application of ab-crosslinked polymers composed of styrene/isoprene–siloxane copolymers to...

Journal of Membrane Science 218 (2003) 211–218

Application of AB-crosslinked polymers composed ofstyrene/isoprene–siloxane copolymers to pervaporative

removal of volatile organic compounds from water

Wojciech Kujawskia,∗, Jochen Kerresb, Renata Roszakaa Faculty of Chemistry, N. Copernicus University, ul. Gagarina 7, 87-100 Torun, Poland

b Institut für Chemische Verfahrenstechnik, Universität Stuttgart, Böblinger Str. 72, D-7000 Stuttgart, Germany

Received 22 November 2002; received in revised form 25 March 2003; accepted 31 March 2003

Abstract

Separation and transport properties of AB membranes during pervaporation of water–methyl acetate (water–MeAc) andwater–methyltert-butyl ether (water–MTBE) mixtures were investigated. Membranes were prepared from AB-crosslinkedcopolymer composed of elastomeric oligo or poly H-polysiloxanes and glassy styrene/isoprene prepolymer. Results showedthat the properties depended on the siloxane content in the membrane. The pervaporation properties of AB membranes weremuch better than properties of the commercially available hydrophobic membranes. The percolation threshold of permeabilityand selectivity was observed for AB membranes containing 35–40 wt.% of siloxane.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: AB-crosslinked copolymers; Hydrophobic membranes; Pervaporation; Methyl acetate; Methyltert-butyl ether

1. Introduction

Pervaporation is a membrane-based process whichis used for the separation of liquid mixtures. In thistechnique, the liquid feed mixture is in contact withone side of a membrane, whereas permeate in vaporphase is continuously removed from the other side ofmembrane into the vacuum or sweeping gas[1–3].Pervaporation can be applied for the dehydration oforganics, for the extraction of organics from aqueoussolutions and/or for the separation of the componentsof non-aqueous mixtures[4–7].

Contamination of groundwater and soils withvolatile organic compounds (VOCs) is a problem at

∗ Corresponding author. Tel.:+48-56-611-43-15;fax: +48-56-654-24-77.E-mail address: [email protected] (W. Kujawski).

many industrial and government sites. The level ofvolatile organic compounds allowed in dischargedwastewater or drinking water is lowered every fewyears. Methyl acetate (MeAc) and methyltert-butylether (MTBE) are the examples of VOCs, which canbe found in the effluents from many chemical, phar-maceutical or petrochemical factories all over theworld. MTBE, widely used as an octane enhancer,has relatively high water solubility and vapor pres-sure. It was also proved that MTBE is very toxicand moreover it is also suspected for its carcinogenicproperties[8]. Recently, MTBE has been detected inlakes, reservoirs, and groundwater used as potablewater supplies in concentrations exceeding in somecases allowed levels for taste, odor and human health[9]. The most commonly used technologies for re-moving VOCs from water, like air stripping or carbonadsorption, either transfer the waste to another phase

0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0376-7388(03)00177-7

212 W. Kujawski et al. / Journal of Membrane Science 218 (2003) 211–218

(i.e. from water to air) or generate the secondarywastes (i.e. used carbon).

Pervaporation can be used for effectively removingVOCs from water concentrating them for economi-cal disposal or recycle/reuse using specially designedhydrophobic membranes. Through a hydrophobicmembrane, the VOCs usually permeate orders ofmagnitude faster than water, as a result of which theVOCs are highly concentrated. Membranes in theseapplications are rubbery polymers, such as polybutadi-ene, polyether copolymers and the most frequently—poly(dimethylsiloxane). However, siloxane has poormechanical and film-forming properties, so suchmembranes are improved by different techniqueslike block copolymerization, blending with zeolites,plasma polymerization or formation of interpenetrat-ing polymer networks[10–12].

The aim of this paper was to investigate the selectiveand transport properties of hydrophobic membranesprepared from the styrene/isoprene and siloxanecopolymers. The influence of the polymer compo-sition on the separation and transport properties ofaqueous MeAc and MTBE mixtures was investigated.

