Functional liquid crystalline gels through multi-scale hierarchical self-assembly of LAPONITE and amidodiol

Article abstract of DOI:10.1039/c6nj01080f

Herein we demonstrate the design and synthesis of liquid crystalline gels through multi-scale hierarchical self-assembly of LAPONITE and amidodiol in water. These two components interact with each other through non-covalent interactions, such as hydrogen bonding and ion-dipole interactions, to form highly ordered superstructures in different dimensions and length scales. Effects of concentration of amidodiol and timescale on specific modes of packing were studied using various microscopic and spectroscopic techniques, such as PLM, SEM, TEM, XRD, rheology and FTIR. Presence of hydroxyl groups was confirmed by chemical analysis. A plausible mechanism for the formation of superstructures in functionalized LAPONITE-amidodiol gel (FLAG) was proposed. Electrochemical impedance studies of the FLAG showed low charge transfer resistance (245 Ω) with a stable rectangular electrochemical window (-0.4 V to 1.5 V). Galvanostatic studies revealed good cycling stability with a specific capacitance of 1856 mF g^-1. Results suggest that FLAGs can be exploited as an efficient gel electrolyte in energy storage devices.

Full text of DOI:10.1039/c6nj01080f

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Functional liquid crystalline gels through
multi-scale hierarchical self-assembly of
Cite this:DOI: 10.1039/c6nj01080f
LAPONITEs and amidodiol†
Rohini Kuttiplavil Narayanan, Neethu Kalloor Sadanandhan, Renjith Sasi and
Sudha Janardhanan Devaki*
Herein we demonstrate the design and synthesis of liquid crystalline gels through multi-scale hierarchical
self-assembly of LAPONITEs and amidodiol in water. These two components interact with each other
through non-covalent interactions, such as hydrogen bonding and ion–dipole interactions, to form highly
ordered superstructures in different dimensions and length scales. Effects of concentration of amidodiol
and timescale on specific modes of packing were studied using various microscopic and spectroscopic
techniques, such as PLM, SEM, TEM, XRD, rheology and FTIR. Presence of hydroxyl groups was confirmed by
chemical analysis. A plausible mechanism for the formation of superstructures in functionalized LAPONITEs–
amidodiol gel (FLAG) was proposed. Electrochemical impedance studies of the FLAG showed low charge
transfer resistance (245 O) with a stable rectangular electrochemical window (ꢀ0.4 V to 1.5 V). Galvanostatic
studies revealed good cycling stability with a specific capacitance of 1856 mF gꢀ1. Results suggest that
FLAGs can be exploited as an efficient gel electrolyte in energy storage devices.
Received (in Montpellier, France)
8th April 2016,
Accepted 19th August 2016
DOI: 10.1039/c6nj01080f
www.rsc.org/njc
Amidodiol possesses diamide and dihydroxyl groups and is
expected to form self-organized macroscopic structures through
1 Introduction
Hybrid liquid crystalline materials with self-assembled super- extensive intermolecular hydrogen bond interactions. Liquid
structures and specific modes of packing along with desired crystalline properties exhibited by polyester amide are attributed
functionalities are reported for potential applications in drug to the intermolecular hydrogen bond interactions between
delivery, sensors, display devices, energy storage, etc.1–5 Development amide–amide and amide–ester groups. Studies on hybrid soft
of liquid crystalline functional gels through self-organization of materials formed by the self-organization of low molecular
organic–inorganic molecular systems has recently become a weight organic molecules and inorganic layered materials, such
significant area of research because of the synergistic properties as LAPONITEs, are gaining importance in many biochemical
arising from the molecular level self-assembly of inorganic–organic applications.18,19
hybrid materials.6,7 The supramolecular self-assembly approach
LAPONITEs is a synthetic hectorite clay having layered hydrous
through non-covalent interactions, such as hydrogen bonding, sodium magnesium silicate, and possesses a high degree of
van der Waals’ interactions, ionic bonding, charge-transfer and structural regularity. LAPONITEs is made up of discoid platelets
electrostatic interactions has thriving significance for the design of magnesium lithium silicate (Si8[Mg5.5Li0.4O24.0]H4O24Na0.7
)
of functional hybrid liquid crystalline gels.8,9 The design of liquid with a thickness of approximately 1 nm, and diameter of approxi-
crystalline functional materials using tailor made molecular mately 30 nm (Fig. S1, ESI†).20,21 LAPONITEs discs are endowed
gelators containing amide and hydroxyl linkages offers new with structural features such as hydroxyl groups, Lewis and
features.10,11 Inter and intramolecular hydrogen bonds between Brønsted acidity, exchangeable interlayer cations, and nano-
amide linkages are reported to be responsible for the functional dimensions of the individual layers. The silanol groups located
properties in proteins and in many synthetic polyamides.12–15 at the edges of the LAPONITEs sheets can be modified with
The liquid crystalline formation of polyester–amide prepared organic compounds, such as thiol, amine, or long hydrocarbon
from amidodiol (AMD) was reported earlier from our group.16,17 chains.22,23 Ruzicka and Gabriel et al. extensively studied
LAPONITEs–water interaction and the lyotropic liquid crystalline
phases of LAPONITEs gels.24–26 LAPONITEs clay particles have an
asymmetric charge distribution as there is a net negative charge on
Chemical Sciences and Technology Division, CSIR – National Institute for
Interdisciplinary Science and Technology, Thiruvananthapuram, 695019, India.
