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Summary
The mass transport characteristics of macromolecular networks have been
the subject of intensive research for the past several years. A better
understanding of transport phenomena through polymers is highly
important in order to achieve significant improvement in the areas of
protective coatings, membrane separation processes and packaging
industry. Recently, a combination of improved economics and better
technology have resulted in membrane products that signed a new era in
the commercial use of membranes for liquid and gas separation.
Membrane separation techniques have been widely used for many
industrial applications like desalination of brine and oxygen enriched air.
Pervaporation, another important technique, has been widely used for the
separation of a wide variety of organic liquid mixtures. This chapter
describes the theory and factors governing the transport of molecules
through polymeric networks. The general features of pervaporation,
vapour and gas transport are presented. Finally the aim and scope of the
present investigation are discussed.
Introduction
Chapter 1
2 Chapter 1
1.1. Transport through polymers
Molecular transport through polymers has become a subject with a
variety of challenges and opportunities for practitioners of chemical
engineering and applied chemistry in recent years [1-10]. The knowledge
of the performance of polymers in the environment of hazardous
solvents, vapours and temperature is essential for their successful
applications as structural engineering materials. Researchers are more
and more enthusiastically adopting the use of small molecules as micro-
structural probes for polymers [11].
The diffusion and permeation properties of polymers play a very major
role in designing food packaging materials [12], solvent reservoirs [13],
pervaporators [14], controlled release devices [15, 16] etc. Regenerated
cellulose or cellophane was the first practical glassy film to find large-
scale use in packaging [17]. The packaging of fruits and vegetables
requires inward permeation of CO2 and respiration of oxygen without loss
of water [18]. In meat-wrap and cheese-wrap applications, the polymer
films chosen are those which will allow at least a certain minimum
oxygen diffusion from air and minimize flavour loss respectively. The use
of polymer membranes as liners and storage tanks for hazardous liquids
is increasing [19]. In order to examine the suitability of a membrane as a
barrier material, it is essential to acquire a thorough understanding of its
interactions with liquids and vapours.
Introduction 3
Liquid mixtures can be separated by partial vapourisation through a non-
porous permselective membrane by a technique called pervaporation
[20]. In order to establish pervaporation as a competitive separation
process, research has been concentrated on the development and
optimization of polymeric membranes and their interaction with different
penetrants. For water desalination by reverse osmosis, the most
successful polymer membrane is Loeb-Sourirajan membrane (a cellulose
acetate based membrane) [21]. The ultra thin layer (0.3 micron) gives
excellent salt rejection, and because it is so thin, water diffusion rates are
relatively high. For these types of membranes, it would be advantageous
to have a high permselectivity and a high diffusion rate.
In biomedical applications also polymeric membranes are extensively
used. Polymer films serve, by selective diffusion, to remove certain
components from blood when a damaged kidney can no longer do so.
The components to be removed are urea, uric acid, water, creatine,
phosphates and excess chlorides from blood through the membrane [22].
In recent years, new polymers have entered the area of controlled drug
release. Many of these materials are designed to degrade within the
body; among them are polylactides, polyglycolides, poly (lactide-co-
glycolides), polyanhydrides etc. There are three primary mechanisms by
which active agents can be released from a delivery system: diffusion,
degradation and swelling followed by diffusion. Any or all of these
mechanisms may occur in a given release system. An ideal drug delivery
4 Chapter 1
system should be inert, biocompatible, mechanically strong, comfortable
for the patient, capable of achieving high drug loading, safe from
accidental release, simple to administer and remove and easy to
fabricate and sterilize.
The silicone membranes have a relatively high permeability to carbon
dioxide and oxygen and are permselective to carbon dioxide. One
possible application is carbon dioxide removal from a space cabin during
extended journeys into space [23]. Another intriguing possibility is that an
animal enclosed in a very thin silicone rubber membrane could live under
water by the diffusion of oxygen from water and by the removal of carbon
dioxide through the membrane [24].
1.2. Fundamentals of transport phenomena
The transport mechanism in polymers consists of three steps (a)
adhesion of penetrant species on the surface of the matrix (b) diffusion of
the penetrant species through the matrix and (c) desorption of the
penetrant species from the matrix. A schematic representation of the
transport mechanism in polymers is shown in Figure 1.1.
Introduction 5
Figure 1.1 : Schematic representation of transport mechanism:
(a) Adhesion of species on the surface of the matrix
(b) Diffusion of species through the matrix
(c) Desorption of the species from the matrix
The transport process slowly tries to equalize the concentration difference
or the chemical potential of the penetrant in the phases separated by the
membrane. This process can be described by Fick’s first and second laws
of diffusion.
If a concentration gradient is established across some arbitrary reference
section in the polymer, a net transport of penetrant occurs in the direction
of decreasing concentration. This phenomenon can be described in terms
of Fick’s first law of diffusion according to which the diffusive flux, J (the
amount of penetrant passing through a plane of unit area normal to the
direction of flow during unit time) in the x-direction of flow is proportional to
the concentration gradient c
x
∂∂
as
(a) (c)
(b)
6 Chapter 1
cJ D
x
∂= −∂
(1.1)
where D is the diffusion coefficient and c the concentration of the diffusing
molecule. This equation is applicable when the diffusion is in the steady
state.
On the other hand, Fick’s second law describes the unsteady state
transport process, which is given by the rate of change of the penetrant
concentration at a plane within the membrane, as
t
c
x
cD
∂∂=
∂∂
2
2
(1.2)
This is an ideal case in which the membrane is isotropic and D is independent
of distance, time and concentration. Depending on the boundary conditions,
there are many solutions available for Equation (1. 2).
When the diffusion rate J achieves a steady value (no longer varies with
time) a material balance requires that it also be independent of x, in which
case Equation (1.1) can be integrated to give,
( )2
10
c ch
c c
Jdx D dc=
=
= −∫ ∫ (1.3)
( )2
1
1c c
c c
J D dch
=
=
= − ∫ (1.4)
Introduction 7
Equation (1.4) indicates the steady-state diffusion rate which is inversely
proportional to the overall membrane thickness h for a given set of
boundary conditions. To complete the integration, the variation of D with c
must be known. In some systems, the diffusion coefficient does vary with
concentration because the Van der Waals forces are high between the
diffusing molecules themselves and between the diffusing molecules and
the polymer chains.
When there are relatively weak intermolecular forces of attraction between
the diffusing molecules and the polymer chains, the solubility is low and
the polymer structure is not significantly altered. Under these
circumstances c can vary without affecting D, and Equation (1.4) then
becomes
( )DJ
h= − � � � (1.5)
where c2 and c1 are respectively the concentrations of the diffusing
molecules dissolved at the downstream and upstream polymer-membrane
faces. In many cases, it is the pressures or partial pressures p of a gas or
vapour above the faces of the polymer film, rather than the surface
concentration, which are known. These quantities are related by Henry’s
law, which states that
C = S p (1.6)
8 Chapter 1
where S is the solubility constant for a given gas-polymer system.
The combination of Equations (1.5) and (1.6) gives the well-known
permeation equation,
1 2p pJ DS
h−⎡ ⎤=
⎢ ⎥⎣ ⎦
(1.7)
The product DS is called the permeability coefficient, P, so that
P = D.S (1.8)
1.3. Factors affecting transport phenomena
1.3.1 Nature of the Polymer
Transport properties of a particular polymer depend on the free volume
within the polymer and on the segmental mobility of polymer chains.
Another influencing factor is glass transition temperature (Tg). That is why
transport characteristics of different polymers are different.
Generally speaking, dense polymer membranes have no pores but there
exists the thermally agitated motion of chain segments to generate
penetrant scale transient gaps in the matrix (free volumes) allowing
penetrants to diffuse from one side of the membrane to the other side [25].
Due to internal micro-motions of chain rotation and translation, as well as
vibration, a larger amount of free volume is basically readily accessible for
diffusion in amorphous polymer matrices. On the other hand in the glassy
state, there is restricted chain mobility. Rotation about the chain axis is
limited and motion within the structure is largely vibratory within a frozen
Introduction 9
quazi-lattice. These types of polymers have very dense structures, with
very little internal void space. Ponangi et al. [26] investigated free volumes
of four different polyurethane membranes Spandex, BFG-1, BFG-2 and
BFG-3 by organic vapour diffusion. The computed diffusion coefficient and
free volume at 30°C in benzene are given in Table 1.1. It is clear from the
table that the diffusion coefficient value decreases with decrease in
average fractional free volume values.
Table 1.1. Comparison of diffusion coefficient and free volume (Penetrant: benzene vapour at 30°C) [R. Ponangi, P.N. Pintauro and D. De Kee, J. Membr. Sci., 178, 151 (2000)]
Polyurethane D × 10-7 (cm2/s)Average fractional free
volume
Spandex 7.14 0.0634
BFG-1 2.45 0.0562
BFG-2 0.905 0.04804
BFG-3 0.611 0.03868
High polymer chain segmental mobility is an important factor enhancing
transport process. The segmental mobility of the polymer chains is
affected by many factors which include the presence of bulky groups in the
chains and the Tg.
Van Amerogen [27] studied the diffusion and permeability of hydrogen,
helium, nitrogen, oxygen, acetylene and cyclopropane through a series of
natural and synthetic rubbers and observed that samples containing larger
10 Chapter 1
number of substituent methyl groups had lower diffusion and permeability
coefficients. Van Amerogen [28] observed an inverse relationship between
nitrile content and sorption coefficient in a series of butadiene / acrylonitrile
copolymers (Figure 1.2).
Figure 1.2 : Gas solubility in butadiene – acrylonitrile copolymer at 25oC[G.J. Van Amerogen, J. Polym. Sci; 5, 307 (1950)].
Bulky groups in the polymer chain effectively reduce their segmental
mobility and thereby diffusion ability. The polymers containing this type of
groups in the chains increase their Tg. Polymers with low Tg possess
greater segmental mobility and will have higher diffusivity. The Tg values of
polyethylene (PE) and polypropylene (PP) are -20 and + 5°C respectively.
PE has flexible backbone but in the case of PP, there are -CH3 groups to
Introduction 11
inhibit the freedom of rotation [29]. Strong polar attraction and crystalline
regions also contribute to high Tg values. Table 1.2 demonstrates that for
polymers with the same substitution pattern, it is the flexibility of the
backbone that determines permeation properties [30].
Table 1.2 : Effect of side chain and main chain substitution on oxygen permeability [S.A. Stem, V.M. Shah and B.J. Hardy, J. Polym Sci. Polym Phys. Ed., 25, 1293 (1987)]
Polymer Tg (0C) P×10-17
(mol m-1 s-1Pa-1) a
A -CH2CMe2- -76 0.84
B -CH2SiMe2- -92 0.44
C -CSiMe2- -123 437.0
D
)8
-88 1.0
E -28 0.14
a At 350C
The Si-O backbone allows for rapid chain segmental motion to occur in the
silicone rubber (polymer C) and substitution of the Si-O linkage by Si-CH2
(polymer B) reduces the permeability to a value even less than that of butyl
rubber (polymer A). The insertion of a (CH2)n sequence into a siloxane
backbone (polymer D) also leads to a decrease in permeability. Similarly,
the Si-O backbone substitution of methyl by more bulky substituents
12 Chapter 1
decreases the permeability (polymer E). The substitution of bulky
functional groups in the side chains appear to have a greater influence on
decreasing diffusivity than substitution of these groups in the polymer
backbone [30-32].
