Side-chain-type anion exchange membranes bearing pendant quaternary ammonium groups: Via flexible spacers for fuel cells

Article abstract of DOI:10.1039/c6ta05090e

To realize high performance anion exchange membranes (AEMs) for alkaline fuel cells (AFCs), a series of quaternized poly(ether sulfone)s (PESs) with different lengths of flexible spacers linking cationic groups and the backbone was synthesized via nucleophilic polycondensation, demethylation and Williamson reactions. Atomic force microscopy (AFM) phase images show clear hydrophilic/hydrophobic phase separation for all the side-chain-type AEMs. The PES-n-QA membrane with hexyleneoxy spacers (n = 6) between the cationic groups and backbone (benzene ring) exhibited the maximum conductivity of 62.7 mS cm^-1 (IEC = 1.48 meq. g^-1) at 80 °C. The AEM materials are found to have an improved long-term alkaline stability by extending the length of the flexible spacer (n ≥ 4). The PES-12-QA membrane with a flexible dodeceneoxy spacer demonstrated the highest alkaline stability, where the conductivity and IEC only decreased by 8.1% and 6.9% after immersing in a 1 M aqueous KOH solution at 60 °C for 720 h. Furthermore, the single fuel cell performance test using PES-6-QA as an AEM showed a maximum power density of 108.3 mW cm^-2 at a current density of 250 mA cm^-2 at 60 °C.

