Reversible photo-responsive gel-sol transitions of robust organogels based on an azobenzene-containing main-chain liquid crystalline polymer

Article abstract of DOI:10.1039/c9ra10161f

Stimuli-responsive supramolecular gels have been widely investigated, but the construction of a liquid crystalline gel with a high mechanical property and reversible photo-response still remains a challenge. This is due to the difficulty of designing gelators with liquid crystal properties and gelation abilities in organic solvents simultaneously. In this study, an azobenzene-containing main-chain polyester (Azo-mLCP) with a pendant amide group was synthesized. The organogel of Azo-mLCP via a hydrogen bond in dioxane possessed reversible thermal- A nd photo-responsive behaviours. The organogel exhibited a good self-supporting ability when the concentration of the gelator was more than 7.5 wt%. The rapid trans-to-cis isomerization of Azo-mLCP in solution was studied via UV-Vis absorption spectra. In addition, the gel-to-sol transition of the organogel could be triggered efficiently by an incomplete trans-to-cis conversion strategy. This study opens a way for the main-chain liquid crystalline polymers to serve in potential applications in photo-responsive robust actuators, electro-optical devices, and so on.

Full text of DOI:10.1039/c9ra10161f

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PAPER
Reversible photo-responsive gel–sol transitions of
robust organogels based on an azobenzene-
containing main-chain liquid crystalline polymer†
Cite this: RSC Adv., 2020, 10, 3726
a
a
Jing Wang,a Qian Jiang,a Xingtian Hao,
Hongchao Yan,a Haiyan Peng,
Bijin Xiong,a Yonggui Liao
and Xiaolin Xieab
ab
*
Stimuli-responsive supramolecular gels have been widely investigated, but the construction of a liquid
crystalline gel with a high mechanical property and reversible photo-response still remains a challenge.
This is due to the difficulty of designing gelators with liquid crystal properties and gelation abilities in
organic solvents simultaneously. In this study, an azobenzene-containing main-chain polyester (Azo-
mLCP) with a pendant amide group was synthesized. The organogel of Azo-mLCP via a hydrogen bond
in dioxane possessed reversible thermal- and photo-responsive behaviours. The organogel exhibited
a good self-supporting ability when the concentration of the gelator was more than 7.5 wt%. The rapid
trans-to-cis isomerization of Azo-mLCP in solution was studied via UV-Vis absorption spectra. In
addition, the gel-to-sol transition of the organogel could be triggered efficiently by an incomplete trans-
to-cis conversion strategy. This study opens a way for the main-chain liquid crystalline polymers to serve
in potential applications in photo-responsive robust actuators, electro-optical devices, and so on.
Received 4th December 2019
Accepted 7th January 2020
DOI: 10.1039/c9ra10161f
rsc.li/rsc-advances
with photo-isomerization are capable of tailoring the chemical
and physical properties, which allow the applications of photo-
Introduction
responsive supramolecular organogels in drug deliveries, so
actuators, optical devices, self-healing materials, etc. A self-
healing organogel of azobenzene-graed poly(carboxylic acid)
in DMF driven by hydrogen bonds and cation–p interactions
has been achieved.9 Besides, a photo-responsive dextran
hydrogel constructed via the host–guest interactions between
trans-azo and cyclodextrin in the pendent polymer chains has
been employed for light-controlled protein release.10 In addi-
tion, a gel from poly(amide-triazole) bearing azobenzene units
along the main chain has a reversible gel–sol transformation,
which can be smoothly triggered by UV and visible light irra-
diation, but the gel is so with a saturated storage modulus of
less than 103 Pa.11
In recent years, azobenzene-containing liquid crystalline
polymers (LCPs) have drawn considerable attention because
they combine the intriguing photo-responsive property and
anisotropic liquid crystalline behaviours related to the rod-like
trans-azo mesogens.12–14 So far, a great number of LCPs with
azobenzene mesogens have been reported. The mechanical
properties and photo-responses of azobenzene LCPs are
strongly related to the position of the azobenzene group, i.e.
