Pervaporation of water–dye, alcohol–dye, and water–alcohol mixtures using a polyphosphazene membrane

Journal of Membrane Science 197 (2002) 89–101

Pervaporation of water–dye, alcohol–dye, and water–alcoholmixtures using a polyphosphazene membrane

Christopher J. Ormea, Mason K. Harrupa, John D. McCoyb,Donald H. Weinkaufc, Frederick F. Stewarta,∗

a Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415-2208, USAb Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

c Department of Chemical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

Received 24 April 2001; received in revised form 10 August 2001; accepted 13 August 2001

Abstract

A novel phosphazene heteropolymer (HPP) was synthesized that contained three differing pendant groups: 2-(2-methoxye-thoxy)ethanol (MEE), 4-methoxyphenol, and 2-allylphenol. The resulting polymer is an amorphous elastomer with goodfilm forming properties where MEE and 4-methoxyphenol pendant groups influenced the hydrophilicity and the solventcompatibility of the polymer. Sorption studies were performed to characterize the polymer in terms of Hansen solubilityparameters. Additionally, group contributions were used to predict the Hansen parameters for the polymer and these datacompared favorably with the observed solubility behavior with 15 solvents that ranged from hydrocarbons to water. Ho-mopolymers synthesized from MEE and 4-methoxyphenol were also studied for solubility revealing different behaviors witheach representing a limit in hydrophilicity; MEE formed a water-soluble hydrophilic polymer and 4-methoxyphenol yieldeda hydrophobic polymer. Membranes formed from HPP were characterized for use as pervaporation membranse using fivedifferent feeds: water–dye, methanol–dye, 2-propanol–dye, water–2-propanol, and water–methanol. Fluxes of methanol andisopropanol were greater than for water. For the alcohol–water separations, the alcohol was the favored permeate in all caseswith higher fluxes observed for higher alcohol feed concentrations, however, separation factors declined. © 2002 ElsevierScience B.V. All rights reserved.

Keywords: Liquid permeability and separations; Pervaporation; Polyphosphazene; Polymer functionalization

1. Introduction

Polyphosphazenes are hybrid organic–inorganicmaterials that are comprised of an inorganic back-bone of phosphorus and nitrogen with alternatingdouble and single bonds. However, unlike similarlystructured organic materials, these polymers are notelectrically conductive. At each phosphorus, two

∗ Corresponding author. Tel.:+1-208-526-8594;fax: +1-208-526-8541.E-mail address: [email protected] (F.F. Stewart).

pendant group substituents provide an inherent diver-sity of physical and mechanical properties. Typicalsyntheses of these materials begins with the commer-cially available hexachlorocyclotriphosphazene (1),that undergoes ring opening polymerization to yieldpoly[bis(chloro)phosphazene] (2), see Fig. 1. Poly-mer 2 has a low glass transition temperature (Tg),−66◦C, testifying to a high degree of chain flexibility,however, this polymer is hydrolytically unstable. Thelability of the phosphorus–chlorine bonds can be ex-ploited through nucleophilic substitution to attach lesslabile groups that do not readily undergo hydrolysis,

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(01)00633-0

90 C.J. Orme et al. / Journal of Membrane Science 197 (2002) 89–101

Fig. 1. Synthesis of polymer HPP. The structure for HPP represents two of the six possible mers.

thus stabilizing the polymer. Common nucleophilesused to stabilize the phosphazene backbone includearyloxides, alkoxides, and amines [1].

Many pendant groups have been attached to thepolyphosphazene backbone yielding a variety ofphysical properties such that structure–function rela-tionships can be examined. Poly[bis-(2-(2-methoxye-thoxy)ethoxy)phosphazene] (MEEP), is a polymerthat possesses flexible polyether chains that im-part both a lowTg and water solubility [2]. Con-versely, poly[bis(phenoxy)phosphazene] (PPOP),is a semi-crystalline material with aTg measuredat −8◦C. Additionally, glassy polymers such aspoly[bis(anilino)phosphazene] with aTg measured at91◦C can be formed [1].