Full text of DOI:10.1039/c6ta05090e

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DOI: 10.1039/C6TA05090E
Journal of Materials Chemistry A
ARTICLE
Side‐chain‐type anion exchange membranes bearing pendant
quaternary ammonium groups via flexible spacer for fuel cells
Received 00th January 20xx,
Accepted 00th January 20xx
Chen Xiao Lin, Xiao Ling Huang, Dong Guo, Qiu Gen Zhang, Ai Mei Zhu, Mei Ling Ye, Qing Lin Liu*
To realize high performance anion exchange membranes (AEMs) for alkaline fuel cells (AFCs), a series of quaternized poly
(ether sulfone)s (PESs) with different lengths of flexible spacer linking cationic groups and backbone was synthesized via
nucleophilic polycondensation, demethylation and Williamson reaction. Atomic force microscopy (AFM) phase images
show clear hydrophilic/hydrophobic phase separation for all the side‐chain‐type AEMs. The PES‐n‐QA membrane with
hexyleneoxy spacers (n=6) between the cationic groups and backbone (benzene ring) exhibited the maximum conductivity
of 62.7 mS cm‐1 (IEC=1.48 meq g‐1) at 80 oC. The AEM materials are found to have an improved long‐term alkaline stability
by extending the length of flexible spacer (n≥4). The PES‐12‐QA membrane with flexible dodeceneoxy spacer
demonstrated the highest alkaline stability, where the conductivity and IEC only decreased by 8.1% and 6.9% after
immersing in a 1 M aqueous KOH solution at 60 oC for 720 h. Furthermore, the single fuel cell performance test using PES‐
6‐QA as an AEM showed a maximum power density of 108.3 mW cm‐2 under current density of 250 mA cm‐2 at 60 oC.
DOI: 10.1039/x0xx00000x
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condition and enable the use of non-noble metals (such as Co,
Ni) instead of precious Pt as electrocatalysts, thus significantly
lowering the cost of fuel cell devices.6,7 Furthermore, the fuel
Introduction
Fuel cells are considered as one of the most promising power
conversion devices for stationary and mobile applications due
to their high energy density, high efficiency, and low pollution.1
During the past decades, fuel cells based on proton exchange
membranes (PEMs) have been the subject of intense research.2,3
Solid polymer membranes serve as an ion conductor and a
barrier for fuel and oxidant is a critical material for fuel cells.
PEMs such as Nafion (Dupont) are the most noted solid
polymer membranes due to their high proton conductivity,
robust mechanical properties and excellent chemical stability.4
However, the large-scale commercialization of Nafion-based
proton exchange membrane fuel cells (PEMFCs) is impeded by
their high fuel permeability and strong dependency upon the
expensive noble metal (e.g. Pt) catalysts.5 Compared with
PEMFCs, alkaline fuel cells (AFCs) based on anion exchange
membranes (AEMs) take the advantages of faster electrode
reaction kinetics for oxygen reduction reaction under alkaline
permeation was inhibited because the conducting direction of
hydroxide ions is opposite to fuel crossover.8 Although the
AFCs have many advantages, the practical application of solid
polymer membranes in AFCs still faces the challenges of low
conductivity and poor alkaline stability of most current AEMs.
In recent years, a variety of AEM materials based on
poly(sulfone)s,9,10 poly(arylene ether)s,11 poly(phenylene
oxide)s,12-14
poly(styrene)s,15,16
poly(olefin)s,17,18
polybenzimidazoles,19,20 have been reported in developing high
performance AEMs. In the majority of these reports, AEMs are
prepared via chloromethylation or bromination, followed by the
Menshutkin reaction with tertiary amine. However, there are
some disadvantages that limit their use in preparing AEMs.
Chloromethyl methyl ether is commonly used for
chloromethylation reaction, but the reagent is carcinogenic and
harmful to human health. Although bromination of
benzylmethyl groups using N-bromosuccinimide can avoid the
use of toxic reagent, their quaternary ammonium (QA) groups
are directly attached to benzyl. This will make the QA groups
vulnerable to being attacked by hydroxide ions under alkaline
condition because of the electron-withdrawing effect of
benzene ring.21 Furthermore, the QA groups are connected to
the backbone via short link (-CH2-), which will restrict the local
mobility of the ion groups and prevent the formation of micro-
phase separation morphology which seems to be essential for
fabricating high efficient ion channels.
Department of Chemical & Biochemical Engineering, College of Chemistry &
Chemical Engineering, Xiamen University, Xiamen 361005, China. E‐mail:
qlliu@xmu.edu.cn
† Electronic supplementary information (ESI) available: Quaternization reaction of 1, ω-
dibromoalkanes in THF; 1H NMR spectrum of Br-x-QA (x=3, 4, 6, 8, 12); 1H NMR
spectrum of PES-OCH3 and PES-OH; 1H NMR spectrum of PES-n-QA (n=3, 4, 6, 8,
12); The appearance (a, b), SEM images of (c) cross-section and (d) surface of PES-6-
QA; The number of absorbed water molecules around each QA group (λ) as a function
of flexible spacer length for PES-n-QA membranes at 30, 60 and 80 oC; 1H NMR
spectrum of (a) PES-4-QA, (b) PES-6-QA and (c) PES-8-QA stored in 1 M aqueous
KOH solution at 60 oC for 0 and 720 h, respectively; The proposed degradation
pathways of QA groups for PES-3-QA under alkaline condition. See
Recently, a few researchers focused on preparing side-chain-
type AEMs bearing long flexible spacers between the backbone
and cationic groups since they usually possess a greatly higher
DOI: 10.1039/x0xx00000x
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alkaline stability and conductivity than those AEMs with tetrahydrofuran (AR) (THF) were obtained from Sinopharm
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benzyl-substituted cations.22-25 The electron-withdrawing effect Chemical Reagent Co. Ltd and usedDOaIs: 10r.e1c0e39iv/Ce6dT.A0B5o09ro0nE
on QA groups is weaken because the cationic groups are far tribromide (99.9%), 1,3-dibromopropane (98%), 1,4-dibromo
away from the backbone, which could reduce the degradation butane (98%), 1,6-dibromohexane (97%), 1,8-dibromooctane
of cationic groups. Furthermore, the flexible spacers improve (98%) and 1,12-dibromododecane (98%) were purchased from
the local mobility of ion groups, which is favourable for Aladdin Chemistry Co. Ltd. Other chemical reagents were
fabricating hydrophilic/hydrophobic micro-phase separation obtained from Aladdin and used as purchased.
morphology and thus enhancing conductivity.26 However, it is
Preparation of side‐chain‐type AEMs
difficult to prepare polymer structure that contains long flexible
Synthesis of (ω‐bromoalkyl)trimethylammonium bromide (Br‐n‐QA)
side chain linking the cationic groups and the polymer
backbone.27 Thus, AEMs with such a side-chain-type structure
were rarely reported. Jannasch et al.22 reported a series of
quaternized poly (phenyl oxide) (PPO) with QA groups
attached to the backbone via long flexible side chain. The
hydroxide conductivity and alkaline stability of the membranes
were significantly enhanced over the QA groups located in
benzylic positions. Nevertheless, the chemical grafting using
highly flammable n-BuLi as the lithiation reagent is tedious
which should be carried out in water-free and oxygen-free
Br-n-QA was synthesized according to the literature with some
modifications.28 The synthetic route for preparing Br-n-QA
(n=3, 4, 6, 8 and 12), where n is the number of carbon atoms
linking the -Br groups and QA groups, is shown in Scheme 1.
Taking Br-6-QA as an example, 24.4 g (100 mmol) of 1,6-
dibromohexane and 150 mL of THF were placed into a 500 mL
three-necked round-bottomed flask equipped with a magnetic
stirrer. Trimethylamine gas was produced by heating
trimethylamine solution at a certain temperature and then was
introduced into the flask at a rate of 12 mL min-1 at RT for 4 h
through a grass tube which extended below the surface of the
reaction solution. The reaction mixture was stirred for another
24 h to obtain a white precipitate. The precipitate was filtered
and washed with THF for several times, followed by vacuum
drying at RT for 24 h to give a product. Br-n-QA (n=3, 4, 8 and
12) was synthesized using the same method. The chemical
structure of the products was characterized by 1H NMR spectra
in DMSO-d6.
conditions at a very low temperature (-78 C). Hence, there is
an urgent demand to design a strategy to facilely prepare side-
chain-type AEMs with similar structure. Furthermore, the
structure-property relationship, especially the effect of the
length of flexible spacer on the performance of the membrane,
which is important for designing AEMs with enhanced
conductivity and stability, is unclear.
In the present work, we prepared a series of novel side-chain-
type AEMs with various lengths of flexible spacer linking the
QA groups and the polymer backbone. In addition, hydroxyl-
bearing poly(ether sulfone) backbone was prepared via
nucleophilic substitution polycondensation and demethylation
Synthesis of hydroxyl‐containing poly(ether sulfone) (PES‐OH)
PES-OH was synthesized from methoxyl-containing poly(ether
sulfone) (PES-OCH3) via demethylation reaction as our
previous reports.29 A typical procedure for preparing PES-
OCH3 is described as follows (Scheme 1). To a 100 mL three-
necked flask equipped with a mechanical stirrer, a condenser, a
Dean-Stark trap and a nitrogen inlet/outlet, 5.0850 g of FPS (20
mmol), 1.0087 g of BPHF (3 mmol), 2.3824 g of MHQ (17
mmol), 5.5284 g of K2CO3 (40 mmol), 55 mL of DMAc and 15
reaction,
followed
by
grafting
(ω-
bromoalkyl)trimethylammonium bromides (Br-n-QA) into the
backbone to give side-chain-type copolymers. This synthetic
strategy avoids the use of highly flammable n-BuLi and the
chemical grafting process was carried out via Williamson
reaction, which is much easier than lithiation-grafting reaction.
Herein, Br-n-QA with different lengths of carbon chain was
designed to obtain various flexible spacers. In particular, the
effect of the length of flexible spacer between the QA groups
and backbone on the properties such as water uptake, swelling
ratio, hydroxide conductivity and alkaline stability of the AEMs
were investigated systematically.
mL of toluene were mixed and heated to 140
C for 4 h,
followed by heating up to 155 C for another 18 h. The reaction

