Separation and Purification Technology 111 (2013) 108–118

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Sepa ration and Purification Techn ology

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

D. Gnanasekaran a,b, P. Ajit Walter a, A. Asha Parveen a, B.S.R. Reddy a,⇑a Industrial Chemistry Laboratory, Central Leather Research Institute (Council of Scientific & Industrial Research), Chennai 600 020, India 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 a b s t r a c t

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

Keywords:POSSMembranesFluorinated imide Structure-gas transport property

1383-5866/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.seppur.2013.03.035

⇑ Corresponding author. Tel.: +91 044 2440 4427; fE-mail address: induchem2000@yahoo.com (B.S.R

The purpose of this work is to study the gas permeation rates of O2, N2 and CO 2 gases and selectivity of O2/N2

and CO 2/N2 using synthesized fluorinated poly(urethane-imide) polyhedral oligomeric silsesquioxa ne (FPUI-POSS). FPUI-POSS membranes having different amount s of fluorinated imide were synthesized viasimple condensation reaction of isocyanate terminated prepolyurethane (PU) and anhydride terminated fluorinated prepolyimide (FPI). All the membranes were characterized for structural details [attenuated total reflection 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 fluorinated 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 fluorinated imide content increased in the poly- meric membranes simultaneously increases the surface roughness and thereby lowering the density because of the loss of uniformity and flexibility of the hybrid membranes.

Crown Copyright � 2013 Published by Elsevier B.V. All rights reserved.

1. Introductio n Polyimide (PI) membranes are high-performance polymers with

Gas separation through polymeric membranes are considered to be an effective tool for the separation of gaseous mixtures due to high separation efficiency, 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- ergy conservation and reduction of emission of the environmental pollutants [3]. The polymeric membran es are widely used in air separation, natural gas purification, petrochemical processing, medical, isolation of CO 2 from power plants and chemical indus- tries [4–6]. The selection of proper membrane for different applica- tions is an important task to become technologic ally superior and minimize global warming .

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ax: +91 044 2491 1589. . Reddy).

excellent thermal stability, chemical resistance and mechanical property which find 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 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 flexible 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

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Nomencla ture

P permeabi lity coefficient J steady state fluxDp 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 cd

L dispersio n factor of membrane for a liquid cd

S dispersio n factor of membrane for a solid cp

L polar factor of a liquid cp

S polar factor of a sample cSV total surface energy of membrane h contact angle between the sample and liquid/air inter-

face qfilm density of the filmmair weight of polymer in air

mliquid weight of polymer in liquid qliquid density of the liquid FFV fractional free volume V total specific volume of the polymer VO occupied volume of the polyme rATR-FTIR attenuated total reflection 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 fluorinated prepolyim ide POSS polyhedr al oligomeri c silsesquiox ane FPUI-POSS fluorinated poly(urethane-imide) POSS PDMS poly(dimethyl silsesq uioxane)

D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118 109

properties of the PU-based membranes have shown that the pro- portion of hard and soft segments influence the permeation prop- erties of the membranes [17–19].

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 modification of fillers and matrices has become an expanding field of research since the introduct ion of functional groups can improve dispersion of fillers and improve the chemical affinities of penetrants in the membranes . There is much scope for research and innovation to develop polymer–inor- ganic nanocompo site membranes for gas separation. Many organ- ic–inorganic 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 [21–24].Moore and Koros [25] have summari zed the relationship between organic–inorganic 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 fluorinated 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 define the structure–properties 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-(Hexafluoroisopropylid- ene)dipthalic anhydrid e (Aldrich, 99%) was purified 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 reflectance Fourier transform infrared (ATR-FTIR) spectra by Perkin–Elmer 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 five different locations for all samples and the average was taken to obtain meaningful measure ments.

The thermal stabilities of the polymers were determined using Perkin–Elmer TGA Q50-TA thermal analyzers by taking 10 mg of samples at a heating rate of 10 �C per minute under nitrogen atmosph ere.

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 m�1. 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.

TEM images (recorded on photograp hic film 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 firstsputtered with gold. The SEM pictures were taken on the flat sur- face of the hybrid membran es.

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.