the surface and a partial positive charge on the edges. Hence, they
E-mail: Sudhajd2001@yahoo.co.in
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj01080f will not interact with all the neighbours, but they can lock together
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in lower densities. LAPONITEs gel is stable for a long time, and no techniques, such as PLM, SEM and TEM, rheology, XRD and
aging problem is observed. Such a low-density gel can act as a gel FTIR spectroscopy. Finally, cyclic voltammetric and galvanostatic
electrolyte and could be employed in energy storage devices. It was charge–discharge studies were carried out, which demonstrated
reported that the high aspect ratio of LAPONITEs clay platelets it’s application as electrolyte in energy storage devices.
favors a superior transport property. It was reported that sodium
ions present in LAPONITEs clay can be exchanged with lithium
2 Experimental
2.1 Reagents
ions, and the conductivity can be increased to a large extent, and
they also find application in energy storage devices.27,28
An electrolyte material must possess excellent ionic conductiv-
LAPONITEs RD (Southern Clay Products) was used as received.
ity, mechanical stability, electrochemical and thermal stability and
It has a cation exchange capacity of 60 meq. per 100 g.
g-Butyrolactone (Fluka, Germany), hexamethylene diamine,
not suffer leakage of electrolyte.29 The present liquid electrolyte
systems possess good ionic conductivity, but have a fundamental
isopropanol, acetone and methanol (SD Fine Chemicals Ltd,
limitation for long-term operation due to their evaporation and
Bombay, India), potassium ferricyanide, potassium ferrocyanide,
leakage.30,31 In this respect, solid electrolytes possess special
potassium chloride (Merck, India) were used without further
features like compactness, portability, reliability and lack of
purification. All the experiments were carried out using Milli-Q
leakage, apart from superior mechanical properties and long-
purified water.
term stability, but have limitations such as low ionic conductivity
and slow rates of diffusion. Gel electrolytes combine the merit
of liquid electrolytes (high ionic conductivity) and solid electro-
lytes (good mechanical stability, lack of leakage, compactness,
portability). They are considered to be superior candidates as
electrolytes due to their three-dimensional gel network, which
facilitates ion transportation.32,33 Compared with organogels,
hydrogels provide an environmentally friendly green approach
and low cost route for the preparation of electrolyte materials.34
The infiltration ability of polymer gel electrolyte is less favorable
and hence the development of a water-based, environmentally
friendly, highly reliable supramolecular hydrogel electrolyte
that resists solvent leakage, and provides good mass transport
is of importance. The self-assembled liquid crystalline materials
forming well defined anisotropic conductive pathways can be
considered as attractive alternatives for the electrolyte material.
Liquid crystalline materials exhibit both temperature- or
solution-induced 1D, 2D and 3D ordered mesophases with
typical textures such as columnar, smectic and bicontinuous
cubic phases with anisotropic channels capable of forming con-
ductive pathways for the transport of charges and ions.35–38 Liquid
crystalline hydrogels are considered as better candidate electro-
lytes for energy storage systems because of their special features,
such as compactness, portability, reliability, lack of leakage and a
green approach in addition to well defined anisotropic pathways
for charge conduction. Organic and polymer gel electrolytes were
reported to be used in dye sensitized solar cells and lithium ion
batteries.39–41 LAPONITEs based gel electrolyte is a new area of
research and may be a prospective alternative for organic based
gel electrolytes. Thus, by combining the advantages of LAPONITEs
and amidodiol, the development of hybrid liquid crystalline
supramolecular hydrogels with self-assembled anisotropic struc-
tures can be considered as alternatives for gel electrolytes in
energy storage devices.
