Separation and Purification Technology 107 (2013) 54–60

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Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Poly(acrylamide-co-acrylic acid) hydrophilization of porous polypropylenemembrane for dehumidification

Sagar Roy, Chaudhery M. Hussain, Somenath Mitra ⇑Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA

a r t i c l e i n f o

Article history:Received 24 September 2012Received in revised form 11 December 2012Accepted 12 December 2012Available online 11 January 2013

Keywords:Water vapor removalPoly(acrylamide-co-acrylic acid)Hydrophilized composite membranePermeanceWater activity

1383-5866/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2012.12.014

⇑ Corresponding author. Tel.: +1 973 596 5611; faxE-mail address: mitra@njit.edu (S. Mitra).

a b s t r a c t

In this paper we report for the first time the development of poly(acrylamide-co-acrylic acid) (PAMAC)hydrophilized porous polypropylene composite membrane for dehumidification applications. The sorp-tion of water vapor on PAMAC functional groups was found to be quite high, and reached nearly a gramper gram of the membrane. This allowed high water vapor permeance and percent water removal thatreached 2 � 10�9 gm mol/cm2 min cm Hg and 37%, respectively.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The removal of water vapor from gas streams is important formany industrial operations such as the dehydration of naturaland flue gases, drying of compressed air and the storage of fruitsand vegetables [1–6]. In addition, dehumidification is an importantfunction in air conditioning, aviation and space flights [7]. Severalapproaches to water vapor removal have been explored, and theseinclude condensation by cooling below the dew point and the useof hygroscopic materials to remove moisture. While the former isenergy intensive, the latter requires regeneration steps and the ex-hausted hygroscopic materials are often discarded after absorbingwater vapor [8–10].

Selective water vapor transport through polymeric membranesis attractive because the instrumentation has small footprint, iseasy to operate and requires less energy [11–14]. Membrane baseddehumidification processes have used composite membranes con-sisting of a porous, non-selective support to provide mechanicalstrength and a micron thick selective layer to enhance selectivity[15–18]. Membranes based on cellulosic polymers, polyamide,polyimide, polyacrylonitrile, polyethylene, polypropylene, poly-sulfone, polydimethylsiloxane have been used in dehumidificationoperations [1,19–26]. Many of these membranes have their advan-tages and disadvantages. Polyacrylonitrile and polyimide haveshown high water vapor selectivity but low water permeability[23,24], polydimethylsiloxane exhibits very high water permeabil-

ll rights reserved.

: +1 973 596 3586.

ity with poor water vapor selectivity [26], cellulose acetate andsulfonated polyetheretherketon membranes have shown reason-able water vapor permeability with high selectivity [1,26], mem-branes prepared from sulfonated polyetheretherketon areexpensive, cellulose membranes are susceptible to water accumu-lation during extended use, and the use of LiBr liquid membranehave led to the formation of unstable membranes as the aqueousphase of liquid membrane evaporated, allowing feed gas mixtureto pass through the porous matrix [27]. Nafion is a popular mem-brane for dehydration applications due to its higher water perme-ability. Sorption of water swells the hydrophilic domains in theNafion membrane comprised of sulfonic acid groups, and providesa path for water diffusion [28]. However, the presence of hydro-phobic tetrafluoroethylene and perfluoro ethers do not help to ab-sorb any significant amount of water at low water vaporconcentration.

Polyacrylamide based highly hydrophilic polymers and copoly-mers have been used as sorbents that can absorb water many moretimes to their own mass [29–31]. Pervaporative dehydration withpolyacrylamide copolymer membrane has been studied [32,33].Poly(acrylamide-co-acrylic acid) (PAMAC), owing to the existenceof hydrophilic COOH and NH2 groups, has the capability to absorblarge amounts of water. Consequently, PAMAC has been used aselective media for water purification and drug delivery [34,35].However, during water absorption the polymer can lose itsmechanical strength and the prepared membrane may not standalone to hold the operating conditions of dehydration processes.

