Segmented network structures for the separation of water/ethanol mixtures by pervaporation

Polymer International 46 (1998) 117È125

Segmented Network Structures for theSeparation of Water/Ethanol Mixtures by

Pervaporation

Filip E. Du Prez,1,* Eric J. Goethals,1 Rossitza Schue� ,2 Houssain Qariouh2

& Schue� 2,FrancÓ ois

1 Department of Organic Chemistry, Polymer Chemistry Division, University of Ghent, Krijgslaan 281 S4-bis, B-9000 Ghent,Belgium

2 Universite� de Montpellier II, Sciences et Techniques de Languedoc, Laboratoire de Chimie Macromole� culaire, Place E.Bataillon, 34095 Montpellier Ce� dex 5, France

(Received 20 October 1997 ; accepted 13 December 1997)

Abstract : a,u-Acrylate terminated poly(1,3-dioxolane) (polyDXL), was used as ahydrophilic cross-linker of hydrophobic poly(methyl methacrylate) (polyMMA)chains for the synthesis of amphiphilic AB-block copolymer networks. The appli-cation of these segmented networks as membranes for dehydration of water/ethanol mixtures by the pervaporation technique was investigated. Because thecross-links inhibit to a great extent phase separation between the components ofthese materials, as revealed by dynamic mechanical thermal analysis, an optimalcontrol of the membrane characteristics could be achieved by variation of thehydrophilicityÈhydrophobicity balance and the cross-link density. The com-bination of desorption experiments, determination of swelling degrees and calcu-lation of deviation coefficients (e) allowed us to demonstrate in these membranesthe existence of a so-called coupling e†ect. It was shown that polyDXL plays apredominant role in the speciÐc interactions between the membrane and the sol-vents, which cause the preferential water transport in all the membranes over thewhole composition range of the feed mixture. 1998 SCI.(

Polym. Int. 46, 117È125 (1998)

Key words : pervaporation ; membrane ; AB-block copolymer networks ; poly(1,3-dioxolane) ; coupling e†ect

INTRODUCTION

Pervaporation is used world-wide as a membraneprocess for the separation of liquid mixtures.1 During apervaporation experiment, the feed mixture remains incontact with one side of a dense polymeric membrane,while the permeate is removed in the vapour state at theopposite side by vacuum or gas sweeping. Thus, the per-meate undergoes a phase change, from liquid to vapour,during its transport through the membrane.

* To whom all correspondence should be addressed.

The technique has been developed on an industriallevel for applications where the separation by othermore conventional methods is difficult or impossible,e.g. for azeotropic mixtures, close-boiling organic mix-tures or thermosensitive products. The developments inpervaporation processes have been reviewed in somerecent publications.2h4 One of the main industrial inter-ests in this Ðeld is the removal of water from ethanol-rich mixtures, which originates from a possiblesubstitution of fossil fuels by the use of ethanol as abiomass energy source.5,6 The case in which the per-vaporation process has the most marked economic

1171998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

118 F. E. Du Prez et al.

advantage over conventional separation methods, is thedehydration of the ethanol/water azeotropic mixture(4.4% water). Many studies were performed on hydro-philic membranes with the purpose of understandingthe parameters inÑuencing the Ñux (J) and selectivity (aor b). Because a single polymer does not usually possessthe optimum properties for a given separation, di†erentpolymers were frequently combined in the form ofpolymer blends7,8 and block- or graft-copolymers.9h12More recently, copolymer networks and interpene-trating polymer networks (IPNs) have also been usedfor the same purpose.13h16 The mass transport througheach of these polymer blends can be controlled andadjusted by changing the composition, the morphologyand the swelling behaviour. The network structureshave the additional advantage that the chemical cross-links may be used to control the degree of swelling andto improve the dimensional stability of the membrane.