110 D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118

Density measureme nts were carried out using a Mettler AJ100 analytical balance fitted with a Mettler ME-33360 density determi- nation kit based on Archimede’s Principle . The relationship be- tween predicted density, mass and liquid density was given in the following equation:

qfilm ¼mair

mair �mliquidqliquid ð1Þ

where qfilm is the predict ed density , mair and mliquid the masses measured in air and liquid , and qliquid is the density of the liquid.

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:

FFV ¼ V � VO

Vð2Þ

where V = total specific 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].

2.2. Gas permeation measuremen ts

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) fixed 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 flows into the soap bubble flow meter raising the bubble. Steady state permeation (dv/dt)was achieved when soap bubble displacemen t was in linear func- tion of time.

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 coefficient P [cm3(STP) cm/(cm2 s cmHg)]was determined using the following equation:

P ¼ Jd p2 � p1

¼ JdDp

ð3Þ

where J [cm3(STP)/(cm2 s)] is the steady state permeate flux, d is the membrane thickne ss (cm) and p2 and p1 are the feed and permeate pressures (cmHg) respective ly. The steady state permeate flux J wascalculate d using relationshi p (4).

J ¼ ðdv=dtÞA

� �273:15

T

� �Pa76

� �ð4Þ

where dv/dt is the volumetric displacem ent rate of soap film in the soap bubble flow 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.

3. Synthesi s

3.1. Synthesis of fluorinated prepolyim ide (FPI)

4,40-(Hexafluoroisopropylidene)diphthalic anhydrid e (6FDA) in 5 mL of THF was placed in a flask 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 refluxed 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.

3.2. Synthesis of prepolyureth ane (PU)

The NCO terminated prepolyurethane was prepared reacting PDMS and HMDI in a 50 mL two-necked flask. PDMS and Cy-POSS were reacted with HMDI in 3 mL of THF, followed by adding two drops of DBTDL as a catalyst at RT (35 �C) for 6 h. The synthetic route was given in Scheme 1I. The chemical distribution s of syn- thesized PU were given in Table 1.

3.3. Synthesis of fluorinated poly(urethane-imide) POSS hybrid membranes (FPUI-POSS)

The FPUI-POSS membranes were prepared by varying the ratios of prepolyimide and prepolyu rethane in the ratio of 0/100, 10/90, 20/80, 30/70 and 100/0 respectively. THF was used as a solvent for refluxion by using spiral condenser at 90 �C for 6 h. The result- ing viscous solution was transferred into a Teflon coated petridish and was kept at room temperature for 6 h. Then all the membran es were kept in hot air oven at 150 �C for 7 h. The model reaction of this polymer was shown in Scheme 1.

4. Results and discussion

4.1. ATR-FTIR spectrosco py of FPUI-POSS

The ATR-FTIR spectra of fluorinated poly(urethane-imide)-POSSmembran es were given in Fig. 2. The spectra confirmed the forma- tion of imide by the presences of characteri stic asymmetric and symmetr ic stretching vibrations of C@O of imide groups appeared at 1760 and 1728 cm �1 respectively . The sharp peaks at 1385 and 1244 cm �1 were due to the presence of CAN stretching vibration sconfirmed the formatio n of imide. On the other hand, the presence of C@O stretch at 1625 cm �1 corresponds to the presence of ure- thane linkage. The broad band around 3320–3400 cm �1 was due

Scheme 1. Synthesis of fluorinated poly(urethane-imide) POSS membranes.

Table 1Chemical feed compositions of PU and FPUI-POSS membranes.

S. no Sample PU (g) FPI (g) Cy-POSS (g)

HMDI Diol a HMDI DA b

1 PU 0.55 1.40 – – 0.05 2 FPUI-10-POSS 0.45 1.30 0.04 0.16 0.05 3 FPUI-20-POSS 0.35 1.25 0.08 0.32 0.05 4 FPUI-30-POSS 0.30 1.05 0.12 0.48 0.05 5 FPI – – 0.55 1.45 –

a Poly(dimethylsiloxane) bis(hydroxylalkyl) terminated. b (Hexafluoroisopropylidene) dipthalic dianhydride.