2.2 Preparation of 1,6-bis(hydroxybutyramido)hexane
1,6-Bis(hydroxybutyramido)hexane (amidodiol, AMD) was prepared
by the aminolysis of g-butyrolactone using 1,6-hexamethylene
diamine as per the earlier reported procedure. The white crystal-
line solid formed, was filtered and washed several times with
isopropanol, then dried in vacuum and further recrystallized
from 150 mL of 1 : 1 methanol–acetone mixture (yield: 95%,
M.P.: 118.5 1C). The scheme for this is given in Fig. S2 (ESI†).
Formation of AMD was confirmed by 1H-NMR and 13C-NMR,
given in Fig. S3 (ESI†).
2.3 Preparation of functionalized LAPONITEs–amidodiol gel
(FLAG)
0.05 g of LAPONITEs was dissolved in 1 mL of hot distilled water
and then it was thoroughly mixed with 1 mL of 1% amidodiol
in water at 60 1C for 5 minutes. Then, the sample was kept
undisturbed at room temperature for 24 hours and a transparent
soft gel was obtained. FLAGs were prepared by varying the
percentage of AMD (0.4% to 1.0%) and are designated as FLAG1,
FLAG2, FLAG3, and FLAG4, respectively. A control sample was
prepared using the same procedure but in the absence of AMD,
and designated as LAP. Experimental details for the preparation
of the FLAGs are shown in Table S1 (ESI†).
2.4 Electrochemical studies
ITO plates were cut into of 2 ꢁ 2 cm pieces using a glass cutter
and were subjected to 5 minutes of cleaning in acetone, isopro-
panol, and distilled water by ultrasonication and were dried. The
test cells, fabricated by sandwiching FLAG4 hydrogel between
transparent conducting ITO-coated glass electrodes, were used
for cyclic voltammetric (CV) and galvanostatic charge–discharge
(GCD) studies. The specific capacitance values were calculated
from the following equation using GCD results
The present work demonstrates the design of liquid crystal-
line functional gel prepared through non-covalent interactions
of LAPONITEs and amidodiol by a multi-scale hierarchical self-
Specific capacitance = (4IDt)/(mDv)
assembly approach. Effects of time and concentration para- where, I is the discharge current in amperes, Dt is the discharging
meters of amidodiol in the design of liquid crystalline functional time, Dv is the potential window during the discharge process,
gels were studied. They were characterized using microscopic and m is the mass of the electrode materials in grams.
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2.5 Characterizations
studied using various microscopic techniques such as PLM, SEM
and TEM. The observed results were further complemented
with the results generated from XRD, rheology and FTIR. FLAGs
of varying compositions were prepared by mixing aqueous
solutions of LAPONITEs and AMD at room temperature. Studies
showed that gelation time and gel strength of FLAGs depended
on the concentration of the components and time. It has been
observed that dilute aqueous LAPONITEs solutions (o2 wt%)
initially appeared as a sol and later changed into a transparent
gel. However, when the LAPONITEs concentration is increased
to 45 wt%, it immediately transforms into an opaque gel.
Observation of LAPONITEs gels (LAP) under PLM showed the
formation of nematic texture and is depicted in Fig. S4 (ESI†).
Our observation is strengthened by the studies reported by
Ruzicka and Gabriel et al.24,25 Liquid crystalline phase formation
is favored by the ordered arrangement of the discs in a particular
direction and gelation is attributed due to the three dimensional
network formations of discs (Fig. S5, ESI†). The SEM image of
LAPONITEs sol before functionalization exhibited isolated disc-
like morphology and is provided in Fig. S6 (ESI†).
The PLM image of AMD is given in Fig. S7a (ESI†), which
shows the formation of birefringent sheets. The SEM image of
AMD also confirmed the sheet-like morphology, as shown in
Fig. S7b (ESI†). The SEM and PLM studies confirmed that AMD
possess the potential for self-assembly with LAPONITEs.
Effects of AMD on the gelation time, gel strength and formation
of liquid crystalline phase formation were studied. It has been
observed that there is an increase in the gel strength in presence
of AMD. This may be due to the network formation established by
the combined effect of various non-covalent interactions among
the partial charges, Mg–OH and Si–OH groups present on the
surface and edges of the LAPONITEs discs with diamide and
dihydroxyl groups contributed by AMD.
