preparation and characterization of zeolite beta–polyurethane composite membranes

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Preparation and Characterization of Zeolite Beta–Polyurethane Composite Membranes Eda Ayse Aksoy, 1 Burcu Akata, 2 Nurcan Bac, 3 Nesrin Hasirci 4 1 Department of Polymer Science and Technology, Middle East Technical University, Ankara 06531, Turkey 2 Central Laboratory, Middle East Technical University, Ankara 06531, Turkey 3 Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey 4 Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey Received 2 June 2006; accepted 30 November 2006 DOI 10.1002/app.25976 Published online 8 March 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Incorporation of zeolite into polyurethane (PU) membranes was investigated by using as-synthesized and calcined zeolite beta particles at two different loading contents (0.1 and 1 wt %). The chemical interaction between the zeolite beta crystals and PU was observed by ATR-FTIR spectroscopy. The SEM results suggested that the calcined zeolite beta crystals were more homogene- ously dispersed in the composite membranes than the as- synthesized zeolite beta crystals. DMA results demon- strated that all composite membranes had higher storage modulus in the rubbery state and higher stability towards thermal and mechanical degradation with respect to the control groups. Tensile testing results also showed increased tensile strength and elongation at break for all composite membranes. This study suggests that incorpo- rating zeolite beta in its as-synthesized or calcined forms and at different amounts can be applied as an alternative method for tailoring the mechanical properties of PU membranes without changing its structural character- istics. Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3378– 3387, 2007 Key words: zeolite beta; polyurethanes; composites;- mechanical properties INTRODUCTION Polymer based organic/inorganic composites are becoming increasingly important due to the fact that the resultant material shows superior performance in terms of mechanical toughness, permeability, selectiv- ity, and photoconductivity for various applications. 1–8 There are different types of inorganic materials used as fillers, such as clays, 1 SiC particulates, 3 ceramic microspheres, 5 and particularly silica particles. 7,8 Pure silica does not contain framework charge since silicon is tetravalent. On the other hand, aluminosilicates have negatively charged oxide frameworks that require balancing of the extra framework positive ions. Thus, using zeolites as fillers could be challeng- ing, since zeolites offer different parameters (i.e., Si/ Al ratio, ion-exchange properties) that can be used as tools for tailoring the properties of the composite membranes for the desired purpose. Zeolites are microporous crystalline aluminosili- cates and are being used as fillers in organic/inor- ganic composites mostly to improve the gas separa- tion performances because of their uniform molecu- lar-sized pores. 9–11 They are also being used in many further applications due to their high thermal and mechanical stabilities. 12,13 Generally preparation of a stable zeolite involves the calcination of the as- synthesized materials. 14–17 Calcination is usually done by heat treating the zeolite sample at 5008C or at higher temperatures. Zeolites offered various chal- lenges in different areas with their unique proper- ties; however, the powder forms of these materials limit their use in several applications especially in the manufacturing field. Their incorporation into polymers creates a very promising field of investiga- tion in the field of nanotechnology. Polyurethanes (PUs) are one of the most com- monly used polymers in technological and medical applications due to their extensive structure/prop- erty diversity. They can be synthesized from diiso- cyanates and polyols in many different forms includ- ing foams, adhesives, coatings, fibers, resins, and elastomers. 18,19 Physical properties, mechanical strength, and surface structure as well as the chemis- try of the membranes can be very important depend- ing on the usage area. These properties can be altered and tailored by varying the preparation com- position, molecular weight of polyol component, type and the structure of diisocyanates, and with addition of inorganic fillers such as zeolites. Correspondence to: B. Akata ([email protected]). Contract grant sponsor: METU; contract grant number: BAP-2005-07-02-00-92. Journal of Applied Polymer Science, Vol. 104, 3378–3387 (2007) V V C 2007 Wiley Periodicals, Inc.

