Proton conductive cationic nanoporous polymers based on smectic liquid crystal hydrogen-bonded heterodimers

Article abstract of DOI:10.1039/c8tc00857d

The fabrication of a cationic nanoporous smectic liquid crystal network (LCN) based on hydrogen bonded heterodimers is presented. The method relies on a supramolecular complex made from a pyridyl bearing reactive mesogen hydrogen bonded to a non-reactive benzoic acid template. Upon addition of a cross-linker, a smectic liquid crystalline phase is obtained that can be fixed by photopolymerization. It was found that the lamellar structure was maintained after template removal when 25 wt% or more cross-linker was used, yielding a nanoporous LCN. After H₃PO₄ immobilization in the pores of the LCN, a cationic 2D nanoporous polymer is obtained showing high and anisotropic anhydrous proton conductivity.

Full text of DOI:10.1039/c8tc00857d

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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JPolueransael odfoMnaotteraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/C8TC00857D
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Proton conductive cationic nanoporous polymers based on
smectic liquid crystal hydrogen-bonded heterodimers
Dirk-Jan Mulder,a,e,† Ting Liang,a,b,c,† Yifei Xu,d Jeroen ter Schiphorst,a Luc M. W. Scheres,a,h Bernette
Received 00th January 20xx,
Accepted 00th January 20xx
M. Oosterlaken,d Zandrie Borneman,b,g Kitty Nijmeijer,*,b,g Albertus P. H. J. Schenning*,a,f
DOI: 10.1039/x0xx00000x
The fabrication of a cationic nanoporous smectic liquid crystal network (LCN) based on hydrogen bonded heterodimers is
presented. The method relies on a supramolecular complex made from a pyridyl bearing reactive mesogen hydrogen bonded
to a non-reactive benzoic acid template. Upon addition of a cross-linker, a smectic liquid crystalline phase is obtained that
can be fixed by photopolymerization. It was found that the lamellar structure was maintained after template removal when
25 wt% or more cross-linker was used, yielding a nanoporous LCN. After H3PO4 immobilization in the pores of the LCN, a
cationic 2D nanoporous polymer is obtained showing high and anisotropic anhydrous proton conductivity.
www.rsc.org/
Liquid crystals (LCs) have been proven to be promising candidates for
the construction of nanoporous polymer materials with pore
dimensions around 1 nm.12 For the construction of these materials,
Introduction
Nanoporous polymer materials are of scientific and technological
importance because of their large pore surface area to volume ratio
and their specific interactions with ions and molecules on the
nanometer scale.1,2 This makes these polymers suitable for
applications such as separation, catalysis, sensing and
conductance.3–6 The properties of nanoporous materials depend on
the pore size, porosity, pore distribution, pore alignment and pore
chemistry. Together these characteristics determine the potential
applications.7,8 The fabrication of nanoporous materials with tailored
pores and programmed properties is, however, still a main challenge.
9 For example, in the field of proton exchange membrane fuel cells,
proton conductive nanoporous membranes that can be applied at
intermediate temperatures (120 – 200 °C) are required while most
materials exhibit high performance under humid condition but lose
conductivity when the temperature rises above 100 °C.10,11
Therefore, anhydrous proton conductive polymers with ordered
protic nanopores are highly demanded.5,6
columnar, lamellar and bicontinuous cubic phases have been used
leading to 1D, 2D and 3D nanoporous polymers, respectively.13–15
Currently, many of these nanoporous materials are based on
hydrogen bonded building blocks where these non-covalent
interactions have been used to position a non-reactive template
molecule16,17 and where polymerization is used to create a network.
The non-reactive template acts as a so-called porogen, which after
removal creates nanopores with dimensions in compliance with the
shape of the template.18–22 So far, most of the pores are negatively
charged carboxylates and often columnar or cubic phases are
used.19,23–26 and to date, there is only one example where the
templating method has been applied to smectic LC materials yielding
2D nanoporous polymers with negatively charged pores27. For the
use in e.g. separation and transport processes, however, it is of
upmost interest to fabricate positively charged cationic 2D
nanoporous polymers, as this will broaden the range of applications.
a.Department of Functional Organic Materials and Devices, Chemical Engineering
and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB,
Eindhoven, The Netherlands.
b.Membrane Materials and Processes, Chemical Engineering and Chemistry,
Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The
Netherlands.
c. School of Chemical Engineering, Sichuan University No. 24 South Section 1, Yihuan
Road, Chengdu 610065, China.
d.Laboratory of Materials and Interface Chemistry, Chemical Engineering and
Chemistry, Eindhoven University of Technology, PO box 513, 5600 MB, Eindhoven,
The Netherlands.
e. Dutch Polymer Institute (DPI), PO Box 902, 5600 AZ, Eindhoven, The Netherlands.
f. Institute for Complex Molecular Systems (ICMS), Eindhoven University of
Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands.
g.Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ,
Eindhoven, The Netherlands.