The water sorption behaviours of biphenyl tetracarboxylic dianhydride
(BPDA) based polyimide thin films depending upon the structural isomers
of diamine were studied by Seo and co-workers [33] and is shown in Table
1.3. The polyimide films with para oriented linkages in backbone structure
showed relatively lower diffusion coefficient and water uptake than the
corresponding polyimide films with meta oriented linkages because of the
well developed crystalline structure and good intermolecular chain
ordering.
Table 1.3 : Tg, Diffusion coefficient and water uptake of the BPDA based polyimide thin films [J. Seo, J. Jeon, C. Lee, S. Park and H. Han, J. Appl. Polym. Sci; 79, 2121 (2001)]
Polyimide Structure Tg (oC) Dx10-10(cm2/s) Water uptake (wt%)
BPDA – p PDA 360 1.6 1.52
BPDA – m PDA 375 7.4 4.75
BPDA – p’p’ ODA 295 3 1.62
BPDA – p’m’ ODA 300 6.7 2.32
BPDA – p’p’ DDS 400 7.2 5.14
BPDA – p’m’ DDS 330 12.4 5.25
Introduction 13
The permeability of permeants which interact weakly with functional
groups present in a polymer can be expected to decrease as the cohesive
energy of the polymer increases. For example, by increasing the polarity
of the substituent group on a vinyl polymer backbone, oxygen permeability
is reduced by almost 50,000 times, as shown in Table 1.4. [34].
Table 1.4 : Effect of functional groups on oxygen permeability of vinyl polymers (CH2CHx)n [S. Steingiser, S.P. Nempos and M. Salame, in Encyclopedia of Chemical Technology, Vol. 3 3rd
Edn [Eds. H.F. Mark, et al], Wiley Interscience, New York, P. 481 (1978)]
Functional group P x 10-17 (mol m-1 s-1 Pa-1)a
H 1.3
Ph 1.1
Me 3.97 x 10-1
F 3.96 x 10-2
Cl 2.1 x 10-2
CN 1.08 x 10-4
OH 2.64 x 10-5
1.3.2. Polymer molecular weight
The effect of polymer molecular weight on transport properties is generally
found to be very low. Berens and Hopfenberg [35] studied the diffusion of
several organic penetrants through polystyrene (PS) films of number
14 Chapter 1
average molecular weight varying between 16,100 and 310,000. Several
experimental studies [36-38] revealed that the diffusion and permeability
coefficients were not strongly dependent on the molecular weight
presumably because the samples involved were of sufficiently high
molecular weight.
1.3.3. Distribution of crosslinks
Covalent chemical bonds generated between macromolecular chains are
known as crosslinks. The transport properties of polymers are strongly
dependent on the distribution of crosslinking in them. An uncrosslinked
polymer will usually dissolve in an appropriate solvent. By contrast,
crosslinked polymers will not dissolve. Solvation of chain segments cannot
overcome the effect of the covalent bonds between the macromolecules;
hence crosslinked molecules cannot be carried off into the solution.
Depending on the distribution of crosslinks or crosslink density, however,
such materials may admit significant amounts of solvent, becoming softer
and swollen as they do so. The swelling by fairly lightly crosslinked
materials is generally reversible and, given appropriate conditions, solvent
that has entered a crosslinked structure can be removed and the polymer
can be returned to its original size [29]. Heavily crosslinked polymers have
a dense three-dimensional network of covalent bonds in them, with little
freedom for motion by the individual segments of the molecules involved in
such structures. The mass transport process is very tortuous in these
types of polymer matrices. Bajpai and Giri [39] studied the swelling
Introduction 15
dynamics of a macromolecular hydrophilic network prepared by performing
graft copolymerization of crosslinked polyacrylamide chains onto
carboxymethyl cellulose and poly vinyl alcohol (PVA), using water. The
crosslinker used was N,N’-methylene bis acrylamide (MBA). The increase
in concentration of crosslinker resulted in substantial fall in the swelling
ratio due to the reduction in mesh sizes of the voids available between the
network chains.
In membrane technology, there are two reasons for crosslinking a polymer.
The first reason is to make the polymer insoluble for the feed mixture and
the second is to decrease the degree of swelling of the polymer in order to
maintain selectivity [40]. Ren et al. [41] investigated the separation of
aromatics/aliphatics with crosslinked 4,4 -hexafluoro-isopropylidene
dianhydride (6FDA)-based copolyamide membranes. In order to obtain
high permeability as well as high selectivity, a combination of the diamines,
2,3,5,6-tetramethyl-1, 4-phenylene diamine (4MPD) and 4,4-hexafluoro-
isopropylidene dianiline (6FpDA) and 3,5-diaminobenzoic acid (DABA) as
a monomer with a crosslinkable group was used. The sorption properties
of the crosslinked copolyimides as well as those of the non-crosslinkable
reference polymers 6FDA–4MPD and 6FDA–6FpDA were investigated at
60°C using benzene and toluene as aromatic components and
cyclohexane, hexane and heptane as aliphatic components. It was found
that all the crosslinked polymers had excellent chemical and thermal
stability in pervaporation experiments. In all the cases, conditioning of the
16 Chapter 1
membrane samples with pure benzene was a suitable pretreatment to
enhance flux without decreasing selectivity significantly. Staudt-Bickel and
Koros [42] improved the selectivity of polyimide membranes for CO2/CH4
mixture by chemical crosslinking with ethylene glycol. CO2/CH4 selectivity
increased with an increase in the degree of crosslinking because of
reduced swelling and polymer chain mobility. The CO2 permeability was
not significantly lowered by using ethylene glycol as a crosslinking agent
since the additional free volume caused by the crosslinks compensated
the reduced chain mobility.
1.3.4. Effect of plasticizers
The plasticizers act as spacers at the molecular level. Hence less energy
is required to make free the molecules for the substantial rotation about
the C-C bonds; thus the Tg is lowered [29]. This results in increased
segmental mobility and enhanced penetrant transport. Kazarian et al. [43]
reported the diffusion of azo dyes in glassy polymers plasticized by
supercritical carbon dioxide. The plasticization increased the polymer free-
volume and caused the swelling of the polymer matrix. Arvanitoyannis et
al. [44] investigated the carbon dioxide and water permeabilities of
chitosan/PVA blends plasticized with sorbitol and sucrose. They found that
the permeability values increased with increase in plasticizer content.
Sorption and diffusion of oxygen in plasticized ethyl cellulose films of
varying degrees of substitution were studied by Beck and Tomka [45]. It
Introduction 17
was found that as the plasticizer concentration and temperature increased,
the permeability coefficient also increased.
1.3.5. Nature of penetrants
The transport process through polymers depends very strongly on the
size, shape, phase and molecular weight of the penetrants. An increase in
size in a series of chemically similar penetrants generally leads to a
decrease in their diffusion coefficients due to the increased activation
energy needed for diffusion. The decrease in diffusivity with increase in
size of the penetrant has been reported by many investigators [35, 46].
Harogoppad and Aminabhavi [47] studied the effect of long chain aromatic
hydrocarbon diffusion through different rubbery polymers. The
dependence of diffusion coefficient and other related parameters on the
size and shape of the penetrant molecule has been discussed. It was
found that the transport parameters decreased with an increase in
penetrant molecular size. S.U. Hong [48] explained the effect of solvent
size on the diffusion process for various solvents through natural rubber
and polybutadine in terms of free volume theory.
Generally, permeant size and shape effects are much more marked in
glassy than in rubbery polymers. This arises from the differences in the
penetrant-polymer mixing process. In rubbery polymers, energy is required
to generate sites for the penetrant molecules to occupy but since
increasing permeant size tends to increase the heat of sorption, it follows
18 Chapter 1
that larger penetrant molecules will be readily sorbed leading to enhanced
plasticization of the polymer chains. In general, flattened or elongated
molecules have higher diffusion coefficients than spherical molecules of
equal molecular volume [35, 49].
The permeant phase also influences the transport process. The
permeation of condensable vapours or liquids is generally much faster
than that of permeant gases. Permeants, which are good solvents for the
polymer, swell and plasticize the polymer structure, which gives rise to an
increased mobility of the polymer chain segments leading to enhanced
permeation rates. For example, the permeability in a poly (butadiene-co-
acrylonitrile) rubber increases seven times on varying the permeant from
gaseous nitrogen to liquid methyl ethyl ketone [50].
1.3.6. Effect of fillers
Depending on the degree of adhesion and compatibility of the fillers to
the polymer matrix, permeability of a particular penetrant may either
increase or decrease. If inert filler is used, which is compatible with the
polymer matrix, it will take up the free volume within the polymer matrix
and create a tortuous path for the permeating molecules [51]. F.L.
Tantawy [52] reported the influence of kerosene transport on the physico-
chemical properties of crosslinked butyl rubber filled with TiC ceramic.
Figure 1.3 shows the effect of filler loading on the butyl rubber. The filler
loading effectively restricts the mobility of individual polymer chains, and
thus causes the rubber to exhibit low sorption behaviour. Ismail and
Introduction 19
Suryadiansyah [53] investigated the effects of filler loading on the
properties of polypropylene-natural rubber-recycle rubber powder (PP-
NR-RRP) composites. Swelling behaviour and morphology of PP-NR-
RRP composites at a fixed weight ratio of 70/15/15 were studied. Carbon
black filled PP-NR-RRP composites exhibit lowest resistance towards
swelling in ASTM Oil No.3 than calcium carbonate and silica filled PP-
NR-RRP composites. When fillers are incompatible with the polymer,
voids tend to occur at the interface, which leads to an increase in free
volume of the system and consequently, to an increase in permeability.
Equilibrium swelling in solvents was found to be a very good technique
for assessing rubber-fibre adhesion in fibre filled composites. Lowering of
equilibrium swelling in fibre filled samples indicates excellent fibre-rubber
adhesion. Haseena et al. [54] examined the interfacial adhesion of short
sisal/coir hybrid fibre reinforced NR composites by restricted equilibrium
swelling technique. As fibre content increases, the solvent uptake has
been found to decrease due to the increased hindrance and good fibre-
rubber interaction. Due to the improved adhesion between the fibres and
NR, the ratio of the volume fraction of rubber in dry composite samples to
that in swollen samples has also been found to decrease.
