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제조 및 특성 연구

조 규 진, 문 일 식, 이재석+

순천 대학교 공과대학 화학공학과, 전남 순천시 매곡동 315번지

／광주 과학기술원 신소재공학과, 광주광역시 쌍암동 572번지

Preparation and Characterization of Polypyrrole-Grafted Silica Through

a Self-Assembled Monolayer

Gyoujin Cho, Ilsik Moon, Jae-suk Lee+

Department of Chemical Engineering, Sunchon National University Sunchon, Cheon Nam 540-742 Korea.

／Department of Material Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju, 506-303 Korea

Introduction.

Since the 1970s, much has  been reported regarding for organic conducting polymers (OCPs), (e.g., polyaniline, polypyrrole, polyacetyleac, etc.). These "synthetic metals," which can stwich reversivly from in oxidation state have led to the increasing displacement of both conventional metals and inorganic materials in variety of applications, e.g., optical displays, sensors, rechargeable batteries, membranes, electromagnetic interface shielding, nonlinear optics, and microelectronics.1

The material's futher application has been dwarfed by the technical challenges they posed. For most crucial thing, most OCPs are unstable in air, and all of them are insoluble. However, recently, new process technology has been developed to address some of the material's inherent  problems.2 In addition, development work has been shifted from its original base such as batteries, electronics component, etc. to coatings such as corrosion-resistant coating, electrostatic component dissipation coating, etc.3

Polypyrrole (PPY) thin films has been shown to work quite well in coatings as protective layers, sensors, and adsorbents for protein separations.4,5 However, the adhesion of PPY films on surface is a major obstacle to be solved for using industiral production. In recent report, the adhesion problem in PPY coating can be solved using N-(3-trimethoxyl-silyl) alkyl pyrrole which can chemisorbed on hydrated metal oxide surfaces and □-(N-pyrrole) alkane thiol which can chemisorbed on gold surfaces. However, those chemicals are very expensive for using in industrial scale production. Therefore, the new method of an alternative inexpensive procedure without sacrificing the electrical properties of PPY is highly demanded.

In this paper, the new method (Scheme 1) for PPY coating on silica using less expensive 3-(Trimethoxyl silyl) propylmethacrylate (TSM) self-assembled monolayer as a grafting site has been reported. The resulting PPY coated on both silica particles and glass surface has been characterized using AFM, SEM, and electrical conductivity measurements.

Materials and  Methods

Pyrrole (98%, Aldrich) was purifide by passing it through a short column of basic alumina, activity grade I (Sigma). (p)-xylene, acetone, and 3-(Trimethoxyl silyl) propylmethacrylate (TSM) (Aldrich) was used without further purification. As a particle substrate, silicone dioxide (325 mesh, Aldrich) was used after cleaning the surface by soaking them into 6 M HCl for 24 h, and then washing with distilled water until the washed water showed pH level of 6. As a plate substrate, microscope glass slids (Fisher) was cut (2□ × 1□) and cleaned them by soaking into 6 M HCl solution for 24 h, and then kept them in a large volume of double distilled water. Water, used in the process, was double distilled (registivity≥13 □□-1). Ammonium persulfate (99.99+%, Aldrich) as an oxidative initiator used as received.

Surface modification of silicone dioxide powder has been carried out based on the previously reported method. 1g of cleaned silicone dioxide powder was put into 100 ml round bottom flask with stirring, and then added 70 mL of xylene solution (contained 50 mmole of TSM ). The mixtwre was refluxed for 24 h with vigorous stirring and equipped with 80℃ water condenser (to remove generated methanol from the  reaction). The resulting silicone dioxide particles were filtered out and washed several times with acetone and dried under vacuum oven at 30℃ for 48 h. Same procedure was adopted to modify the surface of glass with  10 mmole of TSM. The contact angles were determined with sessile drops on the surface modified glass and pure glass substrate in a microscope equipped with an environment-controlled transparent chamber. The advancing water contact angle □ (H2O) was obtained by measuring the tangent to a 2 □L drop at its intersection with the surface.6 The average value of six measurement taken at different location on the surface showed a maximum standard deviation of ±3o. FT-IR (BIO-RAD 3240-spc instrument) was used to prove that grafting site (vinly group of TSM) is survived after the surface modification reaction. Both surfaces modified silicone dioxide particles and glasses were characterized after grafting polymerization of pyrrole on the surfaces. The grafting polymerization of pyrrole on aqueous solution and surface were initiated by adding an equimolar amount of ammonium persulfate (based on pyrrole) into the sample. In this work, we used 1 g of  surface modified silicone dioxide powders or glass plates with 10 mL of water and various pyrrole concentations. For comparison, polypyrrole films were deposited on both bare silicone dioxide particles and glass plates using same conditions. Scaning electron microscopy (SEM) was used to measure the thickness of the PPY films grafted on glass plates. The morphology of the PPY grafted on glass plates was characterized using AFM. The electrical conductivities of PPY films deposited on both silicone oxide particles and glass plates were measured using contact conductivity method and 4-probe method respectively.

Result and Discussion

Surface modification. After the surface reaction of TSM at xylene and silica particle (or glass plate) interfaces, the resulting surfaces of both silica particles and glass plates were became hydrophobic. The contact angle of the surface modified glass was 117o while bare glass surface was 0o. Based on the result of contact angle  measurement, we can apeculate that TSM was