Polyhedral oligomeric silsesquioxane-based fluoroimide-containing poly(urethane-imide) hybrid membranes: Synthesis, characterization and gas-transport properties

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<ul><li><p>ed: </p><p> a,</p><p>b Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa </p><p>a r t i c l e i n f o</p><p>Article history: Received 6 July 2012 Received in revised form 9 February 2013 Accepted 19 March 2013 Available online 27 March 2013 </p><p>Keywords:</p><p>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 .</p><p>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. </p><p>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 </p><p> Corresponding author. Tel.: +91 044 2440 4427; fax: +91 044 2491 1589. </p><p>Separation and Purication Technology 111 (2013) 108118</p><p>Contents lists available at </p><p>i</p><p>.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. </p><p>1. Introductio n</p><p>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- </p><p>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</p><p>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 &amp; Industrial Research), Chennai 600 020, India Polyhedral oligomeric silsesquioxane-baspoly(urethane-imide) hybrid membranesand gas-transport properties </p><p>D. Gnanasekaran a,b, P. Ajit Walter a, A. Asha Parveen</p><p>Sepa ration and Pur</p><p>journal homepage: www013 Published by Elsevier B.V. All uoroimide-containingSynthesis, characterization </p><p>B.S.R. Reddy a,</p><p>SciVerse ScienceDi rect </p><p>cation Techn ology </p><p>sevier .com/locate /seppurrights reserved. </p></li><li><p>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].</p><p>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].</p><p>Nomencla ture </p><p>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- </p><p>face qlm density of the lmmair weight of polymer in air </p><p>D. Gnanasekaran et al. / Separation and PMoore and Koros [25] have summari zed the relationship between organicinorganic membrane morphologies and transport properties.</p><p>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. </p><p>2. Experimen tal </p><p>2.1. Materials and methods </p><p>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. </p><p>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.</p><p>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. </p><p>The thermal stabilities of the polymers were determined using PerkinElmer TGA Q50-TA thermal analyzers by taking 10 mg of </p><p>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 </p><p>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)</p><p>cation Technology 111 (2013) 108118 109samples at a heating rate of 10 C per minute under nitrogen atmosph ere. </p><p>Surface morphology of the polymers was studied using a Nano- scope III AFM instrument and the imaging was done in contact mode at room temperature in air. The membranes were cut into small pieces and placed on a grid. The grid was covered using the commercial tip of Si 3N4 provided by digital instruments .Cantilever length was kept at 200 lm with a spring constant of 0.12 N m1. The scan heads with a maximum range of 250 nm 250 nm and Z scale: 250 nm. AFM images given in this work were reproducibly obtained over at least three points on the sample surface. </p><p>TEM images (recorded on photograp hic lm and digitized with a PC-control led digital camera DXM1200 (Nikon)) were obtained (acceleration voltage = 100 kV) using a JEM 200CX (JEOL) micro- scope. SEM analysis was performed using JEOL 400 microscope by cutting membran es into small pieces and the samples were rstsputtered with gold. The SEM pictures were taken on the at sur- face of the hybrid membran es. </p><p>Dynamic mechanical analysis (DMA) were carried out under nitrogen atmosphere by means of NETZSCH DMA 242 mechanical analyzer (Selb, Germany) on samples of following sizes 25 mm 5 mm 0.5 mm at 0.1 Hz frequenc y and the temperature range of 100 C to 300 C, with a heating rate of 5 C/min. The variation of the storage modulus (E0) was obtained as a function of temperat ure. </p></li><li><p>Density measureme nts were carried out using a Mettler AJ100 analytical balance tted with a Mettler ME-33360 density determi- nation kit based on Archimedes Principle . The relationship be- tween predicted density, mass and liquid density was given in the following equation: </p><p>qfilm mair</p><p>mair mliquid qliquid 1</p><p>where qlm is the predict ed density , mair and mliquid the masses measured in air and liquid , and qliquid is the density of the liquid. </p><p>Density measure ments were performed on the membranes of 1 cm 1 cm dimensio ns and an average of three readings were ta- ken. The density data was used to evaluate the chain packing by calculating the fractional free volume (FFV), from the following equation:</p><p>V VO</p><p>110 D. Gnanasekaran et al. / Separation and PuriFFV V</p><p>2</p><p>where V = total specic volume of the polymer and was obtained from the experime ntally determine d density of the polymer. VO = -occupied volume or zero point volume of the polymer. Typica lly, the occupied volume was estimated to be 1.3 times more than the Van der Waals volume that was estima ted by the method of Bondi [27]using the group contri bution correlation of Van Krevelen and Hofty- zer [28].</p><p>2.2. Gas permeation measuremen ts </p><p>The thickness of the membranes was measured using digital micrometer at ten different positions and the average value was ta- ken for calculatio n. The membran e (B) was then placed in the cen- ter of the permeation cell (C) xed with a rubber O-ring. Initially, the upstream and the downstream side of the apparatus was purged with the penetrant gas via the lower vent line (F), while the upper vent line (G) kept closed and vice versa. The ends of both vent lines were immersed in oil traps (H), which prevent the back- diffusion of air into the apparatus. Closing the vent line (G) termi- nated the purging operation. The complete setup of the permeation apparatus was shown in Fig. 1. The penetrant gas was forced to permeate through the membrane and ows into the soap bubble ow meter raising the bubble. Steady state permeation (dv/dt)was achieved when soap bubble displacemen t was in linear func- tion of time. </p><p>The upstream pressure was varied from 1 atm to 4 atm, whereas the downstream pressure was atmosph eric pressure. Fig. 1. Gas permeation apparatus. The gas permeabilit y coefcient P [cm3(STP) cm/(cm2 s cmHg)]was determined using the following equation: </p><p>P Jd p2 p1</p><p> JdDp</p><p>3</p><p>where J [cm3(STP)/(cm2 s)] is the steady state permeate ux, d is the membrane thickne ss (cm) and p2 and p1 are the feed and permeate pressures (cmHg) respective ly. The steady state permeate ux J wascalculate d using relationshi p (4).</p><p>J dv=dtA</p><p> 273:15</p><p>T</p><p> Pa76</p><p> 4</p><p>where dv/dt is the volumetric displacem ent rate of soap lm in the soap bubble ow meter, A is the membrane area (cm2), T is the tem- perature (K) and Pa is the atmospheric pressure (Bar). The selectiv- ity (aA/B) of the polymeric membrane s for component s A and B wasobtained from the ratio of pure gas permeabili ties. </p><p>3. Synthesi s</p><p>3.1. Synthesis of uorinated prepolyim ide (FPI)</p><p>4,40-(Hexauoroisopropylidene)diphthalic anhydrid e (6FDA) in 5 mL of THF was placed in a ask equipped with a nitrogen inlet and stirred using magnetic stirrer until clear solution was ob- tained. To the clear solution, hexamethyl ene diisocyanate in 3 mL of THF was added followed under stirring. The reaction mixture was reuxed at 90 C in 6 h by connecting to a spiral condenser .The synthetic route was given in Scheme 1A. The chemical compo- sitions of synthesized FPIs were given in Table 1.</p><p>3.2. Synthesis of prepolyureth ane (PU)</p><p>The NCO terminated prepolyurethane was prepared reacting PDMS and HMDI in a 50 mL two-necked ask. PDMS and Cy-POSS were reacted with HMDI in 3 mL of THF, followed by adding two drops of DBTDL as a catalyst at...</p></li></ul>

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