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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