Several researchers have been interested in thedevelopment of phosphazene homopolymer mem-branes for liquid–liquid separations [3]. For ex-ample, Sun et al. probed the sorption and per-meation of several alcohols and ketones intopoly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP)and PPOP [4]. Further work by Sun et al. studied thepermeation of ethanol and water into PTFEP wherewater was the favored permeate [5]. In still anotherinstance, PTFEP was employed to selectively removemethanol from aqueous solutions using pervaporation.

However, in contrast to the ethanol result, methanolremoval was favored [6].

Other liquid separation applications for polyphos-phazene membranes have been explored somewhat.Work by Pintauro and co-workers [7–9] has shown thatsulfonated poly[bis(3-methylphenoxy)phosphazene]and poly[(3-methylphenoxy)(phenoxy)phosphazene]have the requisite characteristics to be employed asproton exchange fuel cell membranes where it is de-sirable to transport protons and reject water. In anunrelated study, tritium and hydrogen were separated(as the oxides) using hydrophobic PPOP and mixedsubstituent phenoxy/carboxylated phenoxy phosp-hazenes [10].

Blends of pendant groups have been shown toprovide a ready avenue for varying polymer physi-cal and chemical properties with a higher degree ofprecision than what is possible using homopolymers.4-Methoxyphenol and 2-(2-methoxyethoxy)ethanol(MEE) have been employed to form phosphazeneterpolymers [11]. By varying the levels of the hy-drophobic 4-methoxyphenol and the hydrophilicMEE, different properties were observed. Thermalanalysis revealed a singleTg for these terpolymersindicating a random substitution pattern along thephosphazene backbone. Additionally, these materials

C.J. Orme et al. / Journal of Membrane Science 197 (2002) 89–101 91

were amorphous and theTg measurement was adirect reflection of the pendant group speciation[12]. Likewise, other mixed pendant group poly-mers have been described in terms of MEE content.These include MEE/diacetone-d-glucofuranose andMEE/2,2,2-trifluoroethanol substituted materials thatexhibit hydrophilicity that directly correlates to theMEE content [12,13]. Furthermore, polyether andalkyl chains terminated with an amino functionalityhave been employed to create film forming mate-rials [14]. Use of mixed pendant group polyphosp-hazenes as gas separation membranes also has beenreported where the materials had a higher perme-ability for CO2 than the permanent gases [15,16].These studies suggest that solubility in these materialscan be adjusted through adroit selection of pendantgroups.

Complex heteropolymers that incorporate threependant groups, see Fig. 1, have been reportedwhere each group serves a distinct purpose withinthe polymer matrix [17]. MEE was used to in-crease the hydrophilicity of phosphazene polymerswhile addition of hydrophobic 4-methoxyphenolgave the materials good film forming propertiesand limited polymer gelation in certain solvents.2-Allylphenol was added into the matrix to pro-vide sites for crosslinking through free radicalprocesses [18]. Crosslinked heteropolymers haveshown a higher level of chemical resistance to ex-tremes in pH and higher levels of physical integritythan corresponding materials without crosslinking[19].

In a recent contribution from this group, we dis-cussed the gas transport behavior of these types ofpolymers where the polar MEE group was shownto have a solubility interaction with carbon dioxide[20]. Permeabilities were found to increase greatlywith increased MEE content in the polymer. In thiswork, we report our findings of the sorption char-acterization of the homopolymers, MEEP and poly-[bis(4-methoxyphenoxy)phosphazene] (PMEOPP),and a heteropolymer, of poly[(4-methoxyphe-noxy)0.96(MEE)0.96(2-allylphenoxy)0.08 phospha-zene] (HPP). Furthermore, HPP was employed asa pervaporation membrane material for the fol-lowing separations: water–dye, alcohol–dye, andwater–alcohol separations, where the alcohols aremethanol and 2-propanol.

