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Preparation, characterization, and properties of polyacrylonitrile-silica gel anion exchange composite fibers.

INTRODUCTION

Organic--inorganic ion-exchange composite materials have the advantages of light weight, flexibility, and mechanical stability because of organic polymer, whereas inorganic part provides thermal stability and inorganic matrices of co-ions for exchangeable counter ions. Such composite materials have excellent ion-exchange behavior and also can be used in separation of toxic metal ions and in electroanalytical applications (1-7). Electrical conducting behavior was an additional advantage of such materials; thus, electrically conducting composite cation exchangers were developed as new class of advance materials (8-10).

A large number of cation exchange materials are reported in the literature, but very few anion exchange materials have been studied so far. The anion exchanger has its own significance in the field of ion-exchange chromatography, because separation and identification of anions are also important from environmental as well as industrial point of view.

Some fibrous anion exchangers were synthesized on the basis of industrially produced PAN fiber (11-18). These materials are industrially produced under trade mark FIBAN (13), VION (12), (18), and NITRON (19). There are numerous reports on polymeric organic-inorganic fibrous cation exchange composite materials (20-22), but little work has been done on the synthesis and applications of polymeric organic-inorganic fibrous anion exchange composite material. Extremely fast rate of sorption, easy permeability of filtering layers for liquids, and high adsorption capacity are the main advantages of fibrous ion exchanger compared with granular resin. Literature survey reveals that the silica gel modified with polymers was proposed to be used in chromatographic ion-exchange separations and can be exploited for the preparation of anion exchange materials (23-27).

It is interesting to prepare a new type of organic-inorganic anion exchange composite fibers (AECFs) based on PAN and silica gel by an ecofriendly route. The following pages summarize the preparation, characterization, and anion exchange behavior of new polyacrylonitrile (PAN)-silica gel composite fibers.

EXPERIMENTAL

Reagents and Chemicals

Following chemicals were used in the preparation of composite fibers. Silica gel-H (E. Merck, India), PAN from Research, Design and Standards Organization, India, and tetrahydrofuran (THF; Qualigens, India). All other reagents and chemicals were of analytical grade (AR).

Preparation of PAN-Silica Gel AECFs

PAN-silica gel AECFs were prepared in various weight ratios of silica with PAN by stirring with magnetic bar (28), (29). A total of 500 mg of PAN was dissolved in THF at room temperature. A controlled amount of silica gel (100, 200, 300, 400, and 500 mg) was dispersed into PAN solution in THF. Mechanical stirring was applied for at least 24 h at room temperature to obtain homogeneous silica gel-dispersed PAN solution. The silica gel-dispersed PAN solution was dropped in vigorously stirring water (demineralized water, DMW) to prepare the fibers using magnetic stirrer. Composite fibers accumulated on the surface of the water. After that the fibers were filtered off, washed with doubly distilled water, and then dried at room temperature. The dried fibers were converted into - form by treating with 1 M NaC1 for 2 days at room temperature (25[degrees]C) with occasional shaking intermittently replacing the supernatant liquid with 1 M NaC1 two to three times. The excess was removed after several washings with DMW and finally dried at room temperature (25[degrees]C).The condition of preparation and the ion-exchange capacity (IEC) of the AECFs are given in Table 1.

TABLE 1. Conditions of preparation and ion-exchange capacity
of various PAN-silica gel anion exchange composite fibers.

Membrane          Amount of  Amount       Amount of  Stirring
no.       polyacrylonitrile      of  tetrahydofuran  time (h)
                       (mg)  silica            (ml)
                                gel
                               (mg)

AECFs-1                 500     100              50         6

AECFs-2                 500     150              50        12

AECFs-3                 500     200              50        12

AECFs-4                 500     300              75        24

AECFs-5                 500     400              75        24

AECFs-6                 500     500              75        24

Membrane  Appearance  Agitation  lon-exchange
no.           of the      speed      capacity
              sample      (rpm)    (mequiv/g)

AECFs-1     Hard and        300          0.80
               white

AECFs-2     Hard and        300          1.25
               white

AECFs-3     Hard and        300          1.39
               white

AECFs-4     Soft and        300          1.40
               white

AECFs-5     Soft and        300          1.45
               milky
               white

AECFs-6     Soft and        300          1.98
               milky
               white


Chemical Composition

The chemical composition of PAN-silica gel AECFs (sample AECFs-6) was determined by using elemental analyzer (Elementary Vario EL III Carlo-Elba, model 1108) and AOAC 18th Edition for carbon, hydrogen, nitrogen, oxygen, and silicon, respectively. The percentage compositions of the material are presented in Table 2.