mixture was cooled to RT and poured into a large excess of
methanol solution to precipitate the copolymers. The product
was filtered and washed with methanol thoroughly and dried
under vacuum at 80 C for 24 h (Yield: 96%).
For preparing PES-OH, 5.000 g of PAES-OCH3 was
dissolved into 100 mL of chloroform to form a solution in a 250
mL three-necked flask with a magnetic stirrer and a nitrogen
inlet/outlet. 4 mL of BBr3 was dropwise added into the PAES-
OCH3 solution at RT. After reacting for 12 h, the resulting
precipitate was filtered and washed thoroughly with a large
excess of methanol. Then, the solid was dried under vacuum at
80 °C for 24 h (Yield: 88%).
Experimental
Materials
Bis(4-fluorophenyl)
methoxyhydroquinone
sulfone
(MHQ)
(FPS)
(98.0%)
(99.0%),
and
2-
4,4'-
(hexafluoroisopropylidene) diphenol (BPHF) (98%) were
purchased from Tokyo Chemical Industry Co. Ltd and dried
under vacuum at room temperature (RT) overnight before use.
Potassium carbonate (K2CO3, 99%), toluene (AR),
trimethylamine solution (30% in water), dimethylacetamide
(AR) (DMAc), dimethyl sulfoxide (AR) (DMSO) and
Synthesis of quaternary ammonium‐functionalized poly(ether
sulfone)s (PES‐n‐QA)
Scheme 1 shows the procedure for preparing PES-n-QA (n=3, 4,
6, 8 and 12). Typically, the synthesis of PES-6-QA is given
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ARTICLE
O
S
CF3
View Article Online
DOI: 10.1039/C6TA05090E
HO
OH
x
+
F
F
+
(1-x)
HO
OH
O
CF3
OCH3
FPS
BPAF
MHQ
DMAc
K2CO3
toluene
140 oC 4 h
155 oC 18 h
O
S
O
CF3
CF3
O
S
O
O
O
O
O
x
1-x
OCH3
PES-OCH3
CHCl3
BBr3
RT 6 h
O
S
O
CF3
CF3
O
S
O
O
O
O
O
x
1-x
OH
PES-OH
DMSO
K2CO3
KI
Br
N+
Br
Br
THF
+
N
+
RT 24 h
Br-
90 oC 24 h
n=3, 4, 6, 8, 12
CF3
O
S
O
O
S
O
O
O
O
x
O
O
1-x
CF3
PES-n-QA
+ Br-
N
Scheme 1 Synthetic route of Br‐n‐QA, PES‐OCH3, PES‐OH and PES‐n‐QA.
as an example. 1 g of PES-OH (2.2858 mmol -OH) and 20 mL (DI) water to remove any residual free ions and immersing in
of DMSO were placed into a 100 mL round-bottomed flask DI water for more than 48 h before use.
equipped with a magnetic stirrer, a condenser and a gas
Characterization
inlet/outlet. The mixture was heated to 90 C with stirring until
all the polymer was completely dissolved to form a brown 1H nuclear magnetic resonance (NMR) spectra were recorded at
homogeneous solution. Then, Br-6-QA (1.0392 g, 3.4287 500 MHz on Bruker AV500 spectrometer using
a
mmol), K2CO3 (0.6318 g, 4.5716 mmol) and KI (0.0379 g, tetramethylsilane (TMS) as the internal reference. The
0.2283 mmol) were added and the reaction was continued at 90 molecular weight of the copolymers was determined by a gel

C for 24 h. After cooling to RT, the copolymer was permeation chromatography (GPC) system (Waters, USA)
precipitated in acetone and washed with acetone and water for using THF as the eluent and polystyrene as the standard. A
several times to remove residual DMSO and other salts, scanning electron microscope (SEM) (Sigma, Zeiss, Germany)
followed by vacuum drying at 80
brown solid (Yield: 95%).
Fabrication of membranes

C for 24 h to yield a light was used to observe the morphology of the membranes. Atomic
force microscopy (AFM) images of the AEMs were captured
on a DI Multimode V microscope (Bruker Co.) in a tapping
mode under ambient temperature with a relative humidity of
~60%. The characterization of the AEMs by small angle X-ray
scattering (SAXS) was carried out on a SAXSee-MC2 X-ray
diffractometer (Anton Paar, Austria) under vacuum at RT. The
The PES-6-QA (1 g) was dissolved in DMF (20 mL) to form a
5 wt% solution. The resulting solution was filtered through a
0.45 μm PTFE syringe filter and was then cast onto four glass
plates and dried at 60 C under vacuum for 24 h to make the
membranes were placed under vacuum at 60
measurements.
C for 24 h before
PES-6-QA membranes in bromide form. The resulting
membranes were peeled off and immersed in a 1 M KOH
solution at RT for 48 h to make the hydroxide form PES-6-QA
membranes, followed by repeatedly washing with deionized
Measurements
Ionic exchange capacity
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1
Ionic exchange capacity (IEC) was measured by both H NMR of the fully hydrated membrane samples was removed. The
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spectroscopy and back titration method. A hydroxide form membrane sample was cooled to -20
membrane was dried under vacuum at 80 C for 24 h and then C with a heating rate of 5
C min-1. The amount of free water
immersed in 30 mL of a 0.1 M HCl standard solution for 48 h. can be calculated by the integral area of the endothermic peaks
Subsequently, the solution was neutralized with a 0.1 M KOH near 0
C using the fusion enthalpy of water (334 J g-1).30,31 The
solution using phenolphthalein as the indicator. The IEC of the amount of bound water can be calculated by subtracting the free

C DaOndI: 1t0h.1e0n39h/eCa6tTeAd05to09200E




membranes can be calculated from equation 1.
water from the total amount of water.
Mechanical property and thermal stability
M1 M 2
IEC 
(1)
m
Mechanical properties were measured on an Instron 3343
universal testing machine with a stretching rate of 5 mm min-1
under ambient condition. The membrane samples were cut into
a dog-bone-shaped (5.0 mm × 2.0 mm in the gauge area). The
wet membranes were obtained by keeping them in DI water for
where
m (g) is the weight of the dry membranes, M1 and M2 are
the initial and final amount of H+ in the HCl solution,
respectively.
Water uptake and swelling ratio
Water uptake and swelling ratio were measured after drying the at least 24 h.
hydroxide form membranes under vacuum at 80 C for 24 h. The thermal decomposition of the membrane samples was

The dried membranes were immersed in DI water at a certain investigated on a thermogravimetric analyzer (SDT-Q600, TA
temperature for 24 h. Subsequently, the mass and instruments, USA). Prior to measurements, the samples were
length/thickness of the membranes were recorded. The water dried in an oven under vacuum at 120

C to a constant weight.
uptake of the membranes is calculated by
The thermal decomposition data were recorded in the
mw  md
temperature range from 50 to 700 C at a heating rate of 10 C
WU 
100%
(2)
min-1 under a nitrogen atmosphere.
md
Alkaline stability
where md is the mass of the dry membranes, mw is the mass of
The alkaline stability of the AEMs was determined by
immersing the membrane samples into a 1 M aqueous KOH
the wet membranes. The number of absorbed water molecules
around each QA group (
λ
) is calculated by
WU(%)10
IEC 18
solution at 60 C. During the test, the hydroxide conductivity
 
