Preparation and Character is at Ion of Poly imide Incorporated Cellulose

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Chemical Engineering Journal 171 (2011) 33–44

Contents lists available at ScienceDirect

Chemical Engineering Journal

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c e j

Preparation and characterisation of poly (amide-imide) incorporated cellulose

acetate membranes for polymer enhanced ultrafiltration of metal ions

S. Rajesh, P. Maheswari, S. Senthilkumar, A. Jayalakshmi, D. Mohan ∗

Membrane Laboratory, Department of Chemical Engineering, Anna University Chennai, Chennai 600 025, India

a r t i c l e i n f o

Article history:

Received 5 November 2010

Received in revised form 7 March 2011Accepted 9 March 2011

Keywords:

Poly (amide-imide)

SEM

AFM

TGA

Polymer enhanced ultra filtration

a b s t r a c t

Polymeric membranes intended to use in industrial separations must maintain excellent thermal and

mechanical properties over their targeted operating conditions. Therefore, cellulose acetate (CA) mem-

branes with superior properties were prepared by phase inversion technique using high performance

thermoplastic poly(amide-imide) (PAI)as themodification agent. Thepreparedmembranes werecharac-

terised using scanning electron microscopy (SEM), atomic force microscopy (AFM), differential scanning

calorimetry (DSC), thermo gravimetric analysis (TGA) and mechanical analysis to understand the influ-

enceof PAIon thepropertiesof modified membranes. TheSEM analysisshowedthat blend CA membranes

have thinner top layer and higher porosity in the sub-layer. The improvement of surface porosity with

an increase in PAI content and decrease in mean pore size was substantiated by the AFM surface rough-

ness analysis data. Themodified membraneswere applied for the separation of metal ions from aqueous

solutions by polymer enhanced ultra filtration. Attempts have been made to correlate the changes in

thermal, mechanical properties and membranes performance with morphology. It is worth mentioning

thatthe outstanding thermal stability and separationefficiency of these membranesarisingfrom the fine

dispersion of PAI in the CA matrix obviously offers immense potential in industrial separations.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Separation and recovery of metal ions from industrial waste

streams is gaining more and more importance because of the ever-

increasing demand for high purity products and of environmental

concerns [1,2]. Conventional techniques such as sorption, chemi-

cal precipitation and biological treatment have been used for the

removal of metalions from aqueous effluents;however,thesetech-

niques suffer from drawbacks such as time, energy, chemical, and

manpower consumption [3,4]. But the membrane separation tech-

nology has emerged as an attractive and effective technique in the

treatment of wastewater and groundwater containing toxic metal

ions, since it offers concentration and separation of metals or valu-

able chemicals without a change of state and without the use of

chemicals or thermal energy [5]. Among the membrane separa-tionprocesses,ultrafiltration (UF) has been widely used for product

recovery and pollution control in the chemical, electro coating,

electronic, metal refining as well as in the food, pharmaceutical

and biotechnological industries [6].

In view of the demand for more versatile and highly tai-

loredmembranes, membranetechnologycurrently employs a wide

rangeof polymericmaterials [7,8]. Developmentof new membrane

∗ Corresponding author. Tel.: +91-044-22359136; fax: +91-044-22350299.

E-mail address: [email protected](D. Mohan).

materials has a crucial role in the growth of this technology and

modern polymer chemistry is highly proficient in tailoring poly-

mers of specific aims in terms of mechanical, thermal, andchemical

stability. Cellulose acetate (CA) is a potentially outstanding mem-

brane material, because of the advantages such as excellent film

forming properties, moderate flux, high salt rejection properties,

easy manufacture, cost effectiveness and renewable source of raw

material.However,celluloseacetateis not suitable for moreaggres-

sive cleaning, has low thermal, oxidation andchemical resistances;

hence the modification of CA membranes is the endless necessity

of time [9–12].

Recently, aromatic poly-imides have attracted interest as

promising membrane material because of their excellent chemical,

mechanical and thermal stabilities as well as good perm selec-

tive properties [13,14]. However, there are some restrictions onselecting suitable solvents in preparing asymmetric membranes

via the traditional phase inversion technique, since the polyimide

materials are normally very resistant to solvent dissolution. The

processability and solubility can be improved by the inclusion

of amide group in to the imide backbone. Thus, aromatic poly

(amide-imide) (PAI), an amorphous thermoplastic, appears to be

of particular interest as a membrane material with remarkable

physical and chemical properties. The aromatic imide units of PAI

provide high performance propertiessuch as considerable mechan-

ical strength, thermal stability and chemical resistance, while the

flexible amide linkages provide good processability [15].

1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.cej.2011.03.033

http://dx.doi.org/10.1016/j.cej.2011.03.033


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34 S. Rajesh et al. / Chemical Engineering Journal 171 (2011) 33–44

Nomenclature

Abbreviations

CA cellulose acetate

PAI polyamideimide

UF ultrafiltration

PEG-600 polyethyleneglycol-600

NMP N-methyl pyrollidone

SEM scanning electron microscopeAFM atomic force microscopy

TGA thermo gravimetric analysis

T g glass transition temperature

T on initial degradation temperature

T max temperature of maximum degradation

DSC differential scanning calorimetry

PUEF polymer enhanced ultrafiltration

PEI polyethyleneimine

AAS atomic absorption spectroscopy

List of symbols

J w pure water flux [lm−2 h−1]

Q quality of pure water permeated [l]

A membrane area [m2

]T sampling time [h]

Rm membrane hydraulic resistance [kPa lm−2 h−1]

P operational pressure [kPa]

C p concentration of metal ion permeate [mg/l]

C f concentration of metal ion feed [mg/l]

Blending of appropriate polymer with CA has been a versatile

techniquefor themodificationof CA membranes. Thus, CA hasbeen

blended with several high performance polymers such as poly-

sulphone, poly (ethersulphone), polyurethane, poly (ether ether

ketone), poly(ether imide) forimprovingthe CA membraneproper-ties and was found to be successful [16–19]. However, an extensive

literature survey revealed that there is no published document

about the exploitation of poly (amide-imide) in the modification

of CA membranes. It is expected that, by bringing together poly

(amide-imide) and cellulose acetate would conserve their superior

properties in the final mixture while concurrently reducing their

poor characteristics.