Evolution of morphological changes in FLAGs were studied
using various microscopic techniques such as PLM, SEM and
TEM. FLAGs exhibited different mesophases depending upon the
amount of amidodiol, LAPONITEs and time. The PLM image of
the initial stage of FLAG1 is observed as birefringent distorted
spherullites dispersed in an isotropic medium. This might be due
to the random dispersion of functionalized LAPONITEs discs in
the aqueous phase (Fig. 1a). The self-assembly of spherullites
in FLAG1 is shown in Fig. 1b. Later, this entity was observed
to be elongated by edge-to-edge interaction of functionalized
LAPONITEs discs and the branched structures formed in FLAG2
are shown in Fig. 1c. Here, the self-assembled bilayer of AMD
co-structures with the LAPONITEs discs in various ways. The
branched quill and feather morphology of FLAG3 is shown in
Fig. 1d and e. With increasing amount of AMD, varieties of
branched nematic tapes were observed in FLAG4 and are shown
in Fig. 1f. Further morphology of the FLAGs was studied by SEM
and the images are shown in Fig. 2a–d. Fig. 2a shows the SEM
image of a single disc with flowery edges of self-assembled
AMD in FLAG1. The SEM image of freshly prepared FLAG2
exhibited thick edged isolated discs and later, these discs piled
FTIR spectroscopic measurements were made with a fully compu-
terized Nicolet Impact 400D FTIR spectrophotometer. Materials
were mixed thoroughly with potassium bromide and compressed
into pellets before recording. All spectra were corrected for the
presence of moisture and carbon dioxide in the optical path.
Wide angle X-ray scattering (WAXS) measurements were performed
using a XEUSS SAXS/WAXS system by Xenocs. Cu Ka, with wave-
length l B 1.54 Å, was used as the source. For SEM measurements,
samples were subjected to thin gold coating using a JEOL JFC-1200
fine coater. The probing side was inserted into a JEOL JSM-5600 LV
scanning electron microscope for taking photographs. TEM
measurements were carried out using an FEI TECNAI G2 30
S-TWIN with an accelerating voltage of 100 kV. For TEM mea-
surements, the samples were drop casted on a formvar coated
copper grid and dried in vacuum at room temperature. Liquid
crystalline phase formation was observed under a polarized light
microscope (PLM) (Olympus BX51 equipped with a camera).
The rheological properties of the sample were measured using
a modular Compact Rheometer (MCR150, Anton Paar) with
cone-and-plate geometry (CP 50-1/Ti d = 49.98 mm, concentri-
city 8 mm and parallelity 5 mm, angle = 0.991, truncation 51 mm)
and a solvent trap was utilized to minimize evaporation at a set
measurement temperature of 25 1C. Each mixture first under-
went an oscillatory strain sweep (g = 0.005–100%) at a constant
frequency of 1 Hz and then conducted frequency sweep measure-
ment at 1% oscillatory amplitude strain. Subsequently, dynamic
or steady shear experiments were performed with the shear rate
ranging from 0.01 to 100 sꢀ1. Electrochemical studies were
carried out with CHI6211B electrochemical analyzer. GCE was
modified with LAP and FLAG4 by drop casting gel on the
electrode, then followed by air drying and were designated as
LAP/GCE and FLAG/GCE, respectively. All the potentials were
recorded using Ag/AgCl as the reference electrode and platinum
wire as the counter electrode. The electrochemical impedance
measurements were performed in a frequency range from 0.1 Hz
to 10 kHz at an open circuit potential of 1 V in a three-electrode,
one-compartment electrochemical cell. The electrochemical
impedance measurements were performed with potassium ferro-
cyanide/ferricyanide solution for GCE, LAP/GCE and FLAG/GCE.
Prior to electrochemical experiments, GCE was polished with 0.03
micron alumina and ultrasonically cleaned for about 5 minutes
in doubly distilled water. Cyclic voltammetric and galvanostatic
studies were carried out using a VSP potentiostat/galvanostat
(Biologic Instruments) by fabricating test cells using ITO electro-
des. All the experiments were carried out at 25 1C ꢂ 5 1C. Before
performing the experiments, the solution was purged with
nitrogen gas for 10 minutes. Ionic conductivity was measured with
a JENWAY 3540 pH and conductivity meter.
3 Results and discussion
3.1 Evolution of supramolecular self-assembly
The effect of composition of the components, such as LAPO- up hierarchically by edge-to-edge etching, as shown in Fig. 2b.
NITEs and amidodiol, on the evolution of the morphology was This observation was supported by PLM studies. As the amount
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Fig. 3 (a) TEM image of FLAG4 showing tape morphology, (b) magnified
image of tape like morphology in FLAG4 showing the Fresnel fringes of
LAPONITEs discs.
are seen with their basal plane parallel to the electron beam.
