Synthesis of recyclable, chemically cross-linked, high toughness, high conductivity ion gels by sequential triblock copolymer self-assembly and disulfide bond cross-linking

Article abstract of DOI:10.1039/c4ra15095c

In this article, we report the synthesis of a disulfide bonded reversibly chemically cross-linked ion gel with high toughness and conductivity by sequential triblock copolymer self-assembly and the subsequent oxidation of thiol groups. Through reversible thiol-disulfide exchange, the ion gels had both high toughness of chemicals and recyclability of physical cross-linked ion gels. The triblock copolymer (SOS-SH) was prepared as follows: first, the RAFT copolymerization of styrene and 4-vinylbenzyl chloride (VBC) using CTA-PEO-CTA as a bi-functional macroRAFT agent was performed to obtain a triblock copolymer (SOS-Cl); then, the chloride group of SOS-Cl was replaced by an azido group to obtain SOS-N₃; and finally, the click reaction of SOS-N₃ with O-ethyl-S-prop-2-ynyl carbonodithioate and subsequent aminolysis were conducted to obtain SOS-SH. The disulfide bonded reversibly chemically cross-linked ion gel could be re-dissolved when mixed with a little amount of mild reducing agent (e.g., DTT) in CH₂Cl₂ with vigorous stirring, which reformed again after the removal of solvent and oxidation of thiol groups. The ion gels could undergo the reduction-oxidation cycle at least twice with a little loss of ionic conductivity and toughness (less than 25%), exhibiting good recyclability. Raman measurements were performed to confirm the existence and the key role of disulfide bond on the recyclability. This journal is

Full text of DOI:10.1039/c4ra15095c

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PAPER
Synthesis of recyclable, chemically cross-linked,
high toughness, high conductivity ion gels by
sequential triblock copolymer self-assembly and
disulfide bond cross-linking†
Cite this: RSC Adv., 2015, 5, 22638
a
b
b
b
b
b
Chengsha Wei, Mingming Chen, Dong Liu, Weiming Zhou, Majid Khan, Xibo Wu,
b
ab
Ningdong Huang and Liangbin Li*
In this article, we report the synthesis of a disulfide bonded reversibly chemically cross-linked ion gel with
high toughness and conductivity by sequential triblock copolymer self-assembly and the subsequent
oxidation of thiol groups. Through reversible thiol-disulfide exchange, the ion gels had both high
toughness of chemicals and recyclability of physical cross-linked ion gels. The triblock copolymer (SOS-
SH) was prepared as follows: first, the RAFT copolymerization of styrene and 4-vinylbenzyl chloride
(
VBC) using CTA–PEO–CTA as a bi-functional macroRAFT agent was performed to obtain a triblock
copolymer (SOS-Cl); then, the chloride group of SOS-Cl was replaced by an azido group to obtain SOS-
and finally, the click reaction of SOS-N with O-ethyl-S-prop-2-ynyl carbonodithioate and
3
N ;
3
subsequent aminolysis were conducted to obtain SOS-SH. The disulfide bonded reversibly chemically
cross-linked ion gel could be re-dissolved when mixed with a little amount of mild reducing agent (e.g.,
Received 24th November 2014
Accepted 6th February 2015
2 2
DTT) in CH Cl with vigorous stirring, which reformed again after the removal of solvent and oxidation of
thiol groups. The ion gels could undergo the reduction–oxidation cycle at least twice with a little loss of
ionic conductivity and toughness (less than 25%), exhibiting good recyclability. Raman measurements
were performed to confirm the existence and the key role of disulfide bond on the recyclability.
DOI: 10.1039/c4ra15095c
www.rsc.org/advances
modication of polymer, impart exibility for designing mate-
rials with desired properties. There are different routes to
Introduction
Ion gels, polymeric networks swollen with a substantial amount prepare ion gels, such as the in situ polymerization of monomer/
5
of ionic liquid (IL), have attracted considerable interest because cross-linker mixtures in ionic liquids, chemically reacting
6
of their use in electrochemical devices, including actuators, difunctional macromolecules with functional cross-linkers,
7
super-capacitors, solid electrolytes, transistors, batteries, solar and the swelling of elastomeric lms. As an alternative way to
cells, gas separation membranes, and organic gate dielectrics, develop ion gels, the self-assembly of triblock copolymers in
in recent years due to their unique physicochemical properties ionic liquids has been proven to be particularly effective and
8–10
such as high ionic conductivity, thin-lm forming ability, ex- versatile.
Usually, the triblock copolymers are ABA types.
ibility, transparency, non-volatility, non-ammability, and wide The ionic liquid insoluble A blocks aggregate to form cross-links
electrochemical window, as well as exceptional thermal and and the soluble B blocks/ionic liquid form a continuous phase.