In this work, knowledge of the synthesis of well-deÐned AB-block copolymer networks was applied toprepare dense membranes for the dehydration of water/ethanol mixtures. In such network structures, polymerB can be bonded to polymer A in two ways : (1) atvarious points along the chains, or (2) at both chainends.17,18 A good example of the applicability of theformer type of networks in membrane technology hasbeen reported by Kerres and Strathmann13 who usedcopolymer networks obtained by a hydrosilylation reac-tion between styrene/isoprene precursor polymers andelastomeric polysiloxanes. The authors used per-vaporation experiments, besides other common polymercharacterization methods, as an original approach toshow the phase-separated morphology and the exis-tence of microphase inversion in their copolymersystems.

In the second type of copolymer networks, for whichthe name “segmented networksÏ was introduced,19,20polymers with two polymerizable end-groups, alsocalled bis-macromonomers, are used as the starting pro-ducts for the synthesis. In this way, networks can bedesigned in which the cross-link density is easily con-trolled by the molecular mass and the fraction of thebis-macromonomer used. To our knowledge, the onlyinvestigation of segmented networks as membranematerials for the separation of aqueous ethanol solu-tions, was made by Okuno et al.14 who prepared densemembranes by using commercially available polyethyl-ene glycol dimethacrylate as hydrophilic cross-linker forhydrophobic poly(benzyl methacrylate) chains. In ourwork, a,u-acrylate terminated poly(1,3-dioxolane)(polyDXL) was used as hydrophilic cross-linker in theformation of segmented networks with hydrophobicpoly(methyl methacrylate) (polyMMA). The bis-macromonomer of polyDXL is obtained by cationicring opening polymerization of 1,3-dioxolane (DXL) inthe presence of a transfer agent.20,21 It was shownbefore20 that the hydrophilicity and the compatibility of

this polymer with certain polymers (such as polyMMA)are two reasons why it is a good starting material fromthe viewpoint of the synthesis of di†erent moleculararchitectures such as copolymer networks and IPNs.

Our aim was to use the polyDXL-based segmentednetworks as model membranes to investigate the inÑu-ence of di†erent parameters such as the degree of swell-ing, cross-link density, hydrophilicityÈhydrophobicitybalance and temperature on the dehydration process ofethanol by pervaporation.

EXPERIMENTAL

Starting materials

1,3-Dioxolane was puriÐed by distillation (74¡C) overand dried on sodium wire under reÑux. Dichloro-CaH2

methane was distilled twice over followed byCaH2 ,drying under reÑux. Dichloromethane was distilledtwice over followed by drying under reÑux onCaH2 ,sodiumÈlead alloy for several hours. The initiator ofthe cationic polymerization, methyltriÑuorometha-nesulphonate (methyl triÑate) was puriÐed by distilla-tion over just before use. Methyl methacrylateCaH2was reÑuxed over in the presence of a radicalCaH2inhibitor, phenothiazine, before distillation. The cross-linking agent tetraethylene glycol diacrylate (TEGDA)was used as delivered by Aldrich Chemical Co.(Bornem, Belgium). The radical low temperature initi-ator of the copolymerization, bis(4-tert-butylcyclohexyl)peroxydicarbonate (Perkadox 16, purity 95%), waskindly given by Akzo Nobel Chemicals (Amersfoort,The Netherlands).

Synthesis of a,u-acrylate terminated poly(1,3-

dioxolane)

The detailed procedure for the synthesis of the macro-monomer can be found in ref. 21. BrieÑy, the cationicring opening polymerization of DXL occurs at roomtemperature in dichloromethane with methyl triÑate asinitiator and methylene bis(oxyethylacrylate) as transferagent. The latter product was prepared by the reactionbetween 2-hydroxyethylacrylate, paraformaldehyde andtriÑic acid :

After stirring the reaction mixture for 6 h, triethylaminewas added and a,u-acrylate terminated polyDXL was

POLYMER INTERNATIONAL VOL. 46, NO. 2, 1998

W ater/ethanol mixture separation by pervaporation 119

precipitated, washed with cold ether, then dried undervacuum. If the initiator concentration is kept lowenough, the molecular weight of the polymer can easilybe varied by changing the concentration of the transferagent and the monomer. In this work, polyDXL with anumber average molecular weight of 4000 was used.