D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118 111

to the presence of urethane NAH stretching. Similar type of obser- vations was reported by Qin et al. [29]. This confirms that the het- erocyclic imide rings were successfully incorporate d into the polyurethan e backbone . The intensity of transmittance for C@O

at 1625 cm �1 and NAH at 3350 cm �1 gradually decreases from PU to FPUI-30- POSS as the proportion of fluorinated imide in- creases in the polymeric membranes .

4.2. Density and fractiona l free volume (FFV)

The density values of the synthesized FPUI-POSS hybrid mem- branes were given in Table 2. The FFV was defined as the ratio of the expansion volume or free volume to the observed volume [30]. The FFV (cm3 of free volume/cm 3 of polymer), was used to characteri ze the efficiency of chain packing in the polymeric mem- brane largely depends on the amount of free volume available in the polymeric matrix [31]. The free volumes were generally pro- duced in the polymer matrices due to conformationa l restraint encounter ed by the polymer chains. This expands the distance

Fig. 2. ATR-FTIR of PU and FPUI-POSS membranes.

Table 2Thermal properties, density and fractional free volume of PU and FPUI-POSS membranes.

Samples T50%a (�C) Char yield

at 600 �C (%)Rq (root mean square) (nm)

Density (g/cm3) FFV

PU 443 14 12 1.33 0.205 FPUI-10-POSS 469 22 41 1.21 0.221 FPUI-20-POSS 480 25 44 1.12 0.235 FPUI-30-POSS 507 30 48 1.02 0.250 FPI 525 28 – – –

a 50% Weight loss.

112 D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118

between the chains resulting in increase of the free volume of the polymer [32]. Usually, a higher decrease in density was observed due to the presence of rigid groups, which obstruct chain packing [33]. In all the FPUI-POSS except PU, the bulky AC(CF3)2A groupswould hinder the chain packing resulting in lowering density. FPUI-30-PO SS with higher imide content shows lowest density of 1.02 g/cm 3. This may be due to the hindranc e in the chain packing of rigid AC(CF3)2A groups and thereby resulting in high free vol- ume. The highest density of 1.33 g/cm 3 was observed for PU with flexible siloxane units.

Fig. 3. TGA curves of PU and FPUI-POSS membranes.

4.3. Thermograv imetric analysis of PU and FPUI-POSS membranes

The TGA curves of all polymeric membranes were shown in Fig. 3. The synthesized membranes were showing two stage decompo sitions around 330–363 �C and above 410 �C. The increase in the decomposition temperature may be due to the presence of POSS and fluorinated imide. It was well observed that POSS and fluorinated imide incorporate d membranes were found to show drastic increase in their thermal property as the percentage of fluo-rinated imide increases from 10 to 30 wt.%. Similar type of obser- vations was earlier reported by Liu and Ma and Song et al. [34,35]. The thermal stability of FPUI-POSS membranes were in- creased from 443 to 507 �C at 50% weight loss as seen from Table 2.The increase in fluorinated imide content in the membranes in- creased thermal stability. Similar observati ons were reported by Gnanarajan et al. [36]. From these observations , we have concluded that the nature of the rigid fluorinated imide groups and bulky POSS molecules were significantly altering the thermal properties of the hybrids.

4.4. Scanning electron microscope of PU and FPUI-POSS membranes

Electron microscopy is a useful technique for morphology char- acterizati on since it provides the details of the position, geometry and gross (50 lm) features of these membran es. The morphologi- cal property of the polymeric membran e depends on both their chemical structure and the aggregation of soft and hard segments .The PDMS exhibits as a soft segment due to the presences of low polar and flexible groups. The urethane and imide moieties display hard segments because of the presence of high polar functiona lgroups. Though PU shows a homogenous surface, the small globule structure s are also present throughout the membrane. It could be explained that PU matrix showing homogenous aggregation of ure- thane units and is uniformly spread in the siloxane continuous ma- trix. The degree of phase separation depends on many factors such as chemical structure, molecular weight, composition and arrange- ment of both hard and soft segments. The polymeric membran es such as FPUI-10-PO SS, FPUI-20-POSS and FPUI-30-POSS showed the partial miscibility of the soft and the hard segments . This also shows the heterogeneous morphology with some sphere-shap ed particles present which may be due to the FPI segments that are phase separated. The microphase separation of these membran es was slightly increased with increase in imide content because of non-comp atibility between FPI and PU units. We have observed similar type of phase separation [37]. The phase separation of FPUI-30- POSS was high compared to the other polymers as ob- served from Fig. 4 and this may be due to the polarity difference between the hard and soft segments. The gray background repre- sents the poly(urethane-imide) matrix and the white particles rep- resent the POSS domains.