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Preparation and Characterization of ZeoliteBeta–Polyurethane Composite Membranes

Eda Ayse Aksoy,1 Burcu Akata,2 Nurcan Bac,3 Nesrin Hasirci4

1Department of Polymer Science and Technology, Middle East Technical University,Ankara 06531, Turkey2Central Laboratory, Middle East Technical University, Ankara 06531, Turkey3Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey4Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey

Received 2 June 2006; accepted 30 November 2006DOI 10.1002/app.25976Published online 8 March 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Incorporation of zeolite into polyurethane(PU) membranes was investigated by using as-synthesizedand calcined zeolite beta particles at two different loadingcontents (0.1 and 1 wt %). The chemical interactionbetween the zeolite beta crystals and PU was observed byATR-FTIR spectroscopy. The SEM results suggested thatthe calcined zeolite beta crystals were more homogene-ously dispersed in the composite membranes than the as-synthesized zeolite beta crystals. DMA results demon-strated that all composite membranes had higher storagemodulus in the rubbery state and higher stability towardsthermal and mechanical degradation with respect to the

control groups. Tensile testing results also showedincreased tensile strength and elongation at break for allcomposite membranes. This study suggests that incorpo-rating zeolite beta in its as-synthesized or calcined formsand at different amounts can be applied as an alternativemethod for tailoring the mechanical properties of PUmembranes without changing its structural character-istics. � 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3378–3387, 2007

Key words: zeolite beta; polyurethanes; composites;-mechanical properties

INTRODUCTION

Polymer based organic/inorganic composites arebecoming increasingly important due to the fact thatthe resultant material shows superior performance interms of mechanical toughness, permeability, selectiv-ity, and photoconductivity for various applications.1–8

There are different types of inorganic materials usedas fillers, such as clays,1 SiC particulates,3 ceramicmicrospheres,5 and particularly silica particles.7,8 Puresilica does not contain framework charge since siliconis tetravalent. On the other hand, aluminosilicateshave negatively charged oxide frameworks thatrequire balancing of the extra framework positiveions. Thus, using zeolites as fillers could be challeng-ing, since zeolites offer different parameters (i.e., Si/Al ratio, ion-exchange properties) that can be used astools for tailoring the properties of the compositemembranes for the desired purpose.

Zeolites are microporous crystalline aluminosili-cates and are being used as fillers in organic/inor-ganic composites mostly to improve the gas separa-

tion performances because of their uniform molecu-lar-sized pores.9–11 They are also being used in manyfurther applications due to their high thermal andmechanical stabilities.12,13 Generally preparation of astable zeolite involves the calcination of the as-synthesized materials.14–17 Calcination is usuallydone by heat treating the zeolite sample at 5008C orat higher temperatures. Zeolites offered various chal-lenges in different areas with their unique proper-ties; however, the powder forms of these materialslimit their use in several applications especially inthe manufacturing field. Their incorporation intopolymers creates a very promising field of investiga-tion in the field of nanotechnology.

Polyurethanes (PUs) are one of the most com-monly used polymers in technological and medicalapplications due to their extensive structure/prop-erty diversity. They can be synthesized from diiso-cyanates and polyols in many different forms includ-ing foams, adhesives, coatings, fibers, resins, andelastomers.18,19 Physical properties, mechanicalstrength, and surface structure as well as the chemis-try of the membranes can be very important depend-ing on the usage area. These properties can bealtered and tailored by varying the preparation com-position, molecular weight of polyol component,type and the structure of diisocyanates, and withaddition of inorganic fillers such as zeolites.

Correspondence to: B. Akata ([email protected]).Contract grant sponsor: METU; contract grant number:

BAP-2005-07-02-00-92.

Journal of Applied Polymer Science, Vol. 104, 3378–3387 (2007)VVC 2007 Wiley Periodicals, Inc.

To assess the proper use of zeolite-PU compositemembranes in certain applications, knowledge oftheir mechanical/thermal properties and their mor-phology is essential. In the current study, zeolitebeta, which is a large-pore and high silica zeolite,possessing a three-dimensional 12-membered ringchannel system20,21 was chosen as the inorganic fil-ler. Unlike other types of zeolites that are currentlyunder investigation in the field of composite mem-branes,9 zeolite beta can be synthesized with a widerange of Si/Al ratios from � 10 to more than 100and is known to be thermally stable showing highresistance to high calcination temperatures withoutloss of crytallinity.12,22 In this study, the effect ofadding zeolite beta particles into PU membranes in as-synthesized or calcined forms at two different loadingcontents were studied by Fourier-transform infraredspectroscopy (ATR-FTIR), scanning electron micros-copy (SEM), dynamic mechanical analysis (DMA), andcharacterized by their mechanical properties. To thebest of our knowledge, this is the first report on thepreparation and characterization of zeolite beta–PUcomposite membranes.