The objective of this research is to explore the use of polyacryl-amide in dehumidification by studying its sorption-permeation

Fig. 1. The schematic presentation of the water vapor removal from air stream.

S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60 55

behavior of water from vapor gas mixture using hollow fiber mem-brane modules.

2. Experimental

2.1. Chemicals, materials and membrane modules

Poly(acrylamide-co-acrylic acid), potassium dichromate(K2Cr2O7), sulphuric acid (95.7% purity, ACS reagent), acetone(99% purity) was obtained from Sigma–Aldrich (St. Louis, MO).High purity N2 (Air Gas, NJ) and deionized water were used in allexperiments.

A hollow fiber membrane module consisting of porous CelgardX-20 polypropylene (PP) hollow fibers (Celgard, Charlotte, NC) wasprepared in a 30 cm long stainless steel casement with ‘‘T’’ connec-tions at each end. The casement and hollow fiber strands weresealed using a fast drying epoxy resin (Resin Technology Group,LLC, S Easton, MA). This prevented intermixing of the lumen andpermeated contents. It also served as the inlet/outlet for the sam-ple and the permeate. The effective surface area of the module wascalculated 94.03 cm2.

2.2. Fabrication of composite membrane

Celgard X-20 PP hollow fibers are highly hydrophobic in nature.Therefore they require surface modification by hydrophilization ofthe porous PP prior to the coating of the dense, homogeneous PA-MAC layer. The modification of the inner surface of the hollow fi-bers was started with the wetting of the fiber pores with acetoneand excess acetone was removed by passing air at low flow ratefrom the lumen side. A chromic acid solution (at 60 �C) was pre-pared by mixing of K2Cr2O7, water and H2SO4 in a proportion of1:20:30 ratio which was circulated slowly through the bore ofthe fibers for 30 min using low flow pump. The hydrophilizedmembranes were then washed thoroughly with distilled water toensure the complete removal of chromic acid from the fibers andthe module.

The PAMAC solution was prepared by dissolving the polymer(3 wt%) in warm distilled water by overnight stirring. Then, thissolution was passed through the bore of hydrophilized membranewith low flow pump for the duration of 10 min. To remove the ex-cess solution from the membranes, air was flushed at low flow rate(with pressure of 5 psig) through the module. The module wasthen placed in an oven at 60 �C. The membrane was further an-nealed at 80 �C for 3 h.

The PAMAC modified composite membrane was characterizedusing scanning electron microscopy (SEM) (Leo 1530 VP, Carl ZeissSMT AG Company, Oberkochen, Germany). This was done by cut-ting the membranes into 0.5 cm long pieces and coating with car-bon films. Thermogravimetric analysis (TGA) was used toinvestigate the degradation of modified membrane materials dur-

ing heating. TGA was carried out using a Perkin–Elmer Pyris 7TGA system with a heating rate of 10 �C/min under air atmosphere.The glass transition temperature (Tg) was measured using a differ-ential scanning calorimetry (DSC) analyzer (model DSC822e, Met-tler Toledo, Switzerland). The temperature range for theseexperiments was 0–250 �C at a scanning rate of 10 �C/min. To ver-ify the hydrophilization, water particles were sprinkled on unmod-ified and modified membranes surfaces for contact angle study,which were mounted on a stage. The particle positions were re-corded using a digital video camera mounted at the top of thestage.

2.3. Dehumidification studies

Sorption studies were carried out using flat PAMAC membraneprepared from its aqueous solution by casting over a Teflon sheet.The dried pre weighed membrane was placed in a temperaturecontrol box. Water vapor at different water activity (aw), which isthe ratio of the vapor pressure of water in the system and the vaporpressure of pure water at the same temperature, was generatedfrom water sample and circulated with constant low flow nitrogen.The weight of the membrane was measured at different time inter-vals till saturation. Sorption experiments were also conducted at30, 40, and 50 �C.