Pronounced Fresnel fringes appeared on each side of the diffract-
ing platelets and are shown in Fig. 3b. These fringes are considered
to originate from the development of various interactions among
the individual moieties present in FLAG4.
The non-covalent interactions between LAPONITEs and
AMD during the self-assembly process leading to the formation
of supramolecular structures in FLAGs were evidenced from the
results obtained by various experimental studies, such as FTIR
spectroscopy and XRD. The FTIR spectra of LAP, AMD and FLAG4
are shown in Fig. 4. The spectrum of LAP exhibited a band at
3640 cmꢀ1 attributed to the presence of Si–OH and Mg–OH
stretching vibrations which are distinct to the varying micro
environments in the LAPONITEs structure. It exhibited bands
at 1640 cmꢀ1 (physisorbed water), 1007 cmꢀ1 (stretching vibration
of Si–O and Si–O–Si moieties), and 640 cmꢀ1 (stretching vibration
of Mg–O linkage). The FTIR spectrum of AMD exhibited character-
istic amide I and amide II bands at 1636 cmꢀ1 (–CQO stretching)
and 1545 cmꢀ1 (–C–N stretching in combination with –N–H
bending). The band at 3300 cmꢀ1 is attributed to –OH and –NH
stretching vibrations of the AMD and the band at 2920 cmꢀ1
corresponds to the stretching vibrations of the –CH2 moiety.
The characteristic spectral bands of Mg–O (1007 cmꢀ1) and
Fig. 1 PLM images of (a) spherullites in the initial stage of FLAG1, (b) self-
assembling and co-structuring of FLAG1, (c) FLAG2, (d and e) feather and
quill like morphology of FLAG3 and (f) branched tape like morphology of
FLAG4.
of AMD increased, these domains transformed into feathers/
plumes and tape like morphologies in FLAG3 and FLAG4,
respectively, as shown in Fig. 2c and d, respectively. Thus, by
controlling the experimental conditions, multi-scale hierarchical
arrangement of the domains can be obtained for various liquid
crystalline phases.
The TEM pictures of FLAG4 taken under different magnifica-
tions are shown in Fig. 3a and b. Micrographs exhibited tape like
morphology and a number of well-defined dark fringes were
observed inside the tape. The contrast in the TEM image strongly
depends on their orientation with respect to the electron beam.
Diffraction contrast is strong when the LAPONITEs nanodiscs
Fig. 2 SEM images of (a) freshly prepared FLAG1 in the initial stage, (b) FLAG2,
(c) branched morphology of FLAG3, (d) the fibrillar tape like morphology of
FLAG4.
Fig. 4 FTIR spectra of LAP, AMD and FLAG4 showing the non-covalent
interactions among LAPONITEs–LAPONITEs
,
LAPONITEs–amidodiol
and amidodiol–amidodiol.
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Si–O–Si (640 cmꢀ1) observed in LAP are shifted to 963 cmꢀ1 and peaks at lower angles between 2y B 21 to 61 revealing the
591 cmꢀ1, respectively, and the distinctive vibrations corres- formation of highly ordered supramolecular ensembles with
ponding to amide bands present in AMD, observed at 1636 the dimensions of 30–40 Å and a shift in the d-spacing of the
and 1545 cmꢀ1, are measured to be shifted to 1585 cmꢀ1 and LAPONITEs disc. The nature and position of this peak varied
1480 cmꢀ1 in FLAG4. Further, the characteristic bands of –OH with the amount of amidodiol. In FLAG1, the peak at 2y B 6.31
and –NH stretching vibrations in LAP and AMD, appearing as a (14.01 Å corresponds to the shift in LAPONITEs d-spacing) and
broad band at 3000–3500 cmꢀ1 in FLAG4, confirmed the exten- the peak at 2y B 2.61 (33.93 Å) agreed with the dimension of
sive hydrogen bonding interaction between the LAPONITEs and the ensemble whereas FLAG2 showed a broad peak at 2y B 5.31
AMD. The shifts in the characteristic bands of LAPONITEs and (16.65 Å shift in the d-spacing of LAPONITEs) and a sharp peak
LAP revealed extensive hydrogen bonding and the non-covalent observed at 2y B 2.41 (36.76 Å, related to the dimension of the
interactions between these two moieties induce the formation of supramolecular ensemble). FLAG3 showed a broad peak at
self-assembled aggregates of mesophase and stable gel phase. 2y B 5.21 (16.97 Å) and a sharp peak observed at 2y B 2.41
Functional hydroxyl groups in FLAGs were estimated by chemical (35.84 Å, related to the dimension of the supramolecular ensemble).
analysis. The hydroxyl value of LAP is 0.94 mmol gꢀ1, which FLAG4 exhibited the peak at 2y = 4.41 (20.05 Å) and the peak at
is increased to 3.04 mmol gꢀ1 in the case of FLAG4 confirming 2y B 2.21 (40.10 Å) agreeing with the dimension of the ensembles.
the enhancement of the functional groups. Thus, FLAGs are The peaks corresponding to AMD is diminished in the FLAGs.
endowed with functional groups of amide and hydroxyl groups, The formation of these supramolecular ensembles was further
apart from the unreacted charges, which can be exploited for supported by themorphological analysis by PLM and SEM.
many applications.