1–4
(
electro)chemical stability. In addition, the various combi- The properties of ion gels, such as gel modulus, minimum gel
nations of different ionic liquid and polymer, as well as the easy concentration and ionic conductivity, may be tuned over broad
ranges by various methods, including the simple adjustment of
the lengths of the IL-philic B and IL-phobic A blocks, the
a
concentration of polymer and variation of the sequences. The
CAS Key Laboratory of So Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, China. E-mail: lbli@ ion gels produced by the self-assembly of ABA triblock copoly-
ustc.edu.cn; Fax: +86-551-5141078
mers were usually physically cross-linked gels with non-covalent
network structure. These physical gels could be processed
b
National Synchrotron Radiation Lab and College of Nuclear Science and Technology,
University of Science and Technology of China, No. 96, Jinzhai Road, Hefei, Anhui
as liquids, then solidied in situ, and made reversible upon
2
30026, China
11
12
external stimulus such as light and temperature. However,
just because of the reversible noncovalent cross-linking, the
†
Electronic supplementary information (ESI) available: FT-IR spectra of SOS-N
3
1
and SOS-triazole; integration of H-NMR spectra of SOS-Cl and SOS-N
3
; GPC
traces of CTA–PEO–CTA, SOS-Cl and SOS-N ; Raman spectroscopy of SOS-SH; physical gels lacked toughness when subjected to large strains,
3
XPS of S2p of SOS-SH and ion gel (0); disulde bond structure of SOS-SH, ion
10,13,14
which was due to the chain pull-out of the A blocks.
To
gel (0), ion gel (1) and ion gel (2); tensile test results. See DOI: 10.1039/c4ra15095c
22638 | RSC Adv., 2015, 5, 22638–22646
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solve this issue, Yuanyan Gu et al. permanently chemically
cross-linked the A block aer the physical ion gels were formed
from an ABA triblock copolymer. Because the chemical cross-
Experimental
Chemical and materials
linking was conned to the non-conducting A domains, the All the commercially available reagents were used as received
ionic transport in the conducting B + IL continuous phase unless otherwise noted. Poly(ethylene oxide) (PEO) (Mn ¼ 35
15
was not affected. However, just because of the irreversible kDa, Đ ¼ 1.04) was purchased from Sigma-Aldrich and dehy-
permanent chemical cross-linking, the ion gels could not be drated by freeze-drying before use. 1-Ethyl-3-methylimida-
recycled any more.
zolium bis-(triuoromethyl) sulfonyl amide ([EMI][TFSA], ionic
Dynamic covalent bonds are covalent bonds, which could liquid), propargyl bromide (80% solution in toluene), dithio-
break and reform reversibly under suitable conditions with threitol (DTT) and 4-vinylbenzyl chloride (VBC) were purchased
16–19
minimal side reactions.
They combine the reversibility of from Aladdin-Reagent. All the other chemicals were purchased
supramolecular non-covalent bond and the robustness of from Sinopharm Chemical Reagent Co., Ltd., China. Styrene (St)
covalent bond, although with generally relatively slower kinetics was puried by distillation under reduced pressure, and
17
of bond cleavage and formation. Dynamic covalent bonds 4-vinylbenzyl chloride (VBC) was passed through activated
have increasingly been exploited for constructing diverse func- alumina columns prior to use. Dichloromethane and triethyl-
tional polymers such as thermo-, chemo-, mechano-, and photo- amine were dehydrated before use.
20–22
responsive dynamic polymers.
As one of the dynamic cova-
lent bonds, disulde bonds are well known to be reduced to
Analyses
thiols in the presence of various reducing agents. The resulting
1
H-NMR spectra were obtained on a Bruker Avance 400
23
thiols can be reversibly oxidized to disulde bonds. Due to this
reversible nature and mild transformation conditions, the
reversible exchange of thiol-disulde bond has widely been
used for the preparation of reversible polymer micelles and
(400 MHz) spectrometer using TMS as an internal reference.
The molecular weight distributions (PDI, M /M ) were deter-
mined by gel permeation chromatography (RI-GPC) with a PLgel
Mixed-C column (Agilent Technologies). Agilent Technologies
w
n
24–26
hydrogels.
It is possible and meaningful to prepare revers-
1100 Series GPC Analysis System with Isocratic G1310A pump
ible chemically cross-linked ion gels with high toughness and
high conductivity as well as with recyclability through the
disulde bonds because of their capacity to reversibly break and
reform.