20 Chapter 1
Figure 1.3 : Mass swelling vs square root of time with different TiC content in kerosene at 25°C (F: the amount of TiC) [ F.L. Tantawy, Polym. Degrad. Stab., 73, 289 (2001)]
Transport properties of the filled polymer films cannot be treated in the
same fashion as semi-crystalline polymers. Although the size and shape of
the filler particles may be better defined than polymer crystallites, they may
be more or less permeable than the surrounding polymer phase, or highly
adsorbent. Further more, filled polymers may contain an additional
dispersed phase in the form of vacuoles. These are expected in any
system where the polymer does not wet the filler particles, and have been
detected, for example, in zinc oxide-natural rubber by density
measurements [55, 56]. The volume fraction of vacuoles can be calculated
if the densities of the filler, polymer and filled film are known.
Introduction 21
1.3.7. Temperature
It is well known that the permeability P of permeants through an
amorphous, homogeneous, dense film increases with temperature,
according to Arrhenius relationship:
(1.9)
where Ep denotes the activation energy of permeation. Ep is the sum of the
activation energy of diffusion, Ed, and heat of sorption Hs.
p d sE E H= − (1.10)
Diffusion through polymers is an activated process; therefore, diffusion
coefficients always increase with temperature, that is Ed is always positive.
Increase in temperature increases polymer segmental mobility and also
kinetic energy of the penetrant thereby generally increasing penetration
rate. Figure 1.4 shows the effect of temperature on the sorption of aliphatic
esters through tetrafluoroethylene/ propylene copolymeric membranes
studied by Aminabhavi et al. [57]. In all the solvents, the sorption values
increase with increase in temperature. Effect of temperature on the liquid
diffusion and permeation characteristics of natural rubber, nitrile rubber,
and bromobutyl rubber was investigated at temperatures between 273 and
313 K by De Kee et al. [58]. As the temperature and elongation increase,
pEP =P* exp -
RT
⎡ ⎤
⎢ ⎥
⎣ ⎦
22 Chapter 1
the steady state flux increases, and the breakthrough time decreases. An
increase in temperature leads to an increase in permeability.
Figure 1.4 : Wt % sorption vs square root of time for polymer with (A) ethyl acetate (B) n-propyl acetate (C) n-butyl acetate (D) n-amyl acetate at 250C(O), 400C (Δ), 550C (•) and 700C (€)
[T.M. Aminabhavi, H.T.S. Phayde, J.D. Ortego and J.M.
Vergnaud, Eur. Polym. J., 32, 1117 (1996)]
Introduction 23
1.4. Transport phenomena in different polymeric systems
1.4.1. Rubbery polymers
The characteristic property of rubbers or elastic materials is that they are
amorphous in nature. The important feature of rubbery polymers is
unsaturation due to which the segments of polymer chains are able to move
so that more free volumes are created. The presence of free volumes and
moving chains are very much favourable for diffusion through them.
The swelling characteristics of sodium chloride filled polybutadiene (PB)
rubber networks in water, water/acetone and water/tetrahydrofuran (THF)
mixtures were investigated by Erdal et al. [59]. The degree of swelling was
observed to increase continuously in water during a two-months period.
The highest rate of diffusion of penetrant was observed for PB/water/THF
ternary system. Diffusion rates of pure water and water/acetone mixture
were much smaller and of comparable magnitude. The mass transfer of
hydrophobic solutes in solvent swollen silicone rubber membranes was
studied by Doig et al. [60]. The degree of swelling was found to be
dependent on the solvents used. Li et al. [61] studied the effect of an
external stress on the barrier properties of natural, bromobutyl and nitrile
rubber (NR, BIIR and NBR) using a modified ASTM permeation method.
They observed a stress enhanced diffusion for most of the rubber/solvent
pairs. Aminabhavi et al. [62] investigated the diffusion and sorption of a
variety of organic liquids through rubbery polymers such as polyurethane
(PU), neoprene (CR), styrene butadiene rubber (SBR), ethylene propylene
24 Chapter 1
diene monomer (EPDM), NBR and NR. The penetrants used were cyclic
compounds, esters and hydrocarbons. The transport mechanism found to
depend, to a great extent, on the type of the solvent molecule and the
barrier material. In all the polymer-solvent systems, the activated diffusion
mechanism was found to be operative and the computed transport
parameters indicated smaller diffusion coefficients and higher activation
energies for bigger solvent molecules. The diffusion mechanism has been
classified to be anomalous.
A novel class of silicon-containing rubbers was proposed by Alentiev et al.
[63] as membrane materials for the separation of hydrocarbons of natural and
associated petroleum gas. Homo-polymers of general formulae –Si-(CH3)2-
CH2– and -Si-(CH3)2-CH2-CH2-CH2– and copolymers with the same randomly
alternating repeat units were prepared by thermal initiated polymerization of
strained silicon-containing four-member cyclic compounds. The amorphous
copolymers exhibited a combination of good film forming properties and
relatively high permeability. Polyakov et al. [64] investigated the transport of
organic liquids in amorphous copolymers of 2, 2-bis-trifluoromethyl-4, 5-
difluoro-1, 3-dioxole and tetrafluoro-ethylene (amorphous Teflons AF). The
co-polymer with a higher content (87 %) of dioxol component, as
distinguished by a larger free volume, was found to be much more permeable
to liquids than the co-polymer containing 65 % of the dioxole co-monomer.
The variation in permeation rates of liquids and gases through these
materials were consistent with a mobility (diffusion) controlled mechanism
Introduction 25
for mass transfer. A detailed study of the transport behaviour of penetrants
through different rubbery polymers has been reported by several researches
[65-68]. Rubbers generally follow Fickian mode of transport. However, there
are cases where the sorption process shows deviation from the typical
Fickian trend [69].
1.4.2. Glassy and crystalline polymers
Glassy polymers are characterized by hard and brittle moiety with
restricted chain mobility and have very little void space (0.2-10 %).
Therefore diffusion in glassy polymers is more complex compared to that
in rubbery polymers. Stannett and Hopfenberg [70] described the various
glassy state transport anomalies, which include (a) time dependent
boundary conditions for vapour transport (b) dual sorption modes even for
inert gases (c) diffusion coefficients characterized by an apparent time
dependence (d) polymer relaxations which provide the rate determining
transport step (e) polymer fracture or micro-fracture (crazing) accompanying
polymer relaxation and (f) a significant change in the transport mechanism as
the Tg is traversed. A significant deviation from the normal or Fickian trend of
transport is observed with glassy polymers [71, 72]. The term Case II
diffusion was first applied by Alfrey et al. [73] to micro-molecular
penetration, into a glassy polymer substrate, characterized by zero order
kinetics and a sharp penetrant advancing front. This phenomenon
appeared to constitute a limiting form of the deviation from the normal
Fickian penetration kinetics commonly exhibited by glassy polymer micro-
26 Chapter 1
molecular penetrant systems. The theoretical studies of Sonopoulou et al. [74]
about the Case II diffusion behaviour are very interesting. Franson and
Peppas [75] examined the influence of copolymer composition on non-
Fickian water transport through glassy copolymers. They have investigated
the swelling behaviour of poly (2-hydroxy ethyl methacrylate-co-methyl
methacrylate) and poly (2-hydroxyethyl methacrylate-co-N-vinyl-2-
pyrolidone) in water. The penetrant front movement indicated that non-
Fickian transport predominated. The mode of transport was found to
change with the copolymer composition. Semenova et al. [76] conducted
interaction experiments of polar gases and vapours such as ammonia,
sulphur dioxide, freons and water with polyamides, derivatives of cellulose
and polyurethanes. A plasticizer effect in the polymer was observed by the
interacting penetrants.
The essential requirement for crystallinity in polymers is some sort of
stereo regularity. The co-existence of crystalline and amorphous regions is
the typical behaviour of many polymers [29]. The crystalline areas act as
impermeable barriers to permeants and they force the permeant molecules
to diffuse along longer path lengths. Excellent reports regarding the effect of
crystallinity on the transport process exist in literature. Van Amerongen [77]
using gutta percha as diffusion medium, was among the first to
demonstrate how the crystallization reduced the diffusion and permeation
coefficients. Lutzow et al. [78] investigated the diffusion of toluene and n-
heptane in polyethylenes of different crystallinity. The results indicated that
Introduction 27
both diffusivity and penetrant solubility decreased with increasing
crystallinity. Further, the tortuosity or the length of the diffusion path
around the crystals was found to increase with the degree of crystallinity.
Prodpran et al. [79] studied the gas transport behaviour of semi-crystalline
syndiotactic polystyrene samples containing amorphous and crystalline
parts. The oxygen and carbon dioxide gas permeabilities, were examined
as a function of crystallinity. These measurements confirmed that more
dense crystalline form was impermeable for the transport of small gas
molecules while less dense crystalline form was highly gas permeable. An
unusual gas transport behaviour of the crystalline form observed was
attributed to the porous crystalline structure containing nano-channels.
Miguel et al. [80] examined the transport of carbon dioxide and water
vapour through bacterial poly (3-hydroxybutyrate) (aPHB)/synthetic poly
(3-hydroxybutyrate) (iPHB) blends. The addition of aPHB to iPHB lead to a
general increment in the sorption levels that was attributed to the decrease
in the overall crystallinity.
1.5. Membrane based transport process
The development of membranes and its utilization for the separation of
liquid and gas mixtures is an important emerging technique in membrane
science and technology. Membrane technologies can be utilized to
separate, fractionate and concentrate contaminants or process
components. In general, they require minimal temperature changes and
chemical addition, operate in either continuous or batch modes, consume
28 Chapter 1
significantly less energy than traditional separation processes and are
easy to integrate into existing process due to their modular nature and
compact size without altering the chemical structure of the processed
materials [81]. During the past two decades, industrial membranes are
being developed for a wide variety of chemical separation application
involving the treatment of industrial liquids, gases and vapours, such as
those involved in waste water treatment [82], pollution control, water
resume and waste recovery, food processing, gas separations [83-85]
petroleum engineering [86], biotechnology and biomedical devices [87-89].
Generally, membrane processes are classified based on various driving
forces. Some use pressure difference (micro filtration, reverse osmosis
and piezodialysis), while others use other driving forces such as
concentration difference (gas separation, pervaporation and dialysis) [90].
1.5.1. Pervaporation (PV)
Pervaporation is the most developed membrane separation method which is
widely in use today. This method could attract the attention of specialists in
chemical and related fields like biochemical and petrochemical industries as
an energy saving and environmentally friendly technology. The main
advantage that makes this method an outstanding one is that it can be used
to separate any liquid mixture in all concentration ranges [91-95].
The term “Pervaporation” indicates “Permeation” and “Evaporation”
(Coined by P.A. Kober in 1917). Pervaporation is the selective evaporation
Introduction 29
of a component from a liquid mixture through a membrane. Pervaporation
differs from other membrane separation methods in the fact that the
material transported through the membrane undergoes a phase change.
Pervaporation involves the separation of two or more components across a
membrane by differing rates of diffusion through a thin polymer and an
evaporative phase change comparable to a simple flash step. A concentrate
and vapour pressure gradient is used to allow one component to
preferentially permeate across the membrane. A vacuum applied to the
permeate side is coupled with the immediate condensation of the
permeated vapours. Figure 1.5 shows an overview of the pervaporation
process [96].