TABLE 2. Percent composition of PAN-silica gel
anion exchange composite fibers.

Element   Weight (%)

Si             30.97
C              35.19
H               5.42
N               7.62
O              19.81


Fourier Transform Infrared Studies

The Fourier transform infrared (FTIR) spectrum of PAN, silica gel, and PAN-silica gel AECFs were obtained using FTIR spectrophotometer (Perkin-Elmer, USA, model Spectrum-BX) in the original form by KBr disc method at room temperature.

X-Ray Analysis

X-ray diffraction (XRD) patterns of PAN, silica gel, and new PAN-silica gel AECFs prepared and described in this work were obtained at a PHILIPS PW1710 instrument equipped with a Cu anode, automatic divergence slit, and a graphite monochromator, under the following experimental conditions: CuK[alpha] radiation, 1.54 [Angstrom]; generator tension, 45 kV; generator current, 40 mA; intensity ratio ([[alpha].sub.2]/[[alpha].sub.1]), 0.500; and 2[theta] range between 5[degrees] and 7[degrees].

Scanning Electron Microscopy Studies

Surface morphology of the original form of PAN, silica gel, and PAN-silica gel AECFs was studied using scanning electron microscopy (SEM; LEO 435-VF) at various magnifications.

Thermal Studies

The degradation process and the thermal stability of PAN, silica gel, and PAN-silica gel AECFs were investigated by thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential thermal gravimetry (DTG) using thermal analyzer-V2.2A DuPont 9900, heating sample from ~ 24 to ~ 1000[degrees]C at the rate of 10[degrees]C/min in the nitrogen atmosphere at the flow rate of 200 ml/min.

Ion-Exchange Behavior of PAN-Silica Gel AECFs

Ion-Exchange Capacity Measurements. In the Cl Form. IECs of the AECFs samples were determined using the Mohr method (30). Accurately weighed dry AECFs were converted to [Cl.sup.-] ionic form through immersion in 1 M NaCl for 2 days. Excess NaCl was washed off and then the AECFs were immersed in 200 ml of 0.5 M [Na.sub.2]S[O.sub.4]. The amount of [Cl.sup.-] was determined using titration with AgN[O.sub.4]; anion exchange values were obtained and expressed as mequiv/g of dry exchanger (In Cl form).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where X is the exchanger phase.

In the Arsenate Form by Indirect Determination. To evaluate the IEC of selected sample (AECFs-6) by indirect determination, accurately weighed dry AECFs were converted to arsenate form through immersion in 0.1 M sodium hydrogen arsenate ([Na.sub.2]2H[A.sub.s][O.sub.4] * 7[H.sub.2]O) for 2 days. Excess sodium hydrogen arsenate was washed off and then the AECFs were immersed in 200 ml of 0.5 M [Na.sub.2]S[O.sub.4].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where X is the exchanger phase.

The amount of arsenic was determined using titrimetric determination of arsenate after precipitation as silver arsenate. An indirect determination of arsenic, based on precipitation, filtration, and washing of silver arsenate, was carried out by dissolving the precipitation in nitric acid and titrating the silver by Volhard's thiocyanate method (31). The anion exchange values were obtained and expressed as mequiv/g of dry exchanger (in arsenate form).

In the Arsenate Form by Direct Determination. Accurately weighed dry AECFs (AECFs-6) were converted to arsenate form through immersion in 0.1 M sodium hydrogen arsenate ([Na.sub.2]HAs[O.sub.4] * 7[H.sub.2]O) for 2 days. Excess sodium hydrogen arsenate was washed off and then the AECFs were immersed in 200 ml of 0.5 M [Na.sub.2]S[O.sub.4]. The amount of arsenic was determined using portable coulometric analyzer (Arsenomate, Estran). The anion exchange values were obtained and expressed in mequiv/g of dry exchanger (in arsenate form).

Effect of Eluant Concentration. To find out the optimum concentration of the eluant for complete elution of Cl ions, a fixed volume (250 ml) of sodium sulfate ([Na.sub.2]S[O.sub.4]) solutions of varying concentration was passed through a column containing 500 mg of the exchanger in the Cl form with a flow rate of ~0.5 ml/min. The effluent was titrated with AgN[O.sub.3] for the eluted Cl ions.