Synthesis of segmented networks

For the preparation of the membranes, chosen amountsof MMA, TEGDA and the bis-macromonomer,depending on the desired weight ratio, were mixedvigorously with 0É2 mol% Perkadox-16 (relative toMMA) at 60¡C for 3 min. The membranes were pre-pared by casting this viscous prepolymerized mixture ina glass plate mould which was kept in an oven for80 min at 60¡C, 3 h at 80¡C and 12 h at 110¡C. Themould was composed of two glass plates separated by aTeÑon spacer (50lm). Before use, the glass plates weretreated with (95%) and a solution (10%) of tri-H2SO4methylsilyl chloride in toluene to facilitate the recoveryof the membrane after preparation. After removal, allmembrane Ðlms (thickness 35È45 lm) were stored for12 h in a vacuum oven at 100¡C.

A piece of each membrane Ðlm was treated in aSoxhlet apparatus with boiling ethanol for 6 h. Thisrevealed a maximum weight loss of 5 wt% in every case,indicating an almost complete copolymerizationbetween MMA and the bis-macromonomer.

Thicker Ðlms were also prepared from the same net-works, using a silicone spacer (thickness 1 or 2 mm)between the glass plates. In this case, the reactionmixture was placed in the mould by means of a hypo-dermic syringe. For the synthesis of the networks with ahigh content of polyDXL, a minimal amount of toluenewas added to obtain a homogeneous and less viscousreaction mixture. The same reaction and puriÐcationprocedure was followed as in the case of the mem-branes.

The networks are coded as illustrated in the followingsample : M2080(x). The numbers 20 and 80 refer, respec-tively, to the weight percentage ratio of polyDXL andpolyMMA. (x) notes the amount (in wt%) of cross-linking agent TEGDA relative to polyMMA.

Pervaporation experiments

An ordinary laboratory scale pervaporation apparatus(see, for example, ref. 13) was used for the separation ofthe ethanol/water mixtures. After placing and sealing adense membrane in a thermostatted permeation cell ona porous Inox support, the liquid feed mixture wasintroduced to the upstream part (capacity 100 ml). Themembrane area in contact with the liquid mixture wasabout 10 cm2. The downstream part of the cell was keptat reduced pressure (2 mmHg) with an oil pump. Thepermeate was condensed with liquid nitrogen in one of

the cooled traps. After sufficient permeate had con-densed in the trap, the vacuum line was switched to thesecond trap by means of valves. This procedure wasrepeated at appropriate time intervals until a constantpermeation rate was reached. This permeation rate (Jp)was calculated by weighing the trapped permeate :

Jp\ mpA *T

(kg m~2 h~1) (2)

where denotes the weight of permeate collected inmpthe cooled trap, A the e†ective membrane surface and*T the permeate time. The permeation rates were nor-malized to a membrane thickness of 10 lm by(J10 lm),assuming the Ñux to be inversely proportional to themembrane thickness (FickÏs law). A relative error on theÑux of 3È5% may occur.

The water content was determined by means of aKarl Fischer coulometer (Metrohm, KF684, Herisau,Switzerland). This technique allows a fast absolute mea-surement with a reproducibility of approximately 0.5%.After determination of the water weight fractions in thefeed mixture (m) and in the permeate (m@), the separationfactors for the water selective membranes wereaEtOHH2Ocalculated as follows :

aEtOHH2O \ m@(1 [ m)m(1[ m@)

(3)

Another dimensionless parameter, the enrichmentfactor b, which can also be used for the calculation ofthe membrane productivity criterion is(Jp(b [ 1))7,deÐned by the ratio of the weight fractions of the fasterpermeant in the permeate and in the feed (m@/m).

Swelling and desorption experiments

A piece of membrane was immersed in a water/ethanolsolution at 25¡C. After swelling, it was weighed within10 s after removal from the solution, surface liquidhaving been Ðrstly removed with a Ðlter paper. Thismeasurement was repeated Ðve times to minimizeexperimental error (5È10%). The swelling experimentswere repeated for each mixture until sorption equi-librium was reached. The degree of swelling Q was cal-culated from:

Q\We[ W0W0

É100o1

(4)

where and respectively, denote the weight of theW0 We ,dry and swollen membrane, and is the density of theo1liquid mixture.