Figure 1 (a) Chemical structures of the monomer mixture containing the hydrogen
bonded 6SzMA•6OB complex and the cross-linker (CL). (b) Schematic representation of
the fabrication of the cationic nanoporous smectic polymer films from the hydrogen
bonded heterodimers and cross-linker.
h.Surfix BV, Bronland 12 B-1, 6708 WH, Wageningen, The Netherlands.
† Both authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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In this work, the fabrication of such a positively charged (cationic) Preparation of the liquid crystal network
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nanoporous smectic polymer, based on hydrogen bonded A mixture containing 6SzMA•6OB and variDoOusI: 1a0m.1o0u39n/tCs8oTfC0C0L8w57aDs
heterodimers using a template method, is reported. This approach used to prepare LCNs (Figure 1). 1 wt% photo initiator (Irgacure 819)
relies on a complex made from a pyridyl reactive mesogen (6SzMA) and 0.5 wt% thermal inhibitor butylated hydroxytoluene (BHT) were
hydrogen bonded to a non-reactive porogen (6OB) and a cross-linker added to perform the photopolymerization. All compounds were
(CL). After cross-linking, upon chemical treatment, the porogen can dissolved in dichloromethane to obtain a homogeneous mixture. The
be removed, resulting in the formation of protic nanopores after acid dichloromethane was removed in vacuum after mixing. 20 μm thick
treatment. When using phosphoric acid the pyridine units are films were prepared by capillary suction of the LC-monomer mixture
protonated, yielding a highly anisotropic 2D proton-conducting in the melt (125 °C) between two accurately spaced glass slides (LC
material.28
cell). After filling of the cell, the mixture was cooled to 30 °C.
Subsequently, the polymerization was performed by UV irradiation
using a EXFO OmniCure® S2000 spot UV curing lamp for 600 seconds
equipped with a 405 nm cut-off filter. To obtain planar and
homeotropic alignment, the glass slides (LC cell) were provided with
rubbed polyimide and octadecyltrimethoxysilane, respectively.
Experimental
Materials
6-Bromohexan-1-ol (> 95 %) was obtained from TCI Europe N.V.,
triethylamine (99 %) was obtained from Acros. Methacryloyl chloride
(97 %), 4-methylpyridine (98 %), 4-hydroxybenzaldehyde (98 %),
acetic anhydride (≥ 99 %), sodium borohydride (> 96 %) and cesium
carbonate (99 %) were obtained from Sigma Aldrich. Cross-linker (CL)
6OB removal
Small pieces of pristine 6SzMA•6OB LCN were exposed to a 0.33 mM
KOH solution in a 2:1 tetrahydrofuran (THF)/water mixture overnight
under continuous shaking. Subsequently, the films were placed in
water to wash off the remaining KOH (Figure 1b). The films were
dried in vacuum prior to further use. To determine the pH at which
the pyridine moieties were protonated the films were immersed in
diluted HCl or citrate buffer, 1 M NaCl solutions of pH 1 to 6.
trans-4-(4-(11-acryloyloxyundecyloxy)
cyclohexanecarboxyloxy)
phenyl-4-(11-acryloyloxyundecyloxy) benzoate (Figure 1a) was
obtained from Syncom B.V. All solvents used were analytical grade
and obtained from Biosolve. Dry ethanol was prepared by storing it
overnight over 3 Å molecular sieves (activated in an oven at 600 °C).
1 M HCl (aq.) was freshly prepared from a 37 % HCl solution obtained
from Sigma Aldrich. All reagents were used as received, without
further purification.
H3PO4 immobilization
Nanoporous polymer films (25 % CL, 6OB removed) were exposed to
a freshly made solution of 0.1 M H3PO4 (68 μl in 10 ml solvent) and
were shaken for 24 hours. Different H3PO4 occupations were
obtained by using different solvents. Occupations of 1.06 equivalents
(eq.) or 2.54 eq. H3PO4 were obtained by using THF/H2O (5 ml: 5 ml)
or pure THF (10 ml) as solvent, respectively. The H3PO4 immobilized
films were washed with ethanol (Figure 1b) and at room temperature
vacuum dried before further characterization.