Figure 1.5 : Overview of the pervaporation process
[M.O. David, R. Gref, T.Q. Nagugen and J. Neel, Trans.1 Chem E. 69, 335 (1991)]
Characteristics of the Pervaporation process include;
• Low energy consumption, minimum energy utilised is the enthalpy
of vaporisation
30 Chapter 1
• No entrainer required, no contamination
• Permeate must be volatile at operating conditions
• Functions independent of vapour/liquid equilibrium
A schematic representation of liquid permeation is given in Figure 1.6 [97]
Figure 1.6 : Liquid Permeation
[V.V. Volkov, Russian Chemical Bulletin, 43, 187 (1994)]
1.5.1.1. Advantages
Comparing to the competing technologies, pervaporation features
the following benefits.
a. Pervaporation is environmentally clean and energy efficient
technology.
Introduction 31
b. Simple equipment design and hence, involves low capital investment.
c. Process is completely enclosed, thereby minimising direct and
fugitive emissions.
d. System is compact, modular and easily transportable.
e. Low operating cost, and easy to scale up for industrial use.
f. Reduces the energy demand because only the fraction of the liquid
needs to be vaporized.
g. Opportunity for recovering concentrated organics.
1.5.1.2. Disadvantages
Besides the advantages, pervaporation process also possesses
several disadvantages. These include;
a. Inadequate membrane material.
b. Fouling of the membrane.
c. Competition with established processes which are seen as safe
options.
1.5.1.3. Industrial Application
Pervaporation offers significant capital and energy savings in
applications that are difficult to separate by conventional techniques such
as azeotropic mixtures or mixtures of close boiling components. The
established industrial applications of pervaporation include [98-101].
32 Chapter 1
a) The treatment of waste water contaminated with organics.
b) Pollution control application.
c) Recovery of valuable organic compounds from process side streams.
d) Separation of 99.5% pure ethanol-water solutions.
1.5.1.4. Membranes
Membrane plays a decisive role in pervaporation process as it functions
as a selective barrier for the mixture to be separated. The word membrane
comes from the Latin word, membrana that means a skin [102]. Today’s word
membrane has been extended to describe thin flexible sheet or films, acting as
a selective boundary between two phases because of its semi permeable
properties. History of synthetic membranes began in 1748 when French Abble
Nolet demonstrated semi permeability for the first time. After a century, Fick
published his phenomenological law of diffusion, which we still use today as a
first-order description of diffusion through membranes. It is an intervening
phase separating two phases and/or acting as an active or passive barrier to
the transport of matter between phases adjacent to it under a driving force. In
other words, it is a discrete thin interface that controls the permeation of
chemical species while in contact with it. This interface may be molecularly
homogenous, that is completely uniform in composition and structure or it may
be chemically or physically heterogeneous.
Introduction 33
1.5.1.4.1. Types of membranes
As a first classification, membranes can be divided into two groups :
biological and artificial membranes [103]. Artificial term can be applied to
all membranes made by man with natural materials and with synthetic
materials (synthetic membranes). Synthetic membrane can be divided
further into organic (made up of polymers) and inorganic (made with
metals, alumina etc.) membranes. Since the effectiveness of the
membranes in applications depends on the detailed morphology and
microstructures of membrane systems, another possible classification of
synthetic membranes can be made as shown in Figure 1.7 [104]
Figure 1.7 : Schematic representation of different membrane morphologies [A. Tavolaro and E. Drioli, Adv. Mater; 11, 975 (1999)]
Synthetic membranes can be classified as either symmetric or asymmetric.
Symmetric membranes are comprised of one material of a single chemical
composition and structural morphology. They are sometimes called
34 Chapter 1
isotropic. Asymmetric membranes are constituted of two or more structural
planes of different morphologies, and the size of the pores changes from
one surface of the membrane to the other. Asymmetric membranes are
sometimes called anisotropic.
On the other hand, asymmetric membranes are characterised by a thin
skin on the surface of the membrane. The method of membrane
preparation can determine if the skin layer is porous or non porous.
Polymeric skin layers resulting from the phase-inversion process are
generally dense and the obtained membranes are called integrally
skinned; those skin layers can also be deposited from the solution or
plasma onto a porous support, in which case the membranes are called
composite membranes.
The membranes used in pervaporation processes are classified according
to the nature of the separation being performed. Hydrophilic membranes
are used to remove water from organic solutions. These types of
membranes are typically made of polymers with Tg above room
temperatures. Poly(vinyl alcohol) (PVA) is an example of a hydrophilic
membrane material. Organophilic membranes are used to recover
organics from solutions. These membranes are typically made up of
elastomers with Tg below room temperature. The flexible nature of these
polymers makes them ideal for allowing organic liquids to pass through.
Examples include nitrile, butadiene rubber and styrene-butadiene rubber.
Introduction 35
1.5.1.5. Membrane material selection
In separating liquid mixtures by pervaporation, an array of feed streams
must be dealt with. For which, three relevant areas such as material
selection, membrane synthesis and system configuration must be
integrated. Right from the early 1960, when work on membrane
separations began, a wide range of materials including dense metals,
zeolites, polymers, ceramics and biological materials are being used for
the manufacturing of membranes. However, polymers are the most widely
used materials for membrane manufacturing at present.
1.5.1.6. Polymer selection for development of membranes
Selection of the polymer materials for separation is based mainly on three
important features: high chemical resistance, sorption capacity and good
mechanical strength of the polymer film in the solution. It should have good
interaction preferably with one of the components of the mixture for separation.
Hence, the solubility parameter [105] and membrane polarity [106, 107] are
the indices of interest in the development of novel membrane materials.
Figure 1.8 shows an example of solubility parameter as an index for
membrane development [107]. As indicated from the figure, methanol is a
good solvent for poly (propylene oxide) PPO (B) membrane, while hexane
is a poor solvent. In the sorption experiment, more methanol was sorbed
into the membrane than hexane and as expected, exceptionally high
selectivity was obtained with this membrane to separate methanol from its
36 Chapter 1
hexane mixture. This result suggests that the solubility parameter is one of
the plausible yard-stick for membrane material selection. However, it is
necessary to know the precise polymer structure to calculate the solubility
parameter.
Figure 1.8 : Solubility parameter of PPO membrane as plotted on two-dimensional grid [M. Yashikawa, N. Ogata and T. Shimidzu, J. Membr. Sci; 26, 107 (1986)]
The membrane polarity also contributes to its separation performance. In
order to separate a particular component from feed mixture, the polarity of
one of the components must be close to the polarity of the membrane. For
example, Shimidzu and Yoshikawa [108], investigated the separation of
water-ethanol mixture using polystyrene membrane. The membrane
polarity (31.7 kcal/mol) was close to the polarity of ethanol (30 kcal/mol)
and as expected the membrane preferably permeated ethanol compared
to water (63.1 kcal/mol).
Introduction 37
In general, selection of polymers compatible with the mixtures for
separation is based on the Hansen solubility parameter (Δ) and Flory-
Huggins interaction parameter (χ). The compatibility among components
‘1’ and ‘2’, which are the liquids in major and minor quantities, respectively,
and polymer ‘3’ is indicated by the relationship [109].
Δ1,3 = 2 2 2, ,3 , ,3 , ,3[( ) ( ) ( ) ]d i d p i p h i hδ δ δ δ δ δ− + − + − (1.11)
where δd, δp and δh are the dispersive, polar and hydrogen bonding
contributions and Δ is the magnitude of the vectorial distance as shown in
three dimensional diagram of δd, δp and δh on x,y and z axes, respectively
(Fig. 1.9), i represents 1 or 2. The greater the compatibility between any
two components, the smaller will be the magnitude of Δ.
Figure 1.9 : Schematic representation of solubility parameter using vectors. [R. Ravindra, S. Sridhar and A.A. Khan, J. Polym. Sci; 37, 1969 (1999)]
38 Chapter 1
Flory-Huggins interaction parameter (χ) also signifies the compatibility of
components with the polymer. The binary interaction parameters, χ1,3 and
χ2,3 between the components and the polymer can be determined from
χi,3 = 23
33 ])1([ln
φφφ +−−
(1.12)
where φ3 is volume fraction of the polymer 3 and i again represents
component 1 or 2. Again the smaller the value of χ (close to 0.5 but not
below), the greater will be the interaction.
1.5.1.7. Membrane structure
Several membrane structures like porous, dense and asymmetric, exist.
The choice for the selection of a good membrane requires a sound
knowledge of the same. In the development of high performance
membranes, there arises a need to adjust the swelling extent occurring
within the membrane while in contact with the feed solution. To achieve
this, the membrane must be crosslinked and the crosslinking degree be
thoroughly controlled. Especially, in the case of a composite membrane it
is rather difficult to ensure controlled crosslinking as composite
membranes are produced by the usual coating-evaporation technique and
chemical crosslinking is achieved during the evaporation period only.
Hence, definite crosslinked polymer networks of appropriate chemical
nature could be realized by means of physical crosslinking. Promising
results concerning the separation of aromatics and saturates using
Introduction 39
physical crosslinking have been reported by B.A. Koenitzer [110]. The
membranes used were made of multi block copolycondensates comprising
alternate flexible (soft) and rigid (hard) sequences.
Highly crystalline polymers do not dissolve easily in many solvents and
furthermore, crystallites due to the lack of flexible groups act as physical
crosslinks which prevent high degree of swelling and thereby show a lower
permeability compared to the amorphous polymer. In pervaporation,
degree of crystallinity also has great influence on the dissolution of the
feed mixture in the membrane, because dissolution generally occurs in the
amorphous part of the polymer. In polymers of the type (-CH2-CHR-)n the
size of the side group-R plays an important role in knowing the
crystallizability of the polymer. The presence of polar groups enables the
polymer to absorb water preferentially compared to organic liquids.
Membranes of this kind are excellent candidates for the separation of
aqueous organic mixtures. Hydrophilic polymers fall into this category.
1.5.1.8. Modification of membranes
There are several methods, which are being used to modify the
membranes to improve the separation performance. They are;
1.5.1.8.1. Crosslinking
In membrane technology, there are two reasons to crosslink a polymer.
The first reason is to make the polymer insoluble in the feed mixture and
the second reason is to decrease the degree of swelling of a polymer in
40 Chapter 1
order to derive good selectivity. Crosslinking can be executed in three
ways. One is via chemical reaction by using a compound to connect two
polymer chains, the second by irradiation and the third, is a physical
crosslinking [111, 112]. A good example of this is the chemically
crosslinked PVA top layer of the GFT composite membrane which shows
excellent resistance to many solvents. On the other hand, excessive
crosslinking has to be avoided as it renders the polymer membrane brittle
with a loss in the dimensional stability thereby making it unsuitable for
pervaporation applications.
1.5.1.8.2. Grafting
Grafting is a polymer modification technique where oligomeric chains are
attached as side chain branches irregularly to the polymer main chain.