EXPERIMENTAL

Material synthesis

Zeolite synthesis

The zeolite beta formulation used (2.2Na2O:Al2O3:60SiO2:4.6(TEA)2O:445H2O, where TEA:tetraethylam-monium) was prepared from two precursor solutions.For this purpose, sodium aluminate precursor solutionwas prepared firstly by dissolving sodium aluminate(52.9 wt % Al2O3, 45.3 wt % Na2O, Riedel de Haen) intetraethylammonium hydroxide (35 wt % TEAOH inwater, Aldrich) solution. To this solution, colloidalsilica (40 wt % suspension in water, Sigma Aldrich,SiO2) was added and the resulting solution was putinto the teflon-lined autoclaves. The autoclaves werekept at 1508C under static conditions for 10 days. Aftercooling, the formed zeolite crystals were filtered,washed, and dried.

The calcined zeolite beta samples were preparedfrom the parent zeolite beta by heat treating the as-synthesized material at 5008C for 10 h in a conven-tional oven. Two different types of zeolites, the as-synthesized and the calcined forms, were used forthe preparation of the zeolite beta–PU compositemembranes. For each type of zeolite beta sample,two different percent loadings (0.1 or 1 wt %) werestudied.

PU synthesis

PU membranes were prepared from toluene diisocy-nate (TDI; Dow Chemical Company, USA as a

mixture of 2,4 and 2,6 toluene diisocynate in the ratioof 80 : 20) and polypropylene ethylene glycol (polyol;Dow Chemical Company; MW � 3500) without addingany other ingredients (solvent, catalyst, or activator) ina closed vacuum system as briefly described previ-ously.23–25 In this process, 20 mL of polyol was putinto the reactor chamber, heated at about 908C, andevacuated for at least 2 h to avoid volatile chemicalsespecially water. Afterwards, 5 mL of TDI was addeddrop wise and the total solution was stirred for 6 h at908C under vacuum. The formed viscous solution waspoured into glass petri dishes, closed, and placed intovacuum oven where they are kept for � 10 days at908C for complete curing.

Preparation of zeolite beta–PUcomposite membranes

The PU membranes were prepared as described ear-lier, except zeolite particles were added after the vis-cous polymer solution was poured into glass petridishes in the form of thin films. Before incorporatinginto the PU, zeolite beta particles were kept in theoven at 1108C for an hour to remove the adsorbedhumidity. Desired amounts were screened (0.1 or1 wt %) using a sieve of 120-mesh to avoid largercrystals to be included into the polymer matrix andthen loaded into the PU solutions. Afterwards, themixtures were kept in the ultrasonic bath for 15 minto obtain a good dispersion. Then they were placedinto vacuum oven and kept at 908C for curing untilthey form solid films (� 10 days). The preparationscheme of the zeolite beta–PU composite membranesis shown in Figure 1, and the summary of the sam-ple codes with their compositions is given in Table I.The procedures were repeated twice for each experi-ment to assure the results were reproducible.

Characterization

Attenuated total reflectance-FTIR (ATR-FTIR) spectrawere obtained on a Bruker IFS 66/S spectrometerequipped with a ZnSe crystal at 458. The sampleswere analyzed over 500–4000 cm�1 range with theresolution of 4 cm�1. All spectra were averaged over32 scans.

SEM was used to study the morphology of the ze-olite beta–PU composite membranes. Electron micro-graphs were obtained using a JSM-6400 ElectronMicroscope (JEOL). Prior to the measurement, thespecimens were coated with gold. Energy dispersiveX-ray spectroscopy (EDX) analysis was carried oututilizing a Phoenix EDAX X-ray analyzer equippedwith sapphire super ultrathin window detectorattached to the JSM-6400 SEM.

DMA measurements were carried out using aPerkin–Elmer Pyris Diamond DMA. The samples were

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measured over a temperature range from �100 to1208C at a heating rate of 58C/min under nitrogenatmosphere. An oscillation frequency of 1 Hz wasapplied. The storage modulus (E0), loss modulus(E00), and tan d values were recorded versus temper-ature. The Tg values of the composite membraneswere obtained from the peaks of tan d curves.