The same swelling procedure was followed on thickerÐlms of the same networks (1È2 mm) for the desorptionexperiments. A weighed amount (1È2 g) of the swollenÐlm was quickly placed in a dry, heated container. Thiscontainer was connected to a vacuum pump to evacuatethe liquid absorbed in the Ðlm. After condensation of



the vapour in a trap cooled with liquid nitrogen, thecomposition of the liquid mixture collected was deter-mined using the Karl Fischer instrument.

Other instruments

Thermogravimetric analysis was performed on aPL-TGA (PL-TG1000, Polymer Laboratories, Lough-borough, UK) under a nitrogen atmosphere.

Values of E@, EA and tan d were measured in bendingmode by a dynamic mechanical thermal analysis(DMTA) instrument (PL-MKII, Polymer Laboratories,Loughborough, UK) on rectangular Ðlms (thickness1 mm) at a heating rate of 2¡Cmin~1 (frequency 1 Hz).

Di†erential scanning calorimetry (DSC) curves wererecorded on a Mettler instrument DSC30 (Mettler-Toledo, ViroÑay, France) with a thermal analysis con-troller TA4000.

RESULTS AND DISCUSSION

Synthesis and physical properties of the segmentednetworks

The segmented copolymer networks are prepared byfree radical copolymerization of the polyDXL bis-macromonomer with MMA. The polyDXL segments,incorporated between the polyMMA-chains, thus act aspolymeric cross-linking agent (Fig. 1).

The content of polyMMA in the networks could bedetermined by thermogravimetric analysis (TGA). Thedi†erence between the degradation temperature ofpolyMMA (260¡C) and the other compounds, polyDXLand TEGDA (160¡C), in the TGA curves is largeenough to di†erentiate both weight losses. In this way,it could be concluded that the di†erence between themeasured and starting “monomerÏ composition of thecomponents never exceeded 2%. A TGA of linearpolyDXL (curve I) and of a network in which theratio polyDXL/polyMMA is 50/50 (curve II) is givenin Fig. 2.

In the case of materials consisting of two or morepolymers, the morphology determines the membranecharacteristics to a great extent. The permeation of het-erogeneous blends is generally dominated by the contin-uous phase in the membranes, while the transportproperties of homogeneous blends change continuouslyover the whole composition range. In the ideal case, in

Fig. 1. Reaction scheme for the synthesis of segmented net-works.

Fig. 2. Determination of the composition of the copolymernetwork with TGA (curve II). The degradation of linearpolyDXL is given in curve I atmosphere, heating rate(N2

6¡Cmin~1).

which both polymers are mixed on a molecular level, anoptimal control of the physical and membrane proper-ties should be possible.

The morphology of the copolymer networks wasinvestigated by DMTA and DSC. The inÑuence of thecomposition of the polymers on the Ðnal morphology ofthe segmented networks, as expressed by the shape oftheir dissipation factor (tan d) versus temperaturecurves, is illustrated in Fig. 3. All curves show one tran-sition between the of polyDXL ([50¡C) and that ofTgpolyMMA (110¡C), indicating the high degree of com-patibility of both polymers in the networks. The tem-peratures, corresponding to the maximum in thetan dÈT curves, are always shifted to higher values thanpredicted by the equation of Fox for random copoly-mers.22 This phenomenon is explained by the mobilitydecrease of the polymer chains in the networks com-pared with the chain mobility in linear polymers.

Even if the broadening of the transition in M5050(curve III) suggests a complicated morphology, it isclear that phase separation is strongly inhibited, up toa polyDXL content of 50%. Higher weight fractions ofpolyDXL result in heterogeneous systems in which thepolyDXL domains are able to crystallize, as shown byDSC experiments. It is well known that the absorption

Fig. 3. Dissipation factor versus temperature for segmentednetworks with di†erent polyDXL/polyMMA ratios : 20/80

(curve I), 40/60 (curve II), 50/50 (curve III).



of the feed mixture in a semi-crystalline membrane islimited to the amorphous part.23 Therefore, the weightfraction of polyDXL in the networks was limited to50%.

