Indian lournal of Chemistry Vol. 41A, August 2002, pp. 1562-1568

In situ preparation of polyaniline/silica composites and study of their adsorption characteristics

AI-Nakib Chowdhury· & M A Yousuf

Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka-IOOO, Bangladesh

and

M. Muhibur Rahman & A Q M QuarnruI Hassan

Department of Chemistry, University of Dhaka, Dhaka-IOOO, Bangladesh

Received 22 Janllary 2002; revised 14 April 2002

Polyaniline/silica, poly-o-toluidine/silica and poly-2-chloroaniline/silica composites have been prepared under a variety of synthetic conditions by polymerizing the respective monomers in the presence of silica prepared in situ from an aqueous sodium silicate solution. The composites, thus synthesized, have been characterized by a wide range of experimental techniques including elemental analysis for silica content, density, infrared spectroscopy and sedimentometry for panicle size determination . When used as column material in the IGC experiments, synthesized composites have been found to be capable of separating the alkanes (CS-C9) from their mixture. The composites have been further examined for their surface free energy and specific surface area. The observed BET surface area of the composites are found to be nearly ten times higher than that of the bulk polymer.

Over the last two decades or so, novel functional material, such as conductive polyaniline (PANI), has been studied extensively because of its unique electrical, optical and chemical properties. These attractive properties, together with the ease of PANI synthesis in aqueous media and its long-term environmental stability, makes it a suitable candidate for a variety of technological applications 1-4. However, the intractable and unprocessable nature of the material makes its practical utilization limited. Preparation of composites of conducting polymer has long been considered to provide a potential solution to the processability problem as well as to enhance the material properties with desirable physical and mechanical characteristics. One way of making these composites involves synthesizing the conductive pol ymer inside the matrices of conventional polymers using chemicals-II as well as electrochemical 12- lg

polymerization. In addition, studies pertaining to the chemical preparation of conductive polymer composites without involving any insulating conventional polymer have also been reported 19-21 . In these approaches, monomer is polymerized in the presence of commercially available colloidal inorganic oxide particles which act as a high surface area colloidal substrate for the precipitating polymer

nuclei leading to the formation of conducting polymer-inorganic oxide composite. These materials have been reported to exhibit relativelv high surface

22?3 d 'f' f 2a d energy . - an specl IC sur ace area compare to conventional polymers .

Choice of synthetic route and conditions are known to be effective means in controlling the surface properties of a material. Instead of using commercial silica as used by the earlier workers, in the present work, attempt was made to prepare PANI / silica composites by oxidizing the monomers in the presence of silica prepared in situ from a silicate solution and the properties of the composites thus prepared are examined for their adsorption capacities in terms of surface free energy and specific surface area.

Materials and Methods Aniline, a-toluidine and 2-chloroaniline purchased

from E. Merck, Germany were purified by distillation under nitrogen atmosphere. Hydrochloric acid (HCl), concentrated sulphuric acid (H2S04), hydrofluoric acid (HF), sodium hydroxide (NaOH), ammoniumpersulphate ((NH4)2S20g), pentane (C5),

hexane (C6), heptane (C7), octane (Cg) and nonane

CHOWDHURY et al.: IN SITU PREPARATION OF POL YANILINElSILICA COMPOSITE 1563

(C9) also from E. Merck, Germany were of analytical grade and used as received. Nitrogen gas of purity 99.997% was supplied by BOC, Bangladesh Limited.

Polymer/silica composites viz., polyaniline/silica (PANVsilica), poly-o-toludine/silica (CH3-