The thermal behaviour of constituent polymers is of much

importance in membrane characterisation and its performance.

Thermo gravimetric data will provide a number of stages of ther-

mal breakdown, weightloss of thematerialin each stage,threshold

temperature, etc [20,21]. Compatibility of the polymer blends can

be studied by differential scanning calorimetry (DSC) which will

measure the glass transition temperature (T g) and melting tem-perature of the polymer. Blending of polymers has been reported

to have much influence on the thermal stability of individual poly-

mers. The present study focus on the development of thermally

stable, high permeable and antifouling ultrafiltration membranes

by incorporating PAI into the casting solution in the absence and

presence of pore former PEG-600. The effect of polymer blend

composition and additive on the morphology, hydraulic resis-

tance, thermal and mechanical properties of the membranes was

analysed.Attempts have been made to correlatethe changein ther-

mal, mechanical properties of the membranes with morphology.

Further, the effect of CA/PAI blend composition and presence of

PEG-600 in the casting solution on the rejection and permeate flux

of toxic heavymetalions such as Cu(II), Ni(II), Zn(II), andCd(II) were

investigated and the results are discussed.

Fig. 1. Chemical structure of TorlonRM 4000T-HV Poly (amide-imide) unit.

2. Experimental

2.1. Materials and methods

Commercial grade MYCEL cellulose acetate CDA5770 (glass

transition temperature 219◦C and acetyl content 39.99%) was

obtained from Mysore Acetate and Chemicals Company Limited,

India, and was used after recrystallization from acetone. Commer-

cial gradepoly (amide-imide),TorlonRM 4000T-HV (glasstransition

temperature 285 ◦C) supplied as gift sample by Solvay Advanced

Polymers, L.L.C., USA, was used as received. Chemical structure of

TorlonRM 4000T-HV PAI unit is shown in Fig. 1.

Analar grade N-Methyl-2-Pyrrolidone (NMP) from SRL Chem-icals Ltd., India, was sieved through molecular sieves (Type-5A◦)

to remove moisture and stored in dry condition prior to use. Ace-

tone of analytical grade was purchased from SRL Chemicals Ltd.,

India, and used for the recrystallization of CA. Sodium lauryl sul-

phate (SLS) of analar grade was obtained from Qualigens Fine

Chemicals Ltd., India, and was used as surfactant in the coagula-

tion bath. Polyethylene glycol 600 (PEG-600) was procured from

Merck (I) Ltd., and was used as supplied, as a non solvent addi-

tive for the whole study. Polyethyleneimine [molecular weight,

Mw = 600,000–1,000,000] 50% aqueous solution was procured from

FlukaChemie AG(France)and wasusedfor thepreparationof 1 wt%

aqueous solution. Copper (II) sulphate (AR), nickel (II) sulphate

(AR) and zinc (II) sulphate (AR) were purchased from Merck (I)

Ltd. (Mumbai, India) and used as such for the preparation of aque-ous metal ion solutions. Cadmium (II) chloride (AR) was obtained

from Qualigens Fine Chemicals Ltd., India and used as such for the

preparation of aqueous metal ion solutions.

2.2. Preparation of blend membranes

The blend solutions based on CA and PAI(total polymer concen-

tration = 17.5wt%) were prepared by dissolving the two polymers

in different compositions in the absence and presence of additive

PEG-600 in NMPunder constant mechanical stirring in a round bot-

tomed flaskfor4 h at40 ◦C. A series of such polymer solutions were

prepared by varying the composition of CA and PAI with PEG-600,

as shown in Table 1. The homogenous solution that obtained was

allowed to stand at room temperature for at least 6 h in an airtightconditionto getrid of airbubbles. Thepreparationmethodinvolved

is thesameas that of the“Phase inversion” methodemployed in our

earlier works as reported by other researchers [22,23]. The casting

environment namely, relative humidity (25±2%) and temperature

(30±1 ◦C) werestandardizedand maintainedfor the preparation of

membranes with better physical properties such as homogeneity,

thickness and morphology. The thickness of the membranes was

maintained at 0.19±0.02mm and measured with a micrometer

having a precision of 0.001 mm.

The casting solution andgelation conditions were kept constant

throughout, since the thermodynamic conditions largely affect the

morphology and performance of the resulting membranes. Prior to

casting, a 2 l gelation bath, consisting of 2% (v/v) NMP(solvent) and

0.2 wt% surfactant, SLS in distilled water (non solvent), was pre-



S. Rajesh et al. / Chemical Engineering Journal 171 (2011) 33–44 35

Table 1

Composition and casting conditions of CA/PAI blend membranes.

Blend

composition

Additive,

PEG-600 (wt%)

Solvent,

NMP (wt%)

Cellulose

acetate (wt%)

Poly

(amide-imide)

(wt%)

100 0 0 82.5

90 10 0 82.5

80 20 0 82.5

70 30 0 82.5

100 0 2.5 80.0

90 10 2.5 80.0

80 20 2.5 80.0

70 30 2.5 80.0

Casting temperature= 30±1◦ C; casting relative humidity= 25±2%; solvent evapo-

ration time= 30 s. Note: total polymer concentration at 17.5 wt%.

pared and kept at 20±1 ◦C. The membranes were cast over a glass

plate using a doctor blade. After casting, the solvent present in the

cast film was allowed to evaporate for 30 s and the cast film along

with theglass plate wasgently immersed in thegelationbath.After

2 h of gelation, the membranes were removed from the gelation

bath andwashedthoroughlywith distilledwater toremove allNMPand surfactant from the membranes. The membrane sheets were

subsequently stored in distilled water, containing 0.1% formalin

solution to prevent microbial growth.

2.3. Experimental setup

The ultrafiltration permeation experiments were carried out in

a batch type, dead end cell (UF cell-8400Model, Amicon, USA) with

an internal diameter of 76 mm fitted with Teflon-coated magnetic

paddle. This cell was connected to a compressor with a pressure

control valve and gauge through a feed reservoir. The schematic

representation of the ultrafiltration experimental set up has given

elsewhere [5,22]. The effective membrane area available for ultra-

filtration was 38.5 cm2

. All the experiments were carried out at30±2 ◦C and at a transmembrane pressure of 345kPa.