Mechanical properties of zeolite beta–PU compos-ite membranes were studied by Lloyd LRX 5K Me-chanical Tester, controlled by a computer runningprogram (WindapR). Zeolite beta–PU compositemembranes (thickness 1.20 6 0.05 mm, width: 10.06 0.05 mm, length 40.0 6 0.05 mm) were attached tothe holders (gauge length: 10 mm) of the instrument.A constant extension rate of 10 mm/min wasapplied. The load deformation curve was printed foreach specimen. The tensile strength was obtainedfrom equation r ¼ F/A, where r is the tensilestrength (MPa), F is the maximum load applied (N)before rapture, and A is the initial area (m2) of the

specimen. The load deformation curve was con-verted to stress–strain curve, where stress is the loadapplied per unit area (F/A) and strain is the defor-mation per unit length. Slope of straight line (elasticregion of the stress-strain curve) is accepted as theYoung’s modulus of the specimen. For each type ofsample, at least five experiments were achieved andthe average values of Young’s modulus, tensilestrength, and percent elongation at break valueswere calculated.

RESULTS AND DISCUSSION

ATR-FTIR analysis

ATR-FTIR spectra of the control PU and zeolite beta–PU composite membranes are shown in Figure 2.There are two regions of interest, which are the��NH absorption and ��C¼¼O stretching regions asshown in Figure 2(a,b). The bands at 3300 and1726 cm�1 are the two typical stretching vibrationsobserved for PUs.4 It was found that each membraneexhibited the peaks for characteristic functionalgroups for PU. This suggests that the incorporationof zeolite beta did not alter the chemical structure ofthe PU membranes.

The stretching band at 3300 cm�1 observed for thecontrol PU and the composite membranes are knownto be the hydrogen bonded N��H stretching vibra-tions. The small shoulder observed at � 3480 cm�1

band is due to free stretching ��NH groups. Asshown in Figure 2(a), the intensity of the free ��NHgroups decreased for samples 1C-PU and 0.1C-PUand disappeared for samples 1AS-PU and 0.1AS-PUupon the incorporation of zeolite beta into PU. Theseresults suggest that N��H groups in all membraneswere partly and sometimes completely hydrogenbonded.1 The band appearing at 1733 cm�1 is due tofree urethane C¼¼O while the peak at 1726 cm�1 isassigned to hydrogen bonded C¼¼O. The 1726 cm�1

band can be explained by formation of hydrogenbondings between the hydroxyl (OH) groups of zeo-lites with oxygen containing carbonyl (C¼¼O) or

TABLE ISample Codes and Zeolite Beta–PU Compositions

Sample codeType and amount

(wt %) of zeolite loading

Control PU Pure PU0.1C-PU 0.1 wt % calcined zeolite beta

loaded PU composite membrane1C-PU 1 wt % calcined zeolite

beta loaded PU composite membrane0.1AS-PU 0.1 wt % as-synthesized zeolite

beta loaded PU composite membrane1AS-PU 1 wt % as-synthesized zeolite

beta loaded PU composite membrane

Figure 1 Preparation flowchart of the zeolite beta–polyur-ethane (PU) composite membranes.

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ether (C��O��C) groups of urethanes.1 The interac-tion between the zeolite and the PU can also beobserved from the slight shift (� 8 cm�1) observedin the peak position of the carbonyl band of the con-trol PU and the composite membranes representedby the 1733 cm�1 band [Fig. 2(b)]. However, a directcorrelation cannot be made in the intensity differen-ces among samples as a function of the loading con-tent (0.1 wt % versus 1 wt %). The FTIR results sug-gest that there is a chemical interaction betweenzeolite beta particles and PU upon the incorpora-tion of zeolite beta while the chemical structure ofPU was maintained.

Morphology

The SEM analysis (Fig. 3) indicated that the as-synthesized zeolite beta particles were predominantlyin the 1–1.5 mm size range. Calcination of the parentzeolite beta did not change the particle size and themorphology of the particles as expected. The EDXanalysis showed identical Si/Al ratio (Si/Al ¼ 306 1) for the as-synthesized and calcined zeolite betasamples.

The surface morphologies of composite mem-branes, upon 1 wt % loading of as-synthesized (AS-PU) or calcined (C-PU) zeolite beta, are presentedin Figures 4 and 5, respectively. All membranes

exhibited smooth surfaces from top and bottom por-tions. Figure 4 shows the morphologies of the 1AS-PU composite membranes from top [Fig. 4(a)], bot-tom [Fig. 4(b)], and cross-sectional [Fig. 4(c)] views.These results show that the as-synthesized zeolitebeta crystals were dispersed in a nonhomogeneousway in 1AS-PU membranes. This is more clearlyobserved from the cross-sectional view in Figure4(c), which showed that some of the as-synthesizedzeolite beta crystals precipitate and accumulatetowards the bottom portion of the composite mem-branes. The reason for the different views betweentop and bottom faces is the precipitation, agglomera-tion of some of the as-synthesized zeolite crystals,and the fact that they were more submerged into thebottom face of the membranes creating a more sphe-roidal view. This was not observed for the 1C-PUcomposite membranes.