By comparing the DMTA curve of a segmentednetwork with that of a blend of the same linear poly-mers (Fig. 4), it can be concluded that the covalentbonds between both polymers in the segmented net-works cause a forced compatibility. While the networkÐlms are transparent materials, the polymer blends areopaque materials, due to a coarse phase separationprocess. Both transitions at the of the pure com-Tgsponents, together with a transition at the melting pointof polyDXL (50È60¡C), can be observed in curve II. Inother words, the heterogeneity of the polymer blendsleads to crystallization of polyDXL domains. Besidesthe lack of mechanical stability, the presence of crys-talline domains is another reason why polymer blendsof polyDXL and polyMMA could not be applied asmembrane materials.

The inÑuence of the cross-link density on the mem-brane properties will be described in the last section ofthe discussion. Keeping the ratio of polyDXL topolyMMA constant, the cross-link density of the net-works could be increased by adding an additional lowmolecular weight cross-linking agent such as TEGDA.The DMTA curves (log E@ versus T ) of three networks,for which the ratio of polyDXL to polyMMA was keptconstant at 20/80, but in which a varying amount of theadditional cross-linking agent TEGDA was added, aregiven in Fig. 5. Applying rubber elasticity theory, i.e. theproportionality between E@ and degree of cross-linkingfor elastomers, the increase of E@ (curves IÈIII) at thestart of the rubbery plateau clearly indicates thedecreasing average molecular weight between cross-links.

One could also consider changing the molecularweight of polyDXL to vary the cross-link density.However, in this case, care should be taken in compar-ing the di†erent materials because some properties ofpolyDXL, such as hydrophilicity and crystallinity,depend on the molecular weight of the polyDXL seg-ments.

Fig. 4. Comparison between the tan dÈT curve of a segment-ed network (curve I) and a blend (curve II) of polyDXL and

polyMMA (w/w 50/50).

Fig. 5. Log E@ÈT curves for segmented networks (polyDXL/polyMMA 20/80) with a varying amount of TEGDA (in wt%relative to polyMMA) : 3 (curve I), 6 (curve II) and 10 (curve

III).

Influence of membrane composition on transportproperties

The inÑuence of the polyDXL content on the mem-brane characteristics of the segmented networks for theazeotropic mixture ethanol/water (95É6/4É4) is shown inTable 1, together with the degree of swelling of themembranes in the pure solvents.

All membranes preferentially permeate water fromthe ethanol-rich feed mixture at room temperature. Thiscould be explained by considering the glassy nature ofthe copolymer membranes at this temperature, whichresults in a decrease of the di†usivity with increasingmolecular size of the permeant (mobility selectivity).However, relatively high degrees of swelling in ethanolwere observed for all membranes (Table 1), suggestingthat the separation is also determined to a great extentby solubility selectivity. The selectivity to water cantherefore be explained by favourable interactionsbetween ethanol and the membrane, leading to an easyincorporation, but to a low rate of di†usion for ethanol.The incorporation of the water molecules is more diffi-cult, but the weak interaction of water with the mem-brane and its smaller molecular size compared withethanol enhance its transport rate. A more detaileddescription of the interactions in these networks will bediscussed in the next paragraph.

The permeation through a homopolymer network ofpolyMMA cross-linked with 3% TEGDA, is virtuallynegligible. However, it increases rapidly with the intro-duction of the hydrophilic polyDXL. At the same time,the selectivity a (and b) drops with increasing fraction ofpolyDXL. The presence of 10% polyDXL is already suf-Ðcient to inÑuence the Ñux and the selectivity consider-ably. This observation is indirect evidence for themiscibility on a molecular level of the two polymer seg-ments in these copolymer systems and is in agreementwith the results obtained by DMTA. The decrease ofthe (Fig. 3) of the whole system with increasingTgpolyDXL content, i.e. an overall higher mobility of the



TABLE 1. Pervaporation properties (J, a and b) and the degrees of swell-

ing (Q) in water and in ethanol of membranes with different polyDXL/-polyMMA ratio (T = 25ÄC)