2.4. Membrane characterisation

2.4.1. Scanning electron microscopy (SEM) analysis

The cross sectional images of the CA and CA/PAI blend mem-

branes were taken by scanning electron microscope (SEM, Cam

Scan MV2300). The membranes were cut into small pieces and

cleaned with filter paper. These membrane pieces were immersed

in liquid nitrogen for 60–90s and were frozen. Frozen fragments of

themembranes were brokenand kept ina desicator. Thedriedsam-

ples were gold sputtered for producing electric conductivity and

photographs were taken in very high vacuumconditions operating

at 15kV.

2.4.2. Atomic force microscopy (AFM) analysis

Atomic force microscopy was used to analyse the surface

morphology and roughness of the prepared membranes (AFM

device was Dual ScopeTM scanning probe-optical microscope, DME

model C-21, Denmark). Small squares of the prepared membranes

(approximately 1 cm2) were cut and glued on a glass substrate.

The membrane surfaces were imaged in a scan size of 5m×5m

and the surface roughness were measured by tapping mode. Three

important components of the surface roughness parameters such

as mean roughness (Ra—the mean value of the surface relative to

the centre plane), the root mean square of the Z data (Rq) and the

mean difference between the highest peaks and lowest valleys (R z)

were calculated. Surface roughness of each membrane was mea-

sured at three different locations and average was reported for

accuracy.

2.4.3. Pure water flux and membrane hydraulic resistance

Membranes after compaction were subjected to a trans-

membrane pressure of 414 kPa, 345kPa, 276 kPa and 207kPa to

measure the purewater flux.The permeabilitywas measuredunder

steady state conditions and the pure water flux is determined by

the following equation:

J w =Q

A(t )(1)

where J w is the water flux (in lm−2 h−1), Q is the quantity of per-

meate collected (in l), t is the sampling time (in h), and A is the

membrane area (in m2).

The hydraulic resistance of the membrane, Rm, was evaluated

from the slope of water flux ( J w) verses transmembrane pressure

difference (P ) using the equation,

J w =P

Rm(2)

2.4.4. Differential scanning calorimetric analysis of the CA/PAI

membranes

Differential scanning calorimetric (DSC)analysisof the prepared

membranes (approximately 6 mg each) were performed on a Uni-

versal V4.5A TA thermal analyser in nitrogen atmosphere. Glass

transition temperature and compatibility of the membranes were

calculated by a two step heating–cooling cycle. In the first cycle

samples were heated from room temperature to 300 ◦C at a heat-

ing rateof 10◦C/min. After thefirst heating cycle,the samples were

cooledto room temperature andonceagainit is heatedup to275 ◦C.

2.4.5. Thermo gravimetric analysis of the CA/PAI membranes

The thermal degradation studies of the blend membranes were

carried out in a Universal V4.5A TA DTG analyser in nitrogen atmo-

sphere. The samples were scanned from room temperature to800 ◦C at a heating rate of 20 ◦C/min. From the TG thermo grams,

the thermal degradation characteristics such as onset of degrada-

tion(T on), temperature of maximum rate of degradation (T max),and

percentage weight losses at different temperatures (300, 400 and

800 ◦C) have been calculated.

2.4.6. Mechanical properties of the CA/PAI membranes

After removing water present in the membranes, mechanical

properties of the prepared membranes were tested according to

ASTM D882 using Shimazu AG-10-TB Universal testing machine.

All samples were cut to the standard shape in the ambient con-

ditions (25 ◦C, relative humidity of 68%) before testing. For each

membrane, three samples were evaluated and the average values

were reported for accuracy.

2.4.7. Rejection of the metal ion solution

Since the pore size of the UF membranes were not suit-

able for the separating heavy metal ions, water soluble polymer

polyethyleneimine (PEI) was used to bind the metals to form

macromolecular complexes. Aqueous solutions of Cu(II), Ni(II),

Zn(II) and Cd(II) were prepared at concentration of 1000ppm in

1 wt% aqueous solution of PEI in deionised water. The pH of these

solutionswas adjusted to 6±0.25 by addingsmallamount of either

0.1 M HCl or 0.1M NaOH. Then solutions were thoroughly mixed

and allowed to stand for 5 days for the completion of binding and

used for the rejection studies. For maintaining constant operating

conditions, each metal ion-PEI complex solution was run in the

ultrafiltration test kit at a transmembrane pressure of 345kPa. The




permeate was collected in graduated tubes in different time inter-

vals and tube contents were analysed for the concentration of the

metal ions using an atomic absorption spectrophotometer (AAS,

Perkin-Elmer 3110). The permeation of metal ions was calculated

by Eq. (1) and rejection was calculated from the concentration of

metal ions in the feed (C f ) and permeate (C p) using the equation,

%Rejection =

1−

C p

C f

× 100 (3)

3. Results and discussion

3.1. Scanning electron microscopic analysis

In order to study the influence of PAI composition on the final

membrane structure, the top surface and cross sections of the pre-

pared membranes were taken using scanning electron microscopy.

The surface and cross section of the thin film ultrafiltration mem-

branes have a critical role in helping to identify the significance of

membrane in themechanism of selectivity andpermeability. It was

known that, the top layer of membrane is responsible for the per-

meation or rejection, whereas the sub layer of the membrane acts

as a mechanical support.

Fig. 2 shows the top surfaces of the pure CA and CA/PAI blend

membranes in the absence and presence of the pore former, PEG-

600. Generally the structure of the top layer is determined by the

instant of phase separation and the ratio of non-solvent inflow to

the solvent outflow. In our preparation, dry–wet casting process

was applied during which evaporation time was 30 s. During the

exposure, NMP evaporates from the casting solution and simulta-

neously it absorbs water from the vapour surroundings. The rate

of absorption of water was high in the case of pure CA membranes

and it showed a tendency to form a liquid layer above the cast-

ing solution. The subsequent immersion of the polymer solution in

the coagulation bath, leads to the formation of relatively immobi-

lized bound layer of water at the polymer-non-solvent interface,

which would inhibit further flow of water into it. This immobilized

bound layer inthe pureCA leads tothe formation of dense top layerwithout pores. But in the case of CA/PAI blend membranes water

absorption was low and it did not shows the tendency to hold the

water at the interface [24,25]. Thus during the immersion process,

the rapid driving force of the solvent with non-solvent at the inter-

face leads to the formation of equally dispersed pores in the skin

layer of the CA/PAI blend membranes.