ꢀ
and RI-G1362A refractive index (RI) detector (set at 35 C) was
used. The eluent was tetrahydrofuran (THF) at a ow rate of
ꢁ
1
1.0 mL min . The polydispersity indexes (PDI, M /M ) for the
w n
sample polymers were calibrated with standard polystyrene
samples. The X-ray photoelectron spectra (XPS) were recorded
on a Thermo-VG Scientic ESCALAB 250 XPS spectrometer
with an MgKa X-ray source (1253.6 eV). The Raman spectrum
was recorded using a RAMALOG 6 (SPEX, USA) with an argon
ion laser at an excitation wavelength of 519 nm. Fourier trans-
form infrared (FT-IR) spectra were recorded at room tempera-
ture using a TENSOR27 IR spectrometer (Bruker, Germany). All
the spectra were recorded at a wavenumber resolution of
Reversible addition–fragmentation chain transfer (RAFT)
polymerization is one of the most important controlled radical
polymerization methods and is widely used to synthesize
various well-dened macromolecular structures (namely, block
27–29
copolymers and star polymers) and functional polymers.
Moreover, as one of the most common click reactions, Cu(I)
catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azides
and alkynes has found wide applications, including the post-
functionalization of synthetic polymers, due to the tolerance
to numerous functional groups, high yields, requirement of
moderate reaction conditions in multiple solvents, and high
ꢁ
1
1
cm . Rheology and impedance measurements are described
in detail below.
30–32
specicity.
Combining RAFT polymerization with azide–
0
Synthesis of the chain transfer agent (CTA), (S)-1-dodecyl-(S )-
alkyne click reaction has been a powerful tool to synthesize
0
00
(
a,a -dimethyl-a -acetic acid) trithiocarbonate
various sophisticated functional polymers with well-dened
33–35
structures.
The chain transfer agent (CTA) was synthesized following a
36
In this article, we prepared a disulde bonded reversibly previously reported procedure with slight modications. In a
chemically cross-linked ion gel with high toughness and typical experiment, 1-dodecanethiol (8.08 g, 0.04 mol), acetone
conductivity by a sequential triblock copolymer self-assembly (19.24 g, 0.33 mol), and Aliquot 336 (tricaprylylmethylammo-
and subsequent oxidation of thiols to disulde bonds. The nium chloride, 0.65 g, 1.6 mmol) were mixed in a three-necked
ꢀ
ABA type triblock copolymer (SOS-SH) was synthesized by round-bottomed ask cooled to 10 C under a nitrogen atmo-
combining RAFT polymerization with azide–alkyne click reac- sphere. Sodium hydroxide solution (50%) (3.34 g, 42 mmol) was
tion. The reversibly chemically cross-linked ion gel could be added over 20 min. The reaction was stirred for an additional
re-dissolved when mixed with a little amount of mild reducing 15 min before carbon disulde (3.04 g, 0.04 mol) in acetone
agent (e.g., DTT) in CH Cl with vigorous stirring, and it (4.04 g, 0.07 mol) was added over 20 min, during which the
2
2
reformed again aer the removal of solvent and oxidation of colour turned red. Aer 10 min, chloroform (7.13 g, 0.06 mol)
thiols. The cross-linked ion gels could undergo the reduction– was added in one portion, followed by dropwise addition of 50%
oxidation cycle at least twice with a slight loss of ionic sodium hydroxide solution (16 g, 0.2 mol) over 30 min. The
conductivity and toughness (less than 25%), exhibiting good reaction was stirred overnight. 60 mL of ionized water was
recyclability.
added, followed by 10 mL of concentrated HCl to acidify the
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aqueous solution. Then, most of the residual acetone was
removed under reduced pressure. The solid was collected with a
Buchner funnel and then stirred in 200 mL of 2-propanol. The
undissolved solid was ltered-off and the 2-propanol solution
was concentrated to dryness. Then, the resulting solid was
re-dissolved in ethyl acetate and passed through a silica column
with ethyl acetate as an eluent. Aer the solvent was completely
removed under vacuum, the obtained crude product was recrys-
tallized from hexane to afford a yellow crystalline solid (about
1
5
0% yield). H-NMR (in CDCl ) of this CTA is shown in Fig. 3A.
3
Synthesis of O-ethyl-S-prop-2-ynyl carbonodithioate
O-Ethyl-S-prop-2-ynyl carbonodithioate was synthesized accord-
37
ing to a previously reported procedure with slight modica-
tions. In a typical experiment, potassium ethyl xanthogenate
ꢁ
2
(
5.00 g, 3 ꢂ 10 mol), 80% solution of propargyl bromide in
ꢁ2
toluene (5.10 g, 3.5 ꢂ 10 mol), and THF (50 mL) were placed in
a 150 mL round-bottomed ask covered with aluminum foil. The
reaction mixture was stirred overnight at room temperature. A
white precipitate was formed (KBr). Aer ltration, the reaction
mixture was diluted with 200 mL of deionized water. The product
was extracted with diethyl ether (2 ꢂ 100 mL). The etherous layer
was dried with anhydrous magnesium sulfate and concentrated.