2. Experimental

2.1. General

Hexachlorocyclotriphosphazene was obtained fromEsprit Chemicals and sublimed prior to use. MEE,4-Methoxyphenol, 2-allylphenol, benzoyl peroxide,pentane, octane, diethyl ether, tetraethylene glycol,2-(2-ethoxyethoxy)ethanol, 1,4-dioxane, and sodiumhydride (60% suspension in mineral oil) were ob-tained from Aldrich and used as received. Tetrahy-drofuran (THF), hexane, 2-propanol, ethylene glycol,chloroform, methylene chloride, and methanol werepurchased from Fisher and used as received. Tolueneused for synthesis was azeotropically distilled priorto use. NMR analyses were performed using a BrukerInstruments DMX300WB spectrometer operating at7.04 T field strength.31P NMR spectra were collectedat 131 MHz and referenced to external H3PO4. 1HNMR data were collected in CDCl3 solvent using ex-ternal tetramethylsilane (TMS) as a reference (0 ppm)and a delay of 60 s was used between scans (16) onspectra used for integration to ensure total relaxation.Thermal analyses were determined using a TA Instru-ments Model 2910 DSC and TA Instruments Model2950 TGA. Laser light scattering was employed tomeasure polymer molecular weights using a WyattTechnologies Dawn-DSP system that uses polarizedlight having a wavelength of 633 nm and measuresscattered light intensities at 18 angles ranging from22.5 to 147◦.

2.2. Polymer synthesis

2.2.1. Synthesis and characterization of HPPPoly(dichloro)phosphazene ((PNCl2)n) was syn-

thesized from hexachlorocyclotriphosphazene (1)according to the method of Allcock et al. [21]. HPPwas synthesized using the following method wherethe three pendant groups were added sequentiallyto (PNCl2)n. A solution of (PNCl2)n (2) (32.9 g,0.28 mol) was made with toluene (200 ml) in a 2 lthree-neck flask equipped with a mechanical stirrer.In a separate flask, a solution of 2-allylphenoxidewas made from the addition of 3.4 g of sodium hy-dride (90 mmol) to a solution of 11.4 g (90 mmol) of2-allylphenol in 200 ml of toluene. The phenoxide so-lution was stirred at room temperature until the sodium


hydride was completely consumed, approximately30 min, after which it was added to the (PNCl2)n solu-tion and mechanically stirred at room temperature for2 h. Sodium hydride (5.7 g, 150 mmol) was added to asolution of 4-methoxyphenol (17.6 g, 150 mmol) and700 ml of THF and the resulting solution was stirredat 65◦C for 2 h to ensure complete consumption ofthe sodium hydride. This solution was then added tothe (PNCl2)n solution and stirred at 57◦C for 14 h.A fourth separate solution was prepared using 81.7 g(680 mmol) of MEE and 26.6 g (680 mmol) of sodiumhydride in 600 ml of THF. This solution was stirredfor 2 h prior to addition to the (PNCl2)n solution. Thefinal mixture was heated to 65◦C for 5.5 h with reac-tion completion verified by31P NMR spectroscopy.Isolation of the polymer was accomplished throughprecipitation of the mother liquor into hexanes wherea solid material was collected. This solid material wasdried and then dissolved in THF followed by pre-cipitation into water according to a literature method[22]. Upon final drying, 41.2 g of a light brown elas-tomer was isolated in 50% yield. HPP characterizationdata—1H NMR (CDCl3): δ (ppm) 7.3 (brs), 7.0 (brs),6.9 (brs), 6.5 (brs), 6.1 (brs), 4.9 (brs), 3.9 (brs), 3.5(brs) and 3.3 (brs). Integrated1H NMR: MEE 48%,4-methoxyphenol 48%, and 2-allylphenol 4%.31PNMR (CDCl3): δ (ppm) −8, −12, −13, −18; DSCTg −43◦C; TGA Td 288◦C; Mw = (3.1±0.2)×106;polydispersity index(Mw/Mn) = 3.09± 0.49.