This again can be done by chemical reaction or by irradiation. If the
molecules to be grafted contain a functional group that is able to react with
a functional group of the polymer, grafting by chemical reaction can occur.
Grafting by irradiation is a versatile technique for the modification of
insoluble polymer films. Polymers with good chemical resistance can be
made into films by melt extrusion/calendaring followed by modification
through irradiation-based grafting. Neel and co-workers [113] and
Ellinghorst et al. [114] have performed a lot of research on grafting films by
irradiation. They used poly (vinylidene fluoride) (PVDF), poly (vinyl
fluoride) (PVF) and poly (tetrafluoroethylene (PTFE) as basic polymers
Introduction 41
and N-vinylpyrrolidone, 4-vinylpyridine, vinyl acetate, acrylic acid,
N-vinylimidazole as grafting monomers.
1.5.1.8.3. Blending
A mixture of two polymers, which are not covalently bonded, is called a
polymer blend. In principle, blending is an ideal technique for creating
optimum hydrophilicity in the hydrophobic membrane. The optimum-
blending ratio can be determined by mixing the hydrophilic polymer with a
hydrophobic one at various compositions and measuring the permeability
and selectivity. Two types of blends can be distinguished: homogeneous
blends, in which the two polymers are miscible on a molecular scale for all
compositions and heterogeneous blends, in which the two polymers are
not totally miscible. In the latter case, domains of one polymer distributed
within the matrix of the other polymer can be observed. However,
homogeneous blends are considered as potential membrane materials for
PV as heterogeneous blends will not give enough mechanical strength to
the thin membranes [115-117].
1.5.1.8.4. Copolymerization
Copolymerization can be applied for the same reason as the blends are
used, but unlike in blends the two polymers are covalently bonded which
increase the mechanical stability of the membrane. Besides grafted
copolymers, block and random copolymers can be formed by this
technique. An important aspect in copolymerization is the degree of
42 Chapter 1
crystallinity. Random copolymers might be fully amorphous while graft
copolymers contain a certain degree of crystallinity. Membrane made of
random copolymers cannot be used for PV applications, as a certain
degree of crystallinity is required for the membrane to show preferable
sorption towards one of the components [118].
1.5.1.9. Principles of pervaporation
In pervaporation, the liquid mixture is in direct contact with one side of the
membrane and the permeated product is removed as vapour at the down
stream side by applying a low pressure. The material transported
undergoes a phase change. The minimum energy utilized is the enthalpy
of vaporization of the permeate. The permeate composition is mainly
determined by the relative affinities of the components in the feed to the
membrane and their unequal mobilities through the membrane. Therefore
permeate composition will differ from that of feed composition.
There are two approaches to describe the mass transport in pervaporation
(1) the solution-diffusion model and (2) the pore flow model. The solution
diffusion model is accepted by several researchers [119-121]. According to
this mechanism, pervaporation consists of three consecutive steps (i) sorption
of the permeant from the feed liquid to the membrane (ii) diffusion of the
permeant in the membrane and (iii) desorption of the permeant to the vapour
phase on the downstream side of the membrane (Figure 1.10) [121].
Introduction 43
Figure 1.10 : Solution-diffusion transport model [J.G. Wijmans and R.W. Baker, J. Membr. Sci, 107, 1 (1995)].
In general, the solubility and diffusivity are concentration dependent.
Matsuura and co-workers [122-124] have proposed a transport model
applicable to pervaporation on the basis of the pore flow mechanism. It is
assumed that there are bundles of straight cylindrical pores on the
membrane surface. The mass transport by the pore flow mechanism
consists of three steps.
(a) Liquid transport from the pore inlet to a liquid-vapour phase
boundary.
(b) Evaporation at the phase boundary and
(c) Vapour transport from the boundary to the pore outlet (Figure 1.11)
[124]
SORPTION
44 Chapter 1
Figure 1.11 : Pore-flow model [T. Okada, M. Yoshikawa and T. Matsuura, J. Membr. Sci, 59, 151 (1991)].
The distinguishing feature of the pore flow model is that it assumes a
liquid-vapour phase boundary inside the membrane and pervaporation is
considered to be a combination of liquid transport and vapour transport in
series. At present, it would be recognized that the two models represent
two different approaches to the description of pervaporation transport.
1.5.2. Factors affecting membrane performance
There are several factors affecting the membrane performance, which
have to be kept in mind by any investigator before attempting to work on
pervaporation. They are;
Introduction 45
(i) Feed composition and concentration
A change in the feed composition directly affects the degree of swelling at
the liquid membrane interface, as proved by the solution-diffusion principle
and as the diffusion of the components in the membrane is dependent on
the concentration of the components, the permeation characteristics are
hence dependent on the feed concentration as well. An example [125] to
prove this effect is shown in Figure 1.12.
Figure 1.12: Effect of feed concentration on organic-organic pervaporation separation of benzene-cyclohexane mixture
[J.P.G. Villaluenga and A.T. Mohammadi, J. Membr. Sci, 169, 159 (2000)]
(ii) Feed and permeate pressure
The main driving force in pervaporation is the activity gradient of the
components in the membrane. The permeate pressure is directly related
to the activity of the components at the downstream side of the membrane
and strongly influences the pervaporation characteristics. The maximum
46 Chapter 1
gradient can be obtained for zero permeate pressure and thus for higher
permeate pressures, the feed pressure influences the pervaporation
characteristics [99]. This is illustrated in Figure 1.13.
Figure 1.13 : Effect of pressure on pervaporation (for ethanol/benzene
mixture) [B.K. Dutta and S.T. Sikdar, AlchE, J., 37, 581 (1991)]
(iii) Temperature
As the temperature of the feed increases, the permeation rate generally
follows an Arrhenius-type law [126, 127]. The selectivity is strongly
dependent on temperature; in most cases a small decrease in selectivity is
observed with increasing temperature [125] (Figure 1.14).
Introduction 47
Figure 1.14: General trend of flux and selectivity with varying temperature for benzene/cyclohexane mixtures [J.P.G. Villaluenga and A. T. Mohammadi, J. Membr. Sci, 169, 159 (2000)]
(iv) Concentration polarization (CP)
When a binary liquid mixture is permeating through a semi-permeable
membrane, with different individual component permeation rates, an
increase of the less permeable component in the boundary layer near the
membrane surface occurs. This concentration gradient between the more
concentrated boundary solution and the less concentrated bulk is termed
as concentration polarization. Researchers have dealt with problem
related to concentration polarization in pervaporation of organic-water
mixtures and have generally concluded that CP does not play a very
significant role [128-131].
1.5.3. Review on Pervaporation
Separation of organic-organic mixtures using membrane separation
techniques is being investigated extensively owing to its great importance
48 Chapter 1
in chemical and petrochemical industries. Today pervaporation is
considered as a basic unit operation for separation of organic-organic
liquid mixtures because of its efficiency in separating azeotropic and close
boiling mixtures, isomers and heat sensitive compounds. This review
seeks to define the current scientific, and technological factors that govern
the field of application of membranes for separation of organic mixtures.
This review covers separation of a large number of organic mixtures using
various membranes that are widely used in the chemical and
petrochemical fields.
1.5.3.1. Separation of polar/non-polar solvent mixtures
While early demonstrations of pervaporation using cellulose membranes was
dealt way back in 1956 [132], the first real application for the removal of
organics from diluted liquid organic streams was carried out on lab scale in
1960’s by Binning et al. [133]. Hydrophobic materials such as polyethylene
(PE) and polypropylene (PP) were mainly used as membrane materials.
However, these hydrophobic membranes did not show high selectivities for
polar/non-polar organic mixture, as they do not possess any functional groups
to create a difference in interaction between the two components to be
separated in the mixtures, which is vital in PV. Later, in 1976, Aptel et al. [134]
applied pervaporation to separate organic liquid mixtures using a PTFE film
grafted with N-vinylpyrrolidone for separating polar/non-polar mixtures like
methanol/toluene and obtained good selectivity but poor fluxes. Table 1.5
Introduction 49
deals with the performance of various membranes applied for organic
mixtures belonging to this category [135-139].
Table 1.5 : Separation of polar/non polar solvent mixtures [Ref. 135-139]
System (binary liquid mixtures)
Membrane material
Selectivity (α)
Flux (kg μm/m2h)
Temperature (oC)
31% Methanol / benzene
PFSA composite membrane on Teflon
9.6 100.28 45
Methanol (5-90%)/benzene PVA 5-90 0-17
Methanol (5-90%) /toluene
PVA 100-0 0-13
Methanol (1-95%)/toluene
PPY-PTS 5-60 0.05-10 57.5
Methanol (21%)/MTBE
Modified PPO 5.4-7.8 3-4.8 40
1.5.3.2. Separation of aromatic/alicylic mixtures
In the field of separation of aromatic compounds from alicylic compounds
and other aromatics, research had begun in the early 1960’s.
Benzene/cyclohexane (Bz/Cs) is one of the most commonly encountered
mixture which is also the most difficult to separate. Numerous researchers
have focused on this system to assess the pervaporation properties of
membrane materials and interesting studies have been conducted by
scientists for separating this system. Table 1.6 deals with the membranes
applied for different components in this category in association to their
performance [140-145]. Martin and Kelly [140] and Martin et al. [141] used
50 Chapter 1
modified cellulose ester by blending it with 20 wt% of polyphosphonate
ester. The feed mixture constituted of 50 wt% of benzene while the
concentration in the permeate was 73 wt% which indicated reasonably
good selectivity. A flux of 1 kg/m2h was obtained. Cabasso et al. [142]
increased the blending ratio by using 50 wt% of polyphosphonate ester.
The concentration of benzene in the feed was again 50 wt% but the
permeate concentration was 90 wt%. Very high flux values were also
achieved (1.6-2 kg/m2h). This proved that blending in equal proportions
could cause a considerable increase in selectivity and flux. A shift from
dealing with cellulose ester based compounds was brought forth by Elfret
et al. [146] in 1978. Polyurethane membrane of 10μm thickness, with 20
wt% aromatic concentration in the feed resulted in a flux of 0.3kg/m2h with
58 wt% permeate being benzene. Neel and co-workers [134] grafted
membranes made of PVDF, with 4-vinyl pyridine or acrylic acid by
irradiation. Inui et al. [147] used polymethyl methacrylate-co-methacrylic
acid membranes ionically crosslinked by iron (III) and cobalt (II) cations to
attain high flux and good selectivities. With the basic idea of grafting,
Terada et al. [148] synthesized graft copolymer membranes of
2-hydroxyethylmethylacrylate-methylacrylate of 10μm thickness. These
were used on a feed with 50 wt% of benzene. This membrane yielded a
flux of 0.7 kg./m2h and an excellent selectivity (Benzene concentration in
permeate was 100 wt%). Lee and co-workers [149] in 1994, used a blend
membrane of polyvinyl alcohol and polyallyl amine. For a feed containing
Introduction 51
10 wt% of benzene, the blend membrane yielded a flux of 1.3 kg/m2h and
a separation factor of 62.