Synthesis of 4’-(6-methacyloxyhexyloxy-4-stilbazole) (6SzMA)
The methacrylic 6SzMA monomer was prepared by a 4-step
synthesis route (Scheme 1). In the first step (a), 4-acetoxystilbazole
(i) was prepared according to the procedure as described by Shaw et
al.29 and Chiang et al.30 It was found that by adding a small amount
of 4 Å molecular sieves to the reaction mixture, side reactions were
reduced and the yield increased from 30 ~ 40 % to more than 70 %.
Subsequently, the acetic acid ester i was hydrolyzed by reflux in
ethanolic KOH yielding 4-hydroxystilbazole (ii, reaction (b)). In
parallel, 6-bromohexyloxymethacrylate was prepared by the
procedure described by Stumpel et al.31 Finally, 6SzMA was obtained
by a Williamson ether synthesis (reaction (d)) of ii and iii using cesium
carbonate (Cs2CO3) in anhydrous dimethylformamide at room
temperature. The synthesis details are given in the supporting
information.
Characterization
Nuclear magnetic resonance was performed on a 400 MHz Agilent
Technologies 400-MR NMR Spectrometer. Infrared spectroscopy was
performed on a Varian 670 IR spectrometer equipped with a
microscopy setup over a range of 4000 - 650 cm−1 with a spectral
resolution of 4 cm−1 and 100 scans per spectrum. Polarized optical
microscopy was performed on a Leica DM 4 P optical microscope
equipped with polarization filters. A Linkam TMS 600 hot stage was
used for temperature controlled experiments. Differential scanning
calorimetry was performed on a TA Instruments Q1000 calorimeter.
The samples were heated and cooled with 5 °C min−1 between −40 °C
and 130 °C with an isothermal equilibration of 3 minutes after each
heating or cooling ramp. X-ray diffraction experiments were
performed on
a SAXSLAB GANESHA 300 XL system. UV-Vis
spectroscopy experiments were done using a Shimadzu UV-3102
spectrophotometer. Transmission electron microscopy imaging was
performed under 2.5 μm defocus on a FEI-Titan TEM equipped with
a field emission gun (FEG) and operating at 300 kV. Images were
recorded using a 2k×2k Gatan CCD camera equipped with a post
column Gatan Energy Filter (GIF), with an electron dose of 15 and 50
e Å–2 per image for HAuCl4 and H3PO4 infiltrated samples,
respectively. For TEM observations, the membranes were embedded
Scheme 1 Synthesis of 6SzMA. (a) 4 Å molecular sieves, Ac2O, reflux 48 h, Yield:
79 %. (b) 1.1 eq. KOH, EtOH, reflux 16 h, Yield: 77 %. (c) Triethylamine, CH2Cl2,
−20 oC – r.t. 16 h, Yield: 57 %. (d) 3 eq. Cs2CO3, 4 Å molecular sieves, DMF, r.t.
24h, Yield: 86 %.
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in EMS Epofix embedding resin and cured at 70 °C for 1 hour. The °C to ‒15 °C (Figure S2c) indicated by the slightly more VhioewmAortgicelenOenoliunes
DOI: 10.1039/C8TC00857D
embedded sample was cross cut using a Reichert-Jung Ultracut-E texture and the observation of transition bands perpendicular to the
ultramicrotome set to 50 or 70 nm thickness. The cross sections were molecular director at TSmA–SmB by polarized optical microscopy.
transferred to a continuous carbon 200 square mesh copper grid by
using an EMS perfect loop.