The schematic diagram for the experimental system used forwater vapor removal is shown in Fig. 1. Dry air was passed througha water-filled impinger and was mixed with another dry air streamto deliver the water vapor at a pre-specified concentration. Theflow rates of the air lines were adjusted to obtain the desired watervapor concentration. The feed flow rates varied between 1 and5 mL/min and the water vapor concentrations was between 5000and 50,000 ppm. A vacuum pump was connected to the shell-sideof the hollow fiber module, and the water vapor was collected in acold trap immersed in liquid nitrogen. The hydrophilic membraneallowed only the water vapor passed through, which were col-lected into the cold trap and measured. The concentration of watervapor in the feed and collected samples was calculated.

3. Results and discussion

3.1. Characterization of membrane

Hydrogen bonding between water molecules and the mem-brane is shown in Fig. 2. The adsorption mechanism of these kindsof polymeric membranes can be primarily attributed to the attrac-tion force between polar water molecules and the functionalgroups in the polymer which are considered as the primary sites.Then, the pre-adsorbed water molecules can serve as secondarysites to adsorb more water molecules due to further hydrogen-bond attraction.

Fig. 2. Interaction of PAMAC membrane with water molecules through H-bonding.

56 S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60

Fig. 3a and b shows the SEM images of the surface of plain PPsubstrate and modified composite membrane respectively.Fig. 3b shows the formation of a defect free dense layer over thehydrophilized PP support. The cross-sectional SEM image of thecomposite membrane is shown in Fig. 3c.

The thermal stability of the composite membrane was studiedby thermogravimetric analysis (TGA). The TGA curve of the com-posite membrane is shown in Fig. 4a. It is clear from the figure thatthe membrane was quite stable at moderate temperature. The TGAshows that the composite membrane showed its first weight lossat 275 �C followed by a final decomposition at 400 �C, which maybe due to the formation of cross-linking network in the PAMACpolymer layer. Fig. 4b shows the DSC curves of the compositemembrane. A very high glass transition temperature of 211 �Cwas observed.

The change in hydrophobic-hydrophilic behavior of unmodifiedand modified PP support is shown in Fig. 5a and b. The photographof the water drop on unmodified PP membrane had a contact angle92.5� indicating a hydrophobic nature. On the other hand, afterhydrophilization with chromic acid solution, the contact angle re-duced to 61.7� which showed an increase in hydrophilicity, the PA-MAC coating over the modified PP support made the compositemembrane completely hydrophilic and the contact angle reducedfurther to 27.4� (Fig. 5c).

Fig. 3. SEM images of the surface (a) plain PP substrate, (b) modified PAMAC com

3.2. Sorption studies

The thermodynamic equilibrium between the water vapor andpolymer phase at the interface is an important factor. The abilityof a hydrophilic polymer to absorb water vapor depends on thespecific interaction between water molecules and polymer chains(as shown in Fig. 2). According to Flory–Huggins theory, the freeenergy of mixing water molecules and polymer chains is [36],

DGm ¼ kT½nw ln uw þ lnð1�uwÞ þ vnwð1�uwÞ� ð1Þ

where v is water–polymer interaction parameter and nw and uw arethe number of water molecules and the volume fraction of water insorbed polymer, respectively. The chemical potential of watermixed with the membrane matrix lsorbed membrane

w can be deducedfrom Eq. (1), and the basic requirement for the thermodynamicequilibrium is,

lsorbed membranew ¼ lvapor

w ð2Þ

where lvaporw is the chemical potential of water in the vapor phase.

Again, the activity of water (aw) is a measure of the energy statusof the water in a particular system and can be defined as;

aw ¼ff0

ð3Þ

where f is the fugacity or the escaping tendency of water in a sub-stance; and f0 is escaping tendency of pure water. For practical pur-poses, the fugacity is closely approximated by the vapor pressure(f � p) so;

aw ¼ff0� p

p0ð4Þ

where p and p0 are the vapor pressure of water in the system andthe vapor pressure of pure water at the same temperature, respec-tively. The activity of water in the membrane, aw, can be deducedfrom Eqs. (1) and (2), and expressed as,

ln aw ¼ ln uw þ lnð1�uwÞ þ vð1�uwÞ ð5Þ

posite membrane and (c) cross-section of the PAMAC composite membrane.