The product was puried using silica column chromatography
ꢀ
(
petroleum ether (30–60 C): CH Cl ¼ 20 : 1) and freeze dried
2
2
1
3
to obtain a pale-yellow oil (about 60% yield). H-NMR (in CDCl ):
(
d ¼ 1.44 ppm (t, 3H), d ¼ 2.23 ppm (t, 1H), d ¼ 3.87 ppm (d, 2H),
d ¼ 4.67 ppm (q, 2H)).
Polymer synthesis
Fig. 1 Synthesis procedure of bi-functional macroRAFT agent, CTA–
PEO–CTA, and ABA type triblock copolymers, SOS-Cl, SOS-N , SOS-
3
3 3
Synthesis of SOS-N . SOS-N triblock copolymer was
synthesized using a combination of reversible addition frag-
mentation chain transfer (RAFT) polymerization and post-
triazole, and SOS-SH.
15
polymerization reaction (see Fig. 1). First, the chain transfer
0
0
00
agent (CTA), (S)-1-dodecyl-(S )-(a,a -dimethyl-a -acetic acid) tri- by quenching with ice water. The reaction mixture was diluted
thiocarbonate, was attached to a PEO precursor on both the in ca. 25 mL of CH Cl and precipitated into n-hexane twice. The
2
2
ends. In this reaction, CTA (1.62 g, 4.4 mmol) and a drop of DMF product (SOS-Cl) was then collected and freeze-dried under
were mixed with excess of oxalyl chloride (7.0 mL) in 10 mL of vacuum.
dry CH
2
Cl
2
and stirred at room temperature for 2 hours. Excess
SOS-Cl (4.50 g) and NaN
3
(0.80 g, 0.0123 mol) were then re-
ꢀ
reagents were then removed completely under vacuum, and the dissolved in N,N-dimethylformamide (ca. 200 mL) at 60
C
CTA was re-dissolved in 50 mL of dry CH
2
Cl
2
. Subsequently, and the reaction was stopped aer stirring for an additional
dried PEO (16.0 g, 0.45 mmol) and dry CH
added in one portion to the solution followed by nitrogen reaction. Most of the DMF was removed at about 120 C under
2
Cl
2
(60 mL) were 40 hours. Deionized water (ca. 5 mL) was added to quench the
ꢀ
bubbling for 60 minutes. The reaction was allowed to proceed reduced pressure. The contents were re-dissolved in CH Cl and
2 2
for 36 hours. Aer most of the solvent was removed under washed with NaCl aqueous solution twice, followed by the
vacuum, the mixture was precipitated twice in hexane to obtain addition of anhydrous magnesium sulfate to dry the organic
PEO end-capped with CTA.
3
phase. The product (SOS-N ) was obtained by precipitation in
CTA–PEO–CTA was then used as a bi-functional macroRAFT diethyl ether aer most of the solvent was removed under
agent to copolymerize styrene and VBC (molar ratio 4 : 1) into vacuum at room temperature and freeze-dried under vacuum.
the end-blocks. CTA–PEO–CTA (5.00 g, 0.143 mmol), puried
styrene (5.50 g, 0.053 mol) and VBC (2.00 g, 0.013 mol) experiment, SOS-N
Synthesis of SOS-triazole by click reaction. In a typical
(1.00 g), PDMETA (124.5 mg, 0.72 mmol),
3
were added to a 25 mL tube with a stirring bar, degassed via and O-ethyl-S-prop-2-ynyl carbonodithioate (53.0 mg, 0.32 mmol)
ꢀ
three freeze–pump–thaw cycles, sealed under vacuum and were dissolved in DMF (20 mL) at 60 C. Aer the solution was
ꢀ
heated in an oil bath at 60 C for 30 minutes to obtain a cooled to room temperature, CuI (20 mg, 0.105 mmol) was
ꢀ
homogeneous solution. The solution was then heated to 140 C added and the mixture was stirred at room temperature for 24 h.
and polymerization was allowed to proceed for 42 min, followed The crude polymer product was obtained by precipitating it
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twice in diethyl ether. Aer drying under vacuum, the SOS-
triazole was obtained as a brown powder.
Synthesis of SOS-SH. In a typical experiment, SOS-triazole
(0.25 g) and n-butyl amine (1 mL) were dissolved in 50 mL of
CH Cl and stirred for 1.5 h at room temperature. Aer excess
2
2
solvent was removed under vacuum at room temperature, the
SOS-SH was obtained by precipitation in diethyl ether and
drying under vacuum.