The SEM images of cross-sections of the membranes prepared

from pure CA and CA/PAI blend membranes in the absence and

presence of the pore former, PEG-600 were shown in Fig. 3. Gen-

eral structure was very similar for all the membranes consisting

of a top skin layer, an intermediate layer with a sponge like struc-

ture and a bottom layer of fully developed open pores. However,

the membranes prepared from pure CA exhibits a finger like cavi-

ties and the pores were not fully developed. The morphologies of the CA/PAI blend membranes differ from that of pure CA mem-

branes. Comparison between images indicates that incorporation

of PAI in the casting solution produces highly porous membranes

with sponge like structure in the sub layer. The changes in the

morphologies can be attributed to the changes in the properties

of the membrane forming polymer by the addition of the amor-

phous polymer PAI. This difference in morphology of the blend

membraneswas mainly because of thepresence ofamideandimide

groups in PAI and effect of these groups in the membrane forma-

tion. We can expect an enhancement of the surface hydrophilicity

for a given content of PAI, due to the preferential orientation of

these groups towards water during the membrane formation pro-

cess [26]. When we are comparing the CA membranes with CA/PAI,

the spongier like structure was formed in the later and it was

due to the difference in tolerance towards water without phase

separation.

Greater affinity of CA to water, leads to longer time for the sol-

vent exchange between the water and the polymer solution before

gelation and vitrification. Experimentally, several minutes of coag-

ulation time was needed for the “setting” of the film made of pure

CA, while blend polymers were set in 4–5 min. This longer settling

time for the pure CA membrane was due to the “tortuous” path

of the crystalline regions of the CA and this region was consider-

ably decreased in blend films due to the presence of PAI.The longer

exchange timebetween the solvent and non-solventin the coagula-

tion bath result in more developed process of polymer-lean phase

growth and coalescence, thus larger the finger like pores in pure

CA membranes. But in case of blends of CA/PAI, high diffusion rate

and hydrophilicity, results in the formation of tear like structures

with more sponge like areas in the sub layer. Highly open tran-

sition sub-layer structure were difficult to obtain with pure CA,

because the diffusion process associated with the re-dissolution

step were slowed down by the viscosity of the polymer solution

and the immobilized water layer formed on the water–polymer

interface [27,28].

3.2. Atomic force microscopic analysis

To study the influence of PAI on the membrane morphology

and surface roughness, AFM analysis was carried out by tapping

mode. The manner in which these properties correlate with the

surface porosity and filtration performance provide insight into the

structure of the filtration membrane.

Figs. 4 and 5 represent the three dimensional AFM images

of pure CA and CA/PAI blend membranes in the absence and

presence of PEG-600 respectively. All membranes were scanned

at a scan size of 5m×5m, since the scanned area is crucial

in comparing the surface roughness of the membranes [29]. In

these images, the brightest area represents the highest points or

nodules of the membrane surface and dark regions indicate the

valleys or membrane pores. It seems that, in the case of pure

CA membranes, surface contains “nodules” and as the concen-tration of PAI increases the “nodules” are almost disappeared at

80/20 blend composition. This is due to the high viscosity of the

CA casting solution and its highly crystalline structure, restricts

the “valley” formation in pure CA membranes. This high peaks

or nodules are almost completely absent or decreased to a sig-

nificant level at 80/20 blend composition due to the presence of

amorphous PAI. But in the case of CA/PAI blend at 70/30 compo-

sition, the “nodules” are reappeared once again and it is larger

in size. This is due to the segmental gap formed between the

componentsin the blendsystem because of its incompatibility. Fur-

ther, it can be observed that when the concentration of dispersed

phase increases the morphology becomes more coarse and unsta-

ble due to the unfavourable interactions of the components at the

interface.Differences in the surface morphology of the CA/PAI blend

membranes, were measured and expressed in terms of surface

roughness, are tabulated in Table 4. From this it is clear that the

surface roughness of the membranes decreased with an increase

in concentration of PAI in the casting solution. Similar trends for

the surface roughness were observed in the Refs. [30,31] and they

have predicted the same tendency for the mean pore size too.

The decrease in the surface roughness means that depressions and

peaksbecome smaller, which in turn indicatesthe decrease inmean

pore size. This change is expected, because the roughness parame-

ters depend on the Z -value, which is the vertical distance that the

piezoelectric scanner moves. When the surface consists of deep

depressions (pores) andhigh peaks (nodules),the tipmovesup and

down over a wide range and the result should be a high roughness




Fig. 2. SEM images of the top surface of CA/PAI blend membranes (w/w): (a) 100/0, without additive (b) 100/0 with additive (c) 80/20, without additive. (d) 80/20, with

additive.

parameter. Theporosityand pore size of themembranes calculated

by the sieving filtration method and included in the first part of

the present investigation was in good agreement with the surface

roughness data.

3.3. Effect of applied pressure on the pure water flux

To investigate the influence of applied pressure and PAI com-

position on the membrane performance, the pure water flux of the

Fig. 3. Cross sectional SEM images of CA/PAI blend membranes (w/w): (a) 100/0, without additive; (b) 100/0 with additive; (c) 80/20, without additive; (d) 80/20, with

additive.




Fig. 4. AFM three dimensional images of CA/PAI blend membranes (w/w) without additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.

prepared membranes were measured at various transmembrane

pressures of 414kPa, 345kPa, 276kPa and 207kPa. The calcu-

lated pure water flux values presented in Table 2, demonstrated

that the pure water flux values were significantly affected by the

applied pressure and showed a decreasing trend with decrease

in applied pressure. Water flux of CA membranes decreased

from109.1 lm−2 h−1 to 26.6 lm−2 h−1 with change in applied pres-

sure from 414kPa to 209 kPa without additive. When the PAI

concentration is increased from 10 to 30%, there was a correspond-

ing increase in flux values for a particular applied pressure and it

decreased with decrease in applied pressure as in the case of pure

CA. The linear increase in flux with increase in PAI content is due

to the hydrophilicity andincreasing immiscible phase behaviour of

the blend, which results from the low interfacial attractive forces

Fig. 5. AFM three dimensional images of CA/PAI blend membranes (w/w) with 2.5 wt% additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.