Fig. 4. (a) Thermogravimetric analysis (TGA) analysis of PAMAC composite membrane, (b) DSC curves of the composite membrane.

Fig. 5. The photograph of the water drop on: (a) unmodified PP membrane, (b) after hydrophilized PP support and (c) PAMAC coating over the modified PP support.

S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60 57

Equilibrium sorption of PAMAC copolymer membrane wasmeasured and plotted against activity of water as shown inFig. 6. The graph shows no significant change in sorption with tem-perature at particular water activity. This is significant for mem-brane application of PAMAC because overall permeability as afunction of temperature is less dependent on the partition coeffi-cient and more on diffusion coefficient; thus allowing higher tem-perature operation. It is clear from the figure that at low wateractivity, the amount of water absorb is relatively low and increaselinearly. However, the curve upturns significantly at higher wateractivities. The large degree of upturn at higher activities is possiblydue to the plasticization of the polymer by the absorbed watermolecules and clustering of water molecules into polymer matrix

[37,38]. The suitability of Flory–Huggins equation approach is sup-ported by the full lines in the graph which closely resembles withthe experimental values as shown in the figure.

3.3. Water vapor removal from N2 gas mixture

Permeation through a dense polymeric membrane usually oc-curs by solution-diffusion mechanism [39]. The process comprisesof the sorption of the solute molecules into the polymer surface onthe feed side, followed by the diffusion of the absorbed gas mole-cules through the polymer matrix, and finally the evaporation ofthe solute in the permeate side. The variation in permeability re-sults from the differences in solubility due to the specific

Fig. 6. Sorption isotherms of PAMAC copolymer at different temperatures. The fulllines represent the prediction from Flory–Huggins theory.

58 S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60

physico-chemical interactions, and the differential diffusivitythrough the polymer matrix. The efficiency of the whole processwas determined on the basis of the removal of water vapor fromthe gas stream, which could be expressed as, percent removal ofwater vapor (% R),

Removal ð% RÞ ¼ ðF iCi � F0C0ÞF iCi

� �� 100 ¼ FpCp

F iCi

� �� 100 ð6Þ

Fig. 7a and b. (a) The water vapor flux and (b) % removal of water vapor as afunction of water activity at 1.0 mL/min feed flow rate.

Fig. 7c and d. (c) The water vapor flux and (d) % removal of water vapor as afunction of feed flow rate.

where Fi, F0 and Fp are the volumetric flow rate at inlet, outlet andpermeate (in cm3/min) respectively, Ci, C0 and Cp are the molar con-centrations at inlet, outlet and permeate (gm mol/cm3) respectively.The rate of transport of water vapor molecules across the mem-brane area A (cm2) in time t (min) can be expressed in terms of fluxcan be expressed as,

Flux of water vapor ðJw gm mol cm�2 min�1Þ

¼ ðF iCi � F0C0ÞA � t

� �¼ FpCp

A � t

� �ð7Þ

Again, according to solution diffusion theory,

Jw ¼Q w

dmðpw;f � pw;pÞ

where pw,f and pw,p are the water vapor pressure at feed and perme-ate side, respectively.

As the concentration of water vapor in the permeate side is neg-ligible, the membrane permeability coefficient for water vapor(Qw) of membrane thickness (dm) can be described as,

Qw

dm¼ Jw

pw;fð8Þ

The water vapor removal was studied as a function of wateractivity at different temperatures and shows in Fig. 7a and b. Thewater vapor flux and the % removal of water vapor increased withthe increase in water activity in feed. This was attributed to the en-hanced driving force. It is clear from the figure that with increase intemperature, the flux and % removal increases at a particular con-centration. Typically, in a solution diffusion model, the solubility ofa component decreases with temperature while the diffusion

S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60 59

coefficient increases with temperature. In the temperature rangestudied, the increase in flux and water removal is attributed tothe increase in diffusion coefficient.

