweep measurements showed that both the moduli are relatively
0
constant, with G being typically an order of magnitude greater
00
than G . Notably, no typical yielding stress point was found within
the entire range of stress amplitudes (Fig. 3a, Fig. S3b and c, ESI†).
However, when the stress reached the limit of an applied stress of
3000 or 5000 Pa, the sample spilled and collapsed rather than
Fig. 4 (a) The solvent-healing and deformation recovery process of the
broken gel 1 (4%, 1 mL DMF), (b) the large area damage of gel 1 (3%, 0.5 mL
Fig. 2 (a–i) Shape-persistent, free-standing objects moulded from gel 1
4%), (j) a gel sculpture made from 4% DMF solution of gel 1.
DMF) was repaired by adding H
healable process of cross rupture.
2
O (0.25 mL), (c) the sonication-assisted
(
1
2848 | Chem. Commun., 2014, 50, 12847--12850
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
assist the self-healing process. A similar healing behavior was also
observed for both the moulded blocks and the solvent-healed gel
blocks (Fig. 5d and e). Thus, the self-healing properties of this
metallogel can be accomplished by different approaches. Notably,
gels 2–4 also displayed such self-healing capability.
In conclusion, we have described the self-healing and mold-
able materials using a readily prepared supramolecular metal-
logel system. The material exhibited remarkable deformation
recovery and bottom-up load-bearing capability. Furthermore,
the macroscopic self-healing process could be achieved using
multiaccessible approaches when damaged. So far no other
self-assembled supramolecular gels have been reported to
possess all these features. These results could give key clues
to the design of self-healing materials based on non-covalent
Fig. 5 (a, b) A bridge constructed by connecting freshly prepared blocks
of gel 1 can be suspended horizontally and (c) held vertically, (d) a bridge
constructed by connecting moulded gel blocks and (e) solvent-healed
(DMF) gel blocks. Red objects correspond to metallogel blocks obtained
upon doping with an aqueous solution of methyl red (ca. 0.1%).
might be associated with the polymer-like feature of the self- bonds. In addition, we foresee that this kind of metallogel
assembled metallogel because the flow of cross-linked polymers could impart outstanding chemical and physical properties of
14
is always accompanied by deformation. Generally, supramolecular transition-metal ions, and efforts are now in progress to investi-
gels formed by noncovalent bonds are extremely sensitive to external gate its catalytic applications.
stimuli and mostly exhibit reversible sol–gel transition when sub-
This work was supported by grants from the National
jected to shaking or heating. As far as we know, examples of such a Natural Science Foundation of China (No. 21102118).
supramolecular gel with solvent or mechanical stress-induced heal-
ing, deformation recovery ability and polymeric features have not
been reported yet. Further investigation demonstrated that the new
Notes and references
1
S. Burattini, B. W. Greenland, D. Chappell, H. M. Colquhoun and
W. Hayes, Chem. Soc. Rev., 2010, 39, 1973.
solvent-healed gel almost lost its viscoelastic response; however, the
viscoelastic moduli were restored to 30% of the original values after
aging for several days (Fig. S3d, ESI†). The results presented here
indicate that the healing of this diluted gel quite possibly involved a
minor proportion of recombination of broken bonds and networks
for maintaining the gel phase firstly to a major proportion after
aging. Interestingly, the recovered gel after aging still retained its
moldability and self-healing ability (Fig. 5e). The large area of
2
(a) B. J. Blaiszik, S. L. B. Kramar, S. C. Olugebefola, J. S. Moore,
N. R. Sottos and S. R. White, Annu. Rev. Mater. Res., 2010, 40, 179;
(
b) K. Imato, M. Nishihara, T. Kanehara, Y. Amamoto, A. Takahara
and H. Otsuka, Angew. Chem., Int. Ed., 2012, 51, 1138.
3
(a) P. Cordier, F. Tournilhac, C. Soulie-Ziakovic and L. Leibler, Nature,
2008, 451, 977; (b) M. Burnworth, L. Tang, J. R. Kumpfer, A. J. Duncan,
F. L. Beyer, G. L. Fiore, S. J. Rowan and C. Weder, Nature, 2011, 472, 334;
(c) M. Nakahata, Y. Takashima, H. Yamaguchi and A. Harada, Nat.
Commun., 2011, 2, 511; (d) N. Holten-Andersen, M. J. Harrington,
H. Birkedal, B. P. Lee, P. B. Messersmith, K. Y. C. Lee and J. H. Waite,
Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 2651; (e) J. E. Martin, A. J. Patil,
M. F. Butler and S. Mann, Adv. Funct. Mater., 2011, 21, 674; ( f ) M. Zhang,
D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng and F. Huang, Angew. Chem.,