Membranea J10 lm

b ab bb Qwater

(%) Qethanol

(%)

(g/m2 h)

PolyMMA À0 ? ? 5 40

(½3% TEGDA)

M1090 300 16 10 6 33

M2080 460 14 8·5 6 32

M3070 530 11 7·5 12 33

M5050 1170 6 4·5 33 39

a Designations of the polymer membranes are explained in the experimental part.

b Pervaporation properties (J, a and b) were measured for the transport of the azeo-

tropic mixture water/ethanol.

polymer chains, favours the di†usion of the solvent mol-ecules through the membranes. Speaking in terms of thesolution-di†usion model,24 which is the most widelyaccepted transport model in pervaporation, the activa-tion energy of permeation decreases with increasingpolyDXL content. However, the easier transport ofboth permeants results in a decrease of the selectivity, ascan be observed in Table 1. If a heterogeneous systemhad been formed, i.e. if polyDXL domains were dis-persed in a matrix of polyMMA (as in the case of thepolymer blends), the permeation characteristics, espe-cially at low polyDXL contents, would be governed bythe high continuous polyMMA matrix, resulting inTgvery low permeation rates. In conclusion, there is aqualitative agreement between the pervaporation resultsand the of the dry membrane materials, as mea-Tg ssured by DMTA. This is a further indication of thehomogeneity of the block copolymer networks.

When comparing the permeation rates of di†erentmembranes, the plasticizing e†ect of the solvents on themembrane should also be taken into account. If the feedconstituents swell the membrane to a di†erent extent,the e†ect upon the polymer segment movements andconsequently upon the permeation rate cannot be com-pared. However, as shown in Table 1, the membraneswith a polyDXL content of up to 20% show approx-imately the same degrees of swelling in water and inethanol, which means that the plasticizing e†ect of bothpermeants is similar. Only at higher polyDXL contentsdoes the degree of swelling in water increase, due to thehigher hydrophilicity of the membrane.

Investigation of coupling effect in copolymermembranes

Membrane M2080 was submitted to a detailed investi-gation of its swelling and pervaporation characteristics.Firstly, swelling experiments were done over the wholerange of water/ethanol mixtures (Fig. 6). The maximum

swelling degree (175%) was found at a concentration ofapproximately 20% of water. It can be assumed that theplasticizing e†ect is also at its maximum at this waterconcentration. Initially, a solubility parameter (d)approach was considered to explain the maximum. Insuch a case, the high degree of swelling at an interme-diate solvent composition may be explained by the simi-larity between the d values of the membrane and thesolvent mixture. However, considering that the d valuesof the pure solvents, water (47É9 (MPa)1@2) and ethanol(26 (MPa)1@2),25 are much higher than the d value of themembrane (19È21 (MPa)1@2),26 the above mentionedsimilarity can never be attained.

On the other hand, the maximum degree of swellingat 20% of water can be explained in terms of the so-called “coupling e†ectÏ in the ternary system water/ethanol/membrane. The origin of this e†ect lies in thefact that the rate of di†usion of one permeant is notindependent from that of the other. Attractive molecu-lar interactions between the two migrants, or betweenthe migrants and the membrane, can result in a mutualdrag between them. Attempts have been made todescribe this complex e†ect in a quantitative way.27,28

Considering the degrees to which di†erent mem-branes swell in water, in ethanol and in the solvent

Fig. 6. Degree of swelling Q of the membrane M2080 versusthe water content in the water/ethanol binary mixture

(T \ 25¡C).



TABLE 2. Degrees of swelling of three membranes

with different polyDXL/polyMMA ratios in water,

ethanol and the solvent mixture with 20% of water

(T = 25ÄC)

Membrane Qwater

(%) Qethanol

(%) Q20@80

(%)

PolyMMA 5 40 40

(½3% TEGDA)

M2080 6 32 175

M5050 33 39 125

mixture with 20% of water (Table 2), it can be con-cluded that the presence of polyDXL plays a predomi-nant role in the coupling e†ect. Indeed, only formembranes which contain polyDXL segments, is amaximum in the swelling curve detected at an interme-diate composition of water and ethanol. The complexmorphology of M5050 (Fig. 3), which indicates someorganization of the polymer chains in domains, couldexplain why the degree of swelling in the 20/80 mixtureis lower than for the M2080 membrane. Although theexact nature of the interaction e†ect of polyDXL withthe solvents is not known, it is clear that it provokesextensive plasticizing e†ects at certain compositions ofthe binary mixture.