Fig. 7c shows the water vapor flux as a function of the inlet feedstream flow rate. It was observed from the figure that water vaporflux increased with the feed flow rate. However, the % removal ofwater vapor reduced with increase in feed flow rate due to lowerresidence time as shown in Fig. 7d. It is also observed from the fig-ures that at a given flow rate the flux and % removal of water in-creases with increase in temperature.

Fig. 9. Water vapor removal performances in terms of water vapor flux and % watervapor removal with time.

3.4. Effect of water activity and feed flow rate on water vaporpermeance

Water vapor permeance values (Qwater/dm) for the membranewere calculated from Eq. (8) and plotted against water activityand feed flow rate. It is observed from Fig. 8a that water vapor per-meance was dependent on water activity in feed and increasedslowly with an increase in water activity in feed mixture. However,the rate of increment of the water vapor permeance increased athigher water activity in feed. The swelling of the membrane allowsmore water to absorb into the polymer matrix, hence, higher per-meance of water vapor through the swollen membrane was ob-served. The permeance of water vapor has a significant effect onfeed flow rate. It is observed from Fig. 8b that with increase in feedflow rate water vapor permeance also increased. The permeance isalso dependent on temperature and increases with an increase infeed temperature. The water vapor permeability has been found109 barrer, which is comparable with other similar types of mem-branes, polyvinyl alcohol and polyamide 6 having permeability of19 and 275 barrer, respectively [40,24].

Fig. 8. Water vapor permeance as a function of (a) water activity and (b) feed flowrate (aw = 0.68) at different temperatures.

3.5. Mass transfer coefficient

According to solution diffusion theory, the flux of the compo-nent through the membrane can also be written as,

Jw ¼ kðpw;f � pw;pÞ

As the concentration of water vapor in the permeate side is neg-ligible, the overall mass transfer coefficient (k) can be described as,

k ¼ Jw

pw;fð9Þ

The overall mass transfer is controlled by diffusion through theboundary layer at low flow rates. With increase in flow rate turbu-lence increases that reduces the boundary layer at the membraneinterface. It was observed that that with increase in flow rate masstransfer coefficient increased significantly, from 2.7 � 10�7 m/s at0.5 mL/min to 2.0 � 10�6 m/s at 4.5 mL/min, at 25 �C and 0.68water activity. Whereas, a slight enhancement was observed withincrease in concentration. The overall mass transfer coefficient wasalso increased with increase in temperature as the rate of diffusionincrease.

3.6. Membrane stability

Water vapor removal performances in terms of water vapor fluxand % water vapor removal were plotted against time in Fig. 9. It isclear from the figure that the membrane was quite stable for longterm use.

4. Conclusions

The highly hydrophilic novel PAMAC modified composite mem-brane was synthesized by solution casting of PAMAC on hydroph-ilized porous polypropylene hollow fibers. The membrane wassuccessfully employed to remove water vapor from N2–water va-por mixtures. The high absorption of water vapor on the PAMACgroups led to high partition coefficients and allowed higher watervapor permeance and removal. The water vapor removal increasedwith increase in vapor concentration in feed mixture, and de-creased with flow rate. The PAMAC composite membrane demon-strated several advantages including enhanced recovery at lowconcentrations as low as 5000 ppm (18% RH).

References

[1] H. Sijbesma, K. Nymeijer, R. Marwijk, R. Heijboer, J. Potreck, M. Wessling, Fluegas dehydration using polymer membranes, J. Membr. Sci. 313 (2008) 263–276.

60 S. Roy et al. / Separation and Purification Technology 107 (2013) 54–60

[2] S.R. Reijerkerk, R. Jordana, K. Nijmeijer, M. Wessling, Highly hydrophilic,rubbery membranes for CO2 capture and dehydration of flue gas, Int. J.Greenhouse Gas Control 5 (1) (2011) 26–36.

[3] K.L. Wang, S.H. McCray, D.D. Newbold, E.L. Cussler, Hollow fiber air drying, J.Membr. Sci. 72 (3) (1992) 231–244.

