Reparation and characterization of amidated pectin based polymer electrolyte membranes

Abstract

The work presents the synthesis and characterization of amidated pectin (AP) based polymer electrolyte membranes (PEM) crosslinked with glutaraldehyde (GA). The prepared membranes are characterized by Fourier transform infrared spectroscopy (FTIR), organic elemental analysis, X-ray diffraction studies (XRD), thermogravimetric analysis (TGA) and impedance spectroscopy. Mechanical properties of the membranes are evaluated by tensile tests. The degree of amidation (DA), molar and mass reaction yields (Y(M) and Y(N)) are calculated based on the results of organic elemental analysis. FTIR spectroscopy indicated the presence of primary and secondary amide absorption bands. XRD pattern of membranes clearly indicates that there is a considerable increase in crystallinity as compared to parent pectin. TGA studies indicate that AP is less thermally stable than reference pectin. A maximum room temperature conductivity of 1.098 x 10(-3) Scm(-1) is obtained in the membrane, which is designated as AP-3. These properties make them good candidates for low cost biopolymer electrolyte membranes for fuel cell applications.

Full text

Chinese Journal of Polymer Science Vol. 27, No. 5, (2009), 639
−
646 Chinese Journal of
Polymer Science
©2009 World Scientific
PREPARATION AND CHARACTERIZATION OF AMIDATED PECTIN BASED
POLYMER ELECTROLYTE MEMBRANES
R.K. Mishraa, A. Anisb, S. Mondalb*, M. Dutta and A.K. Banthiab
a Department of Polymer Science, Bundelkhand University, Jhansi 284128, India
b Materials Science Center, Indian Institute of Technology, Kharagpur 721302, India
Abstract The work presents the synthesis and characterization of amidated pectin (AP) based polymer electrolyte
membranes (PEM) crosslinked with glutaraldehyde (GA). The prepared membranes are characterized by Fourier transform
infrared spectroscopy (FTIR), organic elemental analysis, X-ray diffraction studies (XRD), thermogravimetric analysis
(TGA) and impedance spectroscopy. Mechanical properties of the membranes are evaluated by tensile tests. The degree of
amidation (DA), molar and mass reaction yields (YM and YN) are calculated based on the results of organic elemental
analysis. FTIR spectroscopy indicated the presence of primary and secondary amide absorption bands. XRD pattern of
membranes clearly indicates that there is a considerable increase in crystallinity as compared to parent pectin. TGA studies
indicate that AP is less thermally stable than reference pectin. A maximum room temperature conductivity of 1.098 × 10−3
Scm−1 is obtained in the membrane, which is designated as AP-3. These properties make them good candidates for low cost
biopolymer electrolyte membranes for fuel cell applications.
Keywords: Amidated pectin; Proton conducting; Methanol permeability; Impedance spectroscopy; Fuel cell.
INTRODUCTION
Fuel cells have emerged strongly as power sources for stationary and portable applications owing to their high-
energy conversion efficiency, eco-friendly nature and low temperature operation[1, 2]. Methanol has been widely
used as a fuel in direct methanol fuel cell (DMFC) because of its easy handling and low price. It has high energy
density and can be easily generated from natural resources such as coal, gas and biomass. DMFC is expected to
have high and compromising values as clean energy[3, 4]. Among the various types of fuel cells, the polymer
electrolyte membrane fuel cell (PEMFC) has been well established for over five decades and is successfully
used as an electrical power for spacecrafts and submarines[5]. One of the important areas of research in PEMFC
is the development of low cost membranes with low methanol permeability for direct methanol fuel cells
(DMFC). The main reasons for this growing interest is the high cost, limit of low operating cell temperature and
methanol crossover problems associated with presently used plain perflurosulfonic acid membranes[6, 7]. It has
been reported by few workers that, on suitable chemical modification polymers containing hydroxyl and amine
groups exhibit good conductivities[8, 9]. Applications of biopolymers in electrical devices are not only interesting
but also important for environmental safety. In addition to that, production cost is greatly reduced by using a
biopolymer instead of expensive polymers[10]. Recently some natural polymers have been used as a low cost
electrolyte for the development of polymer electrolyte membrane fuel cell (PEMFC)[11, 12]. Pectin is a complex
carbohydrate polymer that plays an instrumental role in regulating the mechanical properties of plant cell wall[13]
and has several industrial applications related to their thickening and gelling properties[14−16]. Chemically, pectin
is a linear chain of poly(
α
-1,4-galacturonic acids), with varying degree of methylation of carboxylic acid
residues. Chemical modification of pectin can influence their chemical and technological properties. Amidated
pectins are firmly entrenched in the food industry, and they are important pectin derivatives with good gelling
* Corresponding author: S. Mondal, E-mail: shampa234@yahoo.co.in
Received May 26, 2008; Revised September 22, 2008; Accepted October 6, 2008