The inÑuence of this plasticizing e†ect on the per-meation characteristics of the membrane M2080 isshown in Fig. 7. As expected, there is a direct corre-lation between the swelling (Fig. 6) and the permeationbehaviour. A more swollen membrane results in ahigher Ñexibility of the polymer chains, and thus in adecrease in the energy required for transport throughthe membrane. However, the selectivity decreases, witha minimum value at the highest permeation rate.

Desorption experiments were performed on the samemembrane to explain why water is predominantly trans-ported over the entire range of feed concentrations inspite of the fact that the membrane is rather hydropho-bic (Fig. 8). It can be observed that the overall per-vaporation selectivity is only partially determined bythe sorption selectivity of the membrane. In fact, in the

Fig. 7. The permeation rate and selectivity a ofJ10 lm (=) (…)the membrane M2080 versus the composition of the feed

mixture (T \ 25¡C).

Fig. 8. Plot of the water content in the membrane(desorption) and in the permeate (pervaporation)(L) (·)versus the water content in the feed mixture for the membrane

M2080 (T \ 25¡C).

range 20È40% water content, the concentration ofwater in the membrane almost coincides with that inthe feed mixture, while the overall selectivity remainsrelatively high (Fig. 7). It can therefore be concludedthat the selectivity is determined to a great extent by theunequal di†usion rates of both solvents through theactive layer of the operating membrane.29 In otherwords, both upstream sorption and downstream di†u-sion play a role in the preferential transport of watermolecules.

A more quantitative description of the actual per-meation of this binary liquid mixture system and of thethermodynamic interactions between the penetrants andthe membrane can be given by the calculation of thedeviation coefficients e, introduced by Drioli et al.29 :

ei\Ji

Ji0 É xi\ Ji

JiI(5)

where and are, respectively, the permeation ratesJi Ji0of component i (in mol m~2 h~1) during the permeationof the binary mixture and of the single component i, JiIis the ideal permeation rate of component i, i.e. in theabsence of coupling e†ects, and the mol fraction ofxicomponent i in the feed mixture. andJwater (Jw) Jethanol

were calculated as follows :(Je)

Jw \ Jtotyw

yw Mw ] (1 [ yw)Me(6)

and

Je \ Jtot1 [ yw

yw Mw ] (1 [ yw)Me(7)

in which represents the experimental measured totalJtotÑux (in kg m~2 h~1), the mol fraction of water in theywpermeate and M the molar mass (kg mol~1) of the sol-vents. Only if exceeds unity, do the interactionseibetween the membrane and the solvents accelerate thepermeation of a component i, compared with the idealpermeation situation in which the solubility and di†usi-vity of this component is independent from that of the



Fig. 9. Plot of deviation coefficients versus feed compositionin pervaporation of water/ethanol mixtures through the mem-

brane M2080 at 25¡C.

other one. The deviation coefficients for the membraneM2080 are drawn in Fig. 9 as a function of feed com-position. High values of are observed over theewaterentire composition range, indicating a large positivedeviation of the ideal permeation rate. The water con-centration in the feed mixture at the highest value for

corresponds to the maximum in Figs 6 (Q) and 7ewater(J). Only for water mol fractions between 0É3 (15 wt%)and 0É6 (37 wt%), does exceed unity, whicheethanolmeans that the interactions between the membrane andthe permeants, or the formation of associated moleculesbetween both permeants, cause a mutual drag. Thisalso explains the lower selectivity in this region. Forall other feed mixtures, is smaller than unity,eethanolindicating that the interaction between ethanol and

TABLE 3. Temperature dependence of per-

vaporation characteristicsa of the membrane M1090

(feed : azeotropic mixture water/ethanol)

Temperature J10 lm

aEth.H2O J

10 lm(b É1)

(¡C) (kg mÉ2 hÉ1) (productivity)

25 0·30 16 2·7

40 0·87 9 5·2

50 1·59 7 7·9

a The meanings of and areJ10 lm

, aEth.H2O J

10 lm(b É1)

explained in the experimental part.