2.2.2. Synthesis and characterization of PMEOPPPoly(dichloro)phosphazene (5.0 g, 43.1 mmol) in a

mixture of anhydrous tetrahydrofuran (THF) (100 ml)and anhydrous toluene (50 ml) was added to a pre-viously prepared solution of 4-methoxyphenoxide inanhydrous tetrahydrofuran. The phenoxide solutionwas prepared by reacting 4-methoxyphenol (14.3 g,115 mmol) with sodium hydride (5.8 g, 145 mmol) in200 ml of THF. To the polymer-phenoxide solutionwas added 5.0 g (18.8 mmol) of tetrabutylammoniumbromide. The resulting mixture was heated at 70◦Cfor 15 h. Using a Dean-Stark trap, 150 ml of THF wasremoved from the solution and replaced with 150 ml ofdiglyme and the temperature was increased to 110◦Cfor 12 h upon which the reaction was determined to becomplete using31P NMR spectroscopy. The polymerwas purified by precipitation into methanol followedby successive precipitations into water and hexane

from THF solution. This procedure yielded 7.0 g of anoff-white powder, PMEOPP, in 56% yield. PMEOPPcharacterization data—31P NMR (CDCl3): δ (ppm)−16.9 (singlet);Mw = (2.9±0.2)×106 g/mol; RMSradius= 124.5 ± 10.6 nm; DSCTg 17◦C.

2.2.3. Synthesis and characterization of MEEPThis polymer was prepared using a literature

method [22]. MEEP characterization data—1H NMR(D2O): δ (ppm) 3.3 (3H), 3.5 (2H), 3.7 (4H), 4.1(2H); 31P NMR: δ (ppm)−6.6. Anal. Calcd.: C, 42.4;H, 7.8; N, 4.9. Found: C, 42.5; H, 7.5; N, 4.8;Mw =(2.7±0.6)×107 g/mol; RMS radius= 79.5±0.4 nm;polydispersity index(Mw/Mn) = 1.42± 0.47; DSCT g = −84◦C.

2.3. Membrane formation

HPP membranes were prepared using a solutioncasting method. Casting solutions were made fromTHF with 2.5% (wt.%) HPP and 2% benzoyl perox-ide (w.r.t. HPP). These solutions were stirred at roomtemperature until all components were dissolved. Thesolutions were then centrifuged to remove any sus-pended particulate matter before use. Membranes wereformed by directly applying casting solutions to What-man Anopore® ceramic membranes (47 mm diameterwith 0.2�m pore size). The THF solvent was allowedto evaporate leaving transparent and defect free thindense films as discrete layers on the ceramic mem-brane. Defect-free films were most easily obtained byslowing the evaporation rate of the THF solvent suchthat bubbles did not appear. Crosslinking was accom-plished by heating at 130◦C for a minimum of 10 min.The crosslinking process did not result in any physi-cally observable changes to either the HPP film or theceramic support.

Morphology of these membranes is best describedas the polymer forming a distinct polymer layer with-out significant penetration of the polymer into thepores of the ceramic support. Thicknesses of the thinfilms were determined using a Mitutoyo micrometerand were between 3 and 15�m.

2.4. Pervaporation experiments

The membranes were loaded into 47 mm filtrationcells obtained from Millipore, Inc. that were modi-


fied for pervaporation experiments with feed entranceand exit ports perpendicular to the circular membranesurface. Feed solution flow rate over the membranewas approximately 50 ml/min. A diaphragm vacuumpump provided the transmembrane pressure differen-tial. Permeate side pressures for all experiments arelisted in Table 3. Feed reservoirs were open to ambientpressure during all experiments. Temperature controlwas provided by using a constant temperature waterbath to heat the feed solution. Temperature of themembrane was monitored using a calibrated thermo-couple attached to the membrane cell. Permeates werecollected cryogenically and quantified gravimetricallyover 7–9 h per experiment. Each flux measurementrepresents four to seven replications unless otherwiseindicated. Transmembrane fluxes are reported in termsof kg/m2 h, where ‘kg’ denotes mass of permeate, ‘m2’the membrane area (0.0017 m2), and ‘h’ denotes ex-perimental time. Determination of alcohol concentra-tions in feeds and permeate was performed using gaschromatography. From these data, separation factors(α) were calculated from the following expression:

α = yalc/yw

xalc/xw(1)

whereyalc and xalc are the weight fractions of alco-hol in the permeate and feed, respectively. Likewise,yw and xw are the weight fractions of water in thepermeate and feed, respectively.