The small angle XRD peaks confirm the formation of self-
The d-spacing between the LAPONITEs discs and the dimension assembled supramolecular ensembles. The SAXS of FLAG1 is
of the aggregates in FLAG was studied by XRD. The XRD spectra provided in the inset of Fig. 5. The d-values calculated are 14.6 Å
of AMD, LAPONITEs and FLAGs are shown in Fig. 5. The and 32.3 Å. These values matched with the d-values calculated
diffraction pattern of LAPONITEs exhibited a broad diffraction from WXRD measurement [in FLAG1, the peak 2y = 6.31 (14.01 Å
peak at 2y = 6.641 corresponding to inter planar d-spacing of corresponds to the shift in LAPONITEs d-spacing) and the peak
B13.3 Å which can be related to the d001 basal plane oriented at 2y = B2.61 (33.93 Å)].
parallel to the substrate surface. The exact position of this peak
The viscoelastic properties of composites depend on the
may vary as a function of relative humidity. XRD of AMD size, shape, entanglement and periodicity of the components.
exhibited well-defined sharp intense peaks at 2y = 101, 151, Viscoelastic properties of gels are functions of critical oscillatory
211, 231, 261, 301 and 321 revealing its highly crystalline nature. strain, phase angle, storage modulus, loss modulus and
Nevertheless, the XRD spectrum of FLAGs exhibited entirely viscosity.42,43 Dynamic amplitude strain sweep experiments at
different XRD patterns in terms of the shape and position of constant stress controlled conditions were conducted to observe
peaks in comparison to LAPONITEs and AMD, suggesting the the linear viscoelastic region in FLAGs, where the applied strain
formation of supramolecular structures. FLAGs exhibited two does not considerably affect the three-dimensional network
structures. The linear regime is identified by plotting the stress–
strain curve given in Fig. S8 (ESI†). The linear regime is observed
for very low strain values B 1 to 2% and is clearly visible in the
plot. The storage modulus/loss modulus versus amplitude strain
sweep profile of FLAGs is shown in Fig. S9 (ESI†). The crossover
point indicates when the elastic nature of the gel is changed into
its viscous nature. The crossover points obtained for FLAG1,
FLAG2, FLAG3 and FLAG4 of 3.5%, 4.2%, 5.6%, and 6.9%,
respectively, confirmed the role of AMD for establishing robust
nature for FLAGs. Angular frequency dependent storage modulus
and loss modulus of FLAGs and LAP were studied at constant
strain (1%). Typical diagrams showing angular frequency versus
storage modulus (G0) and loss modulus (G00) are shown in Fig. 6a.
At the lower frequency region, the variation of G0 and G00 is
independent of angular frequency where the applied frequency
does not considerably affect the three-dimensional structure of
the gels which is a signature of true gel formation. The gap
between storage and loss modulus increases with increase in
concentration of AMD, and reveals that, as the amount of AMD
increases, the elastic modulus is enhanced due to the high extent
Fig. 5 XRD of AMD, LAPONITEs, FLAG1, FLAG2, FLAG3 and FLAG4 showing
of cross-linking density. The difference in the modulus for LAP,
FLAG1, FLAG2, FLAG3 and FLAG4 are observed to be 730 Pa, 938 Pa,
984 Pa, 1073 Pa, and 1290 Pa, respectively. Time-dependent
the self-assembled aggregates of LAPONITEs and AMD while retaining the
crystalline ordering of LAPONITEs in FLAGs. Inset showing the SAXS of FLAG1
confirms the formation of self-assembled aggregates.
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viscosity value. The double logarithmic plot of shear rate versus
viscosity is given in Fig. 6b. The viscosity decreases with increase
in shear rate according to the power law behaviour, Z = Kgn. The
straight line obtained in Fig. 6b showed a fitting to the power law
behaviour. This observation supports the shear thinning behaviour.
