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  • ed:

    a,

    b Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa

    a r t i c l e i n f o

    Article history: Received 6 July 2012 Received in revised form 9 February 2013 Accepted 19 March 2013 Available online 27 March 2013

    Keywords:

    ergy conservation and reduction of emission of the environmental pollutants [3]. The polymeric membran es are widely used in air separation, natural gas purication, petrochemical processing, medical, isolation of CO 2 from power plants and chemical indus- tries [46]. The selection of proper membrane for different applica- tions is an important task to become technologic ally superior and minimize global warming .

    PIs for gas transport is a serious disadvantag e for such applicati ons [10,11]. To overcome the low gas permeabilit y, Lai et al. [12] andOkamoto et al. [13] incorporate d exible segments such as silox- ane and ether linkages.

    Siloxane polymers especially have a much higher permeabilit ythan that of other rubbery materials [14]. Though, it has very poor separation ability for small gas molecules that restrict its applica- tion in gas separation studies [15,16]. On the other hand transport propertie s can be tailored using polyurethan e (PU) materials by varying the polymer microstructure. The studies of gas transport

    Corresponding author. Tel.: +91 044 2440 4427; fax: +91 044 2491 1589.

    Separation and Purication Technology 111 (2013) 108118

    Contents lists available at

    i

    .e lE-mail address: induchem2000@yahoo.com (B.S.R. Reddy).because of the loss of uniformity and exibility of the hybrid membranes. Crown Copyright 2013 Published by Elsevier B.V. All rights reserved.

    1. Introductio n

    Gas separation through polymeric membranes are considered to be an effective tool for the separation of gaseous mixtures due to high separation efciency, low running cost and simple operat- ing procedures compared to conventional separation methods [1,2]. The gas separation techniqu e has recently become a one of the alternative technique to other usual methods because of its en-

    Polyimide (PI) membranes are high-performance polymers with excellent thermal stability, chemical resistance and mechanical property which nd numerous applications in aerospace, micro- electroni cs and membrane technologie s [7,8]. PIs and related poly- mers have generally rigid-cha in structure s resulting in lower the gas permeabilit y [9]. The rigidity of the polymer chain reduces the segmental motion and plays a role in being a good barrier against gas transport properties. However, low permeab ility of POSSMembranesFluorinated imide Structure-gas transport property 1383-5866/$ - see front matter Crown Copyright 2http://dx.doi.org/10.1016/j.seppur.2013.03.035a b s t r a c t

    The purpose of this work is to study the gas permeation rates of O2, N2 and CO 2 gases and selectivity of O2/N2and CO 2/N2 using synthesized uorinated poly(urethane-imide) polyhedral oligomeric silsesquioxa ne (FPUI-POSS). FPUI-POSS membranes having different amount s of uorinated imide were synthesized viasimple condensation reaction of isocyanate terminated prepolyurethane (PU) and anhydride terminated uorinated prepolyimide (FPI). All the membranes were characterized for structural details [attenuated total reection Fourier transform infrared spectroscopy (ATR-FTIR)], thermal stability [thermogravimet ric analysis (TGA)], surface morpholog y and porosity [scanning electron microscopy (SEM), transmission elec- tron microscopy (TEM), atomic force microscopy (AFM)], mech anical strength [dynamic mechanical anal- ysis (DMA)], and polarity (contact angle). The density and the fractional free volume (FFV) were determined to study and to correlate the structure-gas transport properties of these membranes. From the surface mor- phology studies, root mean square (RMS) surface roughness value of higher percentag e of uorinated mem- brane (FPUI-30-POSS) showed 48 nm compare to the other membranes. From the dynamic mechanical analysis (DMA), storage modulus decreases with increase in the imide content. Thus, DMA of the membrane with higher imide content (FPUI-30-POSS) shows lower storage modulu s due to decrease in the urethane crosslink density. Higher imi de content membrane has lowest density of 1.02 g/cm 3 and resulting in higher free volume due to the hindrance in the chain packing of rigid AC(CF3)2A groups. There is a strong relation- ship between fractional free volume and the gas permeabili ty. Both FFV and gas permeability can also be further corr elated with density. It was concluded that the uorinated imide content increased in the poly- meric membranes simultaneously increases the surface roughness and thereby lowering the density a Industrial Chemistry Laboratory, Central Leather Research Institute (Council of Scientic & Industrial Research), Chennai 600 020, India Polyhedral oligomeric silsesquioxane-baspoly(urethane-imide) hybrid membranesand gas-transport properties

    D. Gnanasekaran a,b, P. Ajit Walter a, A. Asha Parveen

    Sepa ration and Pur

    journal homepage: www013 Published by Elsevier B.V. All uoroimide-containingSynthesis, characterization

    B.S.R. Reddy a,

    SciVerse ScienceDi rect

    cation Techn ology

    sevier .com/locate /seppurrights reserved.