TABLE 4. Dependence of the pervaporation char-

acteristics of the membrane M2080 on the degree

of cross-linking (feed : azeotropic mixture,

T = 25ÄC)

TEGDA J10 lm

aEth.H2O

(wt% relative to polyMMA) (kg mÉ2 hÉ1)

0 460 13

5 320 16

10 220 21

the membrane hinders the permeation of ethanol andaccelerates the water transport. These observationsconÐrm the results obtained by the desorption experi-ments (Fig. 8).

Influence of temperature on the membranecharacteristics

Membrane M1090 was selected to investigate the inÑu-ence of the operating temperature on membrane per-formance (Table 3). The increase in permeability withtemperature is generally attributed to increasing freevolume and decreasing interactions between the per-meants and the membrane.

It is well known30 that in most cases the temperaturedependence of the permeation can be expressed by anArrhenius-type relationship :

J \ Ae~Ep@RT (8)

where denotes the activation energy of permeation,EpA the pre-exponential factor and T the operating tem-perature.

If the logarithms of the Ñuxes in Table 3 are plottedagainst the reciprocal of the absolute temperature, astraight line (correlation coefficient\ 0É99) is obtained(Ðgure not shown), the slope of which gives a value for

of 53 kJmol~1.EpThe selectivity a declines as temperature is increased,

whereas the membrane productivity, which takes intoaccount both the values of the Ñux and the selectivity,increases signiÐcantly.

Influence of the degree of cross-linking on themembrane properties

In an attempt to enhance the selectivity of the mem-branes, the degree of cross-linking of the membranematerial was increased by introducing an additionalcross-linking agent tetraethylene glycol diacrylate(TEGDA), during the radical copolymerization. Table 4shows the pervaporation results for the transport of anazeotropic mixture of water and ethanol throughM2080 obtained with di†erent amounts of TEGDA. Itis clear that the Ñux decreases and a increases, as thedegree of cross-linking increases (see also Fig. 5). This isrelated to an increasing limitation of swelling, whichprovokes mobility selectivity.

CONCLUSIONS

Segmented polymer networks, in which a bis-macromonomer of hydrophilic polyDXL acts as cross-linker of hydrophobic polyMMA chains, have beenused for the preparation of dense polymer membranes.The hydrophilicity and the cross-link density of the net-works were varied by changing the ratio of polyDXL to



polyMMA and the amount of a low molecular weightcross-linker, TEGDA, during the synthesis procedure. Itwas revealed by DMTA measurements that the pres-ence of covalent bonds between both components leadsto a much higher compatibility compared with the cor-responding polymer blends. When these networks areused for the dehydration of ethanol-rich mixtures by thepervaporation technique, this homogeneity, togetherwith the mechanical stability of the segmented networkstructure, lead to control of the permeability and theselectivity to water. A direct relation between the ofTgthe dry membrane and the pervaporation results wasobserved. Desorption experiments and the calculationof deviation coefficients demonstrated the presence ofinteractions between the permeants and polyDXL-containing membranes which accelerate the water trans-port over the whole composition range of thewater/ethanol mixture. Finally, the selectivity of themembranes could be enhanced considerably by increas-ing the degree of cross-linking.

ACKNOWLEDGEMENTS

F.E.D.P. thanks the F.W.O. (Fonds voor Wetenschap-pelijk Onderzoek-Vlaanderen) for Ðnancial support forpostdoctoral research at the University of MontpellierII. The Belgian Program on Interuniversity AttractionPoles initiated by the Belgian State, Prime MinisterÏsOffice, Science Policy Programming is acknowledgedfor Ðnancial support.

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Segmented network structures for the separation of water/ethanol mixtures by pervaporation