Thermal-healable and shape memory metallosupramolecular poly(n-butyl acrylate-co-methyl methacrylate) materials

Article abstract of DOI:10.1039/c4ra02843k

A big challenge in developing stimuli-responsive materials is to integrate multiple functionalities such as shape memory property, healable ability, recyclability into a single-component material. With this purpose, we designed a novel poly(n-butyl acrylate-co-methyl methacrylate) bearing a side group 2,6-bis(1′-methylbenzimidazolyl)pyridine ligand, which is dynamically crosslinked by the metal salt zinc trifluoromethanesulfonate to obtain the metallosupramolecular polymer. The shape recovery and healing is achieved upon application of a thermal or light stimulus due to the specific metal-ligand interactions which not only serve as an "inert" crosslink network at low temperature to produce the shape recovery, but also dissociate at high temperature for healing. The healing rate is quick and the healing efficiently is close to ～90%. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02843k

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Thermal-healable and shape memory
metallosupramolecular poly(n-butyl acrylate-co-
methyl methacrylate) materials†
Cite this: RSC Adv., 2014, 4, 25486
*
Zhenhua Wang, Wenru Fan, Rui Tong, Xili Lu and Hesheng Xia
A big challenge in developing stimuli-responsive materials is to integrate multiple functionalities such as
shape memory property, healable ability, recyclability into a single-component material. With this
purpose, we designed a novel poly(n-butyl acrylate-co-methyl methacrylate) bearing a side group
2,6-bis(10-methylbenzimidazolyl)pyridine ligand, which is dynamically crosslinked by the metal salt zinc
trifluoromethanesulfonate to obtain the metallosupramolecular polymer. The shape recovery and healing
is achieved upon application of a thermal or light stimulus due to the specific metal–ligand interactions
which not only serve as an “inert” crosslink network at low temperature to produce the shape recovery,
but also dissociate at high temperature for healing. The healing rate is quick and the healing efficiently is
close to ꢀ90%.
Received 31st March 2014
Accepted 28th May 2014
DOI: 10.1039/c4ra02843k
www.rsc.org/advances
crosslinking.15 Recently, Schubert et al. reported thermally self-
healing coatings based on Fe–terpyridine binding.16 The self-
Introduction
healing property was attributed to the formation of ion clusters.
A big challenge in the development of stimuli-responsive
materials is to integrate multiple functionalities such as healing
ability, shape memory property and recyclability into a single-
component material with good mechanical properties. Healable
materials have attracted increasingly attention due to the
enormous potential in the substitution for the articial mate-
rials that always fail aer micro-cracks or macroscopic damage.
Those materials can be broadly classied into two categories:
capsule-based healing systems and intrinsic healing systems. In
the last decade, the intrinsic healable polymers based on
dynamic covalent bonds1–3 and non-covalent reversible inter-
actions such as hydrogen bonding,4 host–guest interaction5 or
p–p stacking6 are of great interest owing to its dynamic nature
which could be utilized to heal for multiple times. Among the
non-covalent dynamic bonds, metal–ligand interactions,
though owns its specic properties in various applications,7–12
have been seldom utilized in developing healable supramolec-
ular materials, particularly for solid materials. Rowan et al.
reported the microphase-separated metallosupramolecular
plastic that has the crack healing function by absorbing ultra-
violet light to induce the dissociation of metal–ligand bond.13
Harrington et al. developed a pH-responsive metal–ligand
crosslinked polymer hydrogel.14 Weng et al. developed healable
metallosupramolecular gels based on polymers bearing 2,6-
bis(1,2,3-trizaol-4-yl)pyridine ligand as side group for
Nie also developed a healable gel based on the complexation of
poly(acrylic acid) and Fe3+
.
17
Encouraging progress in self-healing materials has been
witnessed in the last decade, in some situations, however,
damage in polymers oen occurs in company with macroscopic
deformation. Generally, the crack surfaces cannot autono-
mously come together to close proximity for healing without
manual intervention. In some practical applications, the
damaged and deformed parts cannot even be taken out for
repairing. On the other hand, the lack of self-repair property in
shape memory polymers always leads to limited service life,
energy wasting and environmental issues. To meet this chal-
lenge, the shape memory function is introduced to the healable
polymers. Shape memory alloy wires were embedded into self-
healing polymers to improve the healing efficiency.18 Mather
et al. studied an interpenetrating network composed of cross-
linked PCL, which functioned as the xed phase for shape
memory, and linear semi-crystalline PCL, which acted as the
healing agent for self-healing.19 Recently, a shape-memory
assisted self-healing coating based on semi-crystalline PCL and
epoxy was reported.20 A thermally healable and shape memory
polyurethane containing Diels–Alder bonds was developed by
Wang.21 Rowan and colleagues developed a light-triggered self-
healing and shape memory polymer containing disulde
bonds.22 Zhao investigated a light-triggered shape memory and
healable polymer based on crosslinked crystallizing polymer
and gold nanoparticles.23 Up to now, most of the materials
possessing shape-memory assisted healable property are multi-
component materials. The metal–ligand crosslinked polymer
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute,
Sichuan University, Chengdu 610065, China. E-mail: xiahs@scu.edu.cn
† Electronic Supplementary Information (ESI) available. See DOI:
10.1039/c4ra02843k
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with both shape memory and healable properties has not been
reported.