Table 1.6 : Separation of aromatic/alicyclic mixtures [Ref. 140 – 145].
System (binary liquid mixtures)
Membrane material
Selectivity (α)
Flux (kg μm/m2h)
Temperature (oC)
Benzene (Bz) (50%)/ Cyclohexane (Cx)
CA modified with PPN
2.7 100 80
Bz (53%)/Cx Modified CE 5.2 50.3 80
Bz (50%)/Cx Blend of CA and PPN
9 1.6 80
Bz (10%)/Cx PE 2.5 30 35
Bz (50%)/Cx CA 19 0.34 77.8
Bz (60%)/Cx BP-PEO 9.1 2.1 25-70
1.5.3.3. Separation of aromatic/aliphatic/aromatic hydrocarbons
Separation of aromatic-aliphatic hydrocarbon mixtures was first
investigated 25 years ago, in the frame of a European project. Table 1.7
deals with these separations and their performance [150-152]. The field,
then lay dormant until J.P. Brun and Larchet [153] in 1983 dealt with the
separation of benzene/n-heptane mixtures using elastomers like
poly(butadiene-acrylonitrile rubber) (NBR) and poly(butadiene-styrene)
rubber (SBR) and noticed that the membrane possessed excellent
selectivity towards the aromatic compound. A flux of 0.5 kg/m2h, with the
aromatic constituting 70 wt% of the permeate was obtained.
52 Chapter 1
Table 1.7 : Separation of aromatic / aliphatic hydrocarbons [Ref. 150-152]
System (binary liquid mixtures)
Membrane material
Selectivity (α)
Flux (kg μm/m2h)
Temperature (oC)
Toluene (50%)/ n-octane
Composite based on polyesterimide
70 10 --
Toluene (50%)/ i-octane
Ionically crosslinked copolymers of methyl, ethyl, n-butyl acrylate with AA
2.5-13 20-1000 40
Benzene (20-100%)/ n-hexane
PVA 2.2-28.35 -- 50
1.5.3.4. Separation of isomers
The separation of aromatic isomers has gained extensive significance
since 1980’s. Table 1.8 deals with the membranes applied for various
components in this category in association to their performance [154-157].
Initial inroads were made by Mulder et al. [158] on separation of isomeric
xylenes in 1982 with thin films of cellulose esters treated with an organic
solvent, which provided considerably good fluxes but a low selectivity. For
higher selectivity membranes, McCandless and Downs [159] carried out
modifications of hydrophilic polymers in 1987. About 12 polymeric
membranes for the separation of the C8-aromatics at different
temperatures were tested and the separation factors for p- to m-xylene
were below 1.69, for all the films. Wytcherley and McCandless [160] in
1992, separated m- and p-xylene mixtures using commercial PVA
membrane in the presence of CBr4 as a selective feed complexing agent.
Introduction 53
The pervaporation separation of aromatic C8-isomers was explored by
Westing et al. [161] in 1991 using dense homogeneous polyethylene
membranes. The rate of mass transport across the membrane increased
for the aromatic C8-isomers in the order o-xylene < ethylbenzene < m-
xylene < p-xylene and the flux of the components depended strongly on
the downstream pressure. The very small separation factors obtained
restricted the use of PVA membranes for purification of mixed xylenes on
an industrial level. The separation of aromatic isomers continues to be an
active research area because the present separation methods in use are
both complex and energy intensive.
Table 1.8 : Separation of isomers [Ref. 154 – 157]
System (binary liquid mixtures)
Membrane material
Selectivity (α)
Flux (kg μm/m2h)
Temperature (oC)
p-Xylene (10%)/ m-xylene
PVA filled with β-cyclodextrin
2.96 0.95 25
p-Xylene (10%)/ o-xylene Silicate, Faujasite 40-150 -- 40-100
n-Hexane (50%)/ 2,2-dimethylbutane
Silicate zeolites membrane 1.1-22.5 -- 90-165
n-Butane (50%)/ i-butane
Zeolites membranes on α-alumina support
11-52 -- 30-200
1.6. Vapour sorption
The transport of a condensable vapour through a dense membrane
consecutive to an activity gradient is referred to as vapour permeation
process. It offers the unique feature from a fundamental point of view, of
54 Chapter 1
studying the transport process of a single permeant through a dense
membrane, under various upstream concentrations [162]. Liquid
permeation cannot be a characteristic means like vapour permeation
where the modification of the upstream activity of a component can be
attained by adding another compound to the mixture. Here the activity of
both components is modified in comparison with the Gibbs-Duhem
relation, which complicates the transport analysis. But, with vapour
permeation process, coupling phenomena are not to be considered. Also,
the calculation of upstream solvent activity needs some complicated
vapour-liquid equilibria methods. For pure solvent vapour permeation
upstream activity can be easily calculated if upstream pressure is
accurately measured.
The sorption of vapours and other small penetrants in polymeric materials
is well documented and the subject of even current developments in the
literature [163]. The solvent vapour permeation study offers direct practical
conclusion for the understanding and rational design of volatile organic
components (VOC) vapour recovery from contaminated air streams [164].
Vapour permeation offers significant opportunities of energy saving and
solvent reuse, compared to classical VOC control processes such as
incineration, oxidation or active carbon adsorption. One disadvantage of
this technology is the lack of in-depth studies of performance capabilities.
A minimum knowledge of the relation between the organic concentration in
the vapour stream and its flux through the membrane is needed for the
Introduction 55
rational design of a vapour permeation unit. A membrane material
showing high organic vapour permeabilities, but low air and water
permeabilities is required for VOC vapour recovery as well as organic
extraction from aqueous streams by pervaporation. Several elastomeric
materials having high degree of permeability and organophilic behaviour
are employed for this purpose. Silicone rubber offers a good compromise
in most cases and is widely used [165].
The permeation of organic vapour through a membrane is considered to
involve three independent physical process; sorption of vapour molecules at
the feed side surface of the membrane, diffusion of the dissolved vapour
through the wall of the membrane and desorption of vapour molecules from
the permeate side surface. A sorption isotherm illustrates the equilibrium
amount of penetrant sorbed by a polymer versus external vapour activity (or
pressure) at a given temperature, and it is governed by the thermodynamics
of the system. The relative strengths of the interactions between the
penetrant molecules and the polymer or between the penetrant molecules
themselves within the polymer determine the isotherm.
Ponangi and Pintauro [166] measured the equilibrium sorption of benzene,
tetrachloroethylene, and hexane vapour in four commercially available
polyurethanes as a function of temperature and organic activity. The four
polyurethanes were composed of the same polyether soft segments, but
contained different amounts of phase-separated, hydrogen-bonded hard
56 Chapter 1
segment domains. For all three organic penetrants, polymer swelling was
inversely proportional to the polyurethane’s hard segment content. Huang
et al. [167] reported a model for non-Fickian gradient diffusion in polymer-
penetrant systems to assess its capability in predicting experimental
results in weakly non-linear differential vapour sorption experiments at
conditions when two-stage sorption kinetics occur. The effects of six
dimensionless parameters in the model on predictions of sorption kinetics
are evaluated systematically by numerical simulations.
D. R. Paul [168] investigated the water sorption and diffusion of structurally
related polymers and miscible blends of hydrophobic/hydrophilic polymer
pairs. Recently, the vapour sorption isotherms of vinyl acetate (VAc) and
methyl methacrylate (MMA) plasma thin polymer films are measured and
compared with those for cast films of the conventionally polymerized
analogs [169].
The permeability of crosslinked blends of a terpolymer of ethylene, propylene
and ethylidene norbornene (EPDM) with two poly (dimethyl-vinylmethyl)
siloxanes to methanol vapour has been studied by Geerts et al. [170].
Hsu et al. [171] used water vapour as a diffusional probe to study the
segmental mobility in hydrogen-bonded polymer blends. A detailed
analysis of the transport of chloroform, isomeric butanols, methanol and
water vapour through dense silicone rubber membranes was done by Neel
and co-workers [172]. Vapour sorption experiments of chlorinated
hydrocarbons in poly (dimethyl siloxane) (PDMS) and zeolites showed that
Introduction 57
the sorption in PDMS is determined by the hydrophobicity of the organic
component, whereas for the sorption in the zeolites the molecular sieving
effect is the dominating factor [173]. The diffusion and sorption of a non-polar
molecule, n-pentane and a more polar molecule, dichloromethane through UV
irradiated low density polyethylene (LDPE) were studied by Naddeo et al. [174].
They found that the formation of polar groups in the irradiated samples
caused a higher sorption of dichloromethane whereas the sorption of n-
pentane depended on the amorphous fraction. This effect demonstrated
the sensitivity of transport properties to the chemical changes of the
polymer structure (Figure 1.15).
Figure 1.15 : Equilibrium concentration of dichloromethane vapour as a function of activities for (•) initial and (Δ) 300h irradiated LDPE samples [C. Naddeo, L. Guadagno, S. De Luca, V. Vittoria and G. Camino, Polym. Degrd. Stab., 72, 239 (2001)].
58 Chapter 1
1.7. Gas Permeation
Transport of gases through polymers is an area of growing interest as
materials with unique transport properties continue to find use in new,
specialized applications ranging from extended life tennis ball to natural
gas separation system [175]. Polymer films are widely used as packaging
materials. For this purpose, the barrier properties are important to protect
the contents from their surroundings. In contrast, large and selective gas
permeabilities are required for separation material. The gas permeation
through a membrane is dependent on the pore structure of the polymer
comprising it. In the case of dense homogenous membranes, which have
no macro pores, permeating molecules migrate through molecular
interstices woven by the polymer chains.
Traditionally the behaviour of gas transport through a gas separation
membrane was presumably dominated by the diffusion of absorbed
molecules. The gas flux was driven by the concentration gradient of
absorbed molecules in polymer matrix. This leads to a simple model
which is known as the solution diffusion model [176-180]. The permeating
species interact with the polymer matrix and selectively dissolves in it,
resulting in diffusive mass transport along a chemical potential gradient.
Besides solution diffusion model, there are viscous flow, Knudson flow and
molecular sieving for explaining gas transport through porous membranes.
As illustrated in Figure 1.16, the mechanism of flow of gas molecules
depends upon the size of the pores in relation to the mean free-path of the
Introduction 59
gas molecules [181]. In viscous flow, flow is inversely proportioned to the
viscosity of the fluid (gas). In contrast, flow is inversely proportional to the
square root of the molecular mass of diffusing species in Knudson flow.
Ultra micro porous molecular sieving membrane has higher productivities
and selectivities than solution diffusion mechanics [182, 183].