2.5. Sorption determinations

Samples of bulk HPP were crosslinked through20 Mrad electron beam irradiation. Determination ofHPP swelling was performed through immersion ofpre-weighed samples of crosslinked polymer for aminimum of 1 week. The degree of swelling (%) wascalculated by

degree of swelling= Ws − Wd

Wd× 100 (2)

whereWd andWs are the weights of the dry and fullyswollen polymer samples, respectively. After weigh-ing, the swollen polymer samples were placed in avacuum chamber until constant mass was obtained.In all cases, the polymer samples exhibited no appre-ciable solubility in any of the solvents; no significantmass loss was measured from the initial dry mass tothe final dry mass.

Samples of MEEP and PMEOPP were tested with-out crosslinking where a solvent/non-solvent determi-nation was made. A solvent determination indicatedcomplete dissolution of the immersed polymer sam-ple. Immersion time was 1 week.

3. Results and discussion

3.1. Polymer sorption

Sorption of 15 different solvents into HPP wasstudied through immersion. Due to the potential forexcessive gelation, crosslinked samples were em-ployed for these tests. HPP was crosslinked using anelectron beam at a dose of 20 Mrad which has beenpreviously shown to impart a moderate degree ofcrosslinking [18]. Greatest degrees of swelling weremeasured for the chlorinated organics (CHCl3 andCH2Cl2), cyclic ethers (THF and 1,4-dioxane), andtoluene. Conversely, hydrocarbons such as pentane,octane, and cyclohexane exhibited the least swelling.Crosslinking of the HPP samples allowed for relativedetermination of swelling without dissolution. For ex-ample, HPP prior to crosslinking was fully soluble inTHF. After crosslinking, immersion of the polymer inTHF formed a gel with a degree of swelling of 116%(w.r.t. desorbed polymer mass). This method wasemployed for 15 solvents whose Hansen solubilityparameters were identified [23]. Solvents were cho-sen such that a variety of Hansen parameters wouldbe represented.

3.2. Hansen parameter analysis

Solubility parameters are an attempt to quantify thechemist’s “rule of thumb” that “like dissolves like”.These parameters are envisioned to follow the rule thatstates if the parameters of two different materials aresufficiently close, then they should be mutually solu-ble. In the simplest case of the Hildebrand solubilityparameters, only a single number,δH, is used. Its valueis simply found for low molecular weight liquids fromthe heat of vaporization,�H̄v and the molar volume,V̄ :

δ2H = �H̄v − RT

V̄(3)


whereR is the gas constant andT is the (absolute)temperature.

Since polymers do not have a�H̄v, their δH areusually determined by using solvent/non-solvent in-formation. For non-polar solvents and polymers, thisapproach works well. If the solvents are arranged inorder of their solubility parameters, it will be seen thatat a particular value of the solventδH, call it δH (low),the polymer becomes soluble and that above anotherδH, call it δH (high), the polymer becomes insolubleagain. TheδH of the polymer is taken to be the mid-point ofδH (low) andδH (high). This crude evaluationcan be made more precise through swelling experi-ments on lightly crosslinked polymers.

In order for the above picture to be correct, thesolvents must cluster about a particularδH with thenon-solvents segregated on either wing. Once polarsolvents, such as water and alcohols, and polar poly-mers, such as HPP, are included, a singleδ cannotdescribe the solubility behavior. The Hansen param-eters address this by having the solubility behaviorbe encompassed by three parameters:δd, the disper-sion component;δP, the polar component; andδh, the

Fig. 2. Hansen solubility parameter plot for polymer HPP ((�) good solvent; (�) moderate solvent; (�) poor solvent; (�) groupcontribution prediction for HPP).

hydrogen-bonding component. The total cohesion pa-rameter,δt, is given by

δ2t = δ2

d + δ2p + δ2

h (4)

andδt should be identical toδH.Graphical representations of behaviors correspond-

ing with Hansen parameters are properly defined in athree-dimensional Cartesian coordinate space. Due tothis limitation, others [24] have usedδv to represent aparameter:

δv = (δ2d + δ2

p)0.5 (5)

The dispersion and polar components are combined,thus allowing for use of a two-dimensionalX–Y coor-dinate plot. Fig. 2 is a plot ofδh vs.δv for 15 solventsthat range from water to light paraffins. This plot sug-gests that there is a narrowly defined region of “good”solubility behavior. In practical terms, “good” solventcharacter for the 20 Mrad electron beam crosslinkedHPP was defined as swelling in solvent in excessof 50%. Likewise, “moderate” solubility behaviorwas characterized by swelling between 10 and 50%.Less than 10% sorption of solvent was categorized as


Table 1Hansen parameters and molar volumes calculated from group contributions

V (cm3/mol) δd (MPa)0.5 δp (MPa)0.5 δh (MPa)0.5

[PN(MEE)2] 230.6 20.6 11.2 8.3[PN(4-methoxyphenoxide)2] 204.6 19.6 5.8 5.0[PN(2-allylphenoxide)2] 263.4 18.8 4.6 4.6HPP calculated 219.4 20.0 8.3 6.6

“poor”. Immersion of HPP in water, methanol, and2-propanol gave degrees of swelling of 37, 47, and25%, respectively.

If the various solubility parameters are viewed asadditive, then knowing the backbone structure, onecan predict the solvent/non-solvent behavior of newpolymers. Of course, this is a crude tool and the valuesof each group contribution [25] need to be updated asnew experimental results become available. From thework of Hansen and Beerbower one has

V =∑

z

zV (6)

δd =∑

zzFd

V(7)

δp =(∑

zzF 2

p

)1/2

V(8)

δh =(∑

z − zUh

V

)1/2

(9)

where the sum overz is a sum over all groups. For thisanalysis,V was the group molar volume. Additionally,zFd, zFp, and−zUh are the group contributions to the

Table 2Group contribution parameters for the phosphazene polymers [25]

V (cm3/mol) zFd (MPa)0.5 zFp (MPa)0.5 −zUh (MPa)0.5

Phosphorus 8.8a 164a 0a 0a

Nitrogen 4.0 164 820a 1759Aliphatic methylene 16.6 270 0 0Aromatic etherial oxygen 3.8 100 401 1467Non-aromatic etherial oxygen 3.6 235 409 2352Methyl group 31.7 419 0 0Bis-substituted phenyl group 60.4 1319 133 205sp2 methyne 12.4 223 70 143sp2 methylene 32.1 403 94 143

a Estimated in this work.

dispersion, polar, and hydrogen bonding parameters,respectively. Application of this method to the threemers, [PN(MEE)2], [PN(4-methoxyphenoxide)2], and[PN(2-allylphenoxide)2] yielded estimated parametersrepresentative of HPP, see Table 1, using the molarfraction of each pendant group within the polymermatrix. Speciation of MEE, 4-methoxyphenoxide, and2-allylphenoxide was determined from integration ofthe 1H NMR spectrum. It should be noted that HPPis a heteropolymer with six mers; the three indicatedabove and the three possible mixed pendant groupmers. However, in this analysis, simply accounting forthe molar fractions of each pendant group was suffi-cient to determine bulk polymer Hansen parameters.

Only the parameters for phosphorus were not avail-able from literature sources and had to be evaluated[26]. The group molar volume was estimated fromthe availableV values for oxygen, sulfur, and nitro-gen using periodicity. The location of phosphorus onthe backbone is a hindered site without lone pairs ofelectrons, thus the polar and hydrogen-bonding com-ponents were thought to be small. For this analysis,zFp, and−zUh were set to zero. Due to the somewhatsimilar hybridization between the nitrogen and phos-phorus in the backbone,zFd for phosphorus was set


to the value for nitrogen. Subsequently,zFp for nitro-gen was reduced to give better agreement with exper-imental solubility data [27] derived from PPOP, seeTable 2.