whether in the main chain or in the side chain.15 The
azobenzene-containing main-chain LCPs, in comparison with
the side chain ones, display more prominent mechanical
properties and more efficient trans-to-cis isomerization. A
nematic monodomain liquid crystalline elastomer (LCE) based
on mixed main- and side-chains showed that the elongation
A stimuli-responsive supramolecular gel is a kind of interesting
material that can respond to external stimuli, such as temper-
ature, light, pH and magnetic elds.1,2 Among these triggers,
light has attracted considerable interest as it is can be localized
in space and precisely controlled without a direct contact.3,4
Moreover, the photochemical processes do not require extra
additives, and by-products are limited in most cases. Tradi-
tionally, a certain class of photochromic compounds, such as
azobenzene, spiropyran5 and diarylethene,6 can be covalently
conjugated with non-photoresponsive building blocks to ach-
ieve photo-tunable molecular assemblies.7 The photo-induced
and thermal geometric isomerization of azobenzene, a wonder-
ful photo-responsive group, has been extensively studied over
the last four decades.8 The key attractive feature is the unique
reversible trans–cis–trans isomerization cycles upon exposure to
UV and visible lights. The conguration changes accompanied
aKey Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of
Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of
Chemistry and Chemical Engineering, Huazhong University of Science and Technology,
Wuhan 430074, China. E-mail: ygliao@mail.hust.edu.cn
bNational Anti-counterfeit Engineering Research Center, Huazhong University of
Science and Technology, Wuhan 430074, China
† Electronic supplementary information (ESI) available: The a, b and p* values for
different solvents; summary of the Ktc, tN and P of UV light irradiation; 1H NMR
spectra of Azo-mLCP in d8-dioxane at different concentrations; POM images of
Azo-mLCP organogel before and aer UV light irradiation. See DOI:
10.1039/c9ra10161f
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increased with the proportion of the main-chain component calculated for [M (C12H10N2O2) + H]+, 215.1; found 215.1. Mp:
versus the side chain one.16 Azobenzene-containing main-chain 204 C.
ꢀ
LCPs displayed a huge response due to the prominent backbone
anisotropy and large coupling constant to the underlying order
parameter.15
Synthesis of Azo-C12
4,4-Dihydroxyazobenzene (5.64 g, 26.40 mmol) and K2CO3
(36.4 g, 264 mmol) were added in 100 mL of dry DMF, and
stirred for 15 min. Then 2-(2-(2-chloroethoxy)ethoxy)ethanol
(13.28 g, 79 mmol) and potassium iodide (0.035 g, 0.21 mmol)
were added to the reaction mixture. Aer reuxing for 24 h, the
reaction was ltered and spun-dried. The residue was dissolved
in dichloromethane, and then separated with dilute hydro-
chloric acid and washed with distilled water three times. The
crude product was recrystallized from chloroform aer the
solvent was removed by evaporation. An orange product was
obtained. (5.02 g 42% yield) 1H NMR (400 MHz, d6-DMSO, TMS):
d (ppm) ¼ 7.85–7.82 (d, 4H, Ar–N), 7.14–7.11 (d, 4H, Ar–O), 4.61–
4.59 (t, 2H, –OH), 4.20 (m, 4H, Ar–O–CH2), 3.76 (t, 4H, –CH2–O),
3.61–3.43 (m, 16H, CH2CH2–O–CH2CH2). 13C NMR (400 MHz,
d6-DMSO, TMS): d (ppm) ¼ 161.3, 146.9, 124.5, 115.4, 72.8, 70.5,
70.3, 69.3, 68.1, 60.7. HRMS (ESI-MS): m/z calculated for [M
Although the gels based on photo-responsive main-chain
LCPs are expected to possess high mechanical properties and
efficient photo-isomerization behaviours, the design and
synthesis of such a polymeric gelator with liquid crystal prop-
erties and gelation abilities is still a formidable challenge.
Herein, we synthesized a novel kind of azobenzene-containing
main-chain liquid crystalline polyester with a pendent amide
group, which could self-assemble into an organogel in dioxane
by hydrogen bonds and p–p interactions. In addition, the
organogel displayed a self-supporting behaviour, which was
attributed to the excellent mechanical property of the main-
chain LCP gelator. Finally, the efficient gel–sol transition
could be achieved by the incomplete trans-to-cis conversion
strategy upon UV light irradiation. Based on these characteris-
tics, many potential applications can be proposed for this kind
of organogel, such as so actuators, molecular switches and
photomechanical systems.12,17–20
+
ꢀ
(C24H34N2O8) + H] , 479.2; found 479.2. Mp: 120 C.