Table 2

Pure water flux of the CA/PAI blend membranes at different applied pressures and membrane hydraulic resistance.

Polymer blend composition (17.5wt%) PEG-600 (wt%) Pure water flux in (lm−2 h−1) Hydraulic resistance in (kPa lm−2 h−1)

At various pressures (kPa)

209 276 345 414

100/0 0 26.6 48.2 80.7 109.1 0.040

90/10 0 48.1 79.2 106.5 144.9 0.027

80/20 0 81.4 105.2 140.8 187.0 0.021

70/30 0 93.0 139.1 167.3 205.7 0.018100/0 2.5 53.1 87.9 119.4 159.3 0.025

90/10 2.5 85.6 141.9 179.1 221.3 0.017

80/20 2.5 134.2 190.8 256.4 310.1 0.012

70/30 2.5 165.7 236.9 313.6 378.7 0.010

between CA and PAI. But, the membrane prepared in the presence

of additive PEG-600, yielded enhanced flux values and this indi-

cates that the leachability of water soluble additive leading to the

formation of higher and larger number of pores.

The addition of PAI in the CA membrane preparation solution

has brought three advantages on the final membranes structure. (i)

Change in the membrane surface and sub-layer morphology, (ii) an

increase in the hydrophilicity of the CA membrane and (iii) change

in the crystallinity of the CA membrane. As shown in SEM analysis,the blend membrane morphology differs significantly from that of

the pure CA membranes and in the former more sponge like struc-

ture with micro voids was formed in the sub-layer and porosity

increased with the increase in concentration of PAI. In the case for

pure CA membranes macrovoids with irregular morphology were

formed and pores did not opened up properly in the downstream

side of the membranes. In AFM analysis, the pore size of the mem-

branes decreased with the addition of PAIevidenced by the surface

roughness analysis data. In accordance to these results, the pure

water flux values from different compositions of CA and PAI were

higher than the membrane without PAI. From this it is clear that

the pore size did not have any profound effect on the membrane

permeability (CA has finger like cavities). As explained in morpho-

logical analysis, pure CA membrane had low water flux due to the

“tortuous” path of the crystalline regions of the CA and this region

were considerably decreased in blend films due to the presence of

amorphous polymer PAI.

3.4. Effect of PAI on the membrane hydraulic resistance

Membrane hydraulic resistance, Rm, an indication of the toler-

ance of a membrane towards hydraulic pressure, was measured by

subjecting themembranes tovarious pressuresfrom207 to414 kPa

and measuring the pure water flux. The resistance in ultrafiltration

membranesis offered byor dueto thedensetop“skin”layer andthe

surface porosity. Rm was calculated from the inverse of the slope

of the corresponding transmembrane pressure verses pure water

flux plots and the results are presented in Table 2.It was evident from table that the pure CA membranes

in absence of additive exhibited a hydraulic resistance of

0.040kPalm−2 h−1 because of its low porosity. In blend mem-

branes in the absence of additive, as the PAI composition was

increased from 10 to 30% the hydraulic resistance decreased to

0.018kPa lm−2 h−1. When PEG was introduced in to the casting

solution of pure CA, there was a significant decrease in resis-

tance of membrane to the water permeation. As explained in

SEM analysis, addition of pore former in the casting solution

results in the formation of macropores on the membrane surface

due to thermo dynamical instability which enhances precipita-

tion of the casting solution. From this study it was clear that,

resistance towards the permeation was high in the case of pure

CA compared to CA/PAI blend membranes due to its low sur-

face porosity because of the delayed de-mixing during membrane

formation.

In pure water permeation, water itself exists in the form of

hydrogen bonded clusters of approximately 100 molecules with

four to six hundred nearest neighbours. The resistance to the flow

of water molecule through the membrane depends on the physical

and chemical nature of the membrane barrier. In the case of CA/PAI

system, polar groups amide and imide may effectively compete

with the tendency of water molecules to associate with the othersof their kind, thereby causing a destructuring of the original water

complexes and facilitating their transport through the membrane.

Butin thecase ofpure CAmembranes, dueto therepulsion from the

bulky phenylene and acetate groups, the competency and destruc-

turing is considerably reduced, which results in high resistance to

the flow. In addition in the case of pure CA, due to the presence

of pendant groups in the backbone increases the internal energy

of intermolecular motion, this in turn inhibits the rotational and

vibrational motion of the parent chain. However, PAI is more flex-

ible due to the sterically ordered structure which in turn increases

pure waterflux and consequently reduces the hydraulic resistances

in PAI incorporated membranes [32,33].

3.5. Differential scanning calorimetric analysis

The DSC thermo grams of the pure CA and CA/PAI blend mem-

branes obtained in a two step heating–cooling cycle are shown in

Fig. 6. From the DSC thermograms it can be observed that a broad

endothermicpeakis present inthe first heating process ata temper-

ature range of 80–105 ◦C and it disappeared in the second heating

process. The absence of this endothermic peak in the second heat-

ing process is an indication of the irreversible nature of the loss of

Fig. 6. Differential scanning calorimetric curves of CA/PAI blend membranes (w/w)

without additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.




some components. Considering the peculiar affinity of CA to water,

it may be concluded that the peak was caused by the loss of mois-

ture absorbed in the membranes. According to Yang et al. [34], the

loss of moisture within this temperature range is due to the relax-

ation of the polymer chain,indicating the existence of water-bound

sites in cellulose acetate. As shown in Fig. 6, the membranes with

0%, 10%, 20%, and 30% PAI content have an endothermic peak at

104.8, 98.5, 94.2 and 91.9 ◦C respectively. The dissimilarity in the

peak position is attributed to the difference in the structure of the

blend membranes resulting from the interactions of PAI with CA.