Figure 5 shows the SEM results of the 1C-PU com-posite membranes. Unlike the differences observedin the 1AS-PU [Fig. 4(a,b)], there were no differences

Figure 2 ATR-FTIR spectra of control PU and zeolitebeta–PU composite membranes of ��NH absorption(a) and ��C¼¼O stretching (b) regions.

Figure 3 SEM image of as-synthesized (a) and calcined(b) zeolite beta particles.

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between the top and bottom appearances of the 1C-PU composite membranes as shown in Figure 5(a,b).These results suggest that the dispersion of the zeo-lite crystals upon calcination was more homogene-

ous in between the top and bottom layers of themembrane with almost no accumulation and precipi-tation of particles. Homogeneous dispersion can alsobe seen from the cross-sectional views given in

Figure 4 SEM images of 1AS-PU composite membranes.

Figure 5 SEM images of 1C-PU composite membranes.

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Figure 5(c,d). Thus, it was observed that agglomera-tion of the calcined zeolite beta crystals were signifi-cantly less in the 1C-PU composite membranes thanthat of the 1AS-PU composite membranes.

The lower loading content of 0.1 wt % seemed toresult in somewhat less precipitation of the as-syn-thesized zeolite beta crystals in 0.1AS-PU compositemembrane. For 0.1C-PU composite membranes,where calcined zeolite beta crystals were loaded, theresult was a more homogeneous dispersion andalmost no precipitation of the zeolite crystals (notshown). The loading content seemed to be more im-portant in the as-synthesized zeolite beta–PU com-posite membranes to obtain a more homogeneousstructure. These results further suggest that if as-syn-thesized zeolite beta crystals are required for anyparticular purpose, a low amount of zeolite betaloading should be more desirable.

Dynamic mechanical behaviors

DMA was carried out to determine the changes inthe dynamic properties (tan d, storage modulus E0,and loss modulus E00) of the PU membranes uponthe incorporation of as-synthesized or calcined zeo-lite beta at two different loadings. All materials wereobserved to exhibit a typical elastomeric polymerbehavior with respect to the E0 versus temperaturecurves (Figs. 6 and 7). The glassy state is observed atlow temperatures (approximately �1008C) where theE0 stays at a high modulus plateau and a rubberyplateau is reached with a lower E0 at a temperaturerange of approximately 10–1208C.

Figure 6 compares the elastic behaviors (storagemodulus, E0) of the control PU membranes with the0.1AS-PU and 0.1C-PU composite membranes as afunction of temperature. According to Figure 6, no

significant increase in E0 upon incorporation of0.1 wt % zeolite beta was observed in the glassystate and it was found to be 3740 MPa. On the otherhand, in the rubbery plateau region the incorpora-tion of 0.1 wt % zeolite beta lead to a significantincrease in the E0 of the composite membranes withrespect to the control PU. Among the three samples,it was also seen that the 0.1C-PU exhibited the high-est values for E0 (31 MPa) followed by the 0.1AS-PU(15 MPa) and control PU (8 MPa) in the rubbery pla-teau region of the curves.

E0 is known to be a measure of the material stiff-ness.26 Accordingly, the effect of inorganic reinforce-ment was higher upon the incorporation of the cal-cined zeolite beta with respect to its as-synthesizedform in the rubbery state. Furthermore, the existenceof a region in the rubbery state where the storagemodulus remains constant (i.e., 31, 15, and 8 MPafor 0.1C-PU, 0.1AS-PU, and control PU, respectively)indicates that a stable elastomeric network existednot only in the control PU membranes but also inthe 0.1 wt % zeolite incorporated PU compositemembranes.