Synthesis of Azo-mLCP
Experimental
Azo-C12 (3.20 g 6.69 mmol) and diethyl acetamidomalonate
(1.45 g 6.69 mmol) were placed in a three-necked ask equipped
with a mechanical stirrer, a nitrogen inlet and a nitrogen outlet,
and a distillation trap was connected to a vacuum line. The
solution (10 wt% solution of Ti(OiPr)4 in ethyleneglycol
dimethyl ether) as a catalyst was added into the reaction
mixture. The gas inside the ask was exchanged by argon three
times to remove the oxygen. The reactants were molten at 155 ꢀC
and stirred for 15 min. The inert gas was kept to ow until the
bubbles disappeared for the removal of the by-product ethanol.
The reaction was employed under a low vacuum for 2 h and the
temperature was increased to 160 ꢀC. Then the reaction was
kept under high vacuum for 3 h until the reaction was over. The
crude product was dissolved in DMF and precipitation was
repeated into diethyl ether for purication (Scheme 1).
Materials
4-Aminophenol was obtained from Alfa Aesar Co., Inc. 4,40-
Biphenol, 2-(2-(2-chloroethoxy)ethoxy)ethanol, diethyl acet-
amidomalonate, potassium iodide (KI) and tetrabutyl orthoti-
tanate (Ti(OiPr)4) were supplied by TCI Development Co., Ltd.
Ethyleneglycol dimethyl ether was supplied by Aladdin Bio-
Chem Technology Co., Ltd. Sodium nitrite (Na2NO3), meth-
anol (CH3OH), phenol, hydrochloric acid (HCl, 36%), potassium
carbonate (K2CO3), N,N-dimethylformamide (DMF), dichloro-
methane (CH2Cl2), chloroform were (CHCl3), tetrahydrofuran
(THF) and dioxane were purchased from Sinopharm. DMF was
dehydrated before use. All other reagents were commercial
grade and used as received.
ꢀ
The product was dried at 90 C in a vacuum oven for 48 h.
Synthesis of 4,4-dihydroxyazobenzene (Azo)
(3.69 g 85% yield) 1H NMR (400 MHz, d6-DMSO, TMS): d (ppm)
¼ 8.85–8.83 (d, 1H, NH–C(O)CH3), 7.80–7.79 (d, 4H, Ar–Ar),
7.09–7.07 (d, 4H, Ar–N–), 5.15–5.13 (d, 1H,C(O)–CH2–C(O)),
4.23–4.16 (d, 4H, Ar–O–CH2), 3.75–3.56 (m, 20H, CH2CH2–O–
CH2CH2), 1.91 (s, 3H, NH C(O) CH3); GPC (DMF): Mn ¼ 15 200 g
molꢁ1, Mw/Mn ¼ 1.51.
4-Aminophenol (10 g, 90 mmol) was dissolved in 100 mL of 1 M
HCl solution, and then the mixture was cooled to 0 ꢀC in an ice
bath. Sodium nitrite (6.30 g, 90 mmol) dissolved in 150 mL of
water was added drop-wise slowly at 0 ꢀC and 200 mL pre-cooled
methanol was then poured in. Then, phenol (8.55 g, 90 mmol),
which was dissolved in 32.5 mL of 3 M NaOH solution, was
added drop-wise slowly at 0 ꢀC. The solution was returned to
room temperature and stirred for 2 h. Methanol was removed by
Characterization
evaporation. The precipitation appeared aer acidication with 1H and 13C NMR spectra were recorded on a Bruker AV400 NMR
1 M HCl and was ltered. The residue was washed with water spectrometer using d6-DMSO and d8-dioxane as the solvent and
until neutralized. The crude product was recrystallized from an TMS as the internal reference.
ethanol/water (v/v ¼ 1/2) mixture. An orange solid was harvested
Wide-angle X-ray diffraction was performed using a Xenocs
by ltering and drying. (4.9 g 25% yield) 1H NMR (400 MHz, d6- Xeuss 2.0 model. The X-ray source and the detector were
DMSO, TMS): d (ppm) ¼ 10.13 (s, 2H, –OH), 7.71 (m, 4H, Ar–N), MetalJet-D2, Excillum and Pilatus3R 1 M, Dectris, respectively.