This observations supportedby the SEM micrographimages, shows

that pure CA membrane contains finger like pores which are not

opened up properly. This closed pores act as a domain of water

molecules compared to the tear structures with sponge like areas

in the sub layer of the blend membranes.

The DSC criterion is valuable to understand the compatibil-

ity between the individual polymers in the resultant mixture if

the difference between their glass transition temperature values

is at least 20 ◦C. As reported in the literature [35], glass transition

temperature (T g) of the CA membranes can be estimated by the

peak occurring after the endothermic shift. From the Fig. 6 it was

clear that, for the pure CA membranes has a T g of 214 ◦C and for

the CA/PAI blend membranes these values shifted to higher tem-

perature. The T g values of the blends lies between those of the

components in the mixture, precisely at 221, 227.5 and 231.5 ◦C

respectively for increased PAI content from 10 to 30 wt% compo-

sition. Since all blend membranes show a single glass transition

temperature, we can predict the compatibility of the CA/PAI blend

in the entire composition range, probably due to the strong inter-

action between the components. From the DSC studies of the

membranes, we can also conclude that the CA/PAI blend mem-

branes have higher glass transition temperature than the pure CA

membranes. The thermoplastic PAI with T g of 285 ◦C greatly con-

tribute to the improvement of T gs of the polymer blend and it is

believed to be due to the formation of hydrogen bonds between

the components. It seems very reasonable to speculate that inter-

molecular interactions would suppress concentration fluctuations

in misciblepolymerblends forming hydrogen bonds [36]. The resultimplies that cooperative motions of theCA chains, arenecessary for

the glass transition to take place, are strongly affected by the PAI,

which in turn reduces the mobility of the chain.

3.6. Thermo gravimetric analysis

The thermal stability of polymeric membranes plays an impor-

tant role in determining their working temperature limits and

operational conditions prior to their use in industrial applications.

Thus in order to determine the stability of the CA and CA/PAI blend

membranes, thermal behaviour of the blends were analysed using

TGA. The resultant thermo grams of the CA/PAI blend membranes

in the absence and presence of PEG-600 are shown in Figs. 7 and 8

respectively. In general, it was seen that the degradation of all CAbased membranes occurs in three steps. The first step, from the

room temperature (30 ◦C) to 330 ◦C, represents the deacetylation

and evaporation of residual absorbedwater. Elimination of boththe

secondary acetate groups leads to the initiation of decomposition

of the pyranose ring in CA, since the carbohydrate derivatives with

two olefinic bonds are unstable. The second step, starts at 330 ◦C

and ends at 400 ◦C, represents volatilization of the volatile matter

formed due to the scission of the glycosidic bond of CA. This stage

is considered as the major degradation step in CA and this drastic

decomposition was attributed to the extensive loss of acetic acid

and oxides of carbon. The third step, starts at 400 ◦C, symbolizes

the carbonization of the degraded products to ash. The stable third

step corresponds to the formation of carbonaceous residue of non-

graphitizing type which will condense with neighbouring similar

Fig. 7. Thermo gravimetric curves of CA/PAI blend membranes (w/w) without addi-

tive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.

ones to give extensive lateral growth and these may remain stable

to temperature in the region of 1000 ◦C. These three steps may cor-

respond to thesteps suggestedby Scotney [37,38] representing the

thermal degradation of the CA.The pattern of TGA curve of the blend membranes are similar

to that of pure CA, but a small deviation occurs in the tempera-

ture range of 400–550 ◦C, due to the scission of imide linkage and

this is considered to be the most stable step in the random scission

of PAI ring. As shown in Figs. 7 and 8, the incorporation of PAI in

to CA matrix increased the thermal stability of the CA membranes.

The incorporatedPAI could retardthe thermal decomposition of CA

membranes, resulting from the effective function of imide group

and which acts as physical barrier to hinder the external move-

ment of volatile decomposed products. The increase in thermal

properties could be attributed to the change in morphology of the

blends and/or to the interactions of components before and dur-

ing heating. Table 3 gives, thermal degradation characteristics of

the CA/PAI blend membranes prepared at different compositions.

There is a gradual decrease in weight loss could be noticed in blend

membranes and it was attributed to the incorporation of PAI in CA

matrix. But the initial degradation temperature (T on) of the blends

decreased slightly in compared to pure CA membranes. This is due

to the presence of the acidic sites in PAI and it is supposed to

produce catalytic effect on the initial stage of degradation which

results in a shift of initial degradation temperature to a lower value

[39]. In the case of CA/PAI blend membranes with additive, the

initial degradation temperature value is shifted to lower values,

because of the loosely packed arrangement of the polymer chains.

Fig. 8. Derivative thermo gravimetric curves of CA/PAI blend membranes (w/w)

with 2.5 wt% additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.




Table 3

Effect of blend ratio on the initial degradation temperature, temperature of maximum degradation and weight losses at different temperatures of CA/PAI blend membranes.

Polymer blend

composition

(17.5wt%)

PEG-600 (wt%) Initial degradation

of temperature

(T on) in ◦C

Temperature of

maximum degradation

(T max) in ◦C

Weight loss at different temperature (wt%)

300 ◦C 400 ◦C 800 ◦C

100/0 0 280.4 353.18 4.73 71.73 75.73

90/10 0 276.12 357.12 5.28 64.47 70.54

80/20 0 269.73 360.75 6.46 53.11 63.61

70/30 0 259.31 361.67 6.97 48.35 60.46

100/0 2.5 277.18 352.71 4.58 72.94 76.13

90/10 2.5 272.43 357.62 5.73 63.12 72.19

80/20 2.5 265.21 359.89 6.39 54.56 63.9

70/30 2.5 252.92 361.13 7.21 50.15 61.78

From the thermal behaviour of the blends we can conclude that

CA/PAI blend membranes are thermally more stable than the pure

CA membranes.