2. Experimental

2.1. Membrane preparation

Membranes were prepared from an AB-crosslinkedcopolymer composed of elastomeric oligo/polyH-polysiloxanes and glassy styrene/isoprene prepoly-

Fig. 1. The structure of the AB-crosslinked membrane.

mer. AB-crosslinked copolymers are polymers, whichare produced by mixing a polymer A containingcrosslinkable group a with a polymer B containingcrosslinkable group b, where the cross-linking re-action takes place between groups a and b (Fig. 1)[12,13]. Extent of morphological homogeneity of theresulting AB-crosslinked polymer is dependent on themiscibility in both the components. If both polymercomponents are miscible, homogeneous structuresare formed, while phase-separation takes place ifboth components are incompatible. The size of mi-crophases of phase-separated AB-crosslinked copoly-mers is dependent on the degree of crosslinking. Ifthe cross-linking density is high, small microphasesare formed (in nanometer range), while at lowcross-linking density large microphases are formed(in micrometer range). The synthesis of such polymersystem was described in the details elsewhere[13].Three sets of polymer films were used in these in-vestigations, with the styrene/isoprene prepolymer ofdifferent molecular weight and with the polysiloxanecontent changing from 25 to 90 wt.% of siloxane. Thesiloxane HSi 100 (Wacker-Chemie, Burghausen, Ger-many) with theMn molecular weight of 8510 g/moland SiH content of 1.74 mol% was used as an elas-tomeric component. Data of the composition of thestyrene/isoprene prepolymer are given inTable 1.

Membranes were formed from the copolymersolution according to the following procedure. Par-tially reacted hot copolymer solution was poured intoan alumina bowl and put in a vacuum oven. The sol-vent was evaporated at a temperature of 140◦C and

W. Kujawski et al. / Journal of Membrane Science 218 (2003) 211–218 213

Table 1Composition of styrene/isoprene prepolymers

Prepolymer Proportions of monomers incopolymer

Mw

(g/mol)

Styrene(mol%)

Isoprene(mol%)

Isoprene-3,4(mol%)

I 67.7 32.3 19.4 59743II 67.3 32.7 16.6 71791III 68.0 32.0 19.5 81144

at the pressure of 400 mbar. The residual solvent wasevaporated at 50 mbar vacuum. After that the formedmembrane was carefully pilled of and stored for an-other 48 h in the vacuum oven at 100◦C and 50 mbar.For simplicity of the further notation, membranes ob-tained from the prepolymer I will be denoted as AB-I,from the prepolymer II and III—AB-II and AB-III,respectively.

2.2. Pervaporation experiments

Pervaporation experiments were carried out in thelaboratory-scale pervaporation system presented inFig. 2. System was composed of a temperature con-trolled feed vessel, circulating pump, membrane testcell, cold fingers and vacuum pump. Feed solutionwas pumped to a membrane test cell with a membranearea of 19.62 cm2. Feed was circulated in the systemwith a flow rate of 2.24 l/min. Taking into accountthe fact that membranes were relatively thick, such

Fig. 2. Scheme of pervaporation setup.

flow rate was high enough to assume that the influ-ence of the boundary layers on the properties of theinvestigated membranes was negligibly small. Perme-ate was collected into cold fingers cooled by liquidnitrogen. During experiments the upstream pressurewas maintained at the atmospheric pressure, while thedownstream pressure was kept below 1 mbar by usinga vacuum pump. The pervaporation system was oper-ated at 40◦C. The permeation rates were determinedby weighing permeate collected over a given period oftime in the cold fingers. Composition of both the feedand permeate mixtures was determined by using gaschromatography. VARIAN 3300 gas chromatographequipped with PORAPAC Q packed column and athermal conductivity detector (TCD) was used. JMBSBORWIN software (Le Fontanil, France) was usedto the data acquisition and processing. Samples wereinjected by the direct on-column injection technique.Each sample was analyzed 3–5 times.

Performance properties of a given pervaporationmembrane were defined by the separation factorα

(Eq. (1)) and permeate fluxesJ.

αorg/water =(corg/cwater)permeate

(corg/cwater)feed(1)

where corg and cwater denote the weight fraction oforganic and water component, respectively.

Due to the fact that the prepared membranes wereof different thickness (in the range of 69–330�m), thedetermined permeate fluxes were normalized to the


Table 2Chosen physicochemical data of investigated solvents

Solvent Molecularweight (g/mol)

Molar volume(cm3/mol)

Solubility in water(g/100 g H2O)

Dielectricconstant (D)

Water 18 18.0 – 81.0MeAc 74 92.5 31.9 7.1MTBE 88 119.0 5.1 ≈3

thickness of 1�m, according to the following relation:

JN = Jd (2)

whereJN and d denote the normalized flux and themembrane thickness, respectively. Selectivities andpermeate fluxes were determined with the accuracyof ±9%.