4.5. Transmission electron microscope of FPUI-POSS

Transmission electron microscopic images of FPUI-10-PO SS and FPUI-20- POSS hybrid membran es were shown in Fig. 5. TEM imag- ing analysis of the POSS and imide incorporate d polymeric mem- brane reveals further information about the POSS and imide dispersio n in the membranes. TEM investigation of polymeric membran es indicated that black spots scattered in a matrix might be due to the difference in the transmitted electronic density be- tween organic and inorganic portion viz., POSS moiety. The dark area was assigned to the POSS portion. It was observed that the spherical POSS domains were dispersed in the continuous array matrix with the average size of approximat ely 50 nm. This phase separation might be arising from the difference between hydro- gen-bond interactions between hard, soft segments and Van der

Fig. 4. SEM images of PU and FPUI-POSS membranes (50 lm).

Fig. 5. TEM images of FPUI-10-POSS and FPUI-20-POSS membranes.

D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118 113

Waals interactions between the cyclopen tyl groups of POSS mole- cules. The results from TEM indicated that the nanostructu red POSS molecules were embedde d within the polymeric nanocom- posites and similar type of POSS aggregation s were reported by Wang et al. [38].

4.6. Atomic force microscopy of PU and FPUI-POSS membranes

The effect of POSS and FPI content on the surface morphology of FPUI-POSS membranes were studied by AFM. The three-dimen- sional topograp hical image and phase images were given in Fig. 6a . In fact, four topograp hical images have been obtained for four different types of membranes at scan length (250 nm � 250 nm), z-axis: 250 nm and at constant scan points (128 � 128). The fractal dimension (roughness-vs-scale) of FPUI- 10-POSS and FPUI-30-PO SS were shown in Fig. 6b .

RMS surface roughness values of PU, FPUI-10-POSS, FPUI-20- POSS and FPUI-30-POSS were found to be 12, 41, 44 and 48 nm respectively (Table 2). The incorporation of FPI increased the de- gree of surface roughness fourfold compare d to that of the PU. The elevated regions were attributed to the rubbery PU, whereas the flatter or lower regions were glassy FPI. Similar type of obser- vations was reported by Klinedins t et al. [39]. The extent of protru- sion of one phase and the surface roughness also increased with increase in the FPI content. Therefore, this could be attributed to the existence of non-compatibl e phases. Similar correlation of sur- face roughness to the phase separation has been reported for tetra- methyl bisphenol A polycarbona te and polystyrene blends [40,41].

4.7. Dynamic mechanical analysis of PU and FPUI hybrid membranes

The storage modulus (E0) of hybrids such as PU, FPUI-10- POSS, FPUI-20- POSS and FPUI-30-PO SS were shown in Fig. 7. This

Fig. 6a. AFM images of PU and FPUI-POSS membranes.

Fig. 6b. Fractal dimension (roughness vs scale) images of FPUI-10-POSS and FPUI-30- POSS membranes.

114 D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118

indicates that the increase in the crosslink density increases the storage modulus of the hybrids and the values were given in

Table 3. This may be due to the increased number of urethane con- nections and an increase in the interchain interaction caused by

Fig. 7. Plot of E0 versus temperature for PU and FPUI-POSS membranes.

D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118 115

the hydrogen bonds present in the hybrids. DMA of the membran ewith higher imide content (FPUI-30-POSS) shows lower storage modulus due to decrease in the urethane crosslink density. This is perhaps due to the incompatibil ity nature of imide and urethane. The dynamical mechanical analysis clearly indicates that the stor- age modulus depends on hard segment structure, the number of urethane connections and the interchain interaction between hard and soft segments.