6.89 (m, 4H, Ar–O). 13C NMR (400 MHz, d6-DMSO, TMS):
Electron spray ionization mass spectroscopy (ESI-MS) anal-
d (ppm) ¼ 116.5, 125.1, 146.0, 160.8. HRMS (ESI-MS): m/z ysis was performed on a Bruker microOTOF-QII spectrometer.
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Scheme 1 Synthetic routes of Azo-mLCP.
The phase-transition temperature was measured using
GPC analysis was performed on a Waters 1515-2414 appa-
a DSC Q-ꢁ20100 (TA Instruments, USA) at a heating rate of ratus equipped with a refractive index detector, using mono-
ꢀ
10 C min
.
dispersed PMMA standards to get a calibration curve. The
Rheological properties were measured in a parallel plate with elution solvent was DMF at a ow rate of 1 mL minꢁ1 at 40 ꢀC.
a diameter of 25 mm (PP25 and space 0.5 mm) on a rheometer
MCR 302 (Anton Paar, Austria). For the pattern of oscillatory
shear, we used the semiconductor temperature controller of P-
ꢀ
PTD200. The samples were heated to 90 C and maintained for
Results and discussion
Gelation ability of Azo-mLCP
3 min to make completely sure they were clear and transparent.
They were then cooled to 20 ꢀC and held for 40 min to form
In this study, we synthesized an azobenzene-containing main-
chain LCP (Azo-mLCP) as a gelator for organic solvents. As lis-
a stable gel. Frequency sweeps were carried out at g ¼ 0.05%, with
the angular frequency (u) set ranging from 0.1 to 100 rad sꢁ1
.
ted in Table 1, the gelation ability of Azo-mLCP in common
solvents has been examined. Azo-mLCP can form a gel in
dioxane and THF, but dissolves in chloroform and DMF. In
terms of the Kamlet–Ta parameters, hydrogen-bond-mediated
gelation is primarily dependent on a (the ability of a solvent to
donate a proton), and has a secondary dependence on b (the
ability of a solvent to accept a proton), but exhibits a slight
dependence on p* (the index of solvent dipolarity).21 In Table S1
(ESI†), the a value of chloroform, as well as the p* and b values
of DMF are greater than the corresponding value for THF and
dioxane. Therefore, Azo-mLCP can dissolve in chloroform and
DMF instead of forming a gel. Besides, dioxane has a lower
b value than THF, so the corresponding gel would display
a lower minimum gelation concentration (MGC). As expected,
the MGC of Azo-mLCP in dioxane (6.5 wt%) was indeed lower
The micromorphologies of the xerogel were characterized by
SEM (Sirion 200, FEI) and AFM (Shimadzu SPM-9700).
UV-Vis absorption spectra were recorded on a diode-array
spectrophotometer (Shimadzu UV-2550) at room temperature.
The cell path length was 1.0 cm.
The falling ball method was used to measure the gel–sol
transition temperature (TGS) of the organogel at different
concentrations. Steel balls (f ¼ 1 mm) were placed on the
surface of the gel inside the sample vials (f ¼ 10 mm). The
samples were heated until the steel ball fell to the bottom of the
bottle and the temperature was TGS
.
The morphologies of the organogel and xerogel were
observed on a polarized optical microscope (Zeiss Axio Scope
P1, Germany) with a hot stage (Linkam THMS 600).
Table 1 Gelation ability and MGC of Azo-mLCP
Solvent
Gelation abilitya
MGC (wt%)
Solvent
Gelation ability
MGC (wt%)
Dioxane
G
S
P
P
P
6.5
—
—
—
—
THF
DMF
Acetonitrile
Methanol
Water
G
S
P
P
P
9.5
—
—
—
—
Chloroform
Ethyl acetate
Acetone
Methylbenzene
a
G ¼ gel; P ¼ precipitate; S ¼ solution.
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than that in THF (9.5 wt%). Therefore, we focused on the gel
formed in dioxane for the following investigations.