PANVsilica), and poly-2-chloroaniline/silica (Cl­PANVsilica) were prepared chemically under different synthetic conditions of pH and tem~erature, following the procedure described elsewhere 5 except that silica was prepared in situ by hydrolysis of a silicate solution. In a typical synthesis, 7.5 g of sodium silicate was dissolved in 800 mL de-ionized water and the solution was filtered. The pH of the solution was adjusted to the desired value by adding HCl solution. The solution was continuously stirred using a magnetic stirrer, and its temperature was adjusted to the desired value with the help of a magnetic stirrer cum hot plate. 4.71g of (NILhS20g was then added to the stirred silicate solution and dissolved. 1.0 mL monomer (aniline or its derivatives) was then injected with the help of a syringe to the above silicate solution. The reaction yielded a deep green colloidal product within few seconds. The reaction mixture was stirred occasionally and left over night for the completion of polymerization. The colloidal mixture was then sedimented by centrifugation (in Hettich Universal 16A, Germany). The resulting dark green sediment was redispersed in de-ionized water in an ultrasonic bath. The centrifugation-redispersion cycle was repeated several times in order to remove free silica particles and soluble by-products. The solid mass was then dried at 40°C under vacuum in a controlled heating vacuum oven (Gallencamp, England). The dried mass was ground to powder and then passed through 100 mesh sieve prior to storing under vacuum for characterization.

Five different samples of PANl(silica were synthesized at three pH values of the reaction medium viz. 3.1, 7.0 and 11.3 and at temperatures 0, 27 and 50°C while CHrPANllsilica and CI-PANllsilica were prepared at pH 7 and 27°C following the same procedure described above.

Hydrofluorization method26 was employed to analyze the silica content of the composites. For this purpose, Ig of the sample was treated with 10 mL of HF and 1 mL of concentrated H2S04. Density measurements were carried out in a pycnometer (Micromeritics Multivolume Auto Pycnometer, model no. 1305, USA) following the conventional standard

method. Sedimentation method27 was employed for the determination of particle size of the synthesized composites. For this purpose, the composite samples were dispersed in water and dispersions of known density and viscosity were obtained. The dispersion was then introduced into the sample holder of micromeritics sedigraph (model no. 5100, USA). Data for particle size was made available from the computer interfaced with the sedigraph. Infra red (IR) spectra of all the samples were recorded on an IR spectrophotometer (IR-470, Shimadzu, Japan). For this purpose, samples were mixed and ground with pure KBr and finally compressed to a thin pellet for IR measurement. Synthesized materials stored previously were used as column material for inverse gas chromatography (lGC) measurements. For lGC studies, columns were prepared using 150 mm long stainless steel tubes of Imm internal diameter filled with the composites. The samples were kept in place with the help of glass wool and supporting glass rods. Measurements were carried out using a gas chromatograph (Shimadzu, model GC 14A) fitted with a flame ionization detector. The alkanes, C5, C6,

C7, C8 and C9 were used as probe and nitrogen gas of 99.997% purity was employed as carrier gas. A mixture of 0.5 ilL probe was injected manuall y by a micro syringe. Retention data were recorded graphically at the maximum column sensitivity. During injection and detection of the probes , temperature of the system was maintained at 150°C. The column was conditioned at 120°C with a flow of nitrogen gas for 48 hours prior to chromatographic measurements. Surface free energies of the samples were derived utilizing the experimental data obtained from IGC measurements.

Specific surface areas of the composites were determined by the BET method. For this purpose, isotherms for the adsorption of nitrogen on the composite solids at 78 K were measured with the help of a volumetric apparatus, in which the adsorption was monitored by following the pressure change during the course of adsorption with the help of a mercury manometer.

Results and Discussion When aniline is oxidized in the presence of silica

particles prepared in situ from a silicate solution , colloidal PANllsilica composites are produced. The silica particles act as a high surface area colloidal substrate for precipitating PANI yielding a colloidal

INDIAN J CHEM. SEC A. AUGUST 2002

hydrocarbon. The correctness of assignment lecked by injecting individual hydrocarbons. It s that the PANI/silica composites are suitable 1 materials for the separation of hydrocarbons i mixture. Polypyrrole/silica nanocomposites lIso been reported35 as an effective column iI. It may be mentioned that a si lica-supported talyst with 15% Pt, could not separate arbons from the mixture36

.

is a versatile technique that can provide Idynamic information on both the surface and roperties of materials over a wide temperature In the present work, attempt was made to

te surface free energy by adopting the method .ed earlier by Dorris and Gar/ 7

. According to :thod, a plot of -RTIn(VN) versus the number of atom, (nc) in the alkanes results in a li near

tion with a slope equal to /).}-fH 2, the free of adsorption per methylene group (VN is the