Also, polymer materials with triple shape memory func-
tionality might enable complex, active deformations on
demand, which have potential applications in intelligent
medical devices, textile and assembling systems.24 Though the
great progress in shape memory polymers has been made,25,26
the triple shape memory polymers based on metal–ligand
interactions have not been reported up to now.
Herein, aiming at a single-component metallosupramolecular
polymer which possesses the shape memory, healable and triple
shape memory multifunctions, we introduce the dynamic metal–
ligand crosslinks into molecular structure of the intrinsic shape
memory polymer. As an example to illustrate this concept, the
shape memory poly(butyl acrylate-co-methyl methacrylate) (pol-
y(BA-MMA)) was selected because it has widely applications such
as biomedical stents, coatings, etc.27 The 2,6-bis(10-methyl-
benzimidazolyl)pyridine (Mebip) ligand was used because of its
capability of complexation with various metal ions and the
reactivity of hydroxyl Mebip with the acrylic monomer, as well as
thermal and light stimuli responsiveness.13a Recently, the effect
of different metal salts on the morphology and mechanical
properties of metallosupramolecular poly(n-butyl acrylates) was
systematically studied by Jackson and coworkers.28 However, no
results on the healing properties of this material were reported.
Our system has the following advantages: (1) the material reaches
high healing efficiency (ꢀ90%) aer only 4 min light irradiation
or 25 min thermal treatment. Also, shape memory and healing
can be achieved based on such materials due to following
features: (i) the dynamic characteristic of metal–ligand bond, i.e.
dissociation at higher temperature and re-formation at lower
temperature, can endow the materials the self-healing function.
Scheme
1
Preparation of metallosupramolecular poly(BA-MMA)
copolymer containing Zn–Mebip bonds.
(ii) the dynamic crosslinked network can serve as the xed phase copolymer was mainly produced (Fig. S10†). And the determined
at lower temperature to produce the shape recovery, which is molar mass (Mn) of the copolymer CP-3, CP-5, and CP-7 from
different from the blending system developed by Rowan et al., in GPC is 13515, 9815, and 9781 g molꢁ1 respectively (Table S4†).
which the dynamic metal–ligand bond serves as the reversible The poly(BA-MMA-Mebip) copolymers are further crosslinked by
phase for shape memory, while the covalent crosslinked network adding metal salt zinc triuoromethanesulfonate (Zn(OTf)2) to
was used as the xed phase.12 (2) The incompatibility of metal– obtain the crosslinked metallosupramolecular poly(BA-MMA).
ligand cluster and polymer chain could produce a microphase- The crosslinking by Zn(OTf)2 leads to the formation of a gel via
separated structure consisting of dispersed hard-domain nano- metal–ligand interactions. As a control experiment, we also used
spheres and continuous so polyacrylates matrix, which enables the europium triuoromethanesulfonate (Eu(OTf)3) as a cross-
the triple shape memory. (3) The controllable mechanical prop- linking agent. Aer adding the Eu(OTf)3, the Eu-based copol-
erties and healing efficiency can be realized by varying the ymer mixture turns highly viscous rather than a gel state like Zn-
monomer ratio or the metal salts type.