Elution Behavior. A column containing 500 mg of the exchanger (AECFs-6) in CF form was eluted with [Na.sub.2]S[O.sub.4] solution of this concentration in different 20 ml fractions with minimum flow rate as described above, and each fraction of 20 ml effluent was titrated with AgNO3 for the [Cl.sup.-] ions eluted out.

RESULTS AND DISCUSSION

Various samples of organic-inorganic PAN and silica gel-based AECFs were prepared by stirring with magnetic bar (28), (29) under different stoichiometric ratios. A variety of AECFs by varying the stoicheometry between PAN and silica gel were prepared and highest anion-exchange capacity was observed in 1:1 stoichiometry (Table 1). Because of higher IEC and reproducible behavior, sample AECFs-6 (Table 1) was selected for further studies. All PAN-silica gel AECFs samples were in white color fibers, light weight, and suitable for use in the ion-exchange process. The ion-exchange fibers possessed better [Cl.sup.-] IEC than granular silica gel-based anion exchanger (32). It may be due to more sites available for exchange in fibrous material. The IEC of the selected sample AECFs-6 (Table 1) in the arsenate form by indirect determination was found to be 1.16 mequiv/g and by direct determination using portable coulometric analyzer was found to be 1.56 mequiv/g.

The column elution experiments indicated that the concentration of the eluants depends on the rate of elution. It is evident from Fig. 1 that a minimum molar concentration of [Na.sub.2]S[O.sub.4] was found to be 1.50 M for the AECFs-6 sample for the maximum elution of the CF ions from 500 mg of the anion exchanger. The elution behavior indicates that the exchange was quite fast as only 200 ml of the [Na.sub.2]S[O.sub.4] solution (1.50 M) was enough to release all the exchangeable [Cl.sup.-] from 500 mg of the PAN-silica gel sample (Fig. 2).

The FTIR spectra of the PAN, silica gel, and PAN-silica gel AECFs are shown in Fig. 3. In the spectra of PAN, a band appears at 2243 [cm.sup.-1] shows C[equivalent to]N stretching frequency, whereas in the PAN-silica gel composite fibers the C[equivalent to]N frequency (2361 [cm.sup.-1]) peak is shifted to higher energy side because of field effect. This effect may be attributed to the complexation of PAN and silica gel leading the shift of electron density from N atom toward the silicon. The presence of silica gel in the composite fibers is further strengthened because of the presence of broad band at 3433 [cm.sup.-1], which may be assigned to the vibration of hydroxyl groups which in turn is bonded to Si. Rodriguez et al. (33) have also reported the silanol group (Si--OH) at 3450 [cm.sup.-1]. The broadness of this hump may also be due to the presence of occluded water molecule. Further, some peaks are shifted in the composite fibers from 1070 [cm.sup.-1], 799 [cm.sup.-1] to 1090 [cm.sup.-1], 801 [cm.sup.-1], respectively, indicating the formation of PAN-silica gel AECFs.

The percent composition of Si, C, H, N, and 0 in the composite fibers was found to be 30.97, 35.19, 5.42, 7.62, and 19.81%, respectively. The molar ratio of the Si, C, H, N, and 0 in the composite fibers estimated as 2:5.27:9.77:1:2. A tentative formula of AECFs can be proposed as: [(C[H.sub.2]CHCN).sub.2]Si[O.sub.2] * [H.sub.2]O.

Figure 4a-d shows the SEM image of pure PAN, silica gel, and PAN-silica gel composite fibers at different magnifications, indicating the binding of inorganic material, i.e., silica gel with organic polymer, i.e., PAN. The difference in surface morphology of organic polymer, inorganic material, and composite fibers can be seen clearly. It has been revealed that after binding of PAN with silica gel, the morphology has been changed. Figure 4c and d shows that the composite fibers were randomly distributed in the form of fibrous web. The PAN-silica gel AECFs were observed soft and flexible.