Preparation and characterization of LCNs
Homogeneous planar aligned LCN films with a defined thickness (20
μm) were prepared by photopolymerizing the 6SzMA•6OB•CL
Anhydrous proton conduction
Electrochemical impedance spectroscopy (EIS) was recorded on an monomers (Figure 1a) in home-made LC cells. After UV irradiation for
Autolab potentiostat equipped with hot stage (accuracy: ±0.01 oC) in 10 min, the samples were polymerized as was confirmed by the
a frequency range from 1 Hz to 1 MHz (applied voltage: 10 mV). Gold disappearance of the peaks at 1640 cm−1 (C=C stretching vibration),
electrodes of a set area (0.28 cm2) were applied on both sides of the 1410 cm−1 (–CH2 deformation vibration), and 1290 cm−1 (–CH rocking
polymeric films. The conductivities were measured as a function of vibration) in the FTIR spectrum (Figure S3). The order and alignment
temperature between room temperature (22 °C) and 170 °C with 5 of the material was studied by POM and x-ray scattering
°C intervals. The polymer films were vacuum dried overnight at room measurements. From the small angle region of the pristine polymer
temperature and annealed at 120 °C for 20 min before starting the film x-ray diffractogram, one can see that the smectic layer spacing
measurement. The impedance spectrum can be modelled as an is strongly influenced by the amount of cross-linker. For films
equivalent circuit and divided into imaginary (Z’’) and real (Z’) containing 10, 25, and 50 wt% CL, a layer spacing of 4.05 (q=1.55 nm-
components. The resistance (Rb) was estimated from the 1), 4.40 (q=1.43 nm-1) and 4.82 nm (q=1.30 nm-1) was found,
intersection of the real axis (Z’) and the semicircle of the impedance respectively (Figure 2), which can be directly linked to the d-spacing
spectrum. The proton conductivities σ (S cm−1) are calculated using of the individual compounds and their respective amounts (Figure
the following equation:
2e, dashed line).33–35 The polymer film containing 10 wt% CL displays
a weak small angle signal that is split-up and is extremely broad,
which suggests the demixing of 6SzMA•6OB and CL occurs. The wide
angle area of the 2D x-ray scattering patterns (Figure 2a-c) show
lobes located at q ≈14 nm−1 which become narrower and more
intense with increasing cross-link density suggesting that the
orientational order is increasing (Figure 2f). 33 The radial integration
curves were fitted from θ = −0.5π to 0.5π with the maximum
intensity located at θ = 0 (Figure 2f) giving orientation order
parameters of 0.90, 0.73, and 0.58 for the LCNs containing 50, 25,
and 10 wt% CL.36
ꢁ
ꢀ = ꢂ ꢄ
1
ꢃ
Where d (cm) is the thickness of the film, A (cm2) is the area of the
film and Rb (Ω) is the resistance of the sample to conduction.
Results and discussion
Preparation and characterization of the monomer mixtures
To prepare the 2D nanoporous polymer containing cationic pores,
first the hydrogen bonded complex of reactive mesogen 6SzMA and
porogen 6-hexyloxybenzoic acid (6OB), 6SzMA•6OB, was studied in
detail (Figure S1a). FTIR spectra were taken from both compounds
and the 1:1 complex to study the hydrogen bonding (Figure S1b).
6OB displays a broad ‒OH band at 3000 cm−1 overlapping with the
sp3 hybridized C‒H bands (2860 and 2930 cm−1). The absence of the
broad signal at 3000 cm−1 in the 6SzMA•6OB spectrum and the
presence of two new broad peaks centered at 1900 cm−1 and 2480
cm−1 and a band at 1705 cm-1, attributed to hydrogen bonded O‒H
vibrations, confirm the formation of hydrogen bonds between 6OB
and the 6SzMA.32 Differential scanning calorimetry (DSC) revealed
that 6SzMA•6OB exhibits various liquid crystal transitions (Figure
S1c). By polarized optical microscopy (POM) the mesophases were
identified by slowly cooling from the isotropic phase (Figure S1d-g).
The 1:1 complex forms a smectic A (SmA) mesophase at 140 °C. Upon
further cooling a smectic C (SmC) mesophase is observed from 119
to 110 °C. Two much higher order tilted smectic mesophases are
observed from 80 °C to 62 °C and from 62 °C to −4 °C, where after
crystallization sets in. Subsequently, 6SzMA•6OB mixtures
containing various amounts of cross-linker (CL, Figure 1a) were
studied. The addition of CL led to changes in the phase behavior of
the monomer 1:1 mixture. Increasing the amount of CL resulted in a
decreased clearing temperature and the disappearance of tilted
smectic mesophases. For example, when 50 wt% CL is used, upon
cooling from the isotropic state, a narrow nematic mesophase is
observed at 119 °C ‒ 117 °C (Figure S2a), a SmA mesophase from 117
°C to 59 °C (Figure S2b), and a smectic B (SmB) mesophase from 59
Figure
2 Structural characterization of the pristine polymer films. (a-c) 2D x-ray
diffractograms of LCNs containing 10, 25, and 50 wt% CL, respectively. N.B. The visible
asymmetry and dark region is caused by the partial blocking of the scattered x-rays by
the sample holder. (d) Azimuthal integration of the small angle region of the x-ray
diffractograms obtained from MAXS measurements. (e) d-spacing as a function of xCL. (f)
Radial integration of the wide angle signal (q ≈ 14 nm−1).