based system, indicating the weaker binding of Eu–Mebip
compared with Zn–Mebip. To prepare the solid MSP, the solvent
in the gel was removed in vacuum. The obtained solid polymer
was further compression molded into a rectangle sheet of 1 mm
Results and discussion
The synthesis of the copolymer poly(BA-MMA-Mebip) is illus- thickness. The corresponding crosslinked MSP materials with a
trated in Scheme 1. The detailed procedures are described in the Mebip mole percentage of 3.2%, 5.3% and 7.6% were designated
ESI.† The corresponding copolymer is denoted as CP-3, CP-5 and as MSP-3, MSP-5 and MSP-7 respectively. The prepared control
CP-7 according to the feeding content of the ligand. The mole sample Eu–Mebip bonded poly(BA-MMA) materials was desig-
percentage of Mebip in the nal copolymer chain is ꢀ3.2%, nated as “Control-1”. For comparison, and the normal ethyl-
ꢀ5.3%, and ꢀ7.6% respectively as calculated from proton NMR eneglycol dimethacrylate (EGDMA) chemically crosslinked
spectra (Fig. S8 and Table S1†) and the sequence distribution of poly(BA-MMA) material was also prepared and designated as
BA and MMA monomer was tried to analysize from the 13C NMR “Control-2”. The linear poly(BA-MMA) was designated as
spectra (Fig. S9 and Table S2†). The GPC trace shows one broad “Control-3” which is a owing liquid at 25 ꢂC and cannot be
peak for each CP series copolymer, indicating just one kind of processed into solid material.
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Rheological properties of metallosupramolecular gels
To investigate the role of Zn–Mebip crosslinks within the
polymer, rheometer tests were conducted. The dried metal–
ligand crosslinked polymer (MSP-5) was re-swelled for 24 h in
1,1,2,2-tetrachloroethane to form a transparent gel. Fig. 1a
shows the storage modulus G0 and loss modulus G00 or MSP
series samples with the time. The storage modulus (G0) of MSP-
3, MSP-5, and MSP-7 reaches ꢀ230 Pa, ꢀ2300 Pa and ꢀ5500 Pa
respectively. The metal–ligand interaction plays a vital role in Fig. 2 (a) DSC curves of CP series and MSP series with various ligand
contents. (b) DMTA traces of MSP series with various amounts of
Zn–Mebip bonds, the peaks of tan d were fitted by Gaussian function
the construction of the supramolecular dynamic cross-linking
network. Higher Mebip content results in a stronger storage
to ascertain the Tg2
.
modulus due to a higher crosslinking density. To illustrate the
reversibility of the Zn–Mebip interactions, we conducted the
sol–gel experiments. As shown in Fig. 1b, aer treatment at
140 ꢂC (corresponding to the ow temperature of MSP-5 as
disclosed below) for 10 min, the formed gel turns into a viscous
and owable sol due to the collapse of the supramolecular
network resulted from the cleavage of metal–ligand bonds.
Upon cooling, the gel appears again, due to the rebuilding of the
crosslinking network arising from the recombination of metal–
ligand interactions.
content. The Tg1 signals for solid MSPs in DSC traces become
weaker as increasing the metal–ligand content. The possible
reason is that the crosslinked network will restrict the segment
movement to some degree. For solid MSP series, DMTA curves
(Fig. 2b) reveal two different phase transition temperatures,
together with the viscous ow temperature. The rst one is glass
transition temperature of free so polyacrylate chains (Tg1), and
the second one is dened as Tg2, possibly associated with the
glass transition temperature of the molecular chains in the
metal–ligand rich hard domains resulted from microphase
separation, and the third one is the ow temperature (Tf) which
is associated with the breakage of dynamic metal–ligand
network and polymer liquefying. The three temperatures are key
factors for shape memory and healing and triple shape memory.
The Tg1 determined by DMTA for MSP-3, MSP-5 and MSP-7 are
ꢀ24.0 ꢂC, ꢀ39.0 ꢂC and ꢀ46.8 ꢂC, respectively (Fig. 2b). The Tg2
was determined by a Gaussian tting for the tan(d) peak as
shown in Fig. 2b. The Tg2 for MSP-3, MSP-5 and MSP-7 is
ꢀ68.4 ꢂC, ꢀ79.1 ꢂC for and ꢀ83.0 ꢂC respectively. The Tf can
provide the estimation for the dissociation temperature of the
metal–ligand bond. When the temperature is higher than Tf, the
materials start to ow, leading to a rapid decrease in the storage
modulus. The Tf for MSP-3, MSP-5 and MSP-7 are ꢀ95 ꢂC,
Microphase separation of metallosupramolecular poly(BA-
MMA) copolymer
The effect of Mebip content on the thermal behaviors of the
copolymer CP series and metallosupramolecular MSPs were
studied by differential scanning calorimetry (DSC) and dynamic
mechanical thermal analysis (DMTA). For CP series copolymers,
DSC traces (Fig. 2a) suggests that the glass transition tempera-
ture Tg1 for CP-3, CP-5 and CP-7 is ꢀꢁ0.5 ꢂC, ꢀ6.4 ꢂC and
ꢀ15.2 ꢂC respectively, showing an increase trend with the ligand
ꢂ
ꢂ
ꢀ140 C and ꢀ160 C respectively. All the transition tempera-
ture shows an increasing trend with increasing the metal ligand
content. For the controlled Eu-based MSP sample, the Tg2 is not
observed in the tan d curves of DMTA because of the absence of
microphase separation in Eu-based material (Fig. S11†).