2.3. Water–organic systems investigated

The performance parameters (i.e. selectivities andtransport properties) were determined for all investi-gated membranes in the contact with 2 wt.% MeAcaqueous solution and with 1 wt.% MTBE aqueous so-lution at 313 K (40◦C). Table 2summarizes the cho-sen physicochemical properties of solvents used in thiswork.

Fig. 3. Separation diagram of AB membranes in contact with 2 wt.% MeAc aqueous solution.

3. Results and discussion

3.1. Selectivity of AB copolymer membranes

Selectivity of investigated AB membranes in con-tact with water–MeAc and water–MTBE mixtures arepresented inFigs. 3 and 4. It is seen that for the ABmembranes with low content of the siloxane in thecopolymer, membranes are practically non-selectiveor even water-selective (i.e.αorg ≤ 1). With increas-ing amount of the siloxane in the copolymer, thethreshold in the content of organic component inpermeates is observed. In the case of water–MeAcmixture this threshold occurs at the siloxane contentof 35 wt.%, regardless the molecular weight of thestyrene/isoprene component (Fig. 3). However, in thecase of water–MTBE mixture there is an influence of


Fig. 4. Separation diagram of AB membranes in contact with 1 wt.% MTBE aqueous solution.

the molecular weight of styrene/isoprene componentof AB membranes on the threshold value (Fig. 4,Table 1). For the AB-II and AB-III membranes, theselectivity threshold occurs at 35 wt.% of the siloxanebut for the AB-I membrane (i.e. for the membraneprepared with the styrene/isoprene prepolymer of thelowest molecular weight) the selectivity thresholdoccurs at 40 wt.% of the siloxane. Regardless thekind of the water–organic mixture, for the siloxanecontent exceeding 70 wt.%, the maximum content ofthe organic component in the permeate is obtained(Figs. 3 and 4). Similar results were reported forthe siloxane–phosphazene copolymer membranes incontact with water–n-butanol mixture[14].

In general, all AB membranes are more selective incontact with water–MTBE mixture comparing to thewater–MeAc one.Table 3compares the selectivity ofAB membranes with the selectivity of PDMS mem-branes (PERVAP-1060), zeolite filled PDMS mem-brane (PERVAP-1070) and poly(ether-block-amide)membrane (PEBA-4033)[15,16]. It is seen that theseparation factorαorg for the AB-II membrane in con-tact with water–MTBE mixture is much higher thanfor typical organophilic membranes[15,16]. In thecase of water–MeAc membrane separation factorαorgis close to that found for zeolite filled PDMS mem-brane (PERVAP-1070)[16].

3.2. Permeability of AB copolymer membranes

Transport properties of a given membrane in con-tact with water–organic mixture depend on both thesolubilities of mixture components in the membraneand their diffusivities through the membrane[11,17].

The transport properties of AB copolymer mem-branes in contact with 2 wt.% MeAc aqueous solutionare presented inFig. 5. Membranes with low siloxanecontent (i.e.<40 wt.%) are practically impermeable tothe water–MeAc mixture. With increasing amount ofthe siloxane fluxes of both MeAc and water increase,but the increase of MeAc flux is more pronounced.The flux of MeAc increases over the whole investi-gated range of the siloxane content in AB membranes,

Table 3Comparison of selective properties of AB-II membrane and com-mercially available organophilic membranes

Membrane Water–MeAc(2 wt.%)αMeAc–water

Water–MTBE(1%)αMTBE–water

AB-II (70 wt.% siloxane) 300 3200PERVAP-1060 150 270PERVAP-1070 310 280PEBA 13 10

[15,16].


Fig. 5. Transport properties of AB membranes in contact with 2 wt.% MeAc aqueous solution.

Fig. 6. Transport properties of AB membranes in contact with 1 wt.% MTBE aqueous solution.


Table 4Comparison of transport properties of AB-II membrane and com-mercially available organophilic membranes

Membrane Normalized permeateflux of MeAc(kg �m m−2 h−1)

Normalized permeateflux of MTBE(kg �m m−2 h−1)

AB-II (70 wt.%siloxane)

12.0 36.0

PERVAP-1060 9.3 3.9PERVAP-1070 1.3 2.0PEBA 0.6 0.5

[15,16].

whereas the normalized flux of water reaches the con-stant value for the siloxane content in AB membranehigher than 60–70 wt.% (Fig. 5). It is worth noting thatthe normalized permeate fluxes, within the experimen-tal error, are practically independent on the molecularweight of the styrene/isoprene copolymer (Fig. 5).