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Preparation and characterization of the nanoporous polymers by spacing could be observed indicating that the lamellar structure
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DOI: 10.1039/C8TC00857D
template removal
within the LCN was lost. A d-spacing of 3.98 and 4.47 nm was
obtained for the materials consisting of 25 and 50 wt% CL,
respectively, corresponding to a decrease in layer spacing of 9.5 and
7.2 %. These numbers indicated that the removal of the 6OB led to a
partial disorder of the microscopic lamellar structure. Interestingly,
the d-spacing of the film containing 50 wt% CL is roughly twice the
length of a 6SzMA moiety (2.26 nm), indicating a head-to-head
organization while the sample containing 25 wt% CL, has a spacing
smaller than the length of two 6SzMA moieties, revealing a slight
interdigitation. These results show that for cross-link density above
25 wt%, the internal lamellar structure can be preserved despite a
decrease of the orientational order of the materials after removal of
the 6OB moiety. From the radial integration of the wide angle XRD
signal (Figure S5), the orientational order parameter was estimated,
and values of 0.37, 0.48, and 0.78 for 10, 25, and 50 wt% CL,
respectively, were obtained. TEM image of the 6SzMA•6OB LCN with
50 wt% CL film was taken to visualize the nanostructures. Auric
chloride (HAuCl4) was used as staining agent as it will bind to the
pyridyl moieties (Figure 3f). After the infiltration, first transmission x-
ray diffraction measurements (Figure S6) revealed that the position
of the signals originating from the lamellar spacing did not change a
lot compared to the nanoporous polymer film (q = 1.50 nm−1, d = 4.2
nm). Subsequently, the film was ultramicrotomed perpendicular to
the molecular director and ∼50 nm thick cross-sections were
examined by transmission electron microscopy revealing a regular
lamellar organization with straight and well-defined layers. From the
FFT-transformed micrograph (Figure 3f, inset 1) a well-defined
spacing of 4 nm was found, which is in agreement with the distance
found earlier by x-ray diffraction. The zoom-in TEM image shows the
thickness of the (AuCl4)‒ layers is ~1 nm (Figure 3f, inset 2). This data
shows that 2D nanoporous polymer films are obtained having
straight and uniform pores.
In order to obtain nanoporous polymers, the 6OB template was
removed from the LCN (Figure S4a). In the FTIR spectrum, signals at
2500, 1920, and 1680 cm−1 disappeared, indicating the cleavage of
the hydrogen bonds. Furthermore, the relative decrease in intensity
of the sp3 C–H bonds located at 2930 and 2855 cm−1 indicate that
aliphatic material (6OB tail) is removed from the LCN (Figure 3a). The
quantitative removal of 6OB was confirmed by weighing small pieces
of film before and after the treatment. Indeed, the weight loss
measured corresponds to the initial mass of 6OB added to the
mixture, as seen in Figure 3b. The macroscopic dimensional changes
of the film after removal of 6OB and drying of the film were also
investigated. Perpendicular to the molecular director, a small
expansion (1.8 - 3.4 %) was observed while parallel to the molecular
director a more significant contraction of the film was noticed (Figure
S4b). The contraction is highly dependent on the concentration CL
present (xCL, wt%), and for the films containing 10, 25 and 50 wt% CL
a contraction of 18, 11 and 7 % was measured, respectively.
Fabrication of cationic pores and anhydrous proton conduction
As an important potential application of this well-defined
nanoporous material is in proton conduction (e.g. fuel cells), the
proton conducting properties of the material were investigated.