A few researchers suggested that micro-phase separation in
Mebip-based metallosupramolecular polymer system can
produce hard micro-domains.13,28,29 In order to conrm the
occurrence of micro-phase separation; we conducted trans-
mission electron microscopy (TEM), wide angle X-ray diffrac-
tion (WAXD), and small-angle X-ray scattering (SAXS)
characterization. TEM indeed reveals
a clear two-phase
morphology for sample MSP-5, with nanospheres of an average
size ꢀ60 nm dispersed in a continuous poly(BA-MMA) matrix
(Fig. 3a). The size distribution of nanospheres is shown in
Fig. 3b. This structure is not reported in the former metal-
losupramolecular polyacrylates system16,28 and it is different
with lamellar structure revealed in the main chain metal-
losupramolecular polymers.13a The nanospheres are formed by
microphase separation, i.e. the further aggregation of Zn–Mebip
Fig. 1 (a) The storage modulus G0 and loss modulus G00 as a function of
time (strain ¼ 1%, frequency is 1 Hz) for MSP swelled in 1,1,2,2-tetra-
chloroethane with a concentration of 100 mg mlꢁ1. (b) Thermally
responsive sol–gel transition.
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Table 1 Mechanical and self-healing properties of MSPa
Young's
Tensile
Strain at
Healing
Sample
modulus (MPa) strength (MPa) break (%) efficiency*
MSP-3
MSP-5
MSP-7
3.6 ꢃ 0.2
34.6 ꢃ 2.5
143 ꢃ 10.6
4.4 ꢃ 0.2
9.7 ꢃ 0.6
12.5 ꢃ 1.5
8.3 ꢃ 0.6
9.2
242 ꢃ 12
144 ꢃ 23
63 ꢃ 25
507 ꢃ 51
50
98 ꢃ 12
88 ꢃ 18
65 ꢃ 32
7 ꢃ 3
Control-1 18.1 ꢃ 1.9
Control-2 232.3
0
a
* healing efficiency is calculated based on the recovery in the
elongation at break. Strain rate: 50 mm minꢁ1. Control-1 is Eu-based
MSP material. Control-2 is chemically crosslinked poly(BA-MMA-
EGDMA).
mechanical properties of metallosupramolecular material.
Compared with Zn-based material (MSP-5), the Eu-based
material (Control-1) exhibits lower tensile strength but a better
elasticity due to the weaker binding of Eu–Mebip bond. The
tensile strength and strain at break of MSP-5 are both almost
two-fold of the reported optically healable main-chain metal-
losupramolecular material assembled from poly(ethylene-co-
butylene) functionalized with Mebip.13a On the other hand, for
the chemical crosslinked sample (Control-2), the strain at break
decreases by ꢀ66% compared with that of MSP-5 while the
tensile strength shows almost no difference. The linear poly(BA-
MMA) without any crosslinking prepared in this study is viscous
at room temperature and could not be molded into solid
material. Therefore, it is convinced that the unique mechanical
properties of the Zn-based MSP materials are related to the
formation of microphase-separated structure composed of hard
Zn–Mebip hard micro-domains and so polyacrylate phases.
For previous healable metallosupramolecular materials,13–17
the crack interface cannot autonomously contact together once
damaged with considerable strain at break. Therefore the ability
to automatically bring the separated damaged area together
under external stimulus is of great signicance. Our prepared
MSPs have the ability to realize shape memory and healing
under thermal or light stimulus due to its special structure as
stated above. To testify this, we rstly cut the MSP specimen by
ꢀ70% depth of sample thickness, and then kept the crack
surfaces separated at a bending angle of ꢀ90ꢂ for 10 min at
room temperature. Aer removing the stress, a crack with large
deformation was produced. The shape memory and healing was
realized by simply heating the sample to 140 ꢂC for a certain
time to induce the consequent occurrence of shape recovery,
crack surfaces closure and crack healing. Fig. 4d shows the
Fig. 3 (a) TEM image of MSP-5 sample. (b) Size distribution of hard
domain in MSP-5 sample. (c) WAXD patterns of MSPs with various
Mebip contents. (d) SAXS patterns of MSP series, CP-5 and Control-1.
clusters. The microphase separation is responsible for the extra
glass transition temperature of molecular chains around the
metal–ligand rich hard domains and also it is believed that the
microphase separation contributes to an improvement in the
mechanical properties of the materials.