This is supported by the observation made by Onogi and Asada in
the rheological studies of liquid crystalline materials.44,45
3.2 Mechanism for the formation of mesophases
A plausible mechanism for the formation of various mesophases
was manifested from the studies made by FTIR, microscopic
analysis, XRD and rheology. Water molecules penetrate into the
interlaminar space and push the LAPONITEs discs apart. At high
concentrations, LAPONITEs suspension spontaneously turns
into an opaque gel. At low concentrations, initially fluid like
LAPONITEs turns into gel phase after 24 hours. The nematic
liquid crystalline phase formation is due to the periodicity arising
from the face-to-face mutual orientation among the functional
LAPONITEs discs. Formation of various liquid crystalline phases
in FLAGs can be explained due to changes in the periodicity,
defect formation, molecular packing and nano-segregation during
the mesophase formation, which varies with the experimental
conditions. As we have reported above, FLAG comprises highly
Fig. 6 (a) Double logarithmic plot of angular frequency versus loss and
storage modulus profile for LAP and FLAGs. (a) LAP G00, (b) FLAG1 G00, exfoliated nanodiscs with negative charges on the surface,
(c) FLAG2 G00, (d) FLAG3 G00, (e) FLAG4 G00, (f) LAP G0, (g) FLAG1 G0, (h) FLAG2 G0,
(i) FLAG3 G0, (j) FLAG4 G0. (b) Double logarithmic plot of shear rate versus
partial positive charge on the edges and Mg–OH and Si–OH groups
at the broken edges. AMD contains two amide groups and two
viscosity of FLAGs showing shear thinning behaviour.
hydroxyl groups. Hence, there is competition between various
non-covalent interactions among LAPONITEs–LAPONITEs,
LAPONITEs–AMD and AMD–AMD via hydrogen bonding,
rheological studies were carried out to study and predict the
dipole–dipole and electrostatic interactions of hydroxyl groups,
effect of aging on the gel strength. Freshly prepared gel samples
amide groups and ionic charges. Apart from this, LAPONITEs–
showed storage moduli of 831.3, 1193.7, 1295.26, 1552.12, and
AMD–water interactions also play a crucial role in the liquid
2085.3 Pa for LAP, FLAG1, FLAG2, FLAG3, and FLAG4, respectively.
crystalline phase formation. FLAGs are observed as transparent
However, storage modulus values were found to increase with
time, which attributed to the establishment of a greater density of
gels and a photograph of this is given in Fig. 7.
Initially, all FLAGs observed as sols and under PLM appeared
hydrogen bond interactions between these two entities in various
isotropic due to the random dispersion of both AMD and
fashions. Time-dependent changes in the storage modulus of
LAPONITEs. Later, due to supramolecular interactions, AMD is
FLAG4 were measured as 2341, 2784, 3321, 3890, 4512, 4514,
non-covalently attached to the LAPONITEs edges and they form
4515 Pa, which is quite high elastic and solid like behaviour
spherullites in the isotropic medium. Slowly these spherullites
compared with the freshly prepared sample, and is shown in
elongate by a growth process of etching and self-assembly. They
Fig. S10 (ESI†). After one week, the storage modulus was observed
to make only very small variations. The time-dependent IR spectra
provided in Fig. S11 (ESI†) confirm the increase in the density of
hydrogen bonding with time.
self-assemble in two ways, either zig-zag to form a bush or edge-
to-edge etching to form feather/plume type structures. Later, in
the presence of excess AMD, more self-assembled AMD adheres
on the surface of the branched stems (templates) to form arrays
It has been reported in Ericksen’s theory that liquid crystalline
of highly birefringent feathers or leaves and multi-walled tapes by
materials may exhibit high degree of structural ordering and the
domains can be arranged in a specific fashion to obtain molecular
orientation. Under shear flow, structural rearrangement may
occur in the internal polydomain structure of liquid crystalline
materials. Each domain is having a high degree of molecular
orientation. With increasing shear rate, there is a successive
destruction of the polydomain structure into smaller domains.
Accordingly, the lamellar liquid crystalline phases of FLAGs are
composed of ordered hierarchical structures of polydomains.