Figure 1.16 : Mechanism of flow of gas molecules (a) viscous flow (b) Knudson flow (c) molecular sieving and (d) solution-diffusion [J.R. Fried, Polym. Sci. and Technol., Prentice-Hall, Englewood Cliffs, N.J., USA, 1995]
The gas sorption through a homogenous polymer has been classified into
two categories. For sorption to a rubbery polymer membrane, the sorption
behaviour was described by Henry’s law. For sorption to a glassy polymer
membrane, the sorption isotherm has been characterised by many authors
by a dual sorption model. This combines two isotherms, a Henry-type
isotherm for matrix absorption and a Langmuir-type isotherm for site
60 Chapter 1
sorption [184-188] (Figure 1.17) [189]. The gas transport behaviour for
glassy polymer is then described by a so-called dual mobility model, which
assigns two different diffusivities to the molecules absorbed by different
mechanisms, Langmuir and Henry’s sorption. This model well described
the pressure dependency of gas permeability of a wide range of glassy
polymer membrane [190] and reasonably related the gas solubility to its
permeability.
Figure 1.17 : Typical dual-mode sorption isotherm and its components [S.A. Stern and S. Trohalaki, in Barrier Polymers and Structures (Ed. W.J. Koros), Am. Chem. Soc., Washington, DC, 1990, Ch. 2].
The difference in the transport and solution behaviour of gases in rubbery
and glassy polymers is due to the fact that, the latter is not in a state of
Introduction 61
true thermodynamic equilibrium [71, 191]. Rubbery polymers have very
short relaxation times and respond very rapidly to stress that tend to
change their physical conditions. Thus a change in temperature causes
an immediate adjustment to a new equilibrium state. A similar adjustment
occurs when small penetrant molecules are absorbed by a rubbery
polymer at constant temperature and pressure and adsorption equilibrium
is very rapidly established.
By comparison, glassy polymers have very long relaxation times, hence, in
the presence of a penetrant, the motions of whole polymer chains or of
portions thereof are not sufficiently rapid to completely homogenize the
penetrant’s environment. Penetrant (gas molecules) can thus potentially
sit in holes or irregular cavities with very different intrinsic diffusional
mobilities.
Gas separation by permeation through membranes has been suggested
for many years [192, 193]. For the efficient separation of gas mixtures, the
membranes should have a high selectivity for a particular gas over other
gases. However, with homogenous membranes, high selectivity is usually
associated with low permeability, as in the case of membranes used for
enrichment of air [194]. On the other hand, porous membranes usually
have very high permeabilities but their selectivity is generally low. Kanitz and
Huang [195, 196] investigated the permeation of gases through poly(ethylene
styrene) grafted copolymer membranes prepared by X-ray irradiation. They
62 Chapter 1
also studied separation of nitrogen and methane gases in air using vacuum-
irradiated polyethylene and Teflon FEP films. Rogers [197] studied the
permeabilities and separation of helium and nitrogen gases through a
grafted methyl methacrylate and polyisoprene membrane.
Haraya and Hwang [198] have conducted permeation studies in a series of
19 polymers for selecting appropriate polymer for the separation of O2/Ar
mixtures. The gas permeabilities and diffusivities in all polymer tested are
in the order O2 > Ar > N2. Higher permselectivities are observed in glassy
polymers than in rubbery polymers. Permeability ratios for O2/N2 in glassy
polymers are strongly affected by the contribution of the diffusivity ratios.
Lai and co-workers [199] studied the transport phenomenon of oxygen and
nitrogen across a pure polycarbonate and a cobalt (III) acetylacetonate
[Co(acac)3] containing polycarbonate membrane. Cobalt acetylacetonate
was added into a polycarbonate membrane to enhance its oxygen
solubility. It was found that oxygen permeability increased slightly with
respect to pressure.
A variety of techniques have been used to measure gas permeation
through polymer membranes. In essence, the object of a permeation
experiment is to measure the rate of transport of penetrant through a film
as a function of time. There are three basic methods of measuring
penetrant transmission rates; pressure, volume and concentration change
methods. These are depicted schematically in Figure 1.18 [200]. A
comparison of the three methods in determining the oxygen permeability of
Introduction 63
commercial polymer films has been made by Taylor et al. [201]. The high
vacuum pressure change method was first developed by R.M. Barrer [202]
and later refined by Stannet and co-workers [203] to determine the
diffusion and permeability coefficients of gases and vapours. The film
under test is placed in a stainless steel cell, which is divided into two
chambers. Both sides are evacuated, and the residual vapours frozen out
with liquid nitrogen. The penetrant, which can be a gas, vapour or liquid, is
admitted to one chamber, and the increasing pressure in the other is
measured with a suitable gauge.
Figure 1.18 : Schematic diagram of permeation methods [M. Chainey, Hand Book of Polymer Science and Technology, Vol. 4, Composites and speciality applications (Ed. BN.P. Cheremisionoff) Marcel Dekker, INC, New York, 1989, p. 503].
64 Chapter 1
H. R. Todd [204] developed the volume change method, who employed a
cell having a large volume on one side and small volume on the other.
The large chamber was evacuated and the test gas introduced into the
small chamber at atmospheric pressure. A horizontal capillary connected
to the small chamber contained a mercury plug. As gas permeated into
the large chamber, the mercury plug moved in the capillary so as to reduce
the volume of the small chamber and maintain the pressure. By observing
the speed with which the plug moved, the rate of permeation could be
determined.
The concentration change method avoids the necessity for a total pressure
difference across the film, and relies instead on a difference in
concentration or partial pressure of penetrant. Here again, the permeation
cell is divided into two chambers by the film under test. The penetrant
flows over one side of the film under test, either as a pure gas or as a vapour in
a carrier gas stream, and is termed the ‘span’ gas stream. The permeating gas
is swept away by a second carrier gas stream (the sweep gas stream), which is
then analysed. Chemical methods or modern instrumental methods, capable of
much greater sensitivity, have been used to determine the amount of
permeating gas [205]. A novel permeation apparatus is developed which can
make the on-line measurements of both flux transient and permeate
composition in gas permeation. It is based on the continuous-flow technique
employed previously for liquid permeation. The transient measurement allows
the simultaneous determination of permeation characteristics, such as,
Introduction 65
permeability, diffusion and solubility coefficient, and activation energies with one
experiment.
A serious drawback associated with permeation methods is the problem of
leaks. These can occur at the edges of the film or through pinholes in the
exposed area. The former can be avoided to some extent by sophisticated
design (eg., mercury seals), but leaks remain a frequently undetected
source of error and can only be assumed absent when the two lowest
results are in good agreement.
Membrane gas separation has become quite attractive for the purification
of gases based on selective permeation through asymmetric, composite or
mixed matrix membranes [206-208]. Although the concept of separating
gases with polymeric membranes was developed a long time ago, the
widespread use of gas separation membranes has occurred only within
the past 10 to 15 years. The primary reason for this was that the early
membranes were insufficiently permselective and exhibited insufficient
fluxes so that the cost of membrane processes never successfully
competed with the cost of conventional processes. However, modifications
in the physical and chemical structures of membranes have enhanced
separation characteristics. Now there are several important industrial
applications (as indicated in Table 1.9) where membrane operations can
be competitive with more traditional methods of gas separations, such as
cryogenic separation and pressure-swing adsorption (PSA). These include
66 Chapter 1
oxygen enrichment of air, hydrogen separation from carbon monoxide and
other gases, removal of carbon dioxide from natural gas, and the reduction of
organic vapour concentration in air. Other smaller-scale applications include
the preservation of food such as apples and bananas during transport by
blanketing with low-oxygen-content air, the generation of inert gases for safety
purposes, and the dehydration of gases.
Table 1.9 : Applications of polymeric membranes in gas separations [Y. Tsujitha, in Membrane Science and Technology (Y. Osada, T. Nakagawo, Eds;), Dekker, New York, P. 3 (1992)].
Separation Suitable Polymers
Oxygen / Nitrogen
Silicone rubber
Polysiloxane-block-polycarbonate
Polysulfone
Ethyl cellulose
Poly [1-trimethylsilyl)-1-propyne]
Polypyrrolone
Polytriazole
Polyaniline
Hydrogen from carbon monoxide, methane, nitrogen
Polysulfone
Acid gases (CO2 and H2S) from hydrocarbons (eg., natural gas and enhanced oil recovery)
Cellulose acetate
Poly (vinyl chloride)
Polysulfone
Polyetherimide
Hydrocarbon vapours from air Silicone rubber
Recently, gas separation with polymer membranes has been reviewed by
Maier [209]. The effect of incorporating strong surfactants into hollow fibre
membranes and solution cast films made from polysulfone (PSF) and
asymmetric gas separation was investigated by LeRoux and
Introduction 67
VanSchalkwyk [210]. The influence of the ionic nature of the polymer
surface layer on the gas transport across polyphenylene oxide sulfonate
based composite membrane was described by Polotskaya et al. [211].
The permeation behaviour of aromatic homo and copolyetherimides
prepared from 1,4-bis (3,4-dicarboxyphenoxy) benzene dianhydride with
2,4,6-trimethylphenylenediamine, 3,3’-dimethyl -4,4’ methylene dianiline
and 3,5-diaminobenzoic acid was studied by Feng et al. [212]. They found
a significant change in the permeability and perm selectivity resulting from
the systematic variation in chemical composition (Figure 1.19).
Figure 1.19 : Plots of Permselectivity coefficients versus composition
[P.J. Feng, W.Z. Shi and X.O Li, Eur. Polym. J., 38, 339, (2002)]
A new method for gas separation is reported by Kawakami et al. [213] using
an asymmetric membrane, with an ultrathin and defect-free skin layer. The
structure of the asymmetric membrane, prepared by a dry-wet-phase
68 Chapter 1
inversion process, showed an ultra thin skin layer and sponge like structure
characterised by the presence of macro voids. The gas selectivity of the
membrane increases with a decrease in the thickness of the surface skin
layer. Recent evidences suggest that advantages of a purely polymer-based
approach are reaching diminishing returns for important separations such as
O2 and N2. Zeolites, carbon molecular sieves (CMS), and rigid rod polymers
offer attractive transport properties but are difficult and expensive to process.
Mixed matrix composite (MMC) membranes, incorporating molecular sieving
materials within polymeric substances, may provide economical, high
performance gas separation membranes if defects at the molecular
sieve/polymer interface can be eliminated [214]. In addition, careful matching
of the intrinsic permeability and selectivity of the support matrix and the
molecular sieve domains is necessary.
While membrane technology is still young and has great potential for
further improvements, it has been proven economical and technically
efficient in many applications such as treatment of natural gas. Even
though several polymer membranes are available for separation, only a
few exhibit high selectivity and permeability. Hence the pursuit is still
going on in both laboratory and industry for the development of
membranes with high permeability and high selectivity.
1.8. Nanocomposites
Polymer/clay nanocomposites have attracted much attention as researchers
attempt to further enhance the properties of polymers beyond what is
Introduction 69
achievable from more conventional particulate-filled or microcomposites.