Ion gels preparation
Stock solutions to prepare ion gels were prepared by mixing
weighed amounts of polymer (SOS-SH, 10 wt% of ion liquid) with
an ionic liquid ([EMI][TFSA]) in CH Cl (if the SOS-SH was not
2
2
easily dissolved in CH Cl , a little amount of DTT was added (1.0
2
2
wt% of SOS-SH) and the mixture was subjected to ultrasonication Fig. 2 Schematic of the homemade miniature tensile tester: 1, 2, 3, 4,
and 5 are motor, gears, sensor, clamps, and sample, respectively.
and stirring was performed for 3 hours) and stocked in a desic-
cator. The ion gels were prepared using solvent casting. Weighed
amounts of stock solution were placed in a desiccator for 48
ꢀ
were conducted at room temperature (about 25 C). The samples
hours to remove most of the solvent. The residual solvent was
completely removed by freeze-drying for 1 hour. Completely
cross-linked samples were obtained aer allowing the gel to stand
ꢁ1
were stretched at the same rate (20 mm s ), and the instrument
measured the resulting extensional stress until the samples
broke. The results of tensile tests are listed in Table S1 (ESI†).
ꢀ
at 60 C for an additional 24 h. Samples for extensional rheology
were prepared following the same procedure for the less cross-
linked samples as above, and then pressed into a 1 mm thick
mould and annealed at ca. 60 C for 1 hour (less cross-linked) or
Impedance analysis
ꢀ
Ionic conductivities were measured by AC impedance method
2
4 hours (completely cross-linked). The samples were then care-
fully withdrawn and cut into pieces with the desired dimensions
10 mm long and 5 mm wide). To avoid moisture effects, all the
10
according to the previous literature using a CHI660C electro-
chemistry workstation (Shanghai CH Instruments Co., Ltd.,
China) with a homemade cell. Frequency sweeps were con-
(
ion gel samples were kept in a desiccator under vacuum.
5
ducted from 1 to 10 Hz with an AC amplitude of 10 mV. The cell
was composed of a Teon spacer with an inner diameter of
Reduction and reforming of cross-linking ion gels
0.7 cm and a thickness of 0.24 cm sandwiched between two
The cross-linked ion gels, dithiothreitol (DTT) (5 wt% of SOS-
stainless steel electrodes. Temperatures were controlled using
an oven. The samples were thermally equilibrated for 5 min at
each temperature prior to the measurements. Measurements
SH), and CH Cl2 were charged in a round-bottom ask and
2
subjected to vigorous stirring at room temperature for 24 h.
Subsequently, the re-cross-linked ion gel samples could be
obtained following the same procedure described above. The
second reduction and cross-linking were performed according
to the same steps.
were performed at a series of increasing temperatures from
ꢀ
3
0 to 100 C. Ionic conductivity was determined from the high
frequency plateau of the real part of the complex conductivity.
Results and discussion
Rheology
Syntheses and characterizations of chain transfer agent (CTA)
Dynamic shear rheological measurements were performed on a
rotational rheometer (TAAR2000EX, TA Instruments, New
and CTA–PEO–CTA, SOS-Cl, SOS-N , SOS-triazole, SOS-SH
3
Castle, DE) with a parallel plate geometry of 25 mm in diameter The syntheses of bi-functional macroRAFT agent, CTA–PEO–
and a gap of ca. 1 mm in a nitrogen atmosphere. Dynamic CTA, and ABA type triblock copolymers, SOS-Cl, SOS-N
, SOS-
3
ꢀ
frequency sweep measurements were carried out at 100
C
triazole, and SOS-SH, were carried out according to the route
in the linear viscoelastic regime from the frequency of 100 to shown in Fig. 1.
ꢁ
1
ꢁ1
0
.1 rad s with a strain of 3% and shear rate of 10 rad s . The
The bi-functional macroRAFT agent CTA–PEO–CTA was
less cross-linked gel sample used for the measurement was not synthesized through the esterication of hydroxyl and carbonyl
ꢀ
subjected to the annealing of 1 hour (60 C).
chloride. The successful synthesis of CTA–PEO–CTA was
1
Extensional rheological measurements (tensile tests) were conrmed by the comparison of H-NMR of CTA (Fig. 3A)
performed on a homemade miniature tensile tester at room and CTA–PEO–CTA (Fig. 3B). For the H-NMR of CTA–PEO–
38
1
temperature. A schematic of the homemade miniature tensile CTA, the multiplet peak f was assigned to the methylene group
tester used in this work is shown in Fig. 2. The sample was (–O–CH –) of PEO chain; moreover, the peaks of CTA (peaks at
2
mounted between two clamps, which were driven through the about 3.27, 1.71, 1.67, 1.26, and 0.88 ppm, compared to Fig. 3A)
rotation of the motor and gears. The force was measured through existed as well, indicating the successful attachment of CTA to
the sensor that connected the clamp and a stretching arm. Tests the two ends of PEO chain.
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1
Fig. 3 H-NMR (in CDCl
3
) of (A) chain transfer agent (CTA) and (B) bi-
functional macroRAFT agent, CTA–PEO–CTA.