As shown in Fig. 1, the gels have a reversible responsiveness to
temperature. The gel–sol transition temperature (TGS) was tested
at 44 ꢀC by the falling ball method with 8 wt% Azo-mLCP in
dioxane. When the temperature was greater than TGS, a reddish-
brown clear solution was obtained. A stable gel could be formed
again aer cooling to room temperature. The 1H NMR spectra of
Azo-mLCP in d8-dioxane at different concentrations were studied
to determine the driving force of the gel. Fig. 2a and S1 (ESI†)
show that the double peaks of azobenzene protons and amide
proton could be observed below 4.50 mM, but gradually merged
into a broad single peak with increasing concentration. Mean-
while, the intensities of azobenzene protons and amide proton
decreased with the increase in concentration. It is obvious that
the protons of Azo-mLCP were not equivalent in the gel state.22
The results suggest that the Azo-mLCP chains were in the isolated
state below 2.25 mM, which is consistent with the MGC value of
6.5 wt%, and then started to self-assemble via hydrogen bonding
above 2.25 mM. In addition, the FTIR spectra in Fig. 2b show that
the NH and CO stretching modes were located at 3257 and
1674 cmꢁ1 in the pure polymer, respectively. For the gel, the NH
peak became wider while the CO peak was shied to the lower
wavenumber of 1666 cmꢁ1. That is, both the 1H NMR and FTIR
spectra indicated that the polymeric gelator could self-assemble
to form an organogel in dioxane via hydrogen bonds between
the pendent amide groups.
Rheological properties of the Azo-mLCP organogel
The rheological results of the organogels with different
concentrations at 5.5–10.5 wt% are shown in Fig. 3. The storage
modulus (G0) and the loss modulus (G00) were independent of
the strain in the linear viscoelastic region (LVR), which indi-
cated that the organogels were stable under such a strain range.
Fig. 3a shows that G0 was higher than G00 at low strains, which is
the characteristic for a typical solid-like gel. The network
collapsed when the strain exceeded a critical value. Then, the
frequency sweep was tested within LVR. As shown in Fig. 3b and
c, the mixture was liquid when the Azo-mLCP concentration was
below the MGC, i.e. 5.5 wt%. The gelators started to assemble
into a network with the increase in concentration. Meanwhile,
G0 increased rapidly from 103 Pa to 105 Pa. When the concen-
tration was higher than 7.5 wt%, the gel had a self-supporting
ability with G0 > 104 Pa.23 The good mechanical property of the
organogel was probably related to the microscopic ordered
arrangement of the LCP gelator.
Fig. 2 (a) Partially zoomed 1H NMR spectra of Azo-mLCP in d8-
dioxane at different concentrations. The inset of (a) shows the
molecular structure of Azo-mLCP. (b) FTIR spectra of the polymer and
gel in dioxane.
Photoresponsive behaviours of the Azo-mLCP in solution and
in the organogel
Due to the azobenzene groups in the backbone of Azo-mLCP,
the UV-Vis absorption spectra were used to conrm their
trans–cis isomerization in solution as shown in Fig. 4. The p–p*
absorption peak of the trans-isomer at 358 nm decreased
gradually and the n–p* absorption peak of the cis-isomer at
450 nm increased slightly upon UV light irradiation. The photo-
stationary state was reached aer 78 s under UV light irradia-
tion. Reversibly, visible light irradiation resulted in the recovery
of the trans-azo. Meanwhile, the maximum absorption wave-
length was shied from 358 to 312 nm under UV light irradia-
tion and went back upon visible light irradiation, implying a p–
p stacking between the trans-azo moieties. Two isosbestic
points could be observed at 318 and 426 nm, which conrmed
that only the cis and trans absorbing species were present in
solution.24 The rst-order kinetic equation about the trans-to-cis
isomerization of azobenzene was calculated as follows:25
Fig. 1 Photographs of the dual stimuli-responsive behaviours of the
organogel with 8 wt% Azo-mLCP in dioxane.
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such a low intensity except that the PAA-Azo can reach the
photo-stationary state aer 90 s under UV light irradiation at
0.45 mW cmꢁ2.9 In fact, the obtained Ktc linearly depends on the
light intensity at a specic irradiation wavelength.24 When the
intensity of UV light irradiation was only about one-tenth of the
most intensities listed in Table S2 (ESI†), Azo-mLCP could reach
a photo-stationary state with a smaller tN.9,26–32 Compared with
AN ꢁ At
AN ꢁ A0
ln
¼ ꢁKtct
where A0, At and AN are the absorbance values before irradia-
tion, aer irradiation for t and at the photo-stationary state,
respectively. Ktc is the constant of photo-isomerization.