Nanocomposites refer to the composites in which one of the components has
at least one dimension of about a few nanometers [215-218]. Polymer-clay
nanocomposites are a new class of composites which are made by simply
dispersing nano fillers such as clay or nano silica into polymer matrix. Clay
materials consist of about 1nm thick layers held together in stacks by non
covalent interaction [219]. The fundamental principle of polymer clay
nanocomposite formation is that the polymer should penetrate into the
interlayer galleries of the clay. However, due to strong electrostatic interaction
between negatively charged clay layers and the counter ions (Na+, k+ or Ca2+)
located in inter layer galleries, polymer precursor cannot penetrate into the
clay gallery. So the polymerization of polymer with clay leads to a
conventional micro composite with particle size of about 5 to 15 μm.
Polymer-clay nanocomposite was first discovered by a researcher at Toyota
in 1993 [220], while working for light weight materials for automobile
application. By replacing the hydrophilic Na+ and Ca+2 cation of native
montmorillonite with more hydrophobic onium ions, they were able to initiate
polymerization of caprolactum in the interlayer galleries of montmorillonite
clay to form a nylon clay nanocomposite.
Polymer-clay nanocomposite can exhibit increased modulus, [221, 222]
decreased thermal expansion coefficients [223], reduced gas permeability,
[224,225] increased solvent resistance when compared to the matrix
70 Chapter 1
polymer alone. In general, two idealized polymer-layered silicates structures
are possible, intercalated and exfoliated [226]. The greatest property
enhancements are observed for exfoliated nanocomposites. These consist
of individual nanometer thick silicate layers suspended in matrix resulting
from extensive polymer penetration and delamination of the silicate
crystallites. In contrast, polymer penetration resulting in finite expansion of
the silicate crystallites produces intercalated nanocomposite consisting of
well-ordered multilayers with alternating polymer/silicate layers and a repeat
distance of few nanometers. Schematic representation of conventional
composite, intercalated and exfoliated or delaminated nanocomposites are
shown in Figure 1.20. [219]
Figure 1.20 : Structural differences in conventional, intercalated and exfoliated nanocomposites [D. Ratna and G.P. Simon, J. Polym, Mater. 19, 143 (2002)].
Polymer-clay nanocomposites based on organic polymers and inorganic
clay minerals consisting of silicate layers is the most promising
Introduction 71
nanocomposite system [227]. Nowadays, there are lots of studies on the
properties of polymer nanocomposites [228,229]. The platelet structure
of layered silicates has the ability to improve the barrier properties of
polymer materials according to a tortuous path model in which a small
amount of platelet particles significantly reduce the permeability through
the nanocomposite. In the exfoliated system the individual clay platelets
will have the highest aspect ratio and as a result the barrier properties
get improved.
Chuang and co-workers [230] considered alkyl ammonium for modifying
clay and maleic anhydride for grafting onto polyethylene backbone to
decrease the polarity between the clay surface and polymer. The larger
the size of the clay minerals, the more effectively the polyimide hybrid
properties and gas permeability path way are improved [225]. The
inorganic materials improve the physical and mechanical properties of the
polymer. The enhanced properties are presumably due to the nano scale
structure effects and the interaction between the organic and inorganic
materials.
X-ray diffraction technique is most widely used for the characterisation of
structure of layered silicate and polymer nanocomposites [231]. The
change in interlayer spacing, i.e. the d-spacing of the polymer
nanocomposites is observed from the peak position in the XRD graphs.
The interlayer spacing of polymer nanocomposites is found to increase
72 Chapter 1
with the incorporation of clay. In the case of exfoliated structure, layer
separation associated with the delamination of the silicate structure in the
polymer matrix leading to the disappearance of X-ray scattering. It may be
due to either the presence of an extremely large regular ordered spacing
between the layers, or the nanocomposites no longer have an ordered
layer structure. TEM technique is the most important method for the
analysis of dispersion of layered silicates in polymer nanocomposites. The
dark lines in the transmission electron micrographs show dispersion of
silicates in the matrix [232].
Kim et al. [233] investigated the moisture diffusion and barrier
characteristics of epoxy-based nanocomposites. The moisture absorption
and diffusion behaviours were different depending on the type of organo
clay. The moisture diffusivity of nanocomposites decreased with increasing
clay content for all organo clays. The substantial decrease in moisture
permeability was first reported for nano clay polyamide composites [234].
The gas permeability of rubber-clay hybrids was also reduced by 30% with
4 vol% of exfoliated clay [222]. An 80% decrease in water absorption was
also reported for poly(ε-caprolactone) nano clay composites [225]. The
moisture permeability reduction was attributed to the extremely high
aspect ratio of clay platelets. Wang et al. [235] studied the effect of nano
clay content on pervaporation behaviour of polyamide nanocomposites.
Polyamide/organoclay nanocomposites were used for the pervaporation of
water-ethanol mixture. Compared with neat polyamide membranes, the
Introduction 73
separation factor for the polyamide/SDS-clay membranes exhibited higher
values in the 10-90 wt% ethanol feed concentration range.
1.9. Gap in the field
Among different polymer systems, semi crystalline systems gained
significant attention over the years. This is because of the many peculiar
properties shown by them. In the relam of molecular transport through
polymeric systems, they can show very attractive features. The
development of new membranes and its utilization for the separation of
liquids and gas mixtures is an important technique in membrane science
and technology. High permeability, good selectivity and stability are the
important factors in choosing suitable membranes for pervaporation. It is
noted that, the Science and Technology of Membrane separation is still
in its early stages of development. Tremendous opportunities exist and
will continue to exist, for the penetration of membrane processes in every
facet of chemical engineering and environmental protection through
intensive research and development. The level of research and
developmental activity in the field of Membrane Science and Technology
has been rising dramatically all over the world during the past two
decades. This is happening not only because of the growing recognition of
the commercial potentialities of membrane separation, but also because of
the growing realization of the scientific and engineering community that
synthetic membranes can play a significant role for society and industry.
74 Chapter 1
Poly(ethylene-co-vinyl acetate) (EVA), is a semi crystalline polymer, offers
weather resistance, toughness, chemical resistance and processability.
EVA has very much attracted several researchers due to its interesting
transport characteristics. Its transport characteristics and the effect of
different additives in the matrix are also found in literature. Gozzeliuo and
Malucelli [236] reported the permeation of methanol/methyl-t-butyl ether
mixtures through EVA membranes. The membrane showed a higher
permeability towards methanol for all the composition of the copolymer
and the liquid. Dinh et al. [237] investigated the sorption and transport of
ethanol and water through EVA membranes. They found that for a 37%
vinyl acetate EVA membrane, ethanol permeability was found to increase
exponentially with ethanol activity in the membrane, whereas water
permeability deceased with water activity in the membrane.
Flux and selectivity of a membrane are prime deciding factors in transport
so as to make the process more economical. Therefore, development of
membranes with improved selectivity and flux is a key research area in
membrane technology. The present study aims to increase the flux and
selectivity or both of the polymer. A deep knowledge of the behaviour of
EVA system in aggressive liquid environments is highly essential for
almost all of its practical applications. It is noteworthy that no systematic
study has yet been conducted on the transport features of DCP/BP
crosslinked EVA systems with special reference to the effects of
crystallinity and degree of crosslinking. The influence of different factors
Introduction 75
on transport process through these polymer still needs additional
information.
While polymer/nanoclay composites have been developed for several
decades, their applications in real engineering products are still very
limited due to several reasons. Apart from the mechanical properties, the
excellent barrier capability with significantly reduced permeability of
vapours and gases is one of the most attractive and useful properties that
have not been fully explored yet. The enhanced gas barrier properties of
polymer nanocomposites are now finding some limited applications in
packaging material and containers for a wide variety of food and beverage
products. Meanwhile, the vapour barrier properties of nanocomposites are
yet to be exploited. The clay content effect on pervaporation performance
has not been made clear. Also no systematic studies have been
conducted on the gas barrier properties of polymer membranes containing
layered silicates. These aspects have been covered in the present thesis.
1.10. Scope of the work and major objectives
Poly (ethylene-co-vinyl acetate) is a general purpose polymer which finds
many applications. The polymer is extensively used in many engineering
and industrial areas because of its toughness, chemical resistance and
excellent processability. EVA membranes were prepared. Two vulcanizing
systems viz. dicumyl peroxide (DCP) and benzoyl peroxide (BP) were
used to crosslink the matrix. A detailed investigation on the solvent
76 Chapter 1
sorption behaviour of these membranes is necessary for the possible use
of these membranes as barrier materials. The main objectives of the
present study are the following.
1. The influence of nature of crosslinks, distribution of crosslinks, crosslink density and crystallisation on transport.
EVA was vulcanized by peroxide technique using DCP and BP.
Uncrosslinked membranes were also prepared. The nature as well as the
distribution of crosslinks controls the overall properties of these
membranes. Therefore, the influence of crystallinity, nature of crosslinks,
degree of crosslinking, penetrant size and temperature on the sorption
characteristics of EVA were examined in detail.
2. Influence of penetrant size, shape, molecular weight, solubility parameter and polarity on transport
The penetrant characteristics such as size, shape, molecular weight, solubility
parameter and polarity decide the transport characteristics and the related
coefficients. Three sets of organic liquids viz. aromatic, aliphatic and chlorinated
hydrocarbons which have closer solubility parameter values to that the polymer
have been selected for the investigation. The aromatic hydrocarbons used were
benzene, toluene and p-xylene. The aliphatic hydrocarbons selected were
normal alkanes viz. n-pentane, h-hexane and n-heptane. The chlorinated
hydrocarbons selected were dichloromethane, chloroform and carbon
tetrachloride with different degrees of polarity.
Introduction 77
3. The influence of temperature on transport
The influence of temperature on the transport of liquids through EVA
has also been studied. The temperature susceptibility of the matrix
depends on the nature and distribution of crosslinks. The experimental
observation have been used to estimate different kinetic parameters
such as diffusion coefficient, sorption coefficient, permeability
coefficient and thermodynamic parameters such as enthalpy, entropy,
activation energies and polymer-solvent interaction parameter. These
parameters are highly useful for developing clear theoretical
generalization regarding the nature of the polymer towards liquids.
4. The use of EVA membranes for pervaporation, vapour sorption and gas transport.
The feasibility of utilising EVA membranes for the pervaporation separation
of organic liquid mixtures has been examined. The influence of factors
such as crystallinity, feed composition, nature of crosslinks, degree of
crosslinking and penetrant size on the separation has been analysed in
detail. Sorption, diffusion and permeation of organic vapours through
these membranes were investigated.
Development of membranes with high permeability and high permselectivity
has become an active field of investigation because of an increased interest in
the membrane process for gas separation. Therefore, the permeation
characteristics of EVA membranes were analysed with oxygen and
nitrogen.
78 Chapter 1
5. The influence of nanoclay addition in EVA on transport
Finally a new class of membranes, EVA-clay nanocomposites membranes
were prepared. Transport features of these membranes were analysed.
The motive behind this study is to develop a new class membranes for
pervaporation with improved selectivity. The permeability performance of
nano composite towards organic vapours and gases was also analysed.
Introduction 79
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