~ntion volume, R is the gas constant and T is

0.007 ~

/ 0 . 006

0 .005

0 . 004

0.003

0. 002

0 .001

0 . 0000 300 400 500

Equi li brium Pressure (mm Hg)

~ET isotherm for adsorption of N2 at 75K on PANI/si lica

I at pH 7 and 27°C.

the column temperature). The experimental re: shown in Fig. 4, where the plots of -RT In(VN the number of carbon atoms, nc, obtained at 1 ~ PANI, PANI/silica, CH3-PANl/silica ar PANI/silica composites, show excellent linear results clearly show relatively steeper slope composite samples compared to that of bul indicating higher surface free energies than 0

In fact, surface free energies of the studied n were calculated to be 17.46, 24.14, 19.33 an mJ/m2 for PANI, PANI/silica, CHr PANl/sii CI-PANI/silica, respectively. This finding is agreement with an earlier repore3 suggesting higher surface free energy value of the co materials may be due to the micro porosit) materials.

BET technique was adopted to measure area of the bulk PANI and the PANI/silica cor studied. In order to evaluate the BET surface the materials, adsorption isotherms were fi tted

250

200 ,.-....

M

8 <.) 150 '-'

.> ----N

I 100 ,....... '-'

---N

50

o 0.0 0 .1 0.2 0.3 0 . 4 0.5

Z Fig.f>.-Linear pl ot o f Z/( I-Z)*V versus Z for th

PANI/silica prepared atpH 7 and 27°C.

Table 2-Data for the BET surface area of the studi ed samples

) f the sample Preparation condition Surface area

;ili ca iili ca iilica iilica ii lica !\NI/silica

pH Temp (0C) 7 27 7 0 7 27 7 50

3. 1 27 11.3 27

7 27

(m2/g)

25 .9 160.2 252.7 184.3 127.3 178.4 53.3

CHOWDHUR Y et al.: IN SITU PREPARATION OF POLY ANILlNFJSILICA COMPOSITE 1567

equation38. Surface area was determined for all the

studied matrices from the linear plots of the adsorption data according to BET equation. Figure 5 shows a typical BET isotherm for PANI/silica composite. The monolayer and multi player regions may be noted in the isotherm. Figure 6 shows the plot of ZJ(l-Z)*V versus Z, (where relative pressure, Z=PIPo V is the volume of gas adsorbed per gram of adsorbent at equilibrium pressure P and Po is the saturation pressure of the gas at the temperature of adsorption). The plot seems to be linear well as indicated by the straight line through the data points. Experimentally observed surface area of the studied materials is described in Table 2. It can be seen from the results that the PANI/silica composites have higher surface area than that of the bulk PANI. The higher surface area of PANI/silica samples may arise due to the incorporation of silica into the PANI matrices. In fact, the presence of silica into the matrices has been evidenced by chemical analysis, density and IR measurements as described earlier. It is interesting to note here that the change in synthesis condition e.g .. pH and temperature, yielded solids with different surface area although all the samples contain almost the same amount of silica. This finding suggests that the synthesized materials may differ in porosity and average particle size to exhibit different surface area of the studied samples which were prepared under different conditions. In fact , considerable porosity exhibited by some organic polymer/inorganic oxide composites has also been reported by the early workers3l

. However, when derivative of aniline such as o-toluidine and 2-chloroaniline were used in the preparation of respective polymer/silica composites, BET measurements predicted a significantly lower surface area compared to that of PANI/silica matrix as can be seen in Table 2. This result may indicate the influence of functional group that may be attributable to their lower surface area. However, in the present work, attempt was not made further to clarify their lower surface area values.

To conclude PANI/silica composites are formed when aniline and its derivatives are polymerized in the presence of silica prepared in situ from aqueous sodium silicate solution. The composites thus synthesized showed silica content of 11 % and higher density than that of the bulk polymer. PANI/silica composites can separate the hydrocarbon (C5-C9)

efficiently from their mixture and thus the studied

composites may be used as column materials. The composites also exhibit higher surface energy and BET surface area than those of the bulk polymer studied.

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