(3)
and 1H NMR spectroscopy were monitored. Prior to
measurements, the membranes were washed with DI water and
kept in DI water at RT for at least 48 h to remove residual KOH
solution.
The in-plane and through-plane swelling of the membranes is
estimated by the difference between the wet and dry
dimensions in the length or thickness direction, which can be
calculated by
Membrane electrode assembly fabrication and fuel cell
performance
Lw  Ld
Ld
SRin plane

100%
(4)
(5)
The catalyst ink was prepared by mixing Pt/C catalyst (40 wt%,
Johnson Matthey Co.) with PES-6-QA ionomer solution (5
wt%), water and ethanol. The mass ratio of the dried ionomer to
catalyst was controlled to be 2:8. Afterward, the catalyst inks
were sprayed onto both sides of the PES-6-QA membrane with
a thickness of 45 μm to prepare the catalyst-coated membranes
(CCMs). The Pt loading in both the anode and cathode was 0.5
mg cm-2. The effective area of the electrodes was 4 cm2. The
resulting CCMs were sandwiched between two pieces of carbon
paper (Toray TGP-H-060, Japan), followed by hot-pressing at 5
MPa and RT for 5 min to fabricate the membrane electrode
assembly (MEA). The H2/O2 fuel cell performance of the MEA
Tw Td
Td
SRthrough plane

100%
where Lw and Ld are the length of the wet and dry membranes,
and Tw and Td are the thickness of the wet and dry membranes,
respectively.
Hydroxide conductivity
The hydroxide conductivity (σ, mS cm-1) of the membrane
sample (size: 1 cm×3 cm) is calculated by
d
σ 
(6)
AR
where
1cm), and
The membrane impedance
d
is the distance between the reference electrodes (here
is the cross-sectional area of the membrane (cm2).
(kΩ) was recorded on a Parstat
A
was tested on a fuel cell test system at 60 C. The flow rate of
R
fully humidified H2 and O2 gases was controlled at 100 mL
263 electrochemical workstation (Princeton Advanced
Technology, USA) over the frequency ranging from 0.1 Hz to
100 kHz using the two-point probe alternating current
technique. The hydroxide conductivity measurements were
carried out under a fully hydrated condition by placing the cell
into DI water (degassing treatment) to equilibrate at least for 2
h and then to record the impedance spectrum.
min-1.
Results and Discussion
Synthesis and characterization of Br‐n‐QA and copolymers
(ω-bromoalkyl)trimethylammonium bromide (Br-n-QA) was
synthesized by the Menshutkin reaction of 1,ω-dibromoalkanes
with trimethylamine. Usually, the quaternization reaction can
take place at both ends of the 1,ω-dibromoalkanes depending
on the reagents and reaction condition.20,28 In the present work,
Water state analysis
Water state of the AEMs was studied using a low temperature
differential scanning calorimeter (DSC; HT-DSC 404, Netzsch,
Germany). Before measurement, the excess water in the surface
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Table 1 IEC, water uptake, swelling ratio and λ of the PES‐n‐QA membranes.
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DOI: 10.1039/C6TA05090E
IEC (meq g-1)
Swelling ratio (%)d
Membrane
Theo.a
Cal.b
Exp.c
Water uptake (%)d
In-plane
Through-plane
λd
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
1.62
1.58
1.51
1.45
1.34
1.60
1.56
1.51
1.44
1.34
1.58±0.03
1.55±0.05
1.48±0.03
1.42±0.04
1.31±0.04
53.0±0.9
59.4±1.1
77.5±1.0
58.7±0.9
44.0±0.5
20.1±0.3
21.0±0.4
22.0±0.3
19.7±0.5
15.5±0.4
25.7±0.5
28.1±0.3
29.4±0.4
23.3±0.4
17.9±0.3
18.6
21.3
29.1
23.0
18.7
a
b
1
c
d
Calculated from monomer ratio; Calculated from H NMR spectra. Measured by back titration. Determined at 30
average of three measurements.
C; an
(a)
(d)
(e)
(b)
(c)
Fig. 1 AFM phase images of (a) PES‐3‐QA, (b) PES‐4‐QA, (c) PES‐6‐QA, (d) PES‐8‐QA and (e) PES‐12‐QA. Scan box: 500 nm×500 nm
the quaternization reaction occurred exclusively at one end of 1, (PES-OH) via a Williamson reaction (Scheme 1). An excess of
1
ω-dibromoalkane using THF as the solvent, in which Br-n-QA Br-n-QA was used to enhance the grafting degree. The H
precipitated from the solution when it was formed and thus NMR spectra of PES-n-QA are shown in Fig. S3
†
. Take PES-6-
), the disappearance of the
, Supporting information) proton resonance arising from the phenolic -OH groups as a
indicated that Br-n-QA was obtained with a yield ranging from sharp peak around 10.1 ppm along with the appearance of new
56% to 68% (Table S1 ) without the di-quaternized by-product. peaks at 0.8 1.8 ppm and 3.0 4.0 ppm attributed to proton
Taking Br-6-QA as an example (Fig. S1(c) ), the peak around resonance from quaternary ammonium side chain demonstrated
1
prevented from forming the di-quaternized by-product. The H QA as an example (Fig. S3(c)
NMR spectroscopy analysis (Fig. S1
†
†
†