Similar results were obtained with AB membranesin contact with water–MTBE mixture (Fig. 6). Ingeneral, the normalized permeate flux of MTBE ismuch higher than the normalized permeate flux ofMeAc. The ratio of molar fluxes of MTBE and MeActhrough AB membranes is practically independent onthe membrane composition and equal to 2.2 ± 0.3.The higher transport of MTBE molecules throughAB membranes is caused by the fact that MTBE isless polar than MeAc (Table 2) what should result inthe higher solubility and higher diffusivity of MTBEmolecules[11,17].

The water normalized permeate flux is much lowerthan the flux of organic component and is compara-ble for both the investigated water–organic mixtures(Figs. 5 and 6).

Table 4compares the transport properties of AB-II(70% siloxane) with transport properties of the com-mercially available hydrophobic membranes. It is seenthat the normalized permeate fluxes of organic com-ponents are much higher through the AB membranesthan through commercially available hydrophobicmembranes[15,16].

4. Conclusions

The pervaporation properties of AB membranessynthesized from rigid styrene/isoprene prepoly-mer and flexible polysiloxane were determined for

water–MeAc and water–MTBE mixtures. The per-vaporation properties of AB membranes are muchbetter than properties of the commercially availablehydrophobic membranes. The AB membranes withelastomeric continuous phase have potential appli-cations in the separation solvents from wastewaterand waste gas mixtures. Obtained results proved thatboth the permeability and selectivity of AB mem-branes depended on the siloxane content, showing thethreshold value at 35–40 wt.% of the siloxane.

The properties of AB membranes can be qualita-tively described on the basis of the morphology of thistype of membranes and in terms of percolation con-cept[12–14,18–22].

According to the previous works on AB copoly-mer systems[12–14], it can be stated that for the lowcontent of the siloxane in the membrane (<35 wt.%)the styrene/isoprene prepolymer forms the continuousmicrophase with the elastomeric siloxane microphasedispersed in it. In this region of the siloxane con-tent, membrane is non-selective or even water se-lective (i.e. separation factorα ≤ 1) and practicallynon-permeable for water–organic mixture (Figs. 2–5)[13,18]. In the region of 35–60 wt.% of the silox-ane content, the phase inversion of the microphasestakes place. In this region, both styrene/isoprene andsiloxane form the co-continuous morphology and thethreshold of selectivity and permeability is observed(Figs. 2–5). Eventually for the higher siloxane con-tent in the membrane (i.e. >60 wt.%), the elastomericpolysiloxane forms the continuous microphase withdispersed glassy styrene/isoprene domains in it[12–14,18].

The used siloxane prepolymers are liquids at roomtemperature. By adding the styrene–isoprene, a me-chanically stable thermoplastic rubber is synthesized.Of course, one could use commercial siloxane rub-bers. However, these siloxane rubbers contain fillers,like SiO2, whose influence to the pervaporation prop-erties is hardly to determine and to calculate. Thepreparation of the AB-crosslinked siloxane-containingcopolymers allows fine-tuning of the membrane mor-phology and, therefore, allows to determine the influ-ence of type and size of copolymer microstructure tothe separation properties. This knowledge allows forthe development of better separation membranes. Afurther benefit of the AB-crosslinked copolymers isthe possibility for addition of further functional groups


to the copolymer, if not all of the groups are con-sumed for crosslinking. This could lead to possibilityfor fine-tailoring of membrane separation propertiesfor distinct separation tasks.

The percolation concept is also often used to the de-scription of transport of permeates through membraneswith different rigid and flexible segments[19–22]. Ac-cording to the theoretical calculations, the percolationthreshold value is 0.15 for the 3D continuous systembut this is dependent on both dimensionality and themanner in which the two components are dispersed[19–22]. The threshold values found for the inves-tigated systems were in the range of 0.35–0.40. Asmentioned earlier, similar results were reported for thesiloxane–phosphazene copolymer membranes in con-tact with water–n-butanol mixture[14]. These thresh-old values are close to those calculated by Park et al.[19] and Park and Lee[20] for the permeation processthrough the re-normalized percolation lattice with thestep size equal to 1.5 and by Shiroishi et al.[22] forthe simulation of the redox centers and a charge hop-ping distancero between 1.25 and 1.5 nm. On this ba-sis, we can suggest that the organic compound can betransported also by hopping within a limited distanceto the separated siloxane phase even if the siloxanedoes not form yet a continuous phase[12–14,18–22].

Acknowledgements

Participation of M.Sc. Dorota Bogacz and M.Sc.Mariusz Augustyniak in the experimental part of thiswork is gratefully acknowledged.

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