Fig. 3c shows that two peaks are presented in the WAXD
patterns: the peak 2q ꢀ 20ꢂ corresponds to the amorphous halo;
ꢂ
˚
another peak at d ¼ 10.4 A (2q ꢀ 8.5 ) is assigned to the metal-
to-metal distance within the hard phase on the basis of previous
research.28,29 In contrast to MSP-5, the peak of Control-1 sample
˚
at d ꢀ 10.4 A is weak, indicating a decreased packing of metal–
ligand in the Eu-based material. SAXS was used to conrm the
formation of Zn–Mebip cluster. The scattering patterns for all
MSP series samples (Fig. 3d) reveal a broad scattering peak. The
long period calculated from SAXS curve deceases from 7 nm to
5.2 nm as metal–ligand complexes content increases from 3.2%
(MSP-3) to 7.5% (MSP-7). For the scattering patterns of both Eu-
based sample (Control-1) and copolymer sample (CP-5), no
scattering peaks are observed in the SAXS curves. These results
suggest that the Zn ion plays an important role to the formation
of metal–ligand clusters.
Mechanical and healing properties
The copolymer poly(BA-MMA-Mebip) is an amorphous tacky pictures of the shape memory and healing process, and Fig. 4a
solid with very weak mechanical properties. Aer crosslinking and b are digital microscopy with a depth-of-eld. The proposed
with metal salt, the MSP series samples have good and adjust- mechanism is illustrated in Fig. 4c. For the MSP materials
able mechanical properties. The mechanical properties of MSP suffered damage with large deformation, as the sample
materials and the control samples are summarized in Table 1. temperature was heated above Tg1, the separated crack surfaces
With increasing the metal–ligand amount, the tensile strength start to approach each other. Further heating the sample will
and Young's modulus increase, while strain at break decreases. speed up the shape recovery to close the crack surfaces. When
MSP-3 is most extensible and MSP-7 is the stiffest, while MSP-5 the sample temperature reaches the temperature which is
seems to have a good balance between the strength and elon- sufficient to induce the dissociation of metal–ligand bonds,
gation. The type of metal salt has a strong effect on the network de-crosslinking, material liquefying, and consequent
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Fig. 5 Mechanical and self-healing properties for MSP series samples.
(a) Stress–strain curves of damaged MSP-5 sample healed at various
times, healing temperature T ¼ 140 ꢂC. (b) Stress–strain curves of
original, damaged and healed MSP series samples, healing time is
25 min.
ꢂ
ꢀ130 C, which is much lower than Tf of MSP-5. At a tempera-
ꢂ
ꢂ
ture between 55 C and 95 C, shape recovery is good, but the
healing efficiency is very low and can be negligible. For general
healable metallosupramolecular materials, the healing
temperature is usually high in order to achieve the dissociation
of metal–ligand bonds and better mobility of polymer molec-
ular chains, which are crucial for high healing rate and healing
efficiency. For example, the temperature of optically healable
polymer went up to 190 ꢂC and 220 ꢂC for La-based and Zn-
based system respectively upon UV irradiation.13a In another
work, the temperature required to heal the crack was above
Fig. 4 (a) 3D optical photographs of healing process of MSP-5 via
digital microscope with a depth-of-field. (b) Digital images for healing
process of MSP-5. (c) Schematic illustration for thermal-induced
shape memory assisted healing of the metallosupramolecular mate-
rials. (d) Digital pictures of shape memory assisted self-healing process
of MSP-5. Deformation and cracking at room temperature, healing
temperature is 140 ꢂC, healing time 25 min.
ꢂ
100 C for metal–ligand crosslinked polyacrylate and the heal-
ing time last for several days.16 The healing temperature for
metallosupramolecular materials is relatively high, comparable
to the melting or soening temperature for normal linear
amorphous or crystalline polymers. However the healing
mechanism is different from the thermal welding. For tradi-
fast polymer chain diffusion and re-entanglement will occur. tional polymers, the ability to heal the damage is really poor due
Upon cooling the material cracks will be healed and the to the slow rates of diffusion and re-entanglement of the chains
mechanical properties will be recovered.