†
3.1 ppm is attributed to the -CH3 in the QA groups. The peaks that Br-6-QA was grafted into poly(ether sulfone)s backbone
around 3.3 and 3.5 ppm are assignable to the first -CH2- next to successfully. The integral ratio of the peaks from the QA
the QA groups and -Br groups, respectively. Additionally, the groups to aromatic proton peaks was in good accordance with
signals at 1.3, 1.4, 1.7 and 1.8 ppm are attributed to the the one calculated from feed ratio, indicating the near-
chemical shifts of other -CH2- groups in the alkyl chain. The quantitative reaction of the phenolic -OH groups. The ionic
integration ratio of H4 to H1 was 4.50 and agreed well with the exchange capacity (IEC) measured by back-titration agreed
1
theoretical value of 4.50, confirming the successful synthesis of well with that calculated from H NMR spectroscopy analysis
Br-6-QA without di-quaternized by-product. All other well- (Table 1), indicating that Br- was mostly replaced by OH-.
1
assigned H NMR spectra (Fig. S1
†
) confirmed the chemical
Morphological Characterization
structure of Br-n-QA. A nucleophilic polycondensation reaction
(Scheme 1) was used to synthesize PES-OCH3 with high Taking PES-6-QA as an example, transparent and flexible
molecular weight (Mn=47 kg mol-1, Mw=73 kg mol-1). The membranes were prepared by dissolving the ionic copolymers
demethylation reaction of PES-OCH3 using BBr3 as the in DMF and casting them on a flat Teflon sheet (Fig. S4(a)
demethylation reagent was carried out in chloroform as our and (b) ). The morphology of the as-synthesized AEMs was
previous report,29 and was confirmed by 1H NMR spectroscopy observed using SEM, as shown in Fig. S4(c)
and (d) . It is
analysis. As shown in Fig. S2 , all the peaks ranging from 6.6 seen that a smooth, homogeneous and dense membrane without
to 8.0 ppm are attributed to the aromatic protons on the benzene pores is formed after exchanging to hydroxide form.
rings. The peak around 3.7 ppm associated with proton signals Atomic force microscopy (AFM) was employed to further
†
†
†
†
†
from -OCH3 groups disappeared after demethylation reaction reveal the morphology of the AEMs. The AFM phase images of
and a new peak at around 10.1 ppm attributed to the phenolic- the PES-n-QA membranes with various side chains were
OH groups appeared. This confirms the successful synthesis of recorded in a tapping mode at ambient temperature. As shown
PES-OH.
in Fig. 1(a)-(e), the darker regions represent hydrophilic (ionic)
The side-chain-type PES-n-QA copolymers were synthesized domains, whereas the brighter ones represent hydrophobic
by grafting Br-n-QA into the poly(ether sulfone)s backbone domains.19 Clear hydrophilic/hydrophobic phase separation
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uptake of PES-8-QA and PES-12-QA.23 In addition, the number
of absorbed water molecules per QA grDoOupI: 1(0d.1e0s3i9g/nCa6tTeAd05a0s90λE)
30 oC
60 oC
80 oC
View Article Online
120
120
80
40
0
was calculated to overcome the IEC differences of the AEMs.
As shown in Table 1 and Fig. S6
accordance with water uptake and the value is ranged from 18.6
to 29.1 at 30 C.
†, the tendency of λ is in
80
40

The water in the AEMs is critical for ion dissociation and
fabricating hydroxide conducting channels. The water in the
membranes can be generally classified into two kinds: free
water and bound water.30 We use DSC to obtain the amount of
free water in the membrane because the free water exhibits a
melting point near 0
S7 . In order to analyse the content of free/bound water, the
hydration number was calculated and presented in Fig. S8

C,35 and the DSC curves are shown in Fig.
0
2
3
4
5
6
7
8
9
10 11 12 13
†
Number of carbon atoms in spacer unit (n)
†
.
Fig. 2 Water uptake and swelling ratio as a function of flexible
spacer length for PES‐n‐QA membranes at 30, 60 and 80 oC.
The AEMs exhibit a similar content of bound water near the
QA groups while the tendency for the content of free water is in
line with water uptake. The highest content of free water was
obtained by the PES-6-QA membrane. As shown below, the
water content in the membrane will have a great influence on
the hydroxide conductivity.
To demonstrate the influence of the length of flexible spacer
on the swelling behaviour of the AEMs, we compared the in-
plane swelling ratio of the PES-n-QA membranes at different
temperatures (Fig. 2). It is observed that the tendency of
swelling ratio clearly followed the water uptake. The swelling
ratio increased with increasing the length of flexible spacer
was observed for all of the AEMs. It should be noted that the
hydrophobic poly(ether sulfone)s backbone is immiscible with
the hydrophilic side chain, thus driving side-chain-type
copolymers to self-assemble and fabricate hydrophilic nano-
channels containing QA groups and water, which is beneficial
for conducting hydroxide ions (Fig. 2).32,33 The largest
hydrophilic domain was observed for PES-6-QA with
hexyleneoxy spacers, and not for those with shorter or longer
spacers. It may be that if n≤6 the enhanced local mobility of
when n
≤6. However, the AEMs with longer flexible spacer
cationic groups is beneficial for facilitating micro-phase
separation, while the hydrophobic alkyl chain between the
(n=8 and 12) exhibit lower swelling ratio than PES-6-QA. For
example, the in-plane swelling ratio of PES-6-QA is 22.0% at
backbone and QA groups become too long (n
≥ 6) the
30

C, and is higher than that of PES-8-QA (19.7%) and PES-
decreased hydrophilicity of the side chain may result in the
weakening of the hydrophilic/hydrophobic phase separation
degree. Hence, PES-6-QA exhibited more distinct micro-phase
separation than other membranes. In the SAXS results, however,
no obvious scattering peak was found for the membranes (Fig.
12-QA (15.5%) at 30 C. This can be attributed to the increased

hydrophobicity by introducing many more methylene groups in
the flexible spacer that restrict water adsorption and thus inhibit
the swelling. Additionally, Table 1 shows the dimensional
swelling in the in-plane and thickness directions at 30 C. The
S5†). It should be noted that the poly(ether sulfone)-based
through-plane swelling ratio of the PES-n-QA membranes is
found to be ~1.35-fold higher than the in-plane swelling ratio
indicating a mild degree of anisotropy.
It is necessary to maintain a balanced water uptake because
too much water will result in high swelling and thus decrease
mechanical properties of the AEMs. All the side-chain-type
AEMs show a moderate water uptake and swelling ratio
indicating that these membranes can meet the demands for
MEA application and fuel cell operation.
materials are inherent with the semi-crystalline nature and
display no or weaker characteristic peaks in SAXS than the
crystalline polymers such as poly(phenylene oxides)
derivatives,22,32-34 but the AFM phase images were clear enough
to conduct the morphological analysis for the side-chain-type
AEMs. As shown below, the phase separation morphology has
a strong effect on the hydroxide conductivity.
Water uptake and swelling behaviour
Fig. 2 shows the water uptake and in-plane swelling ratio of the
PES-n-QA membranes in hydroxide form after immersing in
water at 30, 60 and 80 C. As expected, the water uptake of all
the membranes increased with temperature. It is clearly found
that the water uptake depends strongly on the length of flexible
spacer linking the QA groups and backbone. Increasing the
length of flexible spacer, an increase in water uptake was
Hydroxide conductivity
The conductivity of the AEMs was measured under a fully
hydrated condition. Fig. 3(a) shows temperature dependence of
the conductivity of the AEMs in hydroxide form. The tendency
of hydroxide conductivity is found to be similar to water uptake,
suggesting that ion transport is strongly dependent on water
content in the membranes. As expected, the conductivity
observed for n
for 6 12. A maximum value was achieved to be 77.5% at
30 C and 126.5% at 80 C for PES-6-QA with hexyleneoxy as
≤6 and a decrease in water uptake was observed
increased with temperature in the range from 30 to 80 C. This
≤n≤
is attributed to the development of saturating the ion transport
channels as the water content of the membranes increased with
temperature. As shown in Fig. 3(b), the hydroxide transport