[4] G.M. Li, C. Feng, J.F. Li, J.Z. Liu, Y.L. Wu, Water vapor permeation andcompressed air dehydration performances of modified polyimide membrane,Sep. Purif. Technol. 60 (2008) 330–334.

[5] E. Das�, G.C. Gürakan, A. Bayındırlı, Effect of controlled atmosphere storage,Effect of controlled atmosphere storage, modified atmosphere packaging andgaseous ozone treatment on the survival of Salmonella enteritidis on cherrytomatoes, Food Microbiol. 23 (2006) 430–438.

[6] R. Ahvenainen, New approaches in improving the shelf life of minimallyprocessed fruit and vegetables, Trends Food Sci. Technol. 7 (1996) 179–187.

[7] J. Haas, A. Sauterleute, System for dehumidification in air conditioners, UnitedStates Patent 7,017,365 March 28, 2006.

[8] L. Jia et al., An experimental study on vapor condensation of wet flue gas in aplastic heat exchanger, Heat Transfer-Asian Res. 30 (7) (2001) 571–580.

[9] Y.H. Zurigat, M.K. Abu-Arabi, S.A. Abdul-Wahab, Air dehumidification bytriethylene glycol desiccant in a packed column, Energy Convers. Manage. 45(1) (2004) 141–155.

[10] A. Ito, Dehumidification of air by a hygroscopic liquid membrane supported onsurface of a hydrophobic microporous membrane, J. Membr. Sci. 175 (1) (2000)35–42.

[11] L.Z. Zhang, Mass diffusion in a hydrophobic membrane humidification/dehumidification process: the effects of membrane characteristics, Sep. Sci.Technol. 41 (8) (2006) 1565–1582.

[12] K.L. Wang et al., Hollow fiber air drying, J. Membr. Sci. 72 (3) (1992) 231–244.[13] Z.G. Wang, T.L. Chen, J.P. Xu, Gas and water vapor transport through a series of

novel poly(aryl ether sulfone) membranes, Macromolecules 34 (26) (2001)9015–9022.

[14] N. Hengl, A. Mourgues, E. Pomier, M.P. Belleville, D. Paolucci-Jeanjean, J.Sanchez, G. Rios, Study of a new membrane evaporator with a hydrophobicmetallic membrane, J. Membr. Sci. 289 (1–2) (2007) 169–177.

[15] J.S. Cha, R. Li, K.K. Sirkar, Removal of water vapor and VOCs from nitrogen in ahydrophilic hollow fiber gel membrane permeator, J. Membr. Sci. 119 (1996)139–153.

[16] Q. Cheng, F. Pan, B. Chen, Z. Jiang, Preparation and dehumidificationperformance of composite membrane with PVA/gelatin–silica hybrid skinlayer, J. Membr. Sci. 363 (2010) 316–325.

[17] F. Pan, H. Jia, Z. Jiang, X. Zheng, J. Wang, L. Cui, P(AA-AMPS)–PVA/polysulfonecomposite hollow fiber membranes for propylene dehumidification, J. Membr.Sci. 323 (2008) 395–403.

[18] L.-Z. Zhang, Fabrication of a lithium chloride solution based compositesupported liquid membrane and its moisture permeation analysis, J. Membr.Sci. 276 (2006) 91–100.

[19] B. Bolto, M. Hoang, Z. Xie, A review of water recovery by vapour permeationthrough membranes, Water Res. 46 (2012) 259–266.

[20] L.-Z. Zhang, Y.-Y. Wang, C.-L. Wang, H. Xiang, Synthesis and characterization ofa PVA/LiCl blend membrane for air dehumidification, J. Membr. Sci. 308 (2008)198–206.

[21] X.-R. Zhang, L.-Z. Zhang, H.-M. Liu, L.-X. Pei, One-step fabrication and analysisof an asymmetric cellulose acetate membrane for heat and moisture recovery,J. Membr. Sci. 366 (2011) 158–165.

[22] J.R. Du, Li Liu, A. Chakma, X. Feng, Using poly(N,N-dimethylaminoethylmethacrylate)/polyacrylonitrile composite membranes for gas dehydrationand humidification, Chem. Eng. Sci. 65 (2010) 4672–4681.

[23] J.A. Barrie, in: Proceedings of the Fourth BOC Priestly Conference, 1986, pp. 89–113.

[24] S.M. Allen, M. Fujii, V. Stannett, H.B. Hopfenberg, J.L. Williams, The barrierproperties of polyacrylonitrile, J. Membr. Sci. 2 (1977) 153–164.

[25] L. Jia, X.F. Xu, H.J. Zhang, J.P. Xu, Permeation of nitrogen and water vaporthrough sulfonated polyetherethersulfone membrane, J. Polym. Sci., Polym.Phys. Ed. 35 (1997) 2133–2140.

[26] S.J. Metz, W.J.C. van de Ven, J. Potreck, M.H.V. Mulder, M. Wessling, Transportof water vapor and inert gas mixtures through highly selective and highlypermeable polymer membranes, J. Membr. Sci. 251 (1–2) (2005) 29–41.

[27] D.W. Deetz, Stabilized ultrathin liquid membranes for gas separations, ACSSymp. Ser. 347 (1987) (Chapter 11).

[28] Q. Zhao, P. Majsztrik, J. Benziger, Diffusion and interfacial transport of water inNafion, J. Phys. Chem. B 115 (2011) 2717–2727.

[29] J. Zhang, A. Li, A. Wang, Synthesis and characterization of multifunctionalpoly(acrylic acid-co-acrylamide)/sodium humate superabsorbent composite,React. Funct. Polym. 66 (2006) 747–756.

[30] F.S. Aggor, E.M. Ahmed, A.T. El-Aref, M.A. Asem, Synthesis and characterizationof poly(acrylamide-co-acrylic acid) hydrogel containing silver nanoparticlesfor antimicrobial applications, J. Am. Sci. 6 (12) (2010) 648–656.

[31] S. Agrawalt, A. Mishra, J.P. Rai, Synthesis, characterization and flocculationefficiency of poly(acrylamide-co-acrylic acid) in tannery waste-water, Iran.Polym. J. 10 (2001) 85–90.

[32] S. Ray, S.K. Ray, Dehydration of tetrahydrofuran (THF) by pervaporation usingcrosslinked copolymer membranes, Chem. Eng. Process.: ProcessIntensification 47 (2008) 1620–1630.

[33] S. Ray, S.K. Ray, Pervaporative dehydration of dimethyl formamide (DMF) bycrosslinked copolymer membranes, Ind. Eng. Chem. Res. 45 (2006) 7210–7218.

[34] E. Orozco-Guareño, F. Santiago-Gutiérrez, J.L. Morán-Quiroz, S.L. Hernandez-Olmos, V. Soto, W. Cruz, R. Manríquez, S. Gomez-Salazar, Removal of Cu(II)ions from aqueous streams using poly(acrylicacid-co-acrylamide) hydrogels, J.Colloid Interface Sci. 349 (2010) 583–593.

[35] C. Chang, B. Duan, J. Cai, L. Zhang, Superabsorbent hydrogels based on cellulosefor smart swelling and controllable delivery, Eur. Polym. J. 46 (2010) 92–100.

[36] P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca,New York, 1973.

[37] E.El. Shafee, H.F. Naguib, Water sorption in cross-linked poly(vinyl alcohol)networks, Polymer 44 (2003) 1647–1653.

[38] H.L. Flemming, C.S. Slater, Pervaporation-definition and background, in:W.S.W. Ho, K.K. Sirkar (Eds.), Membrane Handbook, VaNostrandReinhold,1992, p. 113.

[39] J.G. Wijmans, R.W. Baker, The solution-diffusion model: a review, J. Membr.Sci. 107 (1995) 1–21.

[40] J.A. Barrie, Water in polymers, in: J. Crank, G.S. Park (Eds.), Diffusion inPolymers, Academic Press, NY, 1968, pp. 259–314.