Journal of Membrane Science 260 (2005) 142–155
Pervaporation separation of water + isopropanol mixtures using novel
nanocomposite membranes of poly(vinyl alcohol) and polyaniline夽
B. Vijaya Kumar Naidu a , Malladi Sairam a , K.V.S.N. Raju b , Tejraj M. Aminabhavi a,∗
a
Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India
b Organic Coatings and Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
Received 21 October 2004; received in revised form 13 March 2005; accepted 14 March 2005
Available online 3 May 2005
Abstract
Novel nanocomposite polymeric membranes containing nanosized (30–100 nm) polyaniline (PANI) particles dispersed in poly(vinyl alcohol) (PVA) were prepared and used in the pervaporation separation of water–isopropanol feed mixtures ranging from 10 to 50 mass% of water
at 30 ◦ C. Of the three nanocomposite membranes prepared, the membrane containing 40:60 surface atomic concentration ratio of PANI:PVA
produced the highest selectivity of 564 compared to a value of 77 observed for the plain PVA membrane. Flux of the nanocomposite membranes was lower than those observed for the plain PVA membrane, but selectivity improved considerably. Membranes were characterized
by differential scanning calorimetry, dynamic mechanical thermal analyzer, X-ray photoelectron spectroscopy, Fourier transform infrared
spectroscopy and scanning electron microscopy. The highest selectivity with the lowest flux was observed for 10 mass% water containing
feed mixture. Flux increased with increasing amount of water in the feed, but selectivity decreased considerably. These results were attributed
to the acid-doped PANI particles in the PVA membrane as a result of change in the micromorphology of the nanocomposite membranes. In
addition, molar mass between cross-links and fractional free volume of the membranes are responsible for the varying membrane performance.
Temperature effect on permeability was investigated for 10 mass% water containing feed with the membrane containing higher concentration
of PANI particles, the presence of which could be responsible for varied effect of water permeation through the membrane. Membranes of
this study could remove as much as 98% of water from the feed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Nanocomposite membrane; Polyaniline; Poly(vinyl alcohol); Diffusion; Pervaporation
1. Introduction
Dehydration of isopropanol by pervaporation (PV)
technique has been widely studied [1–3]. One of the key
successes of PV is that, if suitable membranes can be fabricated with high permeability and good selectivity to water, it
is possible to achieve an excellent separation, particularly at
the azeotropic composition. However, more number of novel
polymeric membranes are needed for a successful operation
夽 This paper is Center of Excellence in Polymer Science Communication
# 57.
∗ Corresponding author. Tel.: +91 836 2215372/2778279;
fax: +91 836 2771275/747884.
E-mail address: [email protected] (T.M. Aminabhavi).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2005.03.037
of the process in view of the fact that PV is environmentally
cleaner than the conventional distillation; moreover, the
process is energy intensive. Literature search indicates
that poly(vinyl alcohol), PVA, has been the widely used
membrane in the PV separation of water–organic mixtures
[3–6], but due to the presence of hydrophilic groups in
PVA, the chain induces excessive swelling during PV.
Therefore, attempts have been made to modify the structure
of PVA by cross-linking, blending, grafting, etc. [3–6]. In
the course of our investigations, we realized that one of
the means to control membrane swelling is to incorporate
nanosized inorganic particles in the polymer matrix [7–9].
Polymers have also been reinforced with the nanosized
cellulose whiskers by using the sol–gel techniques [10,11].
The method involves dissolving the preformed polymer
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
in sol–gel precursor solutions, simultaneous formation of
organic and inorganic phases through the synchronous
polymerization of the organic monomer and the sol–gel
precursors [11]. Recent trends using PV membrane separations involve the development of composite membranes by
incorporating zeolites as the reinforcing fillers [12–14].
Several investigations utilizing the conjugated polymers
as membranes to separate various liquid mixtures have also
been reported in the literature [15–18]. Interest in polyaniline (PANI) as a material for membrane separations stems
for its high selectivity toward liquids since most liquids are
in the size regime of 0.2–1 nm. Another advantage is that
PANI has the ability to be tailored after its synthesis through
doping/undoping processes. Since there is a tremendous driving force for adding protonic dopants to the imine nitrogens in the PANI backbone [18], the polymer chains are
readily pushed apart by the incoming dopants. Thus, doping
would induce morphological changes in the polymer resulting in varying permselectivities. Besides such morphological changes, the undoped and doped forms of PANI exhibit
different characteristics. For instance, the undoped form of
PANI is hydrophobic, while the doped form is hydrophilic
[19,20]. Hence, doped PANI preferentially permeates water
over the organics, such as isopropanol. The above-mentioned
advantages are considered to search for novel membranes containing PANI nanoparticles dispersed in the PVA
matrix.
In an effort to minimize the swelling of PVA membrane
and to increase water selectivity, we have developed a novel
hybrid nanocomposite membrane by in situ polymerization of
aniline in the PVA matrix in acidic media. Aniline monomer
was introduced into the PVA matrix and by carrying in situ
polymerization outside the mesopores of the polymer matrix, a nanocomposite structure was formed. The organic
phase extends along the channels to the openings in the
nanocomposite structure due to strong interactions between
the nanoparticle formed and the continuously polymerized
PANI nanoparticles. Polymeric nanocomposites thus prepared are called “hybrid nanocomposite membranes”, which
consist of an organic polymer matrix in which PANI in the
nanoscale dimension is dispersed. These membranes should
possess improved barrier properties by controlling membrane swelling. In the present paper, three nanocomposite
membranes were prepared by polymerizing aniline in different amounts to obtain the PVA–PANI nanocomposites.