Int. Ed., 2012, 51, 7011; (g) E. A. Appel, X. J. Loh, S. T. Jones,
F. Biedermann, C. A. Dreiss and O. A. Scherman, J. Am. Chem. Soc.,
2
damage of gel 1 at CGC could also be repaired upon adding H O,
which also verified the swelling hypothesis. In addition, when
the gel was thoroughly cut into a cross rupture (Fig. 4c), it can be
healed by adding a few drops of H O to the damaged surface of
2
the gel, and the healing process could be accelerated by sonica-
tion, which likely resulted from increasing the permeating speed
of H O. To test the healing properties, an aqueous solution of
2
2012, 134, 11767; (h) T. Kakuta, Y. Takashima, M. Nakahata, M. Otsubo,
H. Yamaguchi and A. Harada, Adv. Mater., 2013, 25, 2849; (i) L. Huang,
N. Yi, Y. Wu, Y. Zhang, Q. Zhang, Y. Huang, Y. Ma and Y. Chen, Adv.
Mater., 2013, 25, 2224; ( j) J. R. McKee, E. A. Appel, J. Seitsonen,
E. Kontturi, O. A. Scherman and O. Ikkala, Adv. Funct. Mater., 2014,
methyl red (ca. 0.1%) was poured onto the surface of the
recovered gel. Although a very faint trace of the cross rupture
remained, no permeation of the red solution into it was
observed, which suggests the excellent healing properties of
the self-assembled metallogel and the biological similarity to
wound healing (Fig. S5, ESI†).
Moreover, a 3 cm long self-supporting bridge was constructed by
simply joining together the blocks of gel 1 and keeping them in contact
for several minutes at room temperature without any external stimuli
24, 2706; (k) M. Guo, L. M. Pitet, H. M. Wyss, M. Vos, P. Y. W. Dankers
and E. W. Meijer, J. Am. Chem. Soc., 2014, 136, 6969; (l) C. Yu, C.-F. Wang
and S. Chen, Adv. Funct. Mater., 2014, 24, 1235; (m) R. J. Wojtecki,
M. A. Meador and S. J. Rowan, Nat. Mater., 2011, 10, 14.
(a) S. Dong, Y. Luo, X. Yan, B. Zheng, X. Ding, Y. Yu, Z. Ma, Q. Zhao
and F. Huang, Angew. Chem., Int. Ed., 2011, 50, 1905; (b) X. Yan,
D. Xu, X. Chi, J. Chen, S. Dong, X. Ding, Y. Yu and F. Huang, Adv.
Mater., 2012, 24, 362; (c) S. Dong, B. Zheng, D. Xu, X. Yan, M. Zhang
and F. Huang, Adv. Mater., 2012, 24, 3191; (d) X. Yan, D. Xu, J. Chen,
M. Zhang, B. Hu, Y. Yu and F. Huang, Polym. Chem., 2013, 4, 3312;
4
(
e) S. Dong, J. Yuan and F. Huang, Chem. Sci., 2014, 5, 247; ( f ) Z. Qi
(Fig. 5b). Interestingly, the gel displayed excellent light transmittance
and C. A. Schalley, Acc. Chem. Res., 2014, 47, 2222.
A. Vidyasagar, K. Handore and K. M. Sureshan, Angew. Chem., Int. Ed.,
2011, 50, 8021.
P. Sahoo, R. Sankolli, H.-Y. Lee, S. R. Raghavan and P. Dastidar,
Chem. – Eur. J., 2012, 18, 8057.
(a) S. Roy, A. Baral and A. Banerjee, Chem. – Eur. J., 2013, 19, 14950;
(b) S. Basak, J. Nanda and A. Banerjee, Chem. Commun., 2014, 50, 2356;
viewed from the yellow shadow of the blocks (Fig. 5a), and even clear
words could be seen through the gel (Fig. S6a, ESI†). The resultant
bridge is strong enough to hold when suspended horizontally or
vertically from a wire (Fig. 5c). Not only the regeneration of reversible
non-covalent interactions, but also the reconnection of the network
across the interface quite possibly facilitated the autonomic
repair properties. The gel block also showed a preference for
adhering to a razor (Fig. S6b, ESI†), and this sticky feature might
5
6
7
(
c) S. Saha, J. Bachl, T. Kundu, D. D. D ´ı az and R. Banerjee, Chem.
Commun., 2014, 50, 3004.
Q. Wang, J. L. Mynar, M. Yoshida, E. Lee, M. Lee, K. Okuro,
K. Kinbara and T. Aida, Nature, 2010, 463, 339.
8
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 12847--12850 | 12849

View Article Online
Communication
ChemComm
9
(a) D. C. Tuncaboylu, M. Sari, W. Oppermann and O. Okay, Macro- 12 T. D. Hamilton, D.-K. Bucar, J. Baltrusaitis, D. R. Flanagan, Y. Li,
molecules, 2011, 44, 4997; (b) X. Chi, D. Xu, X. Yan, J. Chen,
M. Zhang, B. Hu, Y. Yu and F. Huang, Polym. Chem., 2013, 4, 2767.
S. Ghorai, A. V. Tivanski and L. R. MacGillivray, J. Am. Chem. Soc.,
2011, 133, 3365, and references therein.
1
0 CGCs and other concentrations are expressed in % as 100 ꢀ the ratio 13 S. Banerjee, N. N. Adarsh and P. Dastidar, Soft Matter, 2012, 8,
of the weight of the ligand and Pd(OAc) (g) to DMF volume (mL). 7623.
1 A. Ajayaghosh, R. Varghese, S. Mahesh and V. K. Praveen, 14 M. T. Shaw, Introduction to Polymer Rheology, John Wiley & Sons,
Angew. Chem., Int. Ed., 2006, 45, 7729. Hoboken, New Jersey, 2012.
2
1
1
2850 | Chem. Commun., 2014, 50, 12847--12850
This journal is ©The Royal Society of Chemistry 2014