Figure 7 compares E0 among 1AS-PU, 1C-PU, andthe control PU at a higher loading content (1 wt %)as a function of temperature. The same control plotswere used in Figures 6 and 7. It can be seen that theincorporation of 1 wt % of all zeolite beta sampleslead to an increase of the E0 in the rubbery statewith respect to the control PU membrane. Similar towhat was observed upon the incorporation of 0.1 wt %zeolite beta samples (Fig. 6), the incorporation of1 wt % calcined zeolite beta (1C-PU) also possessedthe maximum E0 (32 MPa) in the rubbery plateauregion. In the glassy region, the 1 wt % calcined zeo-lite beta (1C-PU) incorporated composite membraneslead to the maximum E0 as 4365 MPa (Fig. 7), in con-

Figure 6 Logarithm of storage modulus (E0) versus tem-perature curves for 0.1AS-PU and 0.1C-PU.

Figure 7 Logarithm of storage modulus (E0) versus tem-perature curves for 1AS-PU, 1C-PU, and 0.1C-PU.

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trary to the identical elastic moduli observed amongdifferent composite membranes with 0.1 wt % zeolitebeta incorporation (Fig. 6). Furthermore, the E0 of the1AS-PU composite membrane in the glassy regionwas observed to be 1861 MPa, which was lower thanthe control PU (3740 MPa) and the 1C-PU (4365 MPa)membranes, indicating a lower stiffness. The ob-served decrease in the E0 of the 1AS-PU compositein the glassy region can be due to a decrease in thedensity of the material. The decreased densities canbe attributed to an increased external porosity,which can be interpreted as the increase in the freevolume of the composites caused by a decreasedinteraction between the polymer and the zeolite as aresult of a higher loading content. It seems thatthe incorporation of calcined zeolite beta leads to amore elastic and stiffer composite material once theyare integrated into the PU membranes. Furthermore,the membranes formed by adding the calcined zeolitebeta were observed to form a more homogeneous(well dispersed) composite membrane. These resultsare in agreement with the observed morphologies ofthe composite membranes by SEM (Figs. 4 and 5).

As shown in Figure 7, the decline of the E0 curvefor the 1AS-PU sample did not reach a constant andstable value (T > 08C) upon increasing the filler con-tent to 1 wt %. E0 decreased from 24 to 9.5 MPa asthe temperature varied from 10 to 1208C. This showsthat the 1AS-PU did not show an ideal elastomericbehavior as was the case for a loading content of0.1 wt % (Fig. 6, 0.1AS-PU). The fact that such abehavior was not observed at low loading contentimplies that there might be an upper limit to theamount of filler that can be used and the desiredelastic properties of polymers are still maintained. Ifthe weight percent of filler is too high in reinforcedcomposites, there may not be enough polymer ma-trix to hold the composite together. On the otherhand, calcined zeolite beta–PU composite membranesshowed stable behaviors in both glassy (� 4365 and3740 MPa for 1C-PU and 0.1C-PU, respectively) andrubbery (� 32 MPa) states at the loadings that werestudied. In the current study, the unstable behaviorof 1AS-PU as a function of temperature can be aresult of the ongoing process of the removal of thetemplate from the zeolite structure with an increasein the temperature. On the other hand, the calcinedzeolite beta incorporated PU composite membranesshowed a more significant stability, which again is aresult of the fact that the zeolite was calcined at5008C before it was incorporated into the polymer(precalcination), and thus has a more stable structure.Furthermore, an increase in the filler content from 0.1to 1 wt % did not seem to make a significant differ-ence in the storage moduli of the C-PU compositemembranes and it was found to be 32 MPa in therubbery region as shown in Figure 7. This may also

be a result of precalcination, where the zeolite beta–PU network was better formed with a more uniformdispersion, and thus the polymer could bear a higherloading of the calcined zeolite beta.

Calcination at high temperatures (e.g., 5008C)removes the template molecules from the pores ofthe zeolite. Furthermore, an enhanced thermal stabil-ity, and increased hydrophobicity are known to beamong the major reasons for considering calcinationof zeolites. Increased hydrophobicity is usuallyrelated with the lowered number of Brønsted andincreased number of Lewis sites. This trend in thedecrease and increase in the Brønsted (tetrahedral Alsites) and Lewis sites (distorted Al sites), respec-tively, upon calcination were shown in our previousstudies.15 The polymer used in the current study,PU, is not only hydrophilic but also has hydrophobicproperties. For the organic–inorganic segments to becompatible, the added inorganic material shouldmatch the organic segment of the composite. Hydro-philic particles and hydrophobic polymers are notcompatible, which results in poor interfacial bonding.Usually organic modifiers are used to impart hydro-phobicity of the inorganic material in such cases.27

Zeolites also possess both hydrophilic and hydro-phobic properties, which can be controlled bychanging the Si/Al ratio or by calcining the zeolitecrystals. In the current study, it was observed thatcalcination lead to a better match and miscibilitybetween the organic–inorganic composites formingmore homogeneously dispersed crystals within thepolymer matrix. The reasons for this can be thebetter compatibility of the hydrophilic/hydropho-bic properties of zeolite particles and PU upon cal-cination of zeolite particles, which results in lessagglomeration and better dispersion of calcinedzeolite particles within the polymer matrix.