Figure 5 illustrates the diffractograms of pure PAN, silica gel, and PAN-silica gel composite fibers in the 2[theta] range between 5[degrees] and 70[degrees]. In the diffractogram of pure PAN, there is no sharp diffraction peak, confirming their noncrystalline nature. However, in the case of silica gel some small peaks can be observed at 38[degrees], 41.5[degrees], and 65[degrees]. On the addition of silica gel in the PAN matrix, in the diffractogram at 2[theta] values 41.78[degrees], 43.38[degrees], 48.84[degrees], and 50.69[degrees] (d-spacing around 2.165, 2.08, 1.86, and 1.79 [Angstrom]), some sharp peaks can be observed, indicating the formation of PAN-silica gel composite fibers.

Figure 6 shows the TGA curves of silica gel, pure PAN, and PAN-silica gel composite fibers. Silica gel is stable up to 800[degrees]C, whereas PAN was initially stable up to ~ 150[degrees]C (2.73% mass loss, probably because of physisorbed water molecules evaporated at this temperature). The gradually weight loss with the same rate up to about 415[degrees]C can be ascribed to the degradation of the polymers unsaturated groups, whereas beyond 415[degrees]C pure PAN is stable up to 800[degrees]C with 2% further mass loss. The PAN-silica gel composite fibers (Fig. 6) were found stable up to 450[degrees]C and gradually decomposed at 580[degrees]C because of degradation of PAN and subsequently remain stable up to 800[degrees]C with 2.8% further mass loss. The total mass loss up to 800[degrees]C has been estimated to be about 19.90, 73.05, and 39.89% for silica gel, PAN, and PAN-silica gel composite fibers, respectively, and at 800[degrees]C, the percentage of residual weight of silica gel, PAN, and PAN-silica gel composite fibers is 80.10, 37.95, and 60.11%. As a result, these data confirm that the presence of silica gel in the PAN-silica gel composite fibers is responsible for the higher thermal stability of the composite fibers in comparison with pure PAN.

Figure 7 shows the DTA curve of pure PAN, silica gel, and PAN-silica gel composite fibers. DTA of PAN was found to exhibit two exothermic peaks at 358[degrees]C (56.08 [micro]V) and 414[degrees]C (51.25 [micro]V). The exothermic peaks at 358[degrees]C correspond to decomposition stage of PAN between 150 and 395[degrees]C as shown in TGA, whereas the exothermic peak at 414[degrees]C corresponds to second decomposition stage of PAN from 395 to 500[degrees]as indicated in TGA (Fig. 6). However, PAN-silica gel composite fibers exhibited exothermic peaks at 332[degrees]C (32.98 /N) and 447[degrees]C (43.01 [micro]N), which correspond to decomposition stage between (300-395[degrees]C) and (395-550[degrees]C), respectively, in TGA (Fig. 6).

DTG analysis of pure PAN, silica gel, and PAN-silica gel composite fibers was studied as a function of rate of weight loss ([mu]g/min) versus temperature (Fig. 8). In case of pure PAN, decomposition at 175 and 359[degrees]C was found with 25.12 and 451.53 tg/min weight loss, respectively. However, in the case of PAN-silica gel composite fibers, the decomposition was observed at 371[degrees]C with 525.04 [micro]g/min weight loss. Thus, it could be concluded from the DTG studies that the rate of thermal decomposition was higher in the case of pure PAN-silica gel composite fibers, whereas in the case of pure PAN, the rate of thermal decomposition is lower. Better thermal resistance of pure PAN-silica gel composite fibers was due to incorporation of silica gel in the PAN matrix.

CONCLUSION

In this article, PAN-silica gel AECFs containing different amount of silica gel were prepared by stirring with magnetic bar. IEC, effect of eluant concentration, and analysis of elution behavior were also carried out to understand the IECs of the AECFs. SEM results showed the change in the morphology of the composite fibers, when silica gel was added in PAN. Composite fibers were randomly distributed in the form of fibrous web. FTIR spectra and XRD results confirm the formation of PAN-silica gel composite fibers. The addition of silica gel also changes the thermal properties of PAN-silica gel composite fibers.

ACKNOWLEDGMENTS

The authors are thankful to Department of Applied Chemistry, Z.H. College of Engineering and Technology, A.M.U. (Aligarh) for providing research facilities.

Correspondence to: A.A. Khan; e-mail: asifalikhan2008@gmail.com

Contract grant sponsor: Ministry of Environment and Forest; contract grant number: 19-36/2007-RE; contract grant sponsor: University Grants Commission.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2013 Society of Plastics Engineers

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Asif Ali Khan, Umair Baig

Analytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, Uttar Pradesh, India
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Date:Sep 1, 2013
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