The triblock copolymer SOS-Cl was synthesized by the RAFT
copolymerization of St and VBC using CTA–PEO–CTA as
bi-functional macroRAFT agent. The successful synthesis of
1
SOS-Cl was conrmed by H-NMR of SOS-Cl (Fig. 4A). Compared
1
to the H-NMR of CTA–PEO–CTA (Fig. 3B), other than the peaks
of PEO chain (about 3.64 ppm), additional broad peaks at about
4.516 ppm and 6.234–7.234 ppm also existed, which were
assigned to the methylene group (–CH –Cl) and phenyl group
2
15
of poly (St-co-VBC) attached to the two ends of PEO chain,
respectively.
1
Fig. 4
3
H-NMR (in CDCl ) spectra of ABA type triblock copolymers:
The triblock copolymer SOS-N
3
was synthesized by replacing (A) SOS-Cl and (B) SOS-N3.
the chloride group of SOS-Cl with an azide group using NaN3.
The successful synthesis of SOS-N was conrmed by comparing
3
1
H-NMR of SOS-Cl (Fig. 4A) and SOS-N (Fig. 4B). The peak of prop-2-ynyl carbonodithioate (peaks g and h, 4.49 ppm and 4.63
3
3
7
the methylene group (peak d, Fig. 4) shied from 4.516 ppm ppm, respectively), indicating the transformation of the adjacent
Fig. 4A) to 4.28 ppm (Fig. 4B), inferring the successful azide group to a triazole group, as well as the successful attach-
replacement of chloride group with azide group. The FT-IR ment of O-ethyl carbonodithioate group. All these observations
spectrum of SOS-N also showed the characteristic absorption strongly supported the successful synthesis of SOS-triazole.
(
3
ꢁ
1
peak of the azide group (about 2100 cm , Fig. S1, ESI†), which
The nal ABA type triblock copolymer SOS-SH was synthe-
was another strong supporting evidence.
sized by the aminolysis of SOS-triazole. The successful synthesis
1
The triblock copolymer SOS-triazole was synthesized by the of SOS-SH was conrmed by comparing the H-NMR of SOS-
click reaction of azide groups of SOS-N and alkynyl groups of O- triazole (Fig. 5A) and SOS-SH (Fig. 5B). As shown in Fig. 5B,
3
ethyl-S-prop-2-ynyl carbonodithioate. The successful synthesis the shi of peak g (the resulting peak g was covered by the giant
1
of SOS-triazole was conrmed by H-NMR and FT-IR spectra of peak a) and disappearance of peak h indicated the successful
1
SOS-triazole. As shown in the H-NMR of SOS-triazole (Fig. 5A), aminolysis of SOS-triazole. Because of the residual traces of
the peak of the methylene group (peak d) shied from 4.28 ppm copper salt, the resulting thiol groups were easily oxidised to
39
(
Fig. 4B) to 5.36 ppm, along with the disappearance of the FT-IR disulde bonds in air. Thus, the Raman spectrum of SOS-SH
characteristic absorption peak of the azide group of SOS-triazole did not show the characteristic scattering peak of thiol (about
ꢁ
1
(
Fig. S1, ESI†), indicating the transformation of the adjacent 2600 cm , Fig. S4, ESI†), but it exhibited the peak of disulde
1
ꢁ1
azide group to another functional group. The H-NMR of the SOS- bond (about 460 cm (Fig. 10 and S4†); this wavenumber was
triazole also showed the characteristic peak of triazole group relatively lower than the general scattering peak of the disulde
(peak f, 8.03 ppm) and some peaks were assigned to O-ethyl-S- bond and related discussion is given below in detail).
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in CH
2
Cl
2
. Aer the volatilization of the solvent, a less chemi-
cally cross-linked ion gel formed and total chemical cross-
ꢀ
linking could be obtained aer annealing for 24 hours (60 C).
The dynamic shear rheology, ionic conductivity, and tensile
measurement results of less cross-linked and completely cross-
linked ion gels are shown in Fig. 7, 8 and 9, respectively. The
numbers 0, 1, and 2 in the brackets in Fig. 7, 8 and 9 indicate the
times of reduction–oxidation cycle.
ꢀ
For the rheological measurement (Fig. 7, at 100 C), both less
cross-linked (black curve) and completely cross-linked (0) (red
0
curve) ion gels displayed higher storage modulus (G ) than loss
modulus in the measured temperature range, strongly demon-
strating the formation of gels. Aer complete cross-linking, the
gels showed a larger storage modulus than the less cross-linked
ion gels. In terms of ionic conductivity (Fig. 8), the ionic
conductivity of ion gel aer complete cross-linking (0) (red
points) was higher than the less cross-linked ion gel (black
points) at all the measured temperature points, which maybe
because of the shrinkage of non-conducting end-block domains.