Table II summarizes the E0 at 258C of the controlPU and the zeolite beta incorporated compositemembranes. Table II and Figures 6 and 7 show thatthe composite membranes lead to the highest stiff-ness and elasticity with the highest E0 upon incorpo-ration of calcined zeolite beta when compared withAS-PU and control PU membranes in the rubbery

TABLE IIDynamic Mechanical Properties of Control PUand Zeolite Beta–PU Composite Membranes

Name Tg (8C)E0 (MPa)at 258C

Normalizedtan d values

Control PU �43 8 6 0.3 1.00 6 0.10.1AS-PU �40 15 6 0.3 0.95 60.11AS-PU �44 16 6 1.2 0.57 6 0.10.1C-PU �44 31 6 0.3 0.77 6 0.11C-PU �44 32 6 0.3 0.75 6 0.1

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state and particularly around room temperature. Itwas found that as-synthesized zeolite beta incorpora-tion resulted in a twofold, while calcined zeolite betaincorporation resulted in a fourfold increase in theE0 of the control PU. Thus, in the current study, itwas found that calcination had a major influence onthe dispersion, stiffness, mechanical stability, andthe elastomeric behavior of the zeolite beta–PU com-posite membranes with respect to the AS-PU andcontrol PU membranes.

The fact that larger agglomerates were formed inthe as-synthesized zeolite incorporated compositesmay or may not affect the mechanical properties ofthe composites. It was found that the mechanicaland viscoelastic properties of the polymer compo-sites are particle size independent; however, contra-dictions do exist in the literature for submicron crys-tals.28 In the current study, the aim is not to studythe effect of particle size on the mechanical proper-ties; however, it can be speculated that it did nothave a significant effect for the samples prepared inthis study. First of all, the zeolite crystals used werein the micron range. Furthermore, the formation ofagglomerates was found to strongly increase themoduli of the composites, especially at low frequen-cies.28 This was the case in the current study (1 Hz)and the calcined zeolite incorporated composites stillexhibits higher modulus. Thus, it is believed that theresults observed are not solely due to less agglomer-ation of calcined particles in the polymer matrix, butdue to better compatibility of the organic polymer–inorganic calcined zeolite particles.

Figure 8 compares the changes in the tan d curvesof the composite membranes prepared with incorpo-ration of calcined or as-synthesized zeolite beta athigh (1 wt %) and low (0.1 wt %) loadings as a func-tion of temperature. The temperature corresponding

to the maximum of tan d curve is considered as theglass transition temperature (Tg). As shown in Figure8, only one major a relaxation (single Tg) is observedin each composite membrane indicating a single homo-geneous phase. Tg and the relative differences in thenormalized tan d values of the control PU and thecomposite PU membranes are summarized in Table II.

According to Figure 8 and Table II, the control PUexhibits a well-defined relaxation peak centered at�438C, which is ascribed to the Tg of the PU. 0.1AS-PU showed a slight shift in Tg to a higher value of�408C. This indicates that low concentrations of as-synthesized zeolite beta restricted the segmentalmotion of PU chains. With an increase in the loadingcontent of as-synthesized zeolite beta in the PU(1AS-PU), a decrease in the Tg from �40 to �448Cwas observed. Such a change in the Tg as a functionof loading content was not observed for the C-PUcomposite membranes. The final Tg values for 0.1C-PU, 1C-PU, and 1AS-PU composite membranes arenot significantly different from the control PU asshown in Table II. However, the fact that an initialincrease followed by a decrease in the Tg of theAS-PU composite membranes can be an indicationof the formation of an inhomogeneous compositestructure (i.e., agglomeration of zeolite particleswithin the membrane). Another reason may be thepresence of organic templating agent in the overallstructure of the composite membrane, which maybecome more significant at higher loading of 1 wt %.A decreasing Tg was observed by other researchersas a function of temperature and was suggested thatthe modification agent used in the nano-silica par-ticles were acting as softeners in the macromolecularchains of the polymers.5 The fact that the similar Tg

values for 1C-PU and 0.1C-PU composite mem-branes with the control PU, indicates that the addi-tion of calcined zeolite beta in the correspondingamounts to the PU membranes did not have a signif-icant role in the motion of the macromolecularchains in these particular cases. In general, the lowTg values (<08C) are more favorable with respect tothe ones with higher Tg if elastomeric behavioris desired at room temperatures for any specificpurposes.6