With increasing shear force, they slide upon one another to form
Fig. 7 Photograph of (a) FLAG1, (b) FLAG2, (c) FLAG3 and (d) FLAG4.
smaller domains accompanied by a sudden decrease in the
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Scheme 1 Schematic representation of the formation of various mesophases
via supramolecular self-assembly of LAPONITEs and AMD.
co-structuring and self-assembly processes among functionalized
LAPONITEss and AMD. This observation is supported by our
earlier studies made on polyaniline–clay nanocomposites.46 The
various phases formed by FLAGs are further supported by the
work done by previous researchers.47 They suggested that organo-
modified LAPONITEss can undergo various structural transfor-
mation such as star, vesicular and more disorganized bundle-like
structures based on the experimental conditions.48,49 Thus, by
controlling the concentration and time parameters in FLAG, we
can design mesophases through non-covalent interactions by a
self-assembly approach. This type of synthetic design can be
exploited for the development of multi-scale and hierarchical
functional materials from low molecular weight molecules.
A schematic showing various structural arrangements among
the components in the FLAG are depicted in Scheme 1.
Fig. 8 (a) Nyquist plots of GCE, LAP/GCE and FLAG/GCE. (b) Cyclic voltam-
mograms of FLAG/GCE with scan rates ranging from 0.01 to 0.1 V sꢀ1
.
voltammograms of FLAG4 is shown in Fig. 8b. They showed a
good rectangular electrochemical window between ꢀ1 and 1.5 V
3.3 Electrochemical properties
Electrochemical properties were evaluated using various tech- and hence these gel electrolytes are suitable for application in
niques such as cyclic voltammetry, electrochemical impedance capacitors and batteries. The stability of the electrochemical
spectroscopy and galvanostatic charge–discharge studies. The window is retained even at higher scan rates and with high
electrochemical impedance spectroscopy (EIS) is used to study current density.
the electron transfer properties of the surface modified electrodes.
The galvanostatic charge–discharge measurements were carried
The EIS data were analyzed via Nyquist plot and it consists of two out within the potential window of 0 to 1.5 V at various current
regions – a semicircle region lying in the Z00 axis followed by a densities ranging from 0.02 to 0.41 A gꢀ1 and are shown in Fig. 9a.
straight line. The semicircle region at higher range of frequencies Galvanostatic charge–discharge studies showed symmetrical
corresponds to the electron transfer kinetics and the linear part at charge–discharge profiles suggesting that FLAG4 exhibits ideal
the lower range of frequencies represents diffusion limited elec- electrical double layer capacitive behaviour without any redox
tron transfer processes.50 The Nyquist plots of GCE, LAP/GCE and reaction involved. For a constant current density of 0.02 A gꢀ1
,
FLAG/GCE with Fe(CN)63ꢀ/4ꢀ are shown in Fig. 8a. The equivalent the specific capacitance of the cell was calculated as 1856 mF gꢀ1
circuit diagram used to fit the data is shown in the inset of Fig. 8a illustrating the excellent energy storage capability of the material.
and is composed of solution resistance (Rs), constant phase The specific capacitances of FLAG4 were calculated to be 1856,
element (CPE, Q), electron transfer resistance (Rct) and Warburg 1799, 1676, 1640, 1610 and 1598 mF gꢀ1 at current densities
impedance (W). With LAP/GCE, the diameter of the semicircle of 0.02, 0.04, 0.06, 0.08, 0.16 and 0.41 mF gꢀ1, respectively.
region decreases indicating a low electron transfer resistance. On increasing the current density, we observed a steady low-
In the case of FLAG/GCE, the semicircle region again deceases ering of specific capacitances due to faster discharging. Cycling
and it exhibits very low impedance. The Rct values of bare GCE, performance plays an important role in determining the super
LAP/GCE and FLAG/GCE are 1124 O, 640 O and 245 O, respectively. capacitors for real applications.34,51 Capacity retention of the
The high conductivity of FLAG/GCE may be suggested due to device was analyzed by carrying out repetitive charging and
high anisotropic liquid crystalline ordering of FLAG. The cyclic discharging for 100 cycles. The variation of specific capacitance
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electrochemical window of ꢀ1 to 1.5 V suggesting its utilization
as a gel electrolyte for energy storage devices. Further galvanostatic
charge–discharge studies using FLAG as an electrolyte in a super-
capacitor showed a symmetrical charge–discharge profile, a specific
capacitance of 1856 mF gꢀ1 for a constant current density of
0.02 A gꢀ1, and very good cycling stability. These studies revealed
that FLAG can be considered as an excellent electrolyte material
in the area of energy storage.
Acknowledgements
We are thankful to Dr A. Ajayaghosh, Director, CSIR-NIIST,
Trivandrum, for the constant encouragement and support.
We would also thank to Ms. Lucy Paul, Mr Kiran Mohan and
Dr E. Bhoje Gowd for SEM, TEM and XRD analyses. We are also
thankful for the financial support from CSIR network project
MULTIFUN (CSC0101) and CSIR-SRF fellowship.
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