Remarkably, the ion gel aer complete cross-linking (0) exhibited
almost equal ion conductivity to the pure ionic liquid (namely,
[EMI][TFSA]; magenta points), showing the excellent capability of
ionic conduction. For the tensile measurement (Fig. 9, Table
S1†), the completely cross-linked (0) (red curve) ion gels showed
almost equal average ultimate elongation to the less cross-linked
ion gels (black curve) (about 5.86, 5.9, respectively), but they
exhibited higher average tensile strength (about 93.8 kPa and
75.0 kPa, respectively).
Recyclability investigation of the disulde bonded reversibly
chemical cross-linking ion gel
The recyclability of our ion gels could be evaluated by two
factors: the allowed times of reduction–oxidation cycle and the
performance loss during each time. Our disulde bonded
reversibly chemically cross-linked ion gels could be re-dissolved
at least twice in CH Cl aer being reduced by DTT with
1
3
Fig. 5 H-NMR (in CDCl ) spectra of ABA type triblock copolymers: (A)
SOS-triazole and (B) SOS-SH.
2
2
vigorous stirring, and it could be re-oxidised to form chemically
cross-linked gels aer the solvent was removed, implying that
the ion gels could be recycled at least twice. The performance
loss of ion gels aer the rst and second reduction–oxidation
cycle were assessed by rheology, ionic conductivity and tensile
measurement (Fig. 7, 8 and 9, respectively) and the performance
of the ion gel aer cross-linking (0) was used as the reference.
As shown in Fig. 7, aer the rst and second cycles, the ion
Molecular weights (M ) and polydispersity index (PDI) of the
ABA type triblock copolymer, SOS-Cl and SOS-N3
n
The molecular weights (M ) of the triblock copolymers (SOS-Cl and
n
1
SOS-N ) were calculated from H-NMR spectroscopy (Fig. S2, ESI†)
3
by the integration of the methylene groups in PEO mid-block (peak
a) and the hydrogen on phenyl rings (peak b + c) and the methy-
lene group (–CH
based end-blocks. The polydispersity indexes (PDI) of CTA–PEO–
CTA, SOS-Cl and SOS-N were determined by gel permeation
2
–Cl or –CH
2
–N
3
, peak d) in the poly(St-co-VBC)-
0
00
ꢀ
gels still displayed gel states (G > G ) even at 100 C in the
0
measured frequency range. The storage modulus (G ) of the ion
3
chromatography (GPC, Fig. S3, ESI†). The results are listed in
Table 1.
Table 1
M
n
and PDI of CTA–PEO–CTA, SOS-Cl and SOS-N
3
M
n
(kDa) (kDa)
M
n
Preparation and characterization of disulde bonded
reversibly chemical cross-linking ion gel
Polymer
poly (St-co-VBC)
PEO
PDI
The preparation of disulde bonded reversibly chemically CTA–PEO–CTA
—
3.5
3.5
35
35
35
1.17
1.19
1.27
a
SOS-Cl
cross-linked ion gel was carried out according to the procedure
described above, which is graphically shown in Fig. 6. The tri-
SOS-N
3
a
block copolymer, SOS-SH and the ionic liquid were rst mixed
n
The M of each end-block.
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Fig. 6 Schematic preparation procedure of disulfide bonded cross-
linked ion gel combining the self-assembly of ABA type triblock
copolymer in ionic liquid and reversible chemical cross-linking.
Fig. 9 Stress–strain relationships for less cross-linking (black), after
cross-linking (0) (red), after cross-linking (1) (blue), and after cross-
linking (2) (dark cyan) 10 wt% SOS-SH ion gel at room temperature.
The numbers (0, 1, and 2) in the brackets indicate the times of
reduction–oxidation cycle.
average tensile strength decreased a little to 86.5 kPa (blue
curve) from 93.8 kPa (red curve) and the average ultimate
elongation also decreased by 13% (from 5.9 to 5.1) aer the rst
0
00
Fig. 7 Variations of storage modulus (G ) and loss modulus (G ) as cycle compared to the ones aer cross-linking (0). However,
functions of angular frequency (u) at 100 C for less cross-linking aer the second cycle (dark cyan curve), the tensile strength
ꢀ
(
black), after cross-linking (0) (red), after cross-linking (1) (blue), and
after cross-linking (2) (dark cyan) 10 wt% SOS-SH ion gel. The numbers
0, 1, and 2) in the brackets indicate the times of reduction–oxidation
increased by 4% (from 93.8 to 97.5 kPa) and the ultimate
elongation decreased by 15% (from 5.9 to 5). This showed that
the strength (tensile strength and storage modulus) increased
(
cycle.
(less than 5%) aer the reduction–oxidation cycle, but the
toughness (ultimate elongation) decreased (less than 15%). In
terms of the ionic conductivity (Fig. 8), aer the rst (blue
points) and second cycle (dark cyan points), the ionic conduc-
tivity of the ion gels was maintained almost identical to the one
ꢀ
aer cross-linking (0) at higher temperatures (from 50 C to
ꢀ
1
00 C), but it slightly decreased (less than 25%) at lower
ꢀ
ꢀ
temperatures (from 25 C to 40 C).