which are inversely proportional to the molecular mass.30 For
The healing efficiency is calculated based on the strain normal covalent crosslinked polymer, e.g. sample “Control-2” in
recovery and the data are shown in Table 1. Fig. 5a shows that this study, it is difficult to be self-healed even under high
the stress and strain for the damaged MSP-5 specimen recov- temperature. For MSP materials, the dynamic metal–ligand
ered with the contact time. Aer heating for ꢀ25 min at 140 ꢂC, crosslinks can be broken to form the liqueed linear low-molar-
the healing efficiency of the damaged MSP-5 reaches ꢀ88%. We mass species on exposure to heat. The molecular weight of the
noted that during tensile test some healed samples did not CP series poly(BA-MMA) copolymer which can be produced aer
break at the damaged site but in other places, indicating that de-crosslinking is lower (ꢀ10 000) and thus the rates of chain
the healing efficiency is progressed to a considerable level. diffusion and re-entanglement are much faster compared to
Fig. 5b shows that the healing efficiency of MSP series with normal amorphous polymers (Mn > 100 000). This leads to a
different contents of metal–ligand bond at 140 ꢂC is different. A high healing efficiency for metallosupramolecular polymer
higher amount of metal–ligand bond leads to a lower healing materials.
ꢂ
efficiency at 140 C. This is attributed to the different temper-
All the results indicate that Tf related to the dissociation of
ature of MSP series required for the metal–ligand decom- metal–ligand bond is a key factor for healing. It must be pointed
ꢂ
plexation and material liquefying (Fig. 2b). At 140 C, MSP-3 is out that this result is different from the normal intrinsic healing
near completely liqueed and the molecular chains have the polymer in which an increase in the content of dynamic bonds
best mobility so that the dynamic metal–ligand motifs between will lead to a better healing efficiency, this is because that in our
two crack interfaces have the maximum probability to reach and system, the realization of healing is not only by the re-formation
recombine, resulting in a highest healing efficiency of ꢀ98%. of metal–ligand bonds in the crack surface, but also by the
ꢂ
While for MSP-7, it is only partially liqueed at 140 C and the chain diffusion and re-entanglement caused by the metal–
chain mobility is very limited, leading to a lower healing effi- ligand network de-crosslinking and material liquefying.
ciency of ꢀ65%. It can be noted in Fig. 5b that a lower healing
To illustrate the intrinsic nature of shape memory and
efficiency (ꢀ58%) for MSP-5 was obtained at a temperature of healing process, we also studied the healing behavior for three
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control samples. For covalent crosslinking poly(BA-MMA-
EGDMA) (Control-2), almost no healing can be observed, indi-
cating that the contribution of the molecular chain entangling
to the healing property is very limited. For Eu-based MSP
(Control-1), the healing efficiency is only ꢀ7% which is much
lower than that of Zn-based MSP-5. Europium as lanthanide
group element shows more dynamic and weaker binding with
Mebip ligand. Previous research12 showed that the temperature
(Tf) that is sufficient to disrupt the Eu–Mebip bond and make
material ow is only ꢀ40 ꢂC. The Tg of Control-1 is ꢀ43 ꢂC
(Fig. S10†). The Tf is too close to the Tg, which will hamper the
self-healing of materials. When the sample Contro-1 is heated
above its ow temperature, the Eu–Mebip interactions suffi-
ciently dissociate. Upon cooling below 40 ꢂC, the Eu–Mebip
interactions could not efficiently be recombined because the
molecular chains were frozen below the Tg. Chain mobility is a
key factor for the healing process. Therefore, for MSP materials,
Tf should be much higher than Tg to assure the necessary chain
mobility during healing to achieve the re-formation of metal–
ligand bond. Zn-based MSP material meets such a requirement
while Eu based MSP does not.
Fig. 6 Optical triggered shape memory assisted self-healing of
MSP-7. (a) Optically healing of MSP-7 on exposure to ultraviolet
radiation with a wavelength of 300–400 nm and an intensity of
127 mW cmꢁ2; (b) surface temperature of samples on exposure to
same conditions, as a function of time, determined with an infrared
camera. (c) Stress–strain curves of films of MSP-7, shown are data
for original, damaged, and healed samples (width, 10.1 mm; depth,
0.56 mm).