Fig. 3. Hansen solubility parameter plots for PMEOPP (top) and MEEP (bottom) ((�) solvent; (�) non-solvent; (�) group contributionprediction for each polymer).

To provide additional characterization of the effectof combining pendant groups onto the same poly-mer backbone, homopolymers formed from the MEEand 4-methoxyphenol, MEEP and PMEOPP, respec-


tively, were treated with the same 15 solvents used tocharacterize HPP. In this case, the materials were notcrosslinked, thus solubility behavior was categorizedsimply as solvent/non-solvent. PMEOPP was observedto have a narrow region of solvation, see Fig. 3, wheresolvents such as THF and chlorinated organics effec-tively solvated the polymer. In contrast to this, MEEPhad a much larger solubility range that included thesolvents that dissolved PMEOPP and also includedtoluene, diethyl ether, water, and alcohols. It was cu-rious to note that the two glycols studied, tetraethy-lene glycol and ethylene glycol, were found to benon-solvents for MEEP, although they do swell HPPsuggesting some interaction with MEE.

Determinations of Hansen parameters derivedfrom group contributions for each homopolymer arelisted in Table 1 and plotted in Fig. 3. The calcu-lated parameters for PMEOPP roughly correspondwith the experimental data. However, the signifi-cance of this correlation should be viewed in light ofthe small experimental data set. MEEP gives clearlydifferent parameters consistent with the empiricallyobserved solubility behavior. Specifically, the polarand hydrogen-bonding parameters are considerablyhigher reflecting the more polar nature of MEEP ascompared to hydrophobic PMEOPP.

Fig. 4. Scanning electron micrograph of a 3�m film of HPP solution cast onto a ceramic support. The image is a cross section preparedthrough cryogenic fracturing of the membrane. This membrane was used in a 14-day pervaporation experiment using water–dye as the feed.

3.3. Effect of variable temperature on water,methanol and 2-propanol pervaporation throughHPP membranes

Water–dye separations were employed to determinethe performance of HPP as a membrane material. Inthese experiments, the dye was green food coloring(a mixture of FD&C Blue 1 and FD&C Yellow 5 ina water/propylene glycol base). The presence of thedye assured that pervaporation was the sole mode oftransport. Fifteen replications were conducted with anaverage flux (J) of 0.24± 0.05 kg/m2 h at a constanttemperature of 49◦C. This data compares favorablywith literature values [18]. It should be noted thatthese data were collected with a percent rejection of>99% where no dye was detected in the permeate. Notrend towards decreasing flux over time was observedsuggesting that there was no appreciable creep of thepolymer into the pores of the support. Microscopic ex-amination after the experiment showed no significantphysical stresses on the thin dense HPP film suggest-ing that good mechanical properties are maintainedduring use. An SEM analysis of the membrane crosssection after completion of pervaporation experimentsshows no creep of the polymer into the pores of theceramic support, see Fig. 4.


Table 3Pervaporation performance of a series of variable temperature solvent–dye separations

Permeate Temperature (◦C) Permeate pressure (mmHg) Flux (kg/m2 h) Dye rejection (%)

Water 28 240 0.03± 0.01 >99Water 32 240 0.07± 0.01 >99Water 40 240 0.24± 0.02 >99Water 45 240 0.40± 0.06 >99Methanol 25 90 1.26± 0.02 >99Methanol 29 90 2.89± 0.06 >99Methanol 39 90 4.68± 0.14 >99Methanol 47 90 6.74± 0.09 >992-Propanol 25 240 0.23± 0.02 >992-Propanol 35 240 0.41± 0.04 >992-Propanol 41 240 0.73± 0.01 >992-Propanol 44 240 1.19± 0.05 >992-Propanol 46 240 1.44± 0.03 >992-Propanol 54 240 4.68± 0.14 >99

Water–dye pervaporation experiments were con-ducted where the temperature was varied between28 and 45◦C, see Table 3. Fluxes of water increasedwith increasing temperature. HPP membranes alsowere characterized for pure alcohol (no water) fluxesas solutions spiked with dye. Fluxes for 2-propanolwere observed to be an order of magnitude higher

Fig. 5. Arrhenius plot of the water–dye separation ((�) R = 0.994), methanol–dye ((�) R = 0.948), and 2-propanol ((�) R = 0.971).

than water flux at comparable temperatures. Interest-ingly, methanol fluxes were a factor of 5 higher thanthose observed for 2-propanol.