All these observations showed that aer two reduction–oxida-
tion cycles, the performance of ion gels did not undergo consid-
erable loss (less than 25%), and the strength even increased by
about 5%, inferring the good recyclability of this ion gel.
Raman spectra characterization of the disulde bond in the
ion gels
Although the fact that the ion gels could not be re-dissolved in
CH Cl aer cross-linking and could be re-dissolved in CH Cl
2
2
2
2
Fig. 8 Temperature dependence of ionic conductivity for ionic liquid
aer the mild reducing agent DTT was added strongly demon-
strated that cross-linking was caused by the formation of
disulde bond, more clear evidence was needed. Raman spec-
troscopy is an important method for determining the presence
of disulde bonds. To conrm the existing and key role of the
disulde bond, the Raman spectra of most of the intermediate
(
(
[EMI][TFSA]) (magenta), less cross-linking (black), after cross-linking
0) (red), after cross-linking (1) (blue), and after cross-linking (2) (dark
cyan) 10 wt% SOS-SH ion gel. The numbers (0, 1, and 2) in the brackets
indicate the times of reduction–oxidation cycle.
gel aer the second cycle (the dark cyan curve) was a little larger products were recorded and the result is shown in Fig. 10. The
than the one aer the rst cycle (the blue curve) and both of characteristic scattering peak of S–S stretching vibration in the
ꢁ
1
40
them were also a little larger than the one aer cross-linking (0) Raman spectrum is located in the region of 430–550 cm
red curves). For the tensile measurement (Fig. 9, Table S1†), the Thus, we focused on this region.
.
(
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As shown in Fig. 10, the Raman spectra of SOS-N
3
and SOS- other peaks are compared with the ones of SOS-SH and ion gel
triazole did not display obvious peaks in this region, tting with (0), especially the peaks in the black ellipses in Fig. 10. The shi
no disulde bond structure in their structural formula. The of the peak may be because of the change of the disulde bond
42
obvious peak in this region rst appeared in the spectrum of dihedral angles when its structure changed from that shown in
ꢁ
1
SOS-SH, which was located at about 460 cm . Because of the Fig. S6A (ESI†) to that shown in Fig. S6B (ESI†) aer the addition
presence of residual traces of copper salt, the resultant thiols of DTT. All these observations give the direct evidence of the
were easily oxidised to disulde bonds in air and the spectrum existence of the disulde bond, strongly conrming the key role it
(
Fig. S4, ESI†) did not show the characteristic scattering peak of plays in the cross-linking and recyclability of the ion gels.
ꢁ1
S–H stretching vibration (at about 2600 cm ). Although this
value was relatively lower than the general scattering peak value
Conclusions
ꢁ
1
of disulde bond (about 500 cm ), we still consider it to be
the scattering peak of disulde bond because this value was In conclusion, we have prepared a disulde bonded reversibly
ꢁ
1
located in the desired region (430–550 cm ), and the XPS of chemically cross-linked ion gel by sequential triblock copolymer
S2p of SOS-SH (Fig. S5, ESI†) displayed the peaks assigned to self-assembly and subsequent oxidation of thiols. The triblock
41
–
S–S– or S–H. The spectrum of ion gel aer cross-linking (ion copolymer SOS-SH was synthesized by combining RAFT poly-
1
gel (0)) also exhibited this peak at the same position. The merization and click reaction. H-NMR, FT-IR, Raman spectra,
spectra of ion gels aer the rst and second reduction–oxida- XPS, and GPC strongly conrmed the successful synthesis and
tion cycles (ion gel (1) and ion gel (2)) also displayed the peaks of relative low DPI of SOS-SH. The results of dynamic shear rheo-
ꢁ
1
0
00
disulde bond, but shied to 495 cm (the red circles in logical measurements conrmed the formation of gels (G > G ).
Fig. 10). Although the peaks were not very obvious, we could The rst cross-linking ion gels showed high tensile strength
consider it to be the scattering peak of disulde bond when the (about 0.1 MPa), high ultimate elongation (about 6), and high
ꢁ1
ionic conductivity (about 7 mS cm ). Aer two reduction–
oxidation cycles, the performance loss (toughness and ionic
conductivity) of the ion gels was little (less than 25%) and the
strength even increased by about 5%, inferring the excellent
recyclability of this ion gel. The Raman spectra gave the direct
evidence of the existence of the disulde bond, conrming the key
role it played on the cross-linking and recyclability of the ion gels.
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (51325301, U1232129, 11275205 and 11204285),
9
73 program of MOST (2010CB 934504), special nancial
grant from the China postdoctoral science foundation
2012M521255), and the Fundamental Research Funds for the
(
Central Universities (WK2310000025).
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1
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