R.K. Mishra et al.
640
properties. Amidated pectins often replace low methoxyl pectins, since amidated pectins need less calcium, are
less sensitive to precipitation by high amounts of calcium, and their gels are claimed to be thermo-reversible[17,
18]. Some reports were published dealing with the polyelectrolytic properties of amidated pectin in solution and
their calcium binding ability[19] but no such work has been reported on amidated pectin based membranes, which
are ionically conducting. This paper reports the development of ethanolamine-modified pectin (EAMP) based
membranes. The membranes prepared were characterized by FTIR, organic elemental analysis, XRD, TGA,
impedance spectroscopy and tensile tests.
EXPERIMENTAL
Materials and Methods
Pectin (MW ca. 3.0 × 104−1.0 × 105), methanol (AR) and glutaraldehyde (GA) 25% solution were obtained from
Loba-chemie Indoaustranal Co. Mumbai, India. Ethanolamine was obtained from Sisco Research Laboratories
(SRL), Mumbai, India. Hydrochloric acid 35% pure was obtained from Merck Limited, Mumbai, India. Double
distilled water was used throughout the study. Amidated pectin (AP) was synthesized according to the method
described elsewhere[20].
Preparation of Membranes
The amidated pectin (AP) based membranes crosslinked with GA used in the study were fabricated using
solvent casting technique. A stock viscous solution of APs in water (100 g/L) was prepared by dissolving 2 g of
APs of different concentrations in 20 mL of distilled water and stirring for 2 h at room temperature. To the
dissolved AP solutions 1 mL of glutaraldehyde was added as a crosslinking agent. The resultant mixture was
further stirred for 30 min at room temperature to complete the crosslinking reaction. After that the homogeneous
solution was poured into petri dishes and allowed to dry in a laminar flow chamber. The resulting films were
peeled off from the petri dishes and were characterized. The films so obtained were named AP-1, AP-2, AP-3,
AP-4, AP-5 and AP-6 respectively.
FTIR Spectral Analysis
The FTIR spectra of pectin and AP were taken in the frequency range of 4000−400 cm−1 using attenuated total
reflectance (ATR) mode with the help of an FTIR spectrophotometer (NEXUS−870, Thermo Nicolet
Corporation).
Organic Elemental Analysis
Elemental analysis was obtained from a 2400 series II CHN analyzer (Perkin Elmer). The estimation of only
three elements i.e. carbon, hydrogen and nitrogen was done.
X-ray Diffraction Studies
For the X-ray diffraction studies the raw materials were subjected to X-ray diffraction (XRD-PW 1700, Philips,
USA) using Cu K
α
radiation generated at 40 kV and 40 mA; the diffraction angle, 2
θ
values were in the range
of 10°−70°.
Thermogravimetric Analysis
Thermal stability of pectin and AP membranes was carried out by using NETZSCH TG 209 F1 at a heating rate
of 10 K/min, with nitrogen flushed at 100 mL/min.
Tensile Studies
The tensile studies of the AP based composite membranes were carried out using a Hounsfield H10KS tensile
testing machine.
Crosshead speed: 50 mm/min; Temperature: 18°C; Relative humidity: 60%.
Water Uptake
The water uptake of the hybrid membranes was determined by measuring the change in the weight before and
after hydration. The membrane was immersed in deionized water for 24 h, and the water attached on surface of
the membrane was carefully removed with filter paper. After that, the wetted membrane weight was determined