Since the pyridyl pore surface in the nanoporous LCN is basic,
phosphoric acid, a popularly used proton conducting component28 is
immobilized into the anisotropic lamellae. Before exposing the
nanoporous LCNs to the phosphoric acid, the pKa of the pyridine was
determined to identify under which conditions cationic pores are
formed. Visually, a colour change from nearly colourless to bright
yellow was observed for the films held in low pH solutions (typically
< pH 3), indicating the protonation of the stilbazole.37 After 16 hours,
the films were removed from the solutions and measured by FTIR to
obtain quantitative information (Figure S7). A broad peak cantered
at 2600 cm−1 (attributed to the N–H bond) appeared and was
followed to monitor the protonation. Materials containing 10 and 25
wt% CL had a similar pKa (∼3.1), while for the material containing 50
wt% CL, a pKa of roughly 1.8 was estimated. It is known from
literature that stilbazole derivatives have a pKa of 4.7.38 This suggests
that the cross-linked network influences the acidity of the pyridyl
nitrogen, whereby stronger networks require stronger acids to
protonate the stilbazole. Both films containing 10 wt% and 25 wt%
CL are easily protonated and display a high proton capacity (2.32 and
Figure 3 Removal of the 6OB template. (a) FTIR spectra of a pristine LCN containing 25
wt% CL (short-dash line) and the same film after removing 6OB template (solid line). (b)
Calculated and measured relative weight loss of the samples after removing the 6OB
template of samples containing 10, 25, 50 wt% CL. 2D x-ray diffractograms of
nanoporous LCNs containing (c) 10, (d) 25, and (e) 50 wt% CL, respectively. (f) High
magnification TEM micrograph of the central area of the 50 wt% CL film. Inset 1: FFT of
the micrograph. Inset 2: The zoom-in image of the layers.
XRD measurements were also performed to investigate the impact
of the 6OB removal on the microscopic structure of the material
(Figure 3c-e). For the sample containing only 10 wt% CL, no layer
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1.78 mmol g−1, respectively) as compared to the 50 wt% CL sample The proton conductivity was measured using elVeiecwtrAorctihcleemOnilcinael
(1.05 mmol g−1). The mechanical and structural properties of the impedance spectroscopy (EIS). The impedance spectrum resembles
DOI: 10.1039/C8TC00857D
nanoporous LCNs film containing 10 wt% CL were poor compared to the equivalent circuits containing an interfacial capacitance (CPE1),
the 25 wt% CL films. Therefore, based on these results, only the 25 the sample field capacitance (CPE2), and the sample resistance (Rb)
wt% CL films were used for proton conductivity.
(Figure S10). At each given temperature, the resistance Rb was
estimated from the intersection of the real axis (Z’) and the semicircle
of the impedance spectrum. The proton conduction of planar aligned
LCNs containing 1.06 eq. H3PO4, 2.54 eq. H3PO4, and the initial 6OB
porogen are depicted Figure 4b. The conductivity increases with
increasing temperature. The Arrhenius behavior (linear on the log (σ)
versus T−1 (K) scale) of this curve implies that the proton conduction
is dominated by the “hopping” (Grotthuss) mechanism.39,40 The
conductivity of LCNs containing 1.06 eq. and 2.54 eq. H3PO4 reaches
a maximum conductivity of about 3 × 10−5 S cm−1 and 8 × 10−4 S cm−1
at 170 °C, respectively. This is a great, more than four orders of
magnitude, enhancement compared with the 6SzMA•6OB LCN,
which reaches a maximum conductivity of only 2 × 10−8 S cm−1. The
values are higher than other reported LC based nanoporous polymer
materials41–44. The values are, however, two to three orders of
magnitude lower than the best performing anhydrous proton
conductive materials, i.e. H3PO4 based polymers,45,46 reported so far.
Immobilization of the phosphoric acid in the pores was achieved by
exposing the nanoporous LCN films to a 0.1 M H3PO4 solution in
either pure THF or a 1:1 (v:v) THF/water mixture. After exposure, the
obtained films were examined using transmission FTIR spectroscopy
(Figure S8a). A broad peak was observed ranging from 3400 cm-1 to
2000 cm-1, originating from the stilbazolium N+–H and phosphoric
acid –OH vibrations. Furthermore, the films that contain phosphoric
acid using pure THF as solvent show a higher absorbance than films
where a THF/water mixture was used, indicating a higher degree of
occupation. Figure S8b shows that when using pure THF, and
assuming only electrostatic interactions (1.78 mmol g-1), an
occupation level of 2.54 eq. H3PO4 was obtained. It is known that
phosphoric acid can form strong intermolecular hydrogen bonds in
solution, therefore hydrogen bonded H3PO4 clusters could be
incorporated, explaining the high degree of occupation. When a 1:1
(v:v) THF/H2O mixture was used, an occupation level of 1.06 eq.