The membranes were characterized by differential scanning
calorimetry (DSC), dynamic mechanical thermal analyzer
(DMTA), X-ray photoelectron spectroscopy (XPS), Fourier
transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The PV performance of these membranes was studied at 30 ◦ C in terms of flux and selectivity
for the water–isopropanol feed mixtures ranging from 10 to
50 mass% of water. Finally, the temperature dependence on
PV performance was investigated for the selected feed mixture (i.e., 10 mass% water containing feed mixture) at different temperatures (40 and 50 ◦ C).
143
2. Experimental
2.1. Materials
Laboratory reagent grade PVA (87% degree of hydrolysis)
with a molecular weight 125,000 was procured from s.d. Fine
Chemicals, Mumbai, India. AR grade aniline (Loba Chemicals, Mumbai, India) was vacuum distilled and stored in an
amber colored bottle under cold conditions. Glutaraldehyde
(GA), hydrochloric acid, acetone, isopropanol, ammonium
persulfate and all other chemicals used in this work were
of AR grade samples, purchased from s.d. Fine Chemicals,
Mumbai, India. These were used as received. Deionized water having a conductivity of 20 ␮S/cm was produced in the
laboratory from the Permionics pilot plant (Vadodara, India)
using the nanofiltration membrane module.
2.2. Preparation of PVA–PANI nanocomposite
membranes
Polymerization of aniline in PVA was carried out as per the
published report [21] to obtain the nanocomposite. To prepare
three different nanocomposites, a 3 mass% solution of PVA
prepared in water, 0.6, 0.9 or 1.2 mL of aniline were added.
The pH of the solution was adjusted to 1 by adding dil. HCl.
To this mixture, an aqueous solution of ammonium persulfate
was added at 5 ◦ C under constant stirring by maintaining the
equimolar ratio of aniline to ammonium persulfate. The mixture was stirred for 4 h to obtain the colloidal PANI particles
suspended in PVA. This reaction mixture was then poured
onto a clean glass plate to cast the membranes. The dried
PVA–PANI nanocomposite membranes were cross-linked
with GA by dipping them in 200 mL aqueous acetone mixture containing 1 mL of GA and 1 mL of con. HCl for 12 h.
Membranes were removed from the bath, washed three times
with distilled water and dried in an oven at 40 ◦ C for 4 h. The
nanocomposite membranes prepared with 0.6, 0.9 or 1.2 mL
of aniline were designated as PVA–PANI-I, PVA–PANIII and PVA–PANI-III, respectively. Plain PVA membrane
was prepared by using 3 mass% PVA solution in a similar
manner.
2.3. Characterization
2.3.1. Fourier transform infrared (FT-IR) spectroscopic
studies
FT-IR spectra of the pure aniline, PVA and PVA–PANI
membranes in KBr pellets were recorded on a Nicolet, Model
Impact 410 (Milwaukee, WI, USA) in the wavelength region
of 4000–400 cm−1 .
2.3.2. X-ray photoelectron spectral (XPS) studies
The core level XPS spectra of polyaniline salts and bases
and membranes were recorded using KRATOS AXIS 165
(Shimadzu) with Mg K␣ X-ray source 253.6 eV. The X-ray
power supply was operated at 75 W and 5 mA. Pressure
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in the analysis chamber during scans was kept below
10−8 Torr and the peak area ratios for various elements were
corrected by the experimentally determined instrumental
factors. The N(1s) spectrum, after background subtractions,
was decomposed into suitable components consisting of a
Gaussian line shape with a Lorentzian broadening function.
All fitting parameters including the number of components,
widths and intensities were freely adjustable and determined
for each spectrum with an iterative least squares fitting
routine.
2.3.3. Dynamic mechanical thermal analysis (DMTA)
studies
Dynamic mechanical properties (storage and loss modulus, tan δ) of the plain PVA, uncross-linked and cross-linked
PVA–PANI nanocomposite membranes were measured by
using DMTA IV instrument (Rheometric Scientific, USA) in
a tensile mode at a frequency of 10 or 1.0 Hz and at the heating rate of 3 ◦ C/min. Viscoelastic behavior of the cross-linked
and uncross-linked membranes in nitrogen atmosphere was
investigated in the temperature range of 30–200 ◦ C.
2.3.4. Differential scanning calorimetric (DSC) studies
DSC thermograms of all the uncross-linked and crosslinked membranes were recorded using Rheometric Scientific
(Model DSCSP), UK. The DSC thermograms were recorded
between 25 and 400 ◦ C at the heating rate of 10 ◦ C/min under
nitrogen atmosphere.
2.3.5. Scanning electron microscopic (SEM) studies
Scanning electron micrographs of the PVA–PANI
nanocomposite membranes were recorded using a Joel
electron microscope at 10 kV following the gold sputtering
technique. The prepared nanocomposite membranes were
directly subjected to SEM analysis.