the flexible spacer. It is plausible that the hydrophobicity of the
membranes enhanced greatly by introducing many more
methylene groups into the side chain, which restricted the water
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Journal Name
(a)
ARTICLE
40
30
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
View Article Online
QPPO-2
DOI: 10.1039/C6TA05090E
PES-B100-C1
60
40
20
0
QPAEK-0.6
PES-6-QA
PES-8-QA
SEBS-DMHA
PES-10-PPO
bQAPDHTPE 25
QPAE-X25Y21
QBPES-40
PES-4-QA
PES-3-QA
PES-12-QA
20
10
0
20
30
40
50
60
70
80
90
1.0
1.5
2.0
2.5
3.0
Temperature (oC)
IEC value (meq g-1
)
-2.0
-2.5
-3.0
-3.5
-4.0
Fig. 5 Relative hydroxide conductivity as a function of IEC for
PES‐n‐QA and reported AEMs at 60 C.
(b)
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA

E = 14.31 kJ mol-1
a
and QA groups at 30, 60 and 80 C. The PES-6-QA membrane
E = 14.80 kJ mol-1
(IEC=1.48 mmol g-1) with hexyleneoxy spacers between the
cationic groups and backbone showed the highest conductivity
a
E = 16.32 kJ mol-1
a
of 62.7 mS cm-1 at 80
C, followed by the PES-8-QA
membrane with octyleneoxy spacers. However, the AEMs with
spacer of propyleneoxy (PES-3-QA) and dodecyleneoxy (PES-
12-QA) showed the lowest hydroxide conductivity. The lower
conductivity may be attributed to the lower water uptake in the
membranes, in which the formation of ion conducting channels
is hard, as confirmed by AFM (Fig. 1). It is clearly that the
optimum hydroxide conductivity was achieved when the spacer
between the QA groups and backbone (benzene ring) was
hexyleneoxy (PES-6-QA). This may be attributed to the high
content of free water in the PES-6-QA membrane that the
hydroxide ion migration was greatly enhanced.
For a good comparison of the hydroxide conductivity with
other AEMs, the relative hydroxide conductivity (designated as
η=conductivity/IEC) was calculated to overcome the IEC
difference. Fig. 5 shows that the PES-n-QA membranes have
higher relative hydroxide conductivity than some main-chain-
type AEMs (QPPO-2, PES-B100-C1 and QPAEK-0.6),39-41
some side-chain-type AEMs (SEBS-DMHA and PES-10-
PPO.29,42 The η of the PES-6-QA and PES-80-QA membranes
is even higher than that of some block copolymer AEMs
(bQAPDHTPE 25, QPAE-X25Y21 and QBPES-40),43-45
indicating that side-chain-type AEMs bearing flexible pendant
QA groups is beneficial for enhancing hydroxide conductivity.
E = 17.06 kJ mol-1
a
E = 15.52 kJ mol-1
a
2.8
2.9
3.0
3.1
3.2
3.3
1000/T (K-1)
Fig. 3 (a) Temperature dependence of the conductivity of the
PES‐n‐QA membranes and (b) their Arrhenius plots.
80 oC
60 oC
60
30 oC
40
20
0
2
4
6
8
10
12
Mechanical properties and thermal stability
Number of carbon atoms in flexible spacer (n)
The thermal stability of the PES-n-QA membranes in hydroxide
form was examined via thermogravimetric analysis (TGA)
Fig. 4 Hydroxide conductivity as a function of spacer length for
the PES‐n‐QA membranes at 30, 60 and 80 C.

under a nitrogen atmosphere at a heating rate of 10
to 700 C. As shown in Fig. 6, the membranes show a two-step
degradation process. The first step between 150 and 350
originated from the degradation of the QA groups in the side
chain. The second step above 350 C was probably due to the

C min-1 up

activation energies (Ea) of the AEMs are in the range from
14.31 to 17.06 kJ mol-1, and are similar to the reported
AEMs.36-38 The lowest Ea of PES-6-QA suggests that the lowest
energy is required for ion conducting in the membrane. Fig. 4
shows the conductivity versus the spacer linking the backbone

C

degradation of the copolymer backbone. The degradation
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100
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
100
80
60
40
20
0
View Article Online
DOI: 10.1039/C6TA05090E
80
60
40
20
0
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
100
200
300
400
500
600
700
0
100 200 300 400 500 600 700 800
Time (h)
Temperature (oC)
Fig. 7 Hydroxide conductivity changes of the PES‐n‐QA (n=3, 4,
6, 8, 12) membranes in a 1 M KOH solution at 60 C.
Fig. 6 TGA curves of the PES‐n‐QA membranes under nitrogen
flow.