According to the linear tting, the Ktc for Azo-mLCP in
dioxane could be calculated as 0.035 sꢁ1 when the intensity of
UV light was only 0.28 mW cmꢁ2. There are rarely reports about
Fig. 4 UV-Vis absorption spectra of Azo-mLCP in dioxane (1 ꢂ
10ꢁ4 mol Lꢁ1) (a) irradiated by UV light (365 nm, 0.28 mW cmꢁ2) and
Fig. 3 (a) Strain and (b and c) frequency sweeps for the Azo-mLCP then (b) irradiated by visible light (550 nm, 2.10 mW cmꢁ2). (c) Kinetic
organogels at different concentrations.
curve of the trans-to-cis isomerization of Azo-mLCP in dioxane.
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most azobenzene-containing polymers and small molecules, solution upon UV irradiation.33 Azo-mLCP could not only
our Azo-mLCP displayed larger constant of photo- undergo quick photo-isomerization under such a low UV light
a
isomerization under a lower UV light intensity. This was intensity, but also converted completely from trans-azo to cis-
mainly attributed to the coupling effect between the photo- azo. To the best of our knowledge, this is the rst example for
isomerization reaction and the conformation of the liquid azobenzene-containing polymers. In addition, the exact
crystal polymer chain being stronger when the photo- proportion of cis-isomers to trigger a gel-to-sol transition is
responsive group is located on the main chain.15
important for understanding the construction mechanism of
As shown in Fig. 5a, the 1H NMR spectra of Azo-mLCP photo-responsive gels. As shown in Fig. 5b, aer UV irradia-
before and aer UV irradiation were measured to explore the tion, the peak at 6.73 ppm for cis-azobenzene in the sol
ratio of trans- and cis-isomers of azobenzene at the photo- increased dramatically, while the peaks at 7.77 ppm and
stationary state. Before irradiation, the two double proton 6.96 ppm for trans-isomer decreased simultaneously. There-
peaks of benzene appeared at 6.96 and 7.77 ppm, respectively. fore, the percentage of cis-isomers during the trans-to-cis
Aer irradiation for 10 min to ensure a photo-stationary state, transformation could be calculated as 48% by comparing their
the two double peaks perfectly merged into a quadruple peak integral. This is rational because our organogel was robust.
at a lower chemical shi of 6.73 ppm. In other words, almost Finally, upon visible light irradiation, the peaks for trans-
all the trans-azobenzene was isomerized to cis-azobenzene in azobenzene protons returned to the original state, while the
peak for cis-isomer disappeared. The incomplete trans-to-cis
conversion strategy could efficiently trigger the gel-to-sol
process of the organogel with the external stimulus.34
However, due to the hydrogen bond and p–p interactions
between the polymer chains in the organogel, the movement
space of Azo-mLCP was conned. Thus, the isomerization
process of the macromolecular backbone was depressed,
which resulted in the UV light intensity for the photo-
isomerization for the polymeric gelator in the organogel
being higher than in solution.
In order to further investigate the changes in the morphology
of the gels before and aer UV irradiation, FE-SEM and AFM
Fig. 5 (a) 1H NMR spectra of Azo-mLCP in d8-dioxane before (bottom)
and after UV irradiation (365 nm, 0.28 mW cmꢁ2) for 10 min (top). (b) 1H
NMR spectra compositions of the gel based on Azo-mLCP (bottom),
after UV light irradiation (middle), and subsequent irradiation under
visible light (top). The black squares and the red circle represent the Fig. 6 (a and c) FE-SEM and (e) AFM images of Azo-mLCP xerogels
proton peaks of trans- and cis-isomer of azobenzene, respectively. formed in dioxane without UV irradiation, (b and d) FE-SEM and (f) AFM
The gel is prepared in d8-dioxane at a concentration of 7.5 wt%.
images of xerogels with UV irradiation.