Polymer Electrolyte Membranes Based on Pectin 641
instantaneously. The water uptake was calculated by using the following equation:
Water uptake =
d
dw
M
MM − (1)
where Mw and Md are the weight of the swollen polymer and the dry polymer in grams, respectively[21].
Methanol Permeability Measurements
The measurement of methanol diffusion coefficient through the composite membranes was performed using an
in house built diffusion cell having two compartments, which were separated by the membrane situated
horizontally. Prior to diffusion coefficient measurement, the membranes were equilibrated in aqueous methanol
solution 50% V/V for 24 h, and the experiments were carried out at room temperature (25°C). The methanol
concentration of the receptor compartment was estimated using a differential refractometer (Photal OTSUKA
Electronics, DRM-1021); the differential refractometer was highly sensitive to the presence of methanol. The
change in refractive index of the diffusion samples was averaged over 52 scans in the differential refractometer
to determine the change in refractive index. The methanol diffusion coefficients for the PEMs was calculated
using the following equation[22]:
methanol2
methanol1
exp
sample ln cc
cc
At
lV
D−
−
= (2)
where D is the methanol diffusion coefficient, l is the thickness of the membrane, Vsample is the volume of the
sample solution; A is the cross-section area of membrane and texp is the time interval during which diffusion
occurs, c
methanol is the concentration of methanol fed in the donor compartment, c1 is the concentration of
methanol in receptor compartment at t = 0, and c2 is the concentration of methanol in the receptor compartment
at t = texp.
Impedance Spectroscopy
Conductivity measurements were made at room temperature (25°C) after equilibrating the membrane in
deionized water for 24 h. The conductivity cell was composed of two stainless steel electrodes of 16 mm
diameter. The membrane sample was sandwiched in between stainless steel electrodes. AC impedance spectra of
the membranes were recorded from 40 Hz to 15 MHz with amplitude of 10 mV using an Agilent 4294A
Precision Impedance Analyzer. The bulk resistance of the membrane was determined from the high frequency
intercept of the imaginary component of impedance with the real axis. The conductivity was calculated using the
following equation:
RA
l
=
σ
(3)
Where
σ
, l, R, and A denote the membrane conductivity, membrane thickness, the bulk resistance of the
membrane, and the cross-sectional area of the membrane, respectively[23].
RESULTS AND DISCUSSION
The reaction mechanism of amidation of pectin with ethanolamine (Sinitsya et al.) is depicted below.

R.K. Mishra et al.
642
Elemental Analysis
The results of elemental analysis (C, H, N) of pectin and modified pectin (AP) membrane and calculated values
of DA, DM, YM and YN are listed in Table 1[20]. It is evident from the table that pectin does not show any
significant presence of nitrogen, but in case of AP membranes, the analysis shows a considerable increase in
nitrogen content with increasing concentration of ethanolamine. It can be clearly observed from Table 1 that the
degree of amidation (DA%) of pectin increased with increase in concentration of ethanolamine. This is due to
incorporation of amide groups or amine moieties in a greater extent on the pectin structure after amidation
reaction.
100
73
14
100
14
1
100
73
6
×=
×
=
×
⎥
⎦
⎤
⎢
⎣
⎡−+
⎟
⎠
⎞
⎜
⎝
⎛
+=
A
N
AN
M
N
C
N
DA
D
Y
MM
Y
M
K
M
M
(4)
Where DA is the degree of amidation, YM the mass yield of the reaction, i.e. the relative mass of bonded amine
(%) in reaction product, YN the molar yield of the reaction i.e. the relative content of ester groups substituted by
amine (%), MH the hydrogen content (%), MN the nitrogen content (%) and MC the carbon content (%), MA the
molar mass of amine (gmol−1), 73 is the degree of methylation (DM) of original pectin (%)[24].
Table 1. Results of elemental analysis of AP based membranes
Sample identity MC (%) MH (%) MN (%) DA (%) DM (%) YM (%) YN (%)
Pectin 36.27 5.85 0.39
−
73
− −
AP-1 33.08 6.21 1.11 22.46 31 0.39 6.840
AP-2 38.31 5.56 2.17 33.00 33 1.55 13.69
AP-3 32.56 7.25 2.83 59.61 34 3.03 20.54
AP-4 31.55 7.32 3.13 68.83 32 4.47 27.39
AP-5 33.02 7.71 4.17 88.45 29 7.44 34.24
AP-6 32.92 7.74 4.30 91.48 35 9.21 41.09
Fig. 1 FTIR spectra of (a) pure pectin and (b) amidated pectin based membrane (AP-6)