Table 3 IEC and conductivity of the AEMs before and after
Table 2 Mechanical properties of the membranes in wet state.
immersing in a 1 M KOH solution at 60

C for 720 h.
Tensile
strength
(MPa)
Young’s
modulous
(MPa)
Elongation
at
break
IEC (meq g-1)a
Conductivity (mS cm-1)b
Membrane
(%)
Membrane
Before
After
Before
After
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
13.6±2.1
11.5±1.9
9.2±1.9
11.2±2.2
13.1±2.5
285±15
219±20
158±12
199±16
270±20
28.0±3.2
42.1±4.6
57.6±3.5
38.6±2.8
23.8±2.9
PES-3-QA
PES-4-QA
PES-6-QA
PES-8-QA
PES-12-QA
1.58
1.55
1.48
1.42
1.31
0.49
1.36
1.33
1.30
1.22
18.4
20.2
28.2
22.1
17.3
4.9
16.6
24.9
19.9
15.9
a Estimated by back titration, b Measured at 30 C
temperature of all the membranes was far above the operating
temperature (roughly 23 70 C) of alkaline fuel cells. This
suggests that the as-synthesized membranes have a good
thermal stability.46
The mechanical properties of AEMs are critical for
fabricating robust MEAs and long-term stability of membranes
for fuel cells. The mechanical properties of the wet membranes
are important because AEMs are usually in operation at a fully
hydrated condition. Thus, the membrane samples were
immersed in DI water for at least 24 h before test. Table 2
shows the mechanical properties of the wet membranes. The
tensile strength, Young’s modulus and elongation at break of
of hydroxide ions and/or β-hydrogen (Hofmann) elimination,
leading to a decrease in IEC and conductivity.33,47 Fig. 7 shows
the change in conductivity during stability testing. Moreover,
the IEC and conductivity of the membranes before and after
stability test are given in Table 3. Extending the length of
flexible spacer (n≥4) results in an improved long-term alkaline
stability of AEM materials. As shown in Fig. 7, all the AEMs
with the exception of PES-3-QA retained above 80% of their
initial conductivity after 720 h test. For example, the PES-12-
QA membrane demonstrated the greatest alkaline stability,
where the conductivity and IEC only decreased by 8.1% and
6.9%, respectively. However, the IEC of PES-3-QA decreased
from 1.58 to 0.49 meq g-1, and the conductivity decreased from


the AEMs were in the range of 9.213.6 MPa, 158285 MPa
18.4 to 4.9 mS cm-1 at 30
of the initial value, respectively. Table S2

C, and retained only 26.6% and 31.0%
shows the
and 23.3% 57.6%, respectively. The PES-n-QA membranes

remained adequate tensile strength and could meet the
mechanical property requirements for fabrication of MEA and
application in alkaline fuel cells.
†
mechanical properties after the alkaline stability test. A slight
decrease in the tensile strength over the pristine membrane was
observed. This indicates that the poly(ether sulfone)s backbone
may be degraded slightly during the test.
Alkaline stability
1H NMR spectra (Fig. 8 and Fig. S9
†) were further used to
AEMs with long-term stability under alkaline condition are of
great importance for fuel cell application. In the present work,
the evaluation of long-term alkaline stability for the side-chain-
type AEMs with various flexible spacers between the backbone
and QA groups was performed by immersing the membrane
samples in a 1 M aqueous KOH solution at 60 C for 720 h. It
is known that QA groups have a tendency to degrade in a basic
environment due to the nucleophilic substitution by the attack
examine the change in chemical structure of the membranes.
The length of flexible spacer is found to have a great influence
on the alkaline tolerance of the AEMs. As shown in Fig. 8(a)
for PES-3-QA, the signal at 3.0 ppm assigned to the QA methyl
protons was much weaker than that of the pristine membrane.
The two obvious new peaks around 4.9 and 5.7 ppm were
attributed to the proton resonance from -CH=CH2. This
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ARTICLE
60 oC
80 oC
4
View Article Online
DOI: 10.1039/C6TA05090E
120
1.0
(a)
100
0.8
0.6
0.4
0.2
0.0
DMSO
1
3
2
80
60
40
20
0
4
H2O
Aromatic protons
PES-3-QA
2
3
1
t=0 h
0
50 100 150 200 250 300 350 400 450
Current density (mA
cm-2)
t=720 h

Fig. 9 Fuel cell performance of MEA using the PES‐6‐QA
membrane at 60 and 80
9
8
7
6
5
4
3
2
1
C.
Chemical shift (ppm)
7
method to investigate the degradation pathways of ammonium
cation with different lengths of carbon chain. There is linkage
of C-C or C-O bond between the QA groups and backbone.
Although the linkage of C-C bond between the QA groups and
backbone 24 shows higher alkaline tolerance than the linkage of
(b)
1
2
3
4
DMSO
H2O
5
C-O bond when the length of alkyl groups is short (n3), the
6
3,4
alkaline stability of the AEMs with C-O bond can be greatly
enhanced and is close to the membrane with C-C bond by
extending the flexible spacer (n≥4). When the length of alkyl
groups linking the backbone and QA groups is sufficiently long
(n≥4), the effect of C-C or C-O group, which is far away from
the QA groups, can be negligible. They found that the
ammonium is vulnerable to take Hofmann elimination when the
carbon chain of alkyltrimethylammonium cations extended
from 2 to 4 and became stable when extended from 4 to 6,
which is in line with our results. In addition, the PES-4-QA
Aromatic protons
7
6
1
2,5
PES-12-QA
t=0 h
t=720 h
9
8
7
6
5
4
3
2
1
Chemical shift (ppm)
Fig. 8 1H NMR spectra of (a) PES‐3‐QA and (b) PES‐12‐QA
stored in a 1 M aqueous KOH solution at 60
respectively.
(Fig. S9(a)†), PES-6-QA (Fig. S9(b)†), PES-8-QA (Fig. S9(c)†)
and PES-12-QA (Fig. 8(b)) membranes exhibited a slight
decrease of the signals intensity from the QA groups, and
demonstrated an excellent stability under alkaline condition at
C for 0 and 720 h,
60 C. This is attributed to the increased electron density
indicates that the decomposition takes place via Hofmann
elimination. The new peaks around 0.8, 1.25 and 2.6 ppm may
be attributed to the proton resonance from the product of
nucleophilic substitution reaction. A small peak around 6.0 ppm
may be ascribed to the degradation products of poly(ether
sulfone)s backbone. Above all, the decrease in IEC and
conductivity is mainly come from the degradation of the QA
groups. As noted by Elabd et al.,48 the proposed degradation
pathways of QA groups for PES-3-QA are presented in Scheme
around β-hydrogen by increasing the number of carbon atoms
between the backbone and QA groups, which depressed the
electron-withdrawing effect. As a result, the QA groups become
less susceptible to being attack by hydroxide ions. In summary,
the cation stability can be greatly improved by increasing the
length of flexible spacer (n≥4) between the backbone and QA
groups.
Single fuel cell performance
S1†. It is suggested that the presence of the three carbon atoms
The PES-6-QA membrane was evaluated in a single alkaline
fuel cell to examine the applicability of the AEMs due to its
high conductivity and excellent alkaline stability. The testing
was carried out under humidified H2 and O2 with gas flow rate
of 100 mL min-1. Fig. 9 shows polarization curve of the MEA at
between the QA groups and the electronegative ether oxygen
groups increased the reactivity of β-hydrogen, which will result
in Hofmann elimination reaction.49 However, there is not any
1
new peaks from 4.3 to 5.8 ppm observed in other H NMR
spectra of the AEMs after 720 h test, indicating that the
Hofmann elimination reaction was greatly inhibited via
60 and 80 C. The open circuit voltage (OCV) of the fuel cells
is above 1.0 V, and is in the range of typical alkaline fuel
cells,51,52 indicating that the voltage loss caused by gas
extending the length of flexible spacers (n≥4) linking the QA
groups and backbone. The present findings are in accordance
with Pivovar et al50 who used density functional theory (DFT)
crossover is insignificant. The maximum power density (Pmax
)
for the PES-6-QA membrane at 60