The relative heights of the tan d, ratios of lossmodulus (E00) to storage modulus (E0), are known tobe an in indicator of capability of energy dissipationwithin the microstructures.5 According to Figure 8and Table II, tan d intensities of the composite mem-branes showed a decrease in the peak heights, whichsuggests lower dissipation of energy as heat with theincorporation of zeolite beta into the PU. Theseresults suggest a higher mechanical stability againstthe degradation of the zeolite beta–PU compositemembranes. Furthermore, the tan d intensities wereessentially the same for the 0.1C-PU and 1C-PU

Figure 8 tan d versus temperature curves for 0.1AS-PU,1AS-PU, 0.1C-PU, and 1C-PU.

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composite membranes (� 0.75), while the tan d in-tensity decreased from 0.95 to 0.57 when the loadingcontent was increased from 0.1 to 1 wt % for theAS-PU composite membranes. Thus, the loadingcontent did not have a significant effect on the tan dintensities of the C-PU composite membranes as itdid for the AS-PU composite membranes.

Mechanical properties

Mechanical properties of zeolite beta–PU compositesmembranes are shown in Table III. The compositemembranes showed higher Young’s modulus, tensilestrength, and elongation at break in comparisonwith the control PU sample. For 0.1C-PU samples,these values demonstrated a change from 5 to13.9 MPa, from 1.9 to 4.7 MPa, and from 68.8% to148.5%, respectively. Among all membranes, cal-cined zeolite-PU composites showed the highest me-chanical properties and this is most probably due tobetter dispersion of these particles in the polymermatrix.

Mechanical improvements of the composites ingeneral can be due to the nature of zeolites that al-ready possess OH groups in their nature. There arevarious studies with montmorillonite where surfacemodifications were made to increase the number ofsurface OH groups to achieve better dispersion inPUs.29 It is well known that H-bond formationamong urethane groups greatly contributes to thestrength and modulus of PU’s. This interaction canform between ��NCO groups of PU and especiallythe ��OH groups of calcined zeolite leading to a bet-ter interaction between these two components. Theseresults in general are in agreement with the dynamicmechanical behaviors, SEM, and ATR-FTIR resultsthat again suggest that calcination had a major influ-ence on the dispersion and mechanical properties ofthe composite membranes.29

CONCLUSIONS

In this work, zeolite beta–PU composite membraneswere prepared and investigated. The effect of incor-

poration of as-synthesized (AS) and calcined (C) zeo-lite beta into PU was studied at two different load-ings. In general, the incorporation of zeolite beta intothe PU improved the mechanical properties of thePU against deformation, which was more signifi-cantly observed for the calcined zeolite beta crystals.The C-PU composite membranes lead to the higheststiffness, elasticity with a highest E0, tensile strength,and elongation at break when compared with AS-PUand control PU membranes in the rubbery state. Themechanical properties of the C-PU were also main-tained at a higher loading content. However, if as-synthesized zeolite beta is desired to be used for anyparticular purpose, higher loading contents may notbe practical, since there may be an upper limit to theamount of filler that can be used while the proper-ties of the polymer are still maintained. Taken to-gether, these results suggest that the mechanicalproperties of the PU was changed and improvedupon the incorporation of zeolite beta without alter-ing the chemical structure of the PU membranes.The incorporation of the calcined zeolite beta lead tomore homogeneous, well dispersed, stiffer, andmechanically stronger composite membranes. Fur-ther studies on PU composite membranes usingmodified zeolite beta crystals for desired purposesare currently under investigation.

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TABLE IIIMechanical Properties of Control PU and Zeolite

Beta–PU Composite Membranes

SampleYoung’s

modulus (MPa)Tensile

strength (MPa)Elongationat break (%)

Control PU 5 1.9 68.80.1AS-PU 8 2.3 87.11AS-PU 8.1 2.5 75.130.1C-PU 13.9 4.7 148.51C-PU 12.1 4.1 137.1

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