H3PO4 was obtained. This indicates that the addition of water, leads To demonstrate that the film benefits from the straight pores
to a lower degree of occupation by promoting the dissociation of the through the thickness of the film, homeotropic aligned films that
hydrogen bonds between H3PO4 molecules in solution. A TEM image have the pores oriented parallel to the electrodes were prepared as
of the 2.54 eq. H3PO4 film (with 25 wt% CL) was taken to visualize well. A comparable degree of occupation was achieved using the
the nanostructures (Figure 4a). Well-organized structures having H3PO4 infiltration methods mentioned before. Figure 4c shows the
H3PO4 layers of about 1 nm were again visible. However, it should be proton conduction in both planar and homeotropic aligned films
noted that also less ordered areas (Figure S9) were found in the film. containing 1.06 eq. of H3PO4. At high temperatures, the proton
The latter could also be due to the lower contrast of H3PO4 conduction in homeotropic aligned films was two orders of
compared to the HAuCl4 infiltrated films.
magnitude lower than when using planar aligned films, indicating
that the orientation of the pores contributes significantly to the high
conductivity. In the homeotropic aligned films, the polymer layers act
as insulating layers, hindering the traveling of protons and revealing
that the transport of protons through straight pores is preferred, as
anticipated. Peculiarly, the temperature dependence of the
conductivity does not show Arrhenius behavior. At low temperature,
only a moderate increase in conduction is observed. This is most
likely because the protons have to cross the polymeric layers as
mentioned above. However, once passed the ∼100 °C, the mobility
increases so the protons can more easily cross this barrier.
Thereafter, the temperature dependence of the conduction behavior
is similar to the planar sample, yet, at lower conductivity. The proton
conduction of the homeotropic films containing 2.54 eq. of H3PO4
(Figure 4d) was measured too. Despite a higher conductivity for the
planar sample, the difference in proton conductivity with the
homeotropic aligned films was substantially smaller than when 1.06
eq. H3PO4 were used. In contrast to the films with a lower H3PO4
occupation as discussed before, here both films show the same
temperature dependence curve with a one order of magnitude
difference in conductivity, indicating that the orientation of the
anisotropic pores play a less important role. It is expected that the
excess of phosphoric acid facilitates the transport of protons across
the polymeric layers.
Figure 4 Proton conduction of H3PO4 infiltrated films containing 25 wt% CL. (a) High
magnification TEM micrograph of the 2.54 eq. H3PO4 infiltrated film. Inset 1: FFT of the
micrograph. Inset 2: The zoom-in image of the layers. (b) Anhydrous proton conduction
in a planar aligned pristine (6SzMA•6OB) film (orange open triangles), and planar
aligned nanoporous films containing 1.06 eq. H3PO4 (dark green open squares), and 2.54
eq. H3PO4 (light green open circles), respectively. (c) Anhydrous proton conduction in
planar (dark green open squares), and homeotropic aligned (red open diamonds)
nanoporous films containing 1.06 eq. H3PO4. (d) Anhydrous proton conduction in planar
(light green open squares), and homeotropic aligned (red open triangles) nanoporous
films containing 2.54 eq. H3PO4.
Conclusions
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Journal of Materials Chemistry C
Page 6 of 7
ARTICLE
Journal Name
Cationic 2D nanoporous polymers have been prepared by a
template method based on supramolecular smectic liquid
crystals. It was found that the layered structure was maintained
after template removal when 25 wt% or more cross-linker was 11
used. This facile method can be applied to other porogens and
cross-linkers to tune the pore size by a modular approach.
Proton conductivity measurements show that cationic 12
nanoporous materials exhibit a high and anisotropic proton
conductivity, making them appealing as polymer electrolytes in 13
for example fuel cells. The H3PO4 interacts with the positively
chargeable pyridine groups resulting in ordered protic 14
nanopores that facilitates the proton conduction. Furthermore,
the decrease in conductivity in homeotropic oriented films
shows that the orientation of the 2D plane plays an important 15
role. Our results show that 2D cationic nanoporous polymers
can be fabricated with appealing functional properties. It is
foreseen that also pH independent cationic nanoporous 16
polymers can be fabricated by alkylation of the pyridine units.
Such type polymers are interesting to separate, adsorb or sense
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Acknowledgements
This research forms part of the research program of the Dutch
Polymer Institute (DPI), project 776. Ting Liang is financially
supported by China Scholarship Council (CSC). The authors
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