To explore whether the shape memory and healing process
could be triggered by light, we conducted the experiment under
UV irradiation. Aer the sample was deformed and cracked at
room temperature, it was exposed to ultraviolet radiation with a
wavelength of 300–400 nm and an intensity of 127 mW cmꢁ2. An
exposure of 4 min is sufficient to bring the crack surfaces
ꢂ
heated to 95 C, a temperature higher than Tg2, at which the
together and heal the crack (Fig. 6a). In situ measurements sample was in elastic state, and then it was shaped into a
revealed that the UV irradiation could give rise to a surface temporary twisted “V” shape under external stress. Then the
temperature rise of 185.1 ꢂC (23.9 ꢂC to ꢀ208.0 ꢂC) for MSP-7 in sample was cooled to 55 ꢂC (a temperature higher than Tg1
4 min (Fig. 6b). As the Zn–Mebip content increases, the slope of but lower than Tg2) and maintained for 10 min under external
the curve (which is proportional to the increasing rate in surface stress, aer the stress was released, twisted “V” shape was
temperature) becomes steeper. At a lower Zn–Mebip content xed with internal stress stored in the specimen. The rst
(MSP-3), the surface temperature rise decreased to 120.1 ꢂC,
indicating the vital role of Zn–Mebip interaction in the photo-
temporary shape xation is attained by the Tg2. Secondly, the
specimen was re-shaped from “V” to “S” shape under external
thermal conversion process. This is further evidenced by the stress at 55 ꢂC, the sample was then cooled to 10 ꢂC and aer
surface temperature rise ꢀ90.4 ꢂC (from 22.3 ꢂC to 121.7 ꢂC) for removing the stress, temporary shape “S” was xed with
Control-2 sample without Zn–Mebip motif during 4 min UV internal stress. The second temporary shape xation is
irradiation. Fig. 6c shows that the healing efficiency in 4 min UV attained by the frozen chain below the Tg1
irradiation is very high, close to 100%, The reason is that UV
The shape recovery process is realized by heating (Fig. 7).
induced the surface temperature rise can lead to the decom- When the sample was heated to 55 ꢂC, the frozen chains start
.
plexation of Zn–Mebip, consistent with result of the previous
to move, the temporary shape “S” was changed to twisted
shape “V”, which can be remained at 55 ꢂC. Further
increasing the temperature to 95 ꢂC, the twisted shape “V”
was changed to the original rectangle sheet shape due to the
release of internal stress. For the Eu-based MSP (Control-1),
report.13a
Triple shape memory property
Triple shape memory polymer is an important smart material in triple shape memory cannot be realized due to the lower ow
the shape memory polymers.24–26 Triple shape memory polymer temperature and the absence of microphase separation
can change shapes twice and can x two metastable shapes in (Fig. S11†).
addition to its permanent shape. Triple shape memory has not
been reported in the metallosupramolecular materials up to
now. Two reversible thermal transitions of MSP series materials
including two glass transition, together with metal–ligand bond
crosslinked network below the ow temperature, can lead to a
triple shape memory function.
The temporary shape was xed via a two-step program-
ming as follows: rstly, the rectangle sheet specimen was Fig. 7 Triple shape memory process of metallosupramolecular MSP-5.
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Synthesis of the metallosupramolecular copolymer. Typi-
cally, in a 50 ml beaker the desired amount of the self-made
Conclusions
In summary, we realized the healing and triple shape memory copolymer (CP series) was dissolved in chloroform (100 mg
based on a single-component metal–ligand crosslinked supra- mlꢁ1). The Zn(OTf)2 or Eu(OTf)3 in acetonitrile (100 mg mlꢁ1
)
molecular material. Two main structure criteria for this kind of was added into the copolymer solution. The amount of metal
materials are proposed: (1) the existence of two glass transitions salt added was adjusted to provide for metal : ligand equiva-
(Tg1 and Tg2) of polymer chains resulted from microphase lents of 1 : 2 or 1 : 3 (accounting for 1 : 2 and 1 : 3 binding,
separation; (2) the dissociation temperature of metal–ligand respectively). The mixture turned into gel in 1 min aer adding
band (Tf) must be higher than Tg1 and Tg2. As an example, the Zn(OTf)2. On addition of Eu(OTf)3, the system turned into a
metallosupramolecular copolymer poly(BA-MMA) bearing a viscous uid rather than a gel. Theꢂmixtures were then dried at
ꢂ
Mebip ligand, crosslinked by metal zinc salt was prepared and 70 C for 4 h and overnight at 80 C to remove the solvent in
investigated. This novel material has a microphase separated vacuum. The as-prepared solids were compression moulded at
ꢂ
ꢂ
two-phase structure composed of so polyacrylate phases and 170 C for Zn-based system and 130 C for Eu-based system to
metal–ligand rich hard domains, and has three reversible phase obtain semi-transparent yellow rectangle sheets with a thick-
transitions, which enables the materials with shape memory, ness of ꢀ1 mm (accounting for different bond strength of
healing and triple shape memory functions. The materials have Zn–Mebip and Eu–Mebip).
good mechanical strength and healing efficiency close to 90%,
and those properties can be systematically tuned by a diversity
of molecular parameters. The realization of healing is not only
by the re-formation of metal–ligand bonds in the crack surface,
but also by the chain diffusion and re-entanglement caused by
the metal–ligand network de-crosslinking and material lique-
fying under external thermal or light stimulus. The concept of
integrating healing and triple shape memory into a single-
component material by introducing metal–ligand bond into the
molecular structure of intrinsic shape memory polymer seems
to be applicable to a wide range of other dynamic supramolec-
ular polymer systems.