Polymer Electrolyte Membranes Based on Pectin 643
FTIR Spectral Analysis
The FTIR spectrum of the pure pectin is shown in Fig. 1(a). The spectrum showed peak at 3402 cm−1 due to the
stretching of ―OH groups. The peak at 2939 cm−1 indicated C―H stretching vibration. The peaks at 1460 and
1332 cm−1 could be assigned to ―CH2 scissoring and ―OH bending vibration peaks, respectively. The peak at
1010 and 1647 cm−1 suggested ―CH―O―CH― stretching and C＝O stretching vibration peaks. The peak at
1157 cm−1 suggested the presence of ―CH―OH in aliphatic cyclic secondary alcohol[25, 26]. Figure 1(b) shows
the FTIR spectrum of AP membrane. The carboxyl vibration region 1500−1900 cm−1 is most important for our
analysis. The acid form of modified pectin has two important bands at 1672 cm−1 (amide I) and 1595 cm−1
(amide II)[27]. The presence of these two bands and absence of intense carboxylate stretching bands indicated
that the subsituents were grafted on pectin chain by covalent amide bonds. The intense band near 2925 cm−1 was
due to symmetric C―H stretching vibrations of methylene groups. This effect is due to the increase of C―H
bond content after amidation reaction. The peak at 1402 cm−1 suggested ―CH bending of ―CH2 groups. The
peak at 1016 cm−1 suggested ―OH bending, and the peak at 1074 cm−1 indicated the presence of secondary
alcohol (characteristic peak of ―CH―OH in cyclic alcohol C―O stretch).
X-ray Diffraction Analysis
Figure 2 shows XRD patterns of the pectin and AP based membrane respectively. X-ray diffractogram of the AP
based membrane shows an intense peak at 35.17° (2θ), where as the diffractogram of pectin indicated a single
weak wide diffraction peak at ca. 13.34° (2θ). From the diffractograms of reference pectin and AP based
membranes one can easily justify that the pure pectin is a completely amorphous material because there is no
intense peak except a weak wide diffraction peak at 13.34°, whereas the diffractogram of AP based membrane
shows an intense peak at 2θ equal to 35.17°. The crystallinity of the AP membranes increases after amidation
due to the formation of hydrogen bonding in the amidated pectin. From the XRD plots of pectin and AP
membranes the crystallinity was calculated[28], and it was found to be 0.81% and 49.09%, respectively, which
indicates that AP is a partially crystalline material as compared to reference pectin.
Water Uptake
The water uptakes of AP based membranes with respect to degree of amidation (DA) of prepared membranes
are depicted in Fig. 3. It is evident from the figure that at first there is a considerable increase in water uptake of
the AP membranes with increase in degree of amidation (DA) to 68.83% but further increase in DA leads to
decrease in water uptake of the membranes. The increase in water uptake up to certain level may be attributed to
the hydrophilic nature of AP but after that, there is a considerable decrease in water uptake due to the resistance
offered by the crosslinked networks to further uptake of water by the AP based membranes.
Fig. 2 XRD patterns of (a) pure pectin and (b) AP
based membrane (AP-6) Fig. 3 Water uptake verses degree of amidation of
AP membranes