C was 108.3 mW cm-2
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under current density of 250 mA cm-2, and is much higher than
the Pmax (42.5 mW cm-2) using QA functionalized copolymers
5
M. Tanaka, K. Fukasawa, E. Nishino, S. Yamaguchi, K.
View Article Online
Yamada, H. Tanaka, B. Bae, K. MiyatDaOkeI: 1a0n.1d03M9/.CW6TaAt0a5n0a9b0eE,
J. Am. Chem. Soc., 2011, 133, 10646-10654.
N. Li and M. D. Guiver, Macromolecules, 2014, 47, 2175-
2198.
H. Ono, J. Miyake, S. Shimada, M. Uchida and K. Miyatake,
J. Mater. Chem. A, 2015, 3, 21779-21788.
S. C. Price, X. Ren, A. C. Jackson, Y. Ye, Y. A. Elabd and F.
L. Beyer, Macromolecules, 2013, 46, 7332-7340.
J. Pan, S. Lu, Y. Li, A. Huang, L. Zhuang and J. Lu, Adv.
Funct. Mater., 2010, 20, 312-319.
and Pmax (30 mW cm-2) using an imidazolium-functionalized
6
7
8
9
12,53
PPO membrane at 60

C.
Furthermore, the PES-6-QA
membrane exhibited higher Pmax (117.5 mW cm-2 under current
density of 250 mA cm-2) at 80
C than at 60 C. This is


attributed to the increased conductivity of the membrane at
elevated temperature. This suggests the side-chain-type
membranes hold promising for application in alkaline fuel cells.
Optimization of the MEA construction and operational
condition are underway in our lab.
10 J. Pan, Y. Li, L. Zhuang and J. Lu, Chem. commun., 2010, 46
,
8597-8599.
11 N. Yokota, M. Shimada, H. Ono, R. Akiyama, E. Nishino, K.
Asazawa, J. Miyake, M. Watanabe and K. Miyatake,
Macromolecules, 2014, 47, 8238-8246.
Conclusions
12 J. Ran, L. Wu, J. R. Varcoe, A. L. Ong, S. D. Poynton and T.
Xu, J. Membr. Sci., 2012, 415-416, 242-249.
13 X. Lin, J. R. Varcoe, S. D. Poynton, X. Liang, A. L. Ong, J.
In summary, the poly(ether sulfone)s bearing pendant
quaternary ammonium groups via various lengths of flexible
spacers were prepared via nucleophilic polycondensation,
demethylation and Williamson reaction. All the AEMs exhibit
clear micro-phase separation morphology due to the
immiscibility between the hydrophilic side chain and the
hydrophobic backbone. It is observed that the length of flexible
spacer linking the cationic groups and backbone has a great
influence on the performance of AEMs. The optimum
Ran, Y. Li and T. Xu, J. Mater. Chem. A, 2013, 1, 7262-7269.
14 X. Lin, X. Liang, S. D. Poynton, J. R. Varcoe, A. L. Ong, J.
Ran, Y. Li, Q. Li and T. Xu, J. Membr. Sci., 2013, 443, 193-
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4, 3886-3892.
3
17 Y. Li, Y. Liu, A. M. Savage, F. L. Beyer, S. Seifert, A. M.
Herring and D. M. Knauss, Macromolecules, 2015, 48, 6523-
6533.
18 M. Zhang, J. Liu, L. An, M. D. Guiver and N. Li, J. Mater.
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19 L. Zeng, T. S. Zhao, L. An, G. Zhao and X. H. Yan, J. Membr.
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20 M. P. Kulkarni, T. J. Peckham, O. D. Thomas and S.
Holdcroft, MRS Proceedings, 2014, 1677, 1-12.
conductivity (62.7 mS cm-1, 80
C) was observed for PES-6-
QA with hexyleneoxy spacers, suggesting that too long alkyl
groups in the side chain will increase the hydrophobicity of the
membrane, which is bad for fabricating high efficient ion
conducting channels due to the reduced water content. The
Hofmann degradation of QA groups in the membrane can be
inhibited by increasing the length of flexible spacer (n
≥4)
21 Y. He, J. Pan, L. Wu, Y. Zhu, X. Ge, J. Ran, Z. Yang and T.
between the backbone and QA groups. The PES-12-QA
membrane demonstrated the highest stability after immersing in
a 1 M aqueous KOH solution at 60 C for 720 h. Single cell test
Xu, Sci. rep., 2015,
22 H. S. Dang, E. A. Weiber and P. Jannasch, J. Mater. Chem. A,
2015, , 5280-5284.
5, 13417, 1-7.
3
23 H. S. Dang and P. Jannasch, Macromolecules, 2015, 48
,
showed a moderate performance for the PES-6-QA membrane,
5742-5751.
which hold promising for application in alkaline fuel cells.
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25 W.-H. Lee, Y. S. Kim and C. Bae, ACS Macro Letters, 2015,
, 814-818.
3
Acknowledgment
4
26 N. Li, Q. Zhang, C. Wang, Y. M. Lee and M. D. Guiver,
Financial support from the National Nature Science Foundation
of China (grant no. 21376194 & 21576226), the Nature Science
Foundation of Fujian Province of China (grant no. 2014H0043),
and the research fund for the Priority Areas of Development in
Doctoral Program of Higher Education (no. 20130121130006)
is gratefully acknowledged.
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O
OH-
N+
80 oC
60 oC
30 oC
60
40
20
2
4
6
8
10
12
Number of carbon atoms in flexible spacer (n)