Shape memory and thermal healing test
Thermally induced shape memory and self-healing experiment
was conducted as follows: the MSP specimen was cut by ꢀ70%
depth of sample thickness. The crack depth was determined via
a razor with length scale calibrated by a caliper. The crack
surfaces were then kept separating at a bending angle of ꢀ90ꢂ
for 10 min at room temperature. Aer removing the stress, a
crack with large deformation was produced. The shape memory
and ꢂself-healing was realized by simply heating the sample to
140 C in an oven for a certain time.
Light triggered shape memory and self-healing experiment
was conducted as follows: the sample was cracked and
deformed as the procedures described above, then was exposed
to a ltered light source with wavelengths 300–400 nm at an
intensity of 127 mW cmꢁ2 for 4 min. The surface temperature
rise was in situ measured by an infrared camera.
Materials and methods
Materials preparation
The synthetic route is shown in Scheme 1. The synthesis of 4-
hydroxy-2,6-bis-(10-methyl-benzimidazolyl)pyridine (compound
1, HO-Mebip) and 2-(2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)
pyridin-4-yloxy)-nonan-1-ol (compound 2, BIP-OH) are shown in
the ESI.†
General methods
The NMR spectra were recorded at room temperature on a 400
Synthesis of 2-(2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)- MHz Brucker Avance II 400 spectrometer. Chemical shi were
pyridin-4-yloxy)nonyl acrylate (compound 3, Mebip-Ac). The referenced to the solvent values (for DMSO-d6, d ¼ 2.49; for
self-made compound 2, i.e. BIP-OH (6 g, 15.0 mmol), triethyl- CDCl3, d ¼ 7.25). Molecular weight was measured with gel
amine (3.4 ml, 24.4 mmol), dichloromethane (600 ml) were permeation chromatography (GPC, TOSOH, HLC-8320GPC)
charged to a 1 l round bottom ask, then the solution was kept with THF as the eluent at a ow rate of 0.6 ml minꢁ1 at 40 C.
ꢂ
stirring at 0 ꢂC for 2 h. The acryloyl chloride (2 ml, 24.5 mmol) Molecular weight was calibrated with polystyrene standard. The
dissolved in dichloromethane (20 ml) was added into the above photo-luminesence emission spectra were recorded at FL-4000
solution dropwise and the mixture was le to react for 4 h at (excitation wavelength ¼ 300 nm). Rheological measurements
0 ꢂC, then 24 h at room temperature. The product was collected were measured using parallel plate geometry. The plate used
and was further puried via column chromatography (CH2Cl2/ has a diameter of 25 mm and the gap is set at 1 mm for all
MeOH) using the silica as the stationary phase.
measurements. The measurement of mechanical properties was
Synthesis of poly(BA-MMA) copolymer containing ligand (CP conducted on a universal testing machine (Instron 5567, US) at
series). Butyl acrylate, methyl methacrylate, monomer room temperature with a strain rate 50 mm minꢁ1. DSC curves
compound 3 (Mebip-Ac), initiator, S,S0-bis(R,R0-dimethylacetic were measured on a Q20 instrument (TA instruments, USA). The
acid) trithiocarbonate, were charged into dry DMF solvent, then samples were annealed at 160 ꢂC for 5 min during the rst
ꢂ
the mixture was reacted at 70 C for 24 h under nitrogen. The heating ramp. Aer cooling to ꢁ30 ꢂC at a cooling rate of 10 ꢂC
polymerization recipes are listed in Table S1.† Aer polymeri- minꢁ1, the Tg was measured on the second heating ramp in the
zation, the solution was poured into cold MeOH–H2O (9 : 1) temperature range of ꢁ30–150 ꢂC at a heating rate of 10 ꢂC
mixture to obtain polymer precipitate, and then dried in minꢁ1. The dynamical mechanical tests were performed on a
ꢂ
vacuum at 50 C.
Q-800 instrument (TA Instruments, USA) with tensile mode. The
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