  • uriproperties of the PU-based membranes have shown that the pro- portion of hard and soft segments inuence the permeation prop- erties of the membranes [1719].

    The polymers such as polyimide, poly(amide-imide) and poly(-ether-imide ) have been found to be more successful for gas-sepa- ration. The incorporation of polyhedr al oligomeric silsesquioxane (POSS) macromer into the polyurethan e membrane was found to improve the permeability of gas transport was reported by Madh- avan and Reddy [20]. The modication of llers and matrices has become an expanding eld of research since the introduct ion of functional groups can improve dispersion of llers and improve the chemical afnities of penetrants in the membranes . There is much scope for research and innovation to develop polymerinor- ganic nanocompo site membranes for gas separation. Many organ- icinorganic nanocom posite membranes showed much higher gas permeabilit ies but similar or even improved gas selectivities com- pared to the correspond ing pure polymer membranes [2124].

    Nomencla ture

    P permeabi lity coefcient J steady state uxDp pressure difference d thicknes s of the membr anes A membr ane area T temperat ure Pa atmospher ic pressure aA/B selectivi ty of gas A and BcLV interfacial tension at liquid/air interface cdL dispersio n factor of membrane for a liquid cdS dispersio n factor of membrane for a solid cpL polar factor of a liquid cpS polar factor of a sample cSV total surface energy of membrane h contact angle between the sample and liquid/air inter-

    face qlm density of the lmmair weight of polymer in air

    D. Gnanasekaran et al. / Separation and PMoore and Koros [25] have summari zed the relationship between organicinorganic membrane morphologies and transport properties.

    The objective of our current work is to synthesize and study the structure of poly(urethane-imide) POSS by incorporating different proportions of uorinated prepolyim ide (FPI) to improve the selec- tivity without reduction in the thermal property . The study on the surface morphology about the extent of compatibility of the polar and non-polar groups in the network was carried out in order to dene the structureproperties relationship. We have introduced POSS and bulky A(C(CF3)2A groups into the hybrid membranes by chemically reacting functional groups of POSS molecule s and prepolyimide in order to maintain both selectivity and permeabil- ity with good thermal properties.

    2. Experimen tal

    2.1. Materials and methods

    Heptacycl opentyl tricycloheptas iloxane triol (Cy-POSS) was synthesized in our laboratory and the details were given in our previous publication [26]. Hexamethylene diisocyanate (HMDI,Merk, 95%), poly(dimethylsiloxane), bis(hydroxylalkyl) terminated (Mn = 5600) (PDMS, Aldrich, 99%) and dibutylti n dilaurate (DBTDL,Aldrich, 95%) were used as received. 4,4 0-(Hexauoroisopropylid- ene)dipthalic anhydrid e (Aldrich, 99%) was puried by sublimati on under vacuum and tetrahyd rofuran (THF, Rankem) was distilled using calcium hydride and sodium metal. All other chemicals were analytica l grade and used as received.

    The attenuated total reectance Fourier transform infrared (ATR-FTIR) spectra by PerkinElmer spectrophot ometer was used to analyze the chemical structure of the polymeric membranes .An average of 20 scans was performed for all samples at a resolu- tion of 2 cm 1.

    Contact angles measureme nts were carried out at room temper- ature by Sessile drop method. The surface free energy of PU and FPUI-POSS hybrid membran es were calculated by measuring con- tact angle measureme nts in double distilled water and n-heptadec-ane. The contact angle was measure d at ve different locations for all samples and the average was taken to obtain meaningful measure ments.

    The thermal stabilities of the polymers were determined using PerkinElmer TGA Q50-TA thermal analyzers by taking 10 mg of

    mliquid weight of polymer in liquid qliquid density of the liquid FFV fractional free volume V total specic volume of the polymer VO occupied volume of the polyme rATR-FTIR attenuated total reection Fourier transform infrared

    spectroscopy TGA thermograv imetric analysis SEM scanning electron microscopy TEM transmissi on electron microscop yAFM atomic force microscop yPI polyimid ePU polyurethan eFPI uorinated prepolyim ide POSS polyhedr al oligomeri c silsesquiox ane FPUI-POSS uorinated poly(urethane-imide) POSS PDMS poly(dimethyl silsesq uioxane)

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