R.K. Mishra et al.
644
Mechanical Properties
The tensile strength and elongation at break of the AP membranes are shown in Fig. 4. It is evident from the
figure that the membranes which are designated as AP-1, AP-2, AP-3, AP-4, their tensile strengths are almost
comparable (0.363, 0.267, 0.433 and 0.314 MPa), whereas the tensile strength of the AP-5, AP-6 are quite high
(1.58 and 1.013 MPa) as compared to the other four membranes and the highest strength (1.58 MPa) was
observed in the case of AP-5. Whereas the elongation at break of the AP membranes increases gradually with
increase in degree of amidation (DA) of the membranes but after that a decrease in elongation was observed.
The low tensile strength of the four membranes were attributed to the presence of unreacted pectin that might be
present in the membranes causes reduction in the tensile strength of the prepared AP based membranes.
Fig. 4 Tensile strength and elongation at break of AP based membranes (AP-1 to AP-6)
Thermo Gravimetric Analysis (TGA)
The thermograms of pectin and AP membrane obtained at heating rate of 10 K/min under nitrogen atmosphere
are shown in Fig. 5. It can be observed from the thermogram of pectin that thermal loss in weight starts in the
range of 35−76°C, which is associated with loss of absorbed moisture in the sample, and in second step
decomposition was observed between 195−350°C[29, 30]. In this range the sample rapidly losses 60% of its total
weight up to 350°C. Therefore, it shows a two-step degradation process. Beyond 350°C thermogram shows that
weight loss is slow and gradual up to 697°C. In this temperature range 350−697°C the sample loses 15% of its
original weight. The maximum rate of weight loss was observed at 235°C. Whereas the thermogram of AP
membrane shows a three-step degradation process. The first weight loss starts at 77−194°C, which is due to
moisture vaporization. The second weight loss at 200−350°C is due to thermal degradation of the EAMP
membrane. The third weight loss at 400−500°C is due to the by-product generated by the thermal
Fig. 5 TGA of (a) pure pectin and (b) AP-6 membrane

Polymer Electrolyte Membranes Based on Pectin 645
decomposition of modified pectin (AP) based membrane. Up to 600°C 86% loss in total weight was observed
for AP membrane as compared to 72% weight loss in pectin. This indicates the lower thermal stability of AP as
compared to parent pectin.
Methanol Permeability
Table 2 shows the methanol permeability of amidated pectin (AP) based polymer electrolyte membranes. It can
be observed from the table that there is a regular increase in methanol permeability of the AP based membranes
with increase in DA of the membranes. The methanol permeability was directly correlated with the water uptake
of the membranes because the highest methanol permeability of 1.98 × 10−6 was observed in the membrane AP-
4 that had the highest water uptake. But after that a decrease in methanol permeability was observed. The lower
methanol permeability of 1.22 × 10−6 was observed in the membrane of AP-1.
Table 2. Result of methanol permeability of AP based membranes
Sample identity Membrane thickness (µm) Methanol diff. coefficient (cm2s−1)
AP-1 190 1.22 × 10−6
AP-2 270 1.27 × 10−6
AP-3 270 1.30 × 10−6
AP-4 420 1.98 × 10−6
AP-5 165 1.42 × 10−6
AP-6 245 1.35 × 10−6
Proton Conductivity Analysis
Figure 6 shows the proton conductivity as function of degree of amidation. Here the proton conductivity was of
the order of 10−3 Scm−1. The proton conductivity initially increases with increase in degree of amidation but with
further increase in degree of amidation, a decrease in conductivity of the membranes is observed. The drop in
the conductivity is due to the decrease in water uptake of these membranes because the presence of water assists
in the movement of the protons through the polymer matrix by forming hydronium ions. A maximum
conductivity of 1.098 × 10−3 Scm−1 was obtained for the membrane designated AP-3. The reasonably good
conductivity of the membrane can be attributed to the residual hydroxyl and amine groups present in the
membranes, which give rise to hydrophilic regions in the polymer due to strong affinity towards water. This
hydrophilic area formed around the clusters of side chains leads to absorption of water, which enables easy
proton transfer[31].
Fig. 6 Degree of amidation verses proton conductivity of AP membranes
CONCLUSIONS
In the present study, amidated pectin based polymer electrolyte membranes have been prepared by crosslinking
with glutaraldehyde (GA). FTIR spectra of AP based membranes indicate presence of amide bands (amide I,
amide II) and N-alkyl absorption bands. The data obtained were in good agreement with other analytical
methods (organic elemental analysis). The XRD study indicates that there is considerable increase in

R.K. Mishra et al.
646
crystallinity of pectin after chemical modification with ethanolamine. Mechanical properties of the AP
membranes were found to be sufficient. The water uptake measurement of the membranes shows increase in
water holding capacity of the membranes with increase in degree of amidation of pectin up to 68.83 % after
which a decrease in water holding capacity of the membranes was observed. The proton conductivity of AP
membranes was measured at room temperature, the highest proton conductivity of 1.098 × 10−3 Scm−1 was
observed in the membrane AP-3. Further, due to the high ionic conductivity of the membrane it can be used as
low cost proton conducting biopolymer electrolyte membranes for fuel cell applications.
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