4.8. Surface energy of PU and FPUI-POSS membranes

The contact angle measureme nts provide information about the surface energies of the membranes . The surface energy of poly- meric membrane can be calculated indirectly by measuring equi- librium contact angles of some standard liquids, such as water and higher alkanes, on the air side of membrane surface. The polar and the dispersion factors were calculated using Young and Fowks equation (5).

cLV ð1þ CoshÞ ¼ 2ðcdLc

dSÞ

1=2 þ ðcpLc

pSÞ

1=2 ð5Þ

where cLV is the interfacia l tension at liquid/air interface; cdL and cd

S

are the dispersion factors of the sample for liquid and solid respec- tively; cp

L and cpS are the polar factors of the liquid and sample; h is

the contact angle betwee n the sample and liquid/air interface. The total surface energy cSV of the samples were estimated using the relations hip (6).

cSV ¼ cdSc

PS ð6Þ

The PDMS based polymeric membrane was found to be having higher contact angle in water (i.e., above 80 �) [42]. The surface en- ergy and the corresponding polar/dispersion components of the membranes were calculated and the results were given in Table 3.The maximum reduction in the surface energy of 32.21 mN m�1

was observed for PU. This was lower than the contact angles mea-

Table 3Surface free energy and storage modulus (E0) of PU and FPUI-POSS membranes.

Samples hH2O hC17H36 cds (mN m�1)

PU 99.0 29.96 23.94 FPUI-10-POSS 96.0 32.00 23.47 FPUI-20-POSS 93.80 35.33 22.74 FPUI-30-POSS 90.33 37.10 22.23

suremen ts for other membran es. This could be attributed to the crosslinked nature of the membrane with the rigid groups (POSSand AC(CF3)2A) that might prevent the free mobility of the hydro- phobic groups to lower the surface energy. It was seen in all the systems that increase in the percentage incorporation of the fluo-rinated imide in the membrane results in the correspondi ng in- crease in the surface energy due to the availabili ty of more number of fluorinated imide groups. In each of these systems, the increase in the fluorinated imide alters the polar factor much more than the dispersion factor. The surface energy of the systems follows the order: FPUI-30- POSS > FPUI-20-PO SS > FPUI-10- POSS > PU.

4.9. Pressure dependency of permeability of FPUI-POSS hybrid membranes

The permeability coefficient values of N2, O2 and CO 2 gases were determined for FPUI-POSS membran es at 35 �C under various pres- sures and were shown in Fig. 8. A minimal change in N2 and O2 per-meability with penetrant pressure was observed for a pressure range of 1–4 atm. The graphical analysis of this data of permeabil- ity versus pressure showed a straight line for N2 and O2 gases. The slight reduction in permeabilit y may be due to a reduction in the free volume for the penetrants transport and thereby reduces the penetrant’s diffusion coefficients. In the case of CO 2, it is quite dif- ferent from other two gases such as N2 and O2. This may be due to two factors namely (i) more condensable nature of CO 2 gas and (ii)the plasticizatio n effect of CO 2 gas on the polymer matrices. Re- ports have shown that CO 2 gas molecule plasticize the polymeric membran es when there was increase in the pressure, time and temperat ure aspects [43].

4.10. Effect of hard segment on permeability of PU and FPUI-POSS hybrid membranes

The influence of the fluorinated imide content on the perme- ability coefficients and separation factors under different pres- sures (1–4 atm) at 35 �C were investigated . The results were shown in Fig. 9. The permeabilit y of the gases through PU and FPUI-POSS polymeric membranes strongly depends on their mor- phology, including the size and shape of the dispersed and con- tinuous phases. There seems to have a strong correlation between the efficiency of chain packing and the gas permeab il- ity, which is correlated by the FFV. However, each parameter that could change the chain mobility and the interaction of the soft segment to permeate molecules would affect the gas perme- ation of FPUI-POSS. The fluorinated imide content increased with increasing the phase separation and lowering the FFV be- cause of the loss of uniformity and flexibility of the hybrid membran es.

PU < FPUI-10- POSS < FPUI-20-PO SS. Permeation measureme nts of polymers revealed that the permeabilit y of gases increases with phase separation in polymers. For all the compositions , CO 2showed maximum permeabilit y coefficient and N2 showed the minimum value. In all the membran e systems studied, it was ob- served that the permeability follows the trend: P(CO2) > P(O2) >

cps (mN m�1) csv (mN m�1) E0 (MPa) at 30 �C

8.27 32.21 39 10.79 34.26 26 13.62 36.36 716.48 38.71 4

Fig. 8. Permeability of N2, O2 and CO 2 gases.

Fig. 9. Influence of the fluorinated imide on permeability coefficient (3 Bar).

116 D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118

P(N2). This indicated the existence of a relationshi p between the permeabilit y and the kinetic diameter of the tested gases, i.e., CO2: 3.3 Å, O2: 3.46 Å and N2: 3.64 Å as reported for many poly- meric membranes [44].

4.11. Selectivity of the PU and FPUI-POSS hybrid membranes

The selectivity of PU and FPUI-POSS membranes were given in Fig. 10 . The permsele ctivity of the membranes for P(O2)/P(N2) pair was found to be in the range of 1.8–3.0 and P(CO2)/P(N2) was ob- served in the range of 6.4–12.8. In the case of CO 2/N2 and O2/N2

gas pairs, the selectivity values slightly increased with increase in the fluorinated imide content. This demonst rates that the selec- tivity parameter was controlle d by the diffusion of gas molecule sthrough the hard segment region in the membrane. This may be due to stronger interactions between O2 gas and high imide con- tent/POS S cage. Surprisingly , P(CO2)/P(N2) selectivity of FPUI-POSS was found to be higher when compared to our reported earlier work [3]. Some researchers have reported that the introduction of fluorine atom into side-chains of polymeric membranes could enhance selectivity [45–47]. In addition, cross-linkin g is a simple and effective way to change the structure of PU membrane to im- prove selectivity and to enhance thermal stability. Stern et al. [48]reported that the increase in the rigidity of the polymer chain com- bined with low fractional free volume plays a significant role in selective gas transport property.

5. Conclusi ons

We have prepared PU and a series of FPUI-POSS hybrid mem- branes having various amounts of urethane and fluorinated imide, which combine the advantages of both polyurethan e and

Fig. 10. Selectivity of O2/N2 and CO 2/N2 gases.

D. Gnanasekaran et al. / Separation and Purification Technology 111 (2013) 108–118 117

polyimide. Structural characteristics of the PU and FPUI-POSS hybrids were elucidated by ATR-FTIR analysis by confirming the presence of imide and urethane moieties in the polymer. Observa- tions from TGA showed that the higher fluorinated imide (i.e.,FPUI-30-PO SS) displayin g higher thermal decompositi on tempera- ture compared to the other polymeric membranes. The microphas eseparation of these membranes was slightly increased with fluorinated imide content because of polar/non -polar nature of constituents present in the polymeric membranes. AFM confirmedsignificant changes in the surface roughness at the surface of the membranes because of presence of two different types of soft and hard segments. The gas permeabilit y of O2 and CO 2 as well as the gas selectivity of O2/N2 and CO 2/N2 increased with increase in the fluorinated imide content. These results indicated that the presence of fluorinated imide, POSS and PDMS in the membrane restricts the motion of the chain by the presence of crosslinker (POSS) and rigid AC(CF3)2A group. These factors contribute towards enhancing the transport property. The commercially available polymers were unable to achieve the ideal material prop- erties, because polymers with high permeabilit y usually have low selectivity, and others with high selectivity have low permeabilit y. Here, O2 and CO 2 permeabilit y and O2/N2 and CO 2/N2 selectivity of the synthesized membran es increased simultaneou sly due to the introduction of the fluorine atoms into the polymeric membran es. Finally, physical, thermal and morphological characterization al- lowed us to better understand the relationship between surface morphology and transport properties of membrane by the intro- duction of fluorinated imide content on the properties of generated materials. The synthesized FPUI-POSS membranes are being used towards the enrichme nt of oxygen from air for replacing the supply of oxygen cylinders in the hospital and also in the separation of olefins and alkanes from the cracking process.

Acknowled gements

D. Gnanasekar an thanks Council of Scientific & Industrial Re- search, New Delhi for the Senior Research Fellowship and DST for part-funding (No. SR/S1/PC -45/2011).

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