Even though all blend membranes are stable than the virgin

CA membranes, it is important to note that there is a strong link

between the thermal properties and morphology of the blend

membranes. The percentage increase in the thermal stability of membrane at 70/30 composition is low compared to other blend

compositions. It can be realised from the AFM micrographs that

blend membranes at 90/10 and 80/20 composition exhibit a more

fine,uniformand stable phasemorphology compared to membrane

at 70/30 composition. Thisis attributed to the interfacial interaction

between the amide group in PAI and hydroxyl group in CA leading

to the formationof hydrogen bond,which couldimpart compatibil-

ityin the former. Butin the later,the components areimmiscible to

a smaller extent and it has shown heterogeneous phase morphol-

ogy due to the poor adhesion between components. The schematic

representation of the established hydrogen bond between CA and

PAI in blend membranes is shown in Scheme1. It hasbeen revealed

from this study that stabilisation of morphology through interac-

tion between the components increases the thermally stability of the blend membranes. In CA/PAI blends up to 80/20 composition

the better compatibility at the interface decreases the unfavourable

interfacial interactions between the CA andPAI phases andthereby

PAI decreases the rate of degradation of CA. On the other hand, in

70/30 composition, even though the PAI phase is dispersed in CA

matrix, the unfavourable interactions between the matrix and the

dispersed phases tend to prevent the dispersed phase to protect

the matrix phase from degradation [40].

Scheme 1. Possible H-bonding interactions between the functional groups of the

cellulose acetate and poly (amide-imide) in CA/PAI blend membranes.

3.7. Mechanical properties

The mechanical properties of the ultrafiltration membranes

are of particular interest since they are related to the stresses

encountered by the membrane under operation. In binary blend

membranes with one crystalline component, the final mechani-

cal properties are determined by two competing factors; changein crystallinity due to the incorporation of amorphous compo-

nent and the extent of compatibility between the two polymers.

The mechanical properties of the CA membranes prepared with

different concentrations of PAI in the casting solution are sum-

marised in Table 4. The results indicates that the addition of PAI

phase increased the tensile strength of CA membranes slightly

to a higher value at 90/10 composition and further increase in

PAI content decreased the tensile strength. But, elongation at

break of CA membranes increased considerably up to 80/20 blend

composition and further increase in PAI concentration shows a

negative deviation from the additive line. Interfacial adhesion

between the components is the structural feature responsible

for the elastic properties of the polymer blends. If the attrac-

tion is effective we can consider the binary blend as a singlecomponent system, since the whole volume is penetrated by

less viscous component PAI and forms continuous network with

CA matrix. The incorporation of PAI into the small patches of

crystalline regions of CA changes the tensile strength since the

crystalline regions hinder the rotation of the polymer chain

backbone so that that the distortions occur rapidly in response

to an applied force [41,42]. In the case of CA/PAI membranes

at 70/30 composition, the lack of interfacial adhesion between

the component polymers prevents the polymer chain to snap

back when the energy is removed and leads to lower tensile

strength.

In the case of CA/PAI membranes with additive PEG-600, the

mechanical properties decreased compared to the membranes

without additive. The addition of water soluble additive in to thecasting solution increased the porosity of the membranes and it

caused undesirable changes in the mechanical properties. From

these studies it was clear that the mechanical properties show

similar trend as that of the surface roughness by the addition

of PAI. This is made clear by correlating the roughness values

with the tensile strength of the CA/PAI membranes as given in

Table 4. It can be seen that both roughness and tensile strength

values decrease with addition of PAI content. This decrease in

surface roughness with increase in concentration of PAI is due

to the availability of PAI for the formation of networks with CA,

which decreases the mean pore size of the membrane. This high

porosity with lack of adhesion between the component polymers

leads to premature failure and thus to lower tensile strength

[43].




Table 4

Mechanical properties and AFM surface roughness values of the CA/PAI blend membranes at various compositions.

Polymer blend composition

(17.5wt%)

PEG-600

(wt%)

Mechanical properties AFM surface roughness data

Tensile

strength (MPa)

Elongation at

break (%)

Ra (nm) Rq (nm) Rz (nm)

100/0 0 3.73 18.93 6.38 (0.58) 5.03 (0.31) 31.38 (1.73)

90/10 0 4.68 22.60 5.11 (0.36) 3.62 (0.11) 23.47 (2.23)

80/20 0 4.56 23.14 3.26 (0.53) 2.32 (0.23) 15.57 (1.29)

70/30 0 4.13 17.23 2.27 (0.27) 1.59 (0.29) 9.92 (0.91)

100/0 2.5 3.16 18.27 7.97 (0.82) 6.43 (0.16) 43.38 (1.94)

90/10 2.5 4.17 20.53 6.43 (0.41) 5.31 (0.24) 36.47 (2.49)

80/20 2.5 3.87 21.39 4.72 (0.27) 3.84 (0.37) 27.09 (0.64)

70/30 2.5 3.63 16.21 5.38 (0.59) 4.02 (0.11) 18.11 (1.31)

Values within the parenthesis are standard deviation.

Table 5

Percentage rejection and permeation flux values of the metal ion by the CA/PAI blend membranes.

Polymer blend composition

(17.5wt%)

PEG-600

(wt%)

Metal ion rejection (%) Metal ion permeation flux (lm−2 h−1)

Cu2+ Zn2+ Ni2+ Cd2+ Cu2+ Zn2+ Ni2+ Cd2+

100/0 0 92.08 90.74 89.16 88.38 32.4 36.7 41.1 44.5

90/10 0 96.23 95.16 93.78 93.71 64.7 66.1 67.6 71.9

80/20 0 98.87 98.23 96.89 96.11 61.3 67.3 66.5 68.5

70/30 0 92.49 88.01 88.01 87.48 81.2 84.6 84.9 90.8

100/0 2.5 90.32 90.18 88.91 87.96 51.8 57.5 61.4 89.4

90/10 2.5 95.12 93.27 90.13 91.62 87.9 88.7 89.3 93.4

80/20 2.5 97.92 96.83 94.77 93.84 80.1 81.8 82.6 88.4

70/30 2.5 91.11 86.38 85.83 85.81 98.6 99.5 103.4 109.5

3.8. Metal ion rejection studies

Polymer-enhanced ultrafiltration (PEUF) is considered to be an

effective method for the removal of heavy metal down to desired

concentrations in drinking water purification. In PEUF, pH value

is an important parameter since either protons or hydroxyl ions

wouldcompete withmetalsfor thecomplexationwith thepolymer.