An Arrhenius analysis (InJ versus 1/T) of thesewater fluxes revealed good linear behavior and anactivation energy of transport (Ej ) [28] of 53 kJ/mol,see Fig. 5. A comparison was then performed with


Fig. 6. Water–2-propanol separation performed at 49◦C ((�) separation factor; (�) flux (kg/m2 h)).

Fig. 7. Water–methanol separation performed at 49◦C ((�) separation factor; (�) flux (kg/m2 h)).


the two alcohols, which also revealed linear behav-ior with Ej values measured at 34 and 20 kJ/mol for2-propanol and methanol, respectively. These datasuggest a higher level of temperature susceptibilityfor the less volatile permeate in the temperature rangestudied.

3.4. Pervaporation of alcohol–water mixtures at49 ◦C through HPP membranes

Performance of HPP as a pervaporation mem-brane was assessed using varying compositions of2-propanol and water as the feed, see Fig. 6. Theflux domain is dominated by two general character-istics. At weight percentages in excess of 40%, highfluxes (>1.5 kg/m2 h) were obtained with separationfactors (α) between 2 and 4 favoring 2-propanol. At2-propanol concentrations of less than 40%, fluxes falloff dramatically, however, separation factors favoringthe alcohol increase.

A study of methanol–water pervaporation did notgive the sharp contrast observed for the 2-propanolexperiment, Fig. 7. The trend of higher fluxes athigher feed concentrations of methanol was observed,however. At feed concentrations of higher than 70%methanol, the fluxes were in excess of 2 kg/m2 h withpoor separation factors. At lower concentrations ofmethanol, the fluxes dropped off in a near linearfashion. Correspondingly, the separation factor alsoincreased in a near linear fashion.

4. Conclusion

Pendant group selection plays an important role information of membranes using phosphazene polymers.In this instance, the hydrophilicity of HPP was indi-cated through sorption and pervaporation experiments.Effective and dimensionally stable membranes wereformed through a solution casting and free radicalcrosslinking process using MEE, 4-methoxyphenol,and 2-allylphenol as pendant groups. As homopoly-mers, 4-methoxyphenol forms poor membranes dueto excessive brittleness. Additionally, MEEP is asemi-fluid polymer that requires stabilization to formeffective membranes. However, inclusion of both pen-dant groups onto the polymer forms an amorphouselastomer with good film forming properties. Inclu-

sion 2-allylphenol in small quantities into the polymermatrix allows for facile crosslinking.

Sorption of polar solvents, as demonstrated througha Hansen parameter analysis, was proposed to oc-cur readily due to the polar hydrophilic nature ofthe MEE pendant groups attached to the polymerbackbone. The interactions were probed through per-vaporation of alcohol–dye and water–dye solutionswhere the general trend showed higher fluxes with in-creasing temperature. Further experiments employingalcohol–water mixtures as the feed showed modestseparation factors favoring the alcohols. The Hansenanalysis provided an effective tool to characterizethe effect of blending pendant groups with differingsolvent compatibility on the same polymer backbone.Additionally, this tool may be used to determinethe applicability of phosphazene heteropolymers tospecific separations and could be used to design thependant group distribution.

Acknowledgements

The work described in this paper was supportedby the United States Department of Energy throughContract DE-AC07-99ID13727. The authors thankCatherine Rae of the INEEL for her assistance inthe collection of the gas chromatography data. Addi-tionally, the authors thank Dr. Daniel Goodman andDr. Catherine Byrne (Science Research Laboratory,Somerville, MA) for their assistance with the electronbeam crosslinking.

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Documents

Pervaporation of water–dye, alcohol–dye, and water–alcohol mixtures using a polyphosphazene membrane