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images of the Azo-mLCP xerogels were obtained. The organo-
gels with 10 wt% Azo-mLCP were placed in a vacuum oven at
40 C for 3 days to remove the solvent completely. For the gels
The liquid crystalline behaviour of the Azo-mLCP organogel
The liquid crystalline behaviour of the xerogel was investigated by
DSC and POM. The texture of Azo-mLCP during the cooling
process was observed by POM. It showed a typical focal-conical
texture, which usually corresponds to the smectic phase.37 In
Fig. 7a, theꢀDSC curves of the xerogel showed an endothermic
peak at 91 C, which could be assigned to the phase transition
from smectic to isotropic, while the exothermal peak at 71 ꢀC was
attributed to the isotropic-to-smectic phase transition. In addi-
tion, the wide-angle X-ray diffraction (WAXD) of the xerogel dis-
played a broad peak at 17 nmꢁ1, corresponding to the interaction
of the ether chain and a peak at 21 nmꢁ1, corresponding to the
p–p stacking azobenzene mesogen in Fig. 7b. At room temper-
ature, three peaks could be observed at 4.84 nmꢁ1 (1.30 nm), 9.30
nmꢁ1 (0.68 nm) and 18.45 nmꢁ1 (0.34 nm), respectively. The q
ratio of 1 : 2 : 4 means that the xerogel still displayed a smectic
phase in the gel state. Besides, the q value at 18.45 nmꢁ1 for the
xerogel was higher than the as-prepared Azo-mLCP at 18.10
nmꢁ1, which suggests that the distance between the mesogens
became shorter during the gelation process.38 The birefringence
of the Azo-mLCP organogel disappeared while the isotropic sol
appeared aer UV light irradiation, as seen in Fig. S2 (ESI†),
which indicates the liquid crystalline property of Azo-mLCP could
be destroyed during the gel-to-sol transition.
ꢀ
before UV irradiation, Fig. 6a and c show a wrinkled dry-muddy
sheet in a micrometer scale at the top and irregular aggregates
at the bottom.35 This wrinkled dry-muddy sheet has also been
observed in the gels made of peptides bearing an azobenzene
moiety.36 In addition, the same morphology was observed in the
AFM image as shown as Fig. 6e, so the top wrinkled dry-muddy
sheet should not be ascribed to the collapse under electron
beam. It is interesting that the morphology of the organogel
varied to coffee-bean-like particles with a diameter of about 3
mm aer UV irradiation in Fig. 6b and d, which was consistent
with the AFM observation in Fig. 6f. These results conrmed
that the UV light could change the azobenzene conguration
and polymer backbone conformation, leading to the destruc-
tion of the gel structure.
As shown in Scheme 2, we drew a model for describing the
microstructure changes for the reversible gel-to-sol processes of
the Azo-mLCP organogel upon UV and visible light irradiation.
Smectic Azo-mLCP self-assembled to brils through hydrogen
bonds and weak p–p interactions. These brils then further
formed a gel network to entrap solvents. Aer a part of the
azobenzene groups isomerized from trans to cis congurations
under UV irradiation, the liquid crystallinity of the gelator could
be destroyed. Meanwhile, the backbone conformation and the
ordered stacking microstructures could be changed so that the
solvents could not continue to be anchored. It is interesting that
the network and the organogel could be rebuilt aer visible
light irradiation.
Fig. 7 (a) DSC curves of xerogel tested on the first cooling and second
heating scans in N2 at a heating rate of 10 ꢀC minꢁ1. The inset of (a)
shows a POM photograph of the xerogel captured at 69 ꢀC. P, Scheme 2 Microscopic schematic illustration for the reversible gel–
polarizer; A, analyzer; l, phase retarder. (b) 1D WAXD patterns of sol process of the Azo-mLCP organogel under UV light and visible light
xerogel and Azo-mLCP.
irradiation.
3732 | RSC Adv., 2020, 10, 3726–3733
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Conclusions
In summary, we obtained a photo-responsive liquid crystalline
polymer organogel bearing the azobenzene group in the poly-
mer main chain. The robust organogel could be constructed by
hydrogen bonds of the pendent amide group and p–p interac-
tions of azobenzene. The Azo-mLCP gelator underwent fast
trans-to-cis transformation in solution under UV irradiation
with an ultra-low intensity. In addition, the efficiently reversible
gel-to-sol transition could be triggered when about half trans-
azobenzene transformed to cis-azobenzene. The good mechan-
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duction of the main-chain LCP gelator, while the efficient
photomechanical effect was attributed to the incomplete trans-
to-cis conversion strategy. This work provides some inspiration
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
We acknowledge the nancial support of the National Natural
Science Foundation of China (51773070, 51973072, and
51433002). We greatly appreciate Dr Shitao Fu for the
measurements and analyses of 1H and 13C NMR spectra. We
also thank the HUST Analytical and Testing Center for AFM
measurement.
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