For the sake of reproducibility the metal ion rejections studies and

simultaneous permeate flux measurements were repeated at leastthree times and average was reported.

The separation of metal complexes of Cu(II), Ni(II), Zn(II) and

Cd(II) by the pure CA and CA/PAI blend membranes is shown in

the Table 5. From the rejection values, it was revealed that as the

concentration of PAI in the blend system increases, the rejection

of a particular metal ion increases due to the smaller pore size

of the membrane as shown in AFM analysis. The pure CA mem-

branes in the absence of additive exhibited 92.08% rejection for

Cu(II) ions, which was lower than that of the CA/PAI blend mem-

branes. The other metal ions Ni(II), Zn(II) and Cd(II) had rejections

of 90.74, 89.16 and 88.38% respectively and decrease in the rejec-

tion of these metal ions may have been due to the smaller size of

these metal ion–PEI complex. This observed rejection values are

substantiated by the finger like pores in the pure CA and spongelike structures in theCA/PAI blend membranesas indicatedin Fig.3.

This increase in rejectionof therespective metal ions wasincreased

up to 80/20 blend composition and further increase in PAI content

decreases the rejection. These values at 70/30 blend composition

are lower than that of pure CA membranes due to the hetero-

geneity arising as a result of the higher PAI content creating gaps

between thepolymerchains. Thus therejectionof a given metal ion

by the membranes was in the order CA/PAI (80/20wt%) > CA/PAI

(90/10wt%)> CA (100 wt%) > CA/PAI (70/30wt%).

Introduction of PEG-600 into the casting solution decreases

the rejection of the respective ions and it may be attributed to

the rapid leaching out of PEG-600 during gelation because of

the tendency to lower the free energy of the system [44]. Above

rejections studies showed that the binding capacity of Cu with

PEI is stronger than that of the other metal ions and it is in the

order Cu(II)> Ni(II)> Zn(II)> Cd(II). Further thecomplexingcapacity

depends on the number of functionalgroups in the macromolecular

complex and the atomic size of the metal. The highest separation

of Cu2+ might have been be due to the formation of stronger com-

plexes with stable short bonds, in accordance with the Jahn-Teller

effect [45].

3.9. Metal ion permeate flux studies

The permeate fluxof metal ionsolutionis essential in predicting

the efficiency and economics of the membranesseparation process.

The pure CA membranes in the absence of additive showed a Cu2+

permeate flux value of 32.4 lm−2 h−1. The other metal ions also had

lower flux values because of lower porosity and some of the pores

areclosed in thedownstream side of theCA membranes. Thevalues

werefoundtobe36.7lm−2 h−1,41.1lm−2 h−1 and44.5lm−2 h−1 for

Ni2+, Zn2+ and Cd2+ respectively. The permeation rate of the differ-

ent metal ionsolutionpresented in Table 5 indicated that themetal

ion solution flux values are higher for CA/PAI blend membranes.

This was explained by the fact that in the initial stages of filtra-

tion, the unbound metal ions accumulated in these closed pores

resulting in pore plugging which in turn decreases the flux in pureCA membranes. This low flux are justifiable, comparing the high

surface roughness values of pure CA membranes due to the pres-

enceof large pores in thesurface of themembrane compared to the

CA/PAI blend membranes. The order of flux of the metal chelates

was in the order: Cd(II)> Zn(II) > Ni(II) > Cu(II) and membranes flux

was in the order CA/PAI (70/30wt%) > CA/PAI (90/10 wt%)> CA/PAI

(80/20wt%)> CA (100wt%).

4. Conclusion

In the present investigation, CA ultrafiltration membranes were

prepared by phase inversion technique using high performance

thermoplastic PAI as the modification agent and polyethylene

glycol-600 as the pore former. The effect of blend ratio on the




morphology, membrane hydraulic resistance, compatibility, ther-

mal and mechanical properties of the resultant membranes was

evaluated. Morphological analysis of the blendmembranes showed

that, as the weight percentage of PAI in the CA matrix increases,

defects free thin top layer and spongy sub layer were formed. AFM

studies revealed that, the surface properties and porosity of the

membranes were improved considerably by the addition of PAI in

thecasting solution.It wasobvious that theless viscous component

(PAI) forms smaller dispersed phase in the more viscous matrix

(CA) due to comparatively restricted diffusion and increased shear

stress resulting from the more viscous matrix phase. The hydraulic

resistance of the CA membranes was decreasedwith increasing PAI

content, due to the preferential orientation of the polar groups

towards water during the membrane formation process which

leads to the enrichment of surface with functional groups. This

amide and imide groups on the surface of these blend membranes

effectively compete with the tendency of water molecules to asso-

ciate with the others of their kind, thereby causing a destructuring

of the original water complexes and facilitating their transport

through the membrane, which decreases the resistance to perme-

ation.

Differential scanning calorimetric studies showed a single T gover the entire composition range, indicates the compatibility of

the blend up to 70/30 composition dueto the favourable interfacial

adhesion. The improvement in the thermal stability of membranes

by the incorporation of PAI is due to the fine dispersion of less

viscous PAI in the CA matrix. Overall results suggest that morphol-

ogy, hydraulic resistance, thermal and mechanicalproperties of the

prepared CA/PAI blend membranes improved significantly by the

incorporation of PAI. These modified membranes were applied for

the separation of metal ions from aqueous solutions by polymer

enhanced ultra filtration and the results revealed an augment in

separation efficiency. The optimal combination of blend compo-

nents (CA & PAI) and the addition of non-solvent additive PEG-600,

thus allows the preparation of high performance UF membranes

which are sufficiently dense to retain solute molecules and at the

same time give economically viable fluxes.

Acknowledgements

The authors gratefully acknowledge University Grants Commis-

sion (UGC), New Delhi, India, for financial assistance. The authors

also thank Solvay Advanced Polymers, Alpharetta, USA for provid-

ing poly (amide-imide) (TorlonRM4000T-HV).

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Preparation and Character is at Ion of Poly imide Incorporated Cellulose

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