2.3.6. Particle size measurement
Zeta average diameter of the PANI particles in PVA–PANI
nanocomposite dispersions was measured by using a Zetasizer, Model 3000HS, Malvern, UK.
2.4. Swelling experiments
Dynamic and equilibrium swelling experiments on
PVA–PANI nanocomposite and plain PVA membranes were
performed in water–isopropanol mixtures at 30 ± 0.5 ◦ C in
an electronically controlled incubator (WTB Binder, Model
BD-53, Tuttilgen, Germany) as per the procedures published earlier [22–24]. Circularly cut (diameter = 2.5 cm)
disk-shaped membranes were stored in a desiccator over
the anhydrous calcium chloride maintained at 30 ◦ C for
about 48 h before performing the swelling experiments. Mass
measurements were taken on a digital Mettler microbalance (Model AE 240, Greifensee, Switzerland) sensitive to
±0.01 mg.
2.5. Pervaporation experiments
The procedure used in PV experiments was described
earlier [25,26]. The effective membrane area was 32.43 cm2 .
Weight of the feed mixture taken in the PV cell was 50 g.
Temperature of the feed mixture was maintained constant
by a thermostatic water jacket. Downstream pressure was
maintained below 10 Torr using a vacuum pump (Toshniwal,
Mumbai, India). Before the actual experiment, the test
membrane was equilibrated for about 2 h with the feed
mixture. After establishment of a steady state, permeate
vapors were collected in traps immersed in liquid nitrogen.
PV experiments were performed with the feed mixture of
water–isopropanol taken in different compositions. The
weight of permeate collected in the trap was noted and permeate composition was determined by measuring the refractive
index and by comparing it with the previously established
graph of refractive index versus mixture composition.
Membrane performance was studied by calculating permeation flux, J and selectivity, α using the equations:
J=
α=
W
At
PA
1 − PA
(1)
1 − FA
FA
(2)
Here, FA is mass% of water in the feed and PA is mass% of
water in permeate. The flux (kg/m2 h) was calculated from
the weight, W (kg) of liquids permeated, effective membrane
area, A (taken in m2 ) and measurement time, t (h). At least
three independent measurements of flux and selectivity were
taken under the same conditions of temperature and feed composition to confirm the steady-state permeation.
3. Results and discussion
3.1. Synthesis of nanocomposite membranes
Dispersion polymerization of aniline can be carried out in
the presence of steric stabilizers like poly(vinyl pyrrolidone)
(PVP) [27], PVA [27], hydroxy propyl cellulose (HPC)
[28], carboxy methyl cellulose (CMC) [29], hydroxy ethyl
cellulose (HEC) [30], poly(vinyl methyl ether) (PVME) [31],
etc. Conductivity, particle size and morphology data would
suggest the conductive nature and stabilization. Preparation
of films of PANI with various insulating polymer matrices
and their electrical, mechanical properties have been reported
[21,32,33]. These studies indicated that one could enhance
the processibility of PANI and utilize its conductive nature in
various applications such as electrostatic discharge (ESD),
electromagnetic induction (EMI), etc. Studies utilizing conjugated polymers as membranes to separate ions and liquid
mixtures have also been reported [34–36], but in majority
of cases, pristine conducting polymer membranes were
used to separate gaseous or liquid mixtures. In the present
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
investigation, PVA–PANI nanocomposite membranes were
used for the PV separation of water–isopropanol feed
mixtures. To the best of our knowledge, this is the first kind
of study on such membranes in PV applications dealing with
the separation of water–isopropanol mixtures.
3.2. FT-IR analysis
FT-IR spectrum of aniline, plain PVA and PVA–PANI
nanocomposite membrane is displayed in Fig. 1. For PVA, the
peaks at 2912, 1324 and 843 and 1084 cm−1 are attributed to
C H stretching, C H bending and C O stretching, respectively. A broad high absorption peak observed at 3445 cm−1
is due to O H stretching frequencies of PVA. The band at
1727 cm−1 is attributed to the carbonyl functional groups
due to residual acetate groups remaining after the manufacture of PVA from the hydrolysis of poly(vinyl acetate) or
oxidation during manufacturing and processing. Vibrational
bands observed for PANI are in accordance with the earlier
literature reports [37]. These bands for PANI could be explained on the basis of normal modes of aniline and benzene.
A broad band in the region 3415–3460 cm−1 is assigned to
the N H stretching vibration. Bands at 2915 and 2850 cm−1
are assigned to vibrations associated with the N H moiety in
C6 H4 NH2 C6 H4 group or sum frequency. Bands at 1565 and
1490 cm−1 are due to quinonoid ring (Q) and or benzenoid
ring (B). The bands at 1370 and 1300 cm−1 are assigned to
145
C N stretching vibration in QBQ and QBC, QBB, BBQ,
while a band at 1240 cm−1 is due to C N stretching vibration of the aromatic amine. In the region of 1020–1170 cm−1 ,
the aromatic C H in-plane-bending modes are observed. For
PANI, a strong band appears at 1140 cm−1 due to electronic
band or a vibrational band of nitrogen quinone. A band at
705 cm−1 is assigned to the ring C C bending vibration,
while that at 590 cm−1 is due to the ring in plane deformation.
The C H out-of-plane bending mode has been used as a key
to identify the type of substituted benzene. For PANI, this
mode was observed as a single band at 825 cm−1 , which was
in the range 800–860 cm−1 as reported for 1,4-substituted
benzene. Peaks corresponding to PANI observed at 2850,
1140 and 590 cm−1 confirm the presence of PANI in the PVA
matrix.
3.3. X-ray photoelectron spectroscopic (XPS) analysis
XPS is a powerful tool, which characterizes the doping degree of conducting polyaniline [37]. From the characteristic
binding energies of the photoelectron, the elements involved
can be identified and peak intensity can be directly related to
the atomic concentration in the sample surface. In addition,
various intrinsic redox states of PANI as well as different
neutral and positive nitrogen species can be quantified from
the properly curve fitted N(1s) core level spectrum. XPS was
used to characterize PANI salts and its blends to determine the
dopant ratio and to explain the structure–property relationship [38–40]. It was also used to examine the surface composition of sterically stabilized polypyrrole colloids [41]. In
the present work, PVA–PANI films and their respective salts
were characterized by XPS to find the nitrogen content and
various forms of nitrogen (see Fig. 2). The PVA/PANI ratio
(surface atomic concentration ratio) was determined based on
the reduced atomic concentration of nitrogen in PVA–PANI
nanocomposite membranes with that of pristine PANI prepared in the absence of PVA under similar conditions.
Nitrogen peaks in XPS spectrum of PVA–PANI films
are centered at 400, 402 and 402 eV for PVA–PANII, PVA–PANI-II and PVA–PANI-III films, respectively as
shown in Fig. 2. According to the published report [37], nitrogen peak can be deconvoluted into four peaks corresponding
to imine, amine and the positively charged nitrogen atoms
present in the PANI backbone. In the present study, three
kinds of nitrogens (see Table 1) were observed on deconvoluting N(1s) peak of nitrogen, which corresponds to amine,
cationic radical and cationic nitrogen atoms, respectively.
Table 1
PVA/PANI ratio and deconvolution results of N (1s) XPS spectra of
PVA–PANI films
Fig. 1. FT-IR spectra of plain PANI, plain PVA and PVA–PANI-II nanocomposite membrane.
Sample
PVA/PANI (surface atomic
concentration ratio)
Deconvoluted N(1s)
binding energy (eV)
PVA–PANI-I
PVA–PANI-II
PVA–PANI-III
0.78
0.73
0.52
400.9, 402.0, 402.8
401.1, 401.8, 402.8
400.5, 401.3, 402.4
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B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
membranes were measured at a frequency of 1 Hz. The plots
of loss tangent (tan δ) versus temperature in ◦ C are displayed in Fig. 3(a)–(c) respectively, for the uncross-linked,
cross-linked PVA–PANI, plain PVA and cross-linked PVA
membranes. For PVA, three mechanical dispersions were
reported above −50 ◦ C [43]. A relatively sharp peak in
E with a maximum at 70 ◦ C is assigned to the primary
dispersion (αa ) associated with the glass transition of the
polymer. In this transition temperature region, dynamic
modulus, E decreases markedly from the ‘frozen modulus’,
indicating that the micro-Brownian motions of the main
PVA chains become conspicuous in the amorphous regions.
The presence of the secondary dispersion (βa ) due to the
local relaxation mode of the PVA main chains appear as a
broad shoulder ranging from 0 to 30 ◦ C in the E curve. The
sample gives another dispersion signal above 100 ◦ C due to
relaxation in the PVA crystalline phase.
The tan δ curves of the uncross-linked and cross-linked
PANI–PVA nanocomposite membranes showed two peaks
in the range 55–70 and 100–145 ◦ C, respectively. Peaks
around 50–70 ◦ C are due to the Tg of PVA. The second
peak appearing around 100–145 ◦ C is the α peak, due to
chain relaxation in the crystalline phase of PVA. This α
relaxation was more prominent in the PANI-introduced
nanocomposite membranes compared to the plain PVA
membrane. Simultaneously, the DSC analysis (see Fig. 4 and
Table 2) of the cross-linked PVA showed a decrease in melting temperature, Tm after cross-linking with GA, whereas
Tm of the uncross-linked and cross-linked PVA–PANI
nanocomposite membranes increased compared to the plain
PVA, suggesting a more ordered arrangement of PVA chains
after introducing PANI particles in the matrix.
Storage modulus (E ) versus temperature plots for
PVA, cross-linked PVA, cross-linked and uncross-linked
PVA–PANI films are shown in Fig. 5. The storage modulus measures the stiffness of the polymer. A sharp decrease in
storage modulus was observed for the plain PVA, cross-linked
PVA and PVA–PANI films in the glass transition region,
which later reached a plateau. However, storage modulus of
the plain PVA (1.23 × 107 Pa) has increased after incorporating PANI, indicating an increase in the rigidity of PVA chains.
Fig. 2. Deconvoluted N(ls) spectra of PVA–PANI nanocomposite membranes (a) PVA–PANI-I, (b) PVA–PANI-II and (c) PVA–PANI-III nanocomposite membranes.
3.5. SEM analysis
SEM micrographs of the PVA–PANI nanocomposite
membranes are displayed in Fig. 6. The micrograph
3.4. DSC/DMTA analyses
DMTA provides a sensitive test of physical changes
occurring in polymers over a wide range of temperature
and frequency [42]. In the present study, first frequency
was scanned on the specimen at ambient temperature and
then at the selected frequency, temperature was scanned.
Samples of this study were analyzed from room temperature
to 300 ◦ C. Dynamic mechanical properties of the plain PVA,
uncross-linked and cross-linked PVA–PANI nanocomposite
Table 2
Melting onset temperatures and molar mass between cross-links (Mc ) of
different membranes
Membrane
PVA
PVA–PANI-I
PVA–PANI-II
PVA–PANI-III
Melting onset (◦ C)
Mc ×10−4 (kg/mol)
Uncross-linked
Cross-linked
175
196.1
196.6
194.0
143
193.0
195.7
199.0
886
285
115
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B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
147
Fig. 3. tan δ curves of (a) uncross-linked PVA–PANI, (b) cross-linked PVA–PANI, and (c) plain and cross-linked PVA membranes.
confirmed the uniform distribution of PANI particles in PVA
matrix. Spherical PANI particles were observed at a lower
concentration of aniline, whereas at higher concentration of
aniline (PVA–PANI-III), PANI particles were found to be
agglomerated.
3.6. Particle size analysis
Number average size distribution histograms of PANI
particles in all the three nancomposites are displayed in
Fig. 7. Zeta average diameter of the PANI particles in the
Fig. 4. DSC thermograms of (a) uncross-linked and (b) cross-linked membranes.
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Fig. 5. E curves of (a) uncross-linked PVA–PANI, (b) cross-linked PVA–PANI, and (c) plain and cross-linked PVA membranes.
PVA matrix increased with increasing aniline content. Even
though zeta average diameter ranged between 700 and
900 nm for all the three nanocomposites prepared, a large
number of particles were found to be present in the lower
diameter range i.e., 30–100 nm.
3.7. Molar mass between cross-links
Molar mass, Mc between the cross-links in a polymer matrix is important to know the dimensional stability of the films
in the presence of liquids. This parameter was widely studied
in the literature for both wet and dry polymer films [44,45].
In the absence of solubility parameter values of the polymers,
one could calculate Mc from the DMTA measurements using
the modulus values following the kinetic theory of rubber
elasticity [46,47] as
Mc =
ρ
νe
modulus at a temperature above Tg ) and R is the universal gas constant (8.314 × 107 cal/mol deg). Calculated values of Mc are also given in Table 2. For the plain PVA
membrane, Mc = 886 × 104 kg/mol, the highest among all the
membranes. For PVA–PANI-I, the Mc = 285 × 104 kg/mol, a
value that is almost three-times smaller than that observed
for the plain PVA. Such a decrease in Mc of the nanocomposite membrane could indicate changes in the morphological
setup of the membranes. These changes are directly related
to the pervaporation performance (to be discussed in Section 3.10). It can be seen that with an increasing amount
of PANI particles in the PVA matrix, the Mc values tend
to decrease considerably. Thus, for the PVA–PANI-III, the
Mc = 15 × 104 kg/mol, the smallest of all the membranes prepared.
3.8. Free volume
(3)
where ρ is the density of the film (kg/m3 ), measured by
the benzene displacement method, νe = E /3RT (where E is
Molecular transport through dense polymeric membranes
is influenced by the presence of free volume [48], which arises
as a result of voids created due to poor chain packing during
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
149
Here Vw is the van der Waals volume estimated from the
group contribution method [49]. Within a given family of
polymers, penetrant diffusivity and permeability can be correlated with FFV [50]. The FFV results presented in Table 3
increase with increasing amount of PANI in the matrix. This
increase in free volume space in the nanocomposite membranes with increasing PANI particles could be the result
of morphological changes occurring during the fabrication
of the membranes and/or while carrying out polymerization reaction of aniline in acidic media. Thus, the increased
acid doping could have induced higher free volume spaces
within these matrices. However, for the plain PVA membrane,
FFV was lowest. These results follow the general observations made by Merkel et al. [8] for the nanocomposite blend
membranes prepared from poly(4-methyl-2-pentyne)/fumed
silica.
3.9. Swelling results
Fig. 6. SEM micrographs of (a) PVA–PANI-I, (b) PVA–PANI-II, and (c)
PVA–PANI-III nanocomposite membranes.
the membrane cross-linking process. If the space between
chain segments is large, then more free channels are available
for the faster movement of liquid molecules. The fractional
free volume (FFV) of the membrane was calculated by using:
FFV =
Vsp − V0
Vsp
(4)
Dynamic swelling results of the membranes at 30 ◦ C
obtained in 10 mass% water containing feed mixture are
presented in Fig. 8 and also included in Table 3. Swelling
kinetics is controlled by the diffusion of solvent molecules in
relation to the polymer chain relaxation [51]. In the present
study, swelling increased slightly, but systematically with
increasing amount of PANI in the matrix; this could be due
to the increased void spaces for these membranes with the
decreasing Mc values. The time required to attain equilibrium
swelling was not identical, since it varied depending upon
the nature of the membrane material. For instance, with the
plain PVA membrane, swelling reached equilibrium within
30 min, while for PVA–PANI nanocomposite membranes,
it took 40 min. However, experiments were continued for
longer time to ensure complete equilibration. Thus, polymer
swelling is inversely related to chain morphology as can be
studied by the Mc data. For instance, a matrix with higher
cross-linking will exhibit lower swelling and vice versa. In
addition, free volume and nature of the penetrating liquid
molecules could exert an influence on swelling. The Tg (relaxation effects) of the polymers has shown an effect on swelling.
For instance, with increasing Tg of the nanocomposite membranes swelling increased. Therefore, the PV performance
of the nanocomposite membrane can be explained as due
to the combined effects of several parameters (discussed
before) that are equally important in the selective transport of
water.
Table 3
Fractional free volume of the membranes along with equilibrium swelling
data for different membranes at 10 mass% of water in the feed at 30 ◦ C
Plain PVA
PVA–PANI-I
PVA–PANI-II
PVA–PANI-III
where Vsp is polymer bulk specific volume and V0 is volume
occupied by the polymer chains calculated as [49]:
0.5798
0.5809
Fractional free volume
0.5815
0.5831
V0 = 1.3VW
0.114
0.074
Equilibrium swelling (g)
0.080
0.084
(5)
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Fig. 7. Number average particle size distribution of (a) PVA–PANI-I, (b) PVA–PANI-II, and (c) PVA–PANI-III nanocomposites.
3.10. Pervaporation results
Novelty of the present investigation is that, for the
first time, PANI-doped PVA nanocomposite membranes
were prepared and used to study their PV performance
of water–isopropanol feed mixtures. The PV performance
varied depending upon the amount of aniline dispersed for
the polymerization to occur in the PVA matrix. The dispersed
PANI particles were in the size range of 30–100 nm, even
though some smaller number of particles > 100 nm were
also present depending upon the amount of aniline used
during polymerization. The PV results of the PVA–PANI
nanocomposite membranes are compared in Table 4 with the
plain PVA membrane. Since PANI was synthesized in the
Fig. 8. Plots of swelling vs. time at 10 mass% of water in water containing isopropanol mixtures. Symbols: () pure PVA, (䊉) PVA–PANI-I, ()
PVA–PANI-II and () PVA–PANI-III nanocomposite membranes.
emeraldine oxidation state by the oxidative polymerization
of aniline in an acidic medium, hence the PANI particles
dispersed in the PVA matrix were in the acid-doped form.
The resulting doped films were dark green in color. Flux and
selectivity data are displayed in Figs. 9 and 10, respectively,
Table 4
Pervaporation data of water + isopropanol mixtures at 30 ◦ C
Mass% of water in
Feed
Water flux (kg/m2 h)
Selectivity
Permeate
10
20
30
40
50
89.57
88.2
87.62
85.43
84.63
PVA
0.095
0.216
0.320
0.366
0.398
77.3
29.9
16.5
8.8
5.5
10
20
30
40
50
67.44
78.38
81.04
86.38
89.02
PVA–PANI-I
0.035
0.068
0.091
0.116
0.156
18.6
14.5
10
9.5
8.1
10
20
30
40
50
98.28
87.97
63.02
80.73
87.13
PVA–PANI-II
0.061
0.090
0.084
0.127
0.194
514.3
29.3
4.0
6.3
6.8
10
20
30
40
50
98.43
92.56
84.82
71.75
67.8
PVA–PANI-III
0.069
0.221
0.243
0.219
0.218
564.2
49.8
13.0
3.8
2.1
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
Fig. 9. Water flux vs. mass% of water in feed mixture at 30 ◦ C. Symbols
have the same meanings as in Fig. 8.
for the range of feed compositions investigated. Flux of
water was higher for the plain PVA membrane due to its
hydrophilic nature. The increase in flux from 0.095 to
0.398 kg/m2 h with increasing water content of the feed
mixture could be explained as due to higher hydrophilic
interactions between water molecules and that of the PVA,
leading to an increase in the swelling of the PVA membrane.
Conversely, selectivity to water decreased from 77.3 to
as small as 5.5 with increasing water content of the feed
mixture. The concentration of permeate (water) obtained
from the plain PVA membrane was about 90 mass% of water
but the permeability of isopropanol was much smaller than
water, since water is more polar than isopropanol. In case of
PVA–PANI-I nanocomposite membrane, the amount of water
in the permeate is higher (i.e., 89.02 mass%) for 50 mass%
water-containing feed (a reverse trend to that of the plain
PVA membrane). On the other hand, with PVA–PANI-II and
PVA–PANI-III nanocomposite membranes, 98.43 mass% of
Fig. 10. Selectivity vs. mass% of water in feed mixture at 30 ◦ C. Symbols
have the same meanings as in Fig. 8.
151
water was removed in the permeate for the feed containing
10 mass% water. This could be due to an increased flux of
water, since the higher amount of PANI present in the doped
state of the PVA matrix could absorb higher amount of water
from the feed containing large quantities of water.
In case of PVA–PANI-I nanocomposite membrane, the
flux varied from 0.035 to 0.156 kg/m2 h, while the selectivity
decreased from 18.6 to 8.1 for the feed mixture containing
an increasing amount of water ca. from 10 to 50 mass%. The
flux of the PANI incorporated PVA membranes increased
with increasing concentration of PANI nanoparticles in the
PVA matrix for the obvious reason that more the number of
PANI particles present in the PVA matrix, higher will be the
hydrophilic–hydrophilic interactions. Since PANI is more
of a rigid polymer than PVA and hence, its presence in the
matrix could help to reduce the overall membrane swelling.
This effect could further be compensated by the observed
lower selectivity of PVA–PANI-I nanocomposite compared
to PVA–PANI-II and PVA–PANI-III. High variability in the
permeability of water for the doped PANI (Table 4) may be
due to the relatively large effect that differences in the microscopic membrane morphology have on the permeability of
very small molecule such as water. See for e.g., the Mc values
of the membranes presented in Table 2, wherein with an
increasing amount of PANI in the PVA matrix, the Mc values
decrease, suggesting the morphological changes occurring
in the membranes. These Mc data have a direct effect on
the permeation flux data of water. For instance, in case of
nanocomposite membranes, the flux increased systematically
with decreasing Mc , which is reasonable to assume that with
the lesser number of chain entanglements in the polymer
matrix, more number of diffusional pathways are available,
which would explain the increased flux of the PVA–PANI
membranes. Thus, the optimum selectivity values observed
for 10 mass% water containing feed mixture for all the three
membranes could be the result of increased preferential
interactions of water (compared to isopropanol) with the
doped PANI-incorporated films. However, the decrease in
selectivity at higher amounts of water in the feed could be
due to the preferential escape of isopropanol along with
water. While water’s size (0.28 nm) gives it a high diffusivity,
but in the presence of higher amount of PANI nanoparticles,
such as in case of PVA–PANI-III, the PVA membrane pores
might be blocked leading to a lower selectivity to water.
Overall, the level of doping interaction between PANI and
water is likely to be sensitive to the microscopic morphological change of the film, which may account for the high
variability in the permselectivity of water due to variations of
its composition in the feed. As doping takes place over time,
the morphology properties of the membrane could change.
As the water molecules accumulate in the membrane, some
of the diffusion pathways through the membrane are likely
to be blocked off or become smaller. When this happens,
permselectivity of the membrane begins to decline until
doping in the film stabilizes and permselectivity reaches the
steady-state value. Thus, the decline in selectivity at higher
152
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
compositions of water in the feed is likely due to swelling
as well as doping effects that take place in the membrane
over the course of the PV experiment. Additionally, the
dopant leaching could also play a role in affecting the
permselectivity of water/isopropanol feeds. Large variations
in selectivity of the doped PANI-containing PVA membranes
could be due to highly sensitive dopant leaching effects
[52]. These effects may be caused by the greater role that
solubility selectivity plays on the overall permselectivity of
water–isopropanol in the HCl-doped PANI particles. Since
the solubility of water–isopropanol feeds plays a greater
role in the permselectivity, a slight difference in the extent
of dopant leaching from the HCl-doped PANI-containing
membranes will have much greater effect on the selectivity of this matrix than those mixtures where the overall
permselectivity is dominated by the diffusive selectivity
[53].
In the present study, PVA–PANI-II and PVA–PANIIII nanocomposite membranes were able to extract almost
98 mass% of water from the 10 mass% water containing feed.
This is because when the PANI particles are present in their
doped form, these will induce hydrophilicity as a result of
the separated charges. Thus, the ionic character of the overall membrane could facilitate the transport of water. In the
case of doped PANI, both diffusion (size effect) and sorption
(chemical interaction effect) appear to favor water so that
water is selectively transported through the membranes to a
much greater extent. Additionally, due to the water-adsorptive
nature of the PANI particles, their presence could also help
to provide the free diffusion channels to give an increased
water transport along the void channels of the membrane.
See for e.g., the fractional free volume data presented in
Table 3, which increase with increasing contents of PANI
in the matrix. These data follow the same trends of increase
in permeation flux. However, the exact mechanism of permeation in PV is quite complicated at the molecular level,
yet the sorption–diffusion concept has been widely accepted
to describe the PV performance. The microchanges in the
membrane morphology (i.e., in terms of FFV, Mc and Tg ) are
thus important to explain the molecular transport across the
membranes.
Fig. 11. Comparison of vapor liquid equilibrium curve (), with PV data (䊉)
for water (l)-isopropanol (2) mixtures at 30 ◦ C for PVA–PANI-III membrane.
In process engineering, the purification of isopropanol
has been traditionally achieved through the azeotropic
distillation wherein, benzene is used as an entrainer.
Azeotropic distillation is a energy-consuming process and
the use of entrainer like benzene could cause an unwanted
impurity in the final product as well as the side streams.
Therefore, PV technique could be a better alternate to simple
distillation. Hybrid processes combining simple distillation
with PV have also been recommended [54]. The PVA–PANIIII nanocomposite membrane of this study would be wellsuited for further detailed investigations, since it exhibited
better selectivity at the azeotropic composition of the feed
mixture (12.5 mass% of water) than the other membranes
studied. Fig. 11 displays such a dependence wherein, the PV
curve is always higher than the vapor–liquid equilibrium line
at all the compositions, demonstrating that PVA–PANI-III
nanocomposite membrane selectively permeate water at all
the feed compositions and that the membrane acts as a third
phase, resulting in the effective separation of water.
The PV performance of PVA–PANI-III nanocomposite
membrane is compared in Table 5 with all the other
PVA-based membranes published in the literature [4,55,56]
for separating water–isopropanol mixtures. Compared to
Table 5
Comparison of PV performance of the present nanocomposite membranes with literature data on PVA-based membranes for water + isopropanol mixtures at
30 ◦ C
Membrane
Mass% of water in feed
Flux (kg/m2 h)
Selectivity
Reference
PVA–PANI-III
NaAlg/PVA (75:25)
NaAlg/PVA (50:50)
NaAlg/PVA (25:75)
PVA + KA
PVA + NaA
PVA + CaA
PVA + NaX
PVA cross-linked with glutaraldehyde
PVA cross-linked with citric acid
10
10
10
10
20
0.069
0.025
0.034
0.039
0.179
0.183
0.190
0.216
0.194
0.095
564
195
119
91
410
328
233
133
116
741
Present work
[4]
10
5
PVA: poly(vinyl alcohol); PANI: polyaniline; NaAlg: sodium alginate; KA, NaA, CaA, NaX: zeolites.
[55]
[56]
B. Vijaya Kumar Naidu et al. / Journal of Membrane Science 260 (2005) 142–155
our earlier results [4] on sodium alginate/PVA blend membranes, the present nanocomposite membranes could offer
an improved flux and better selectivity to water. The plain
PVA membrane cross-linked with glutaraldehyde developed
by Burshe et al. [56] had a selectivity of 116 with a flux
of 0.194 kg/m2 h for the separation of water–isopropanol
mixture. On the other hand, 5 mass% water-containing
feed mixture of water–isopropanol had a selectivity of 741,
when the plain PVA was cross-linked with citric acid [56].
Comparing with the PVA membranes incorporated with
KA, NaA, CaA and NaX type zeolites [55], which gave
an improved flux with reasonable selectivites ranging from
410 to 133 at 20 mass% water containing feed mixture, the
present PVA–PANI-III nanocomposite membranes gave
much superior values (see Table 5).
3.11. Temperature-dependent permeation rate
Another method of increasing the permeation rates of
liquids through a membrane module is by increasing the temperature of the permeating solution. Temperature-dependent
permeation rate, selectivity and mass% of water in the
permeate for 10 mass% water and 90 mass% isopropanol
feed mixture through the PVA–PANI-III membrane are given
in Table 6. A two-fold increase in the permeation rate (flux)
is observed upon raising the temperature from 30 to 40 ◦ C,
with an appreciable loss in selectivity from 564 to 181. Upon
further increasing the temperature to 50 ◦ C, an increase in
flux, but not a drastic decrease in selectivity is observed. The
increase in flux over the studied temperature range is likely
due to an increased diffusion rate of the feed molecules.
Additionally, in the complex nanocomposite membrane of
the type prepared in this study, slight changes in the fractional
free volume with increasing temperature could result in their
increased permeation flux. These observations are consistent
with the observed systematic decrease in mass% of water
in permeate with increasing temperature. The Arrhenius
activation energy, EP for the PV process was computed using:
JP = JP0 exp(−EP /RT )
(6)
where JP is permeation flux, JP0 the Arrhenius constant, R
the universal gas constant, and T the temperature in Kelvin.
The estimated EP values for PVA–PANI-III membrane
in case of water (EP = 16.18 kJ/mol) and isopropanol
(EP = 79.48 kJ/mol) for water–isopropanol feed mixtures
indicate the easy energy required to cross the potential
energy barrier in the activated state during the flow process.
Table 6
Pervaporation data of PVA–PANI-III nanocomposite membrane at different
temperatures for 10 mass% water containing feed mixture
Temperature (◦ C)
Water flux
(kg/m2 h)
Mass% of water
in permeate
Selectivity
30
40
50
0.069
0.144
0.158
98.43
95.26
92.87
564.2
180.9
117.2
153
4. Conclusions
The present study addresses the development of novel
nanocomposite membranes of poly(vinyl alcohol) dispersed with the doped polyaniline nanoparticles used for
the pervaporation separation of water–isopropanol feeds
ranging in composition from 10 to 50 mass% of water.
SEM micrographs confirmed the uniform distribution
of polyaniline nanoparticles in the poly(vinyl alcohol)
matrix. The solution-cast membranes were cross-linked with
glutaraldehyde as confirmed by Fourier transform infrared
spectra. Membranes could exhibit an increased selectivity
to water about five-folds compared to the plain poly(vinyl
alcohol) membrane at the expense of reduced water flux. The
PVA–PANI-III nanocomposite membrane could successfully
separate water–isopropanol feed mixture at the azeotropic
composition compared to simple distillation. The membrane
performance was also studied at higher temperatures (40
and 50 ◦ C). The results of this study were explained in terms
of the acid doping effects of the PANI particles in the PVA
matrix. However, the parameters like molar mass between
cross-links, fractional free volume and extent of swelling
are important to explain the pervaporation results.
Acknowledgements
Professor T.M. Aminabhavi and Dr. B.V.K. Naidu (RA)
thank the Council of Scientific and Industrial Research
(CSIR), Grant No. 80(0042)/02/EMR-II for a financial support of this study. Authors thank the University Grants Commission (UGC), New Delhi, India for a major funding (F141/2001/CPP-II) to establish the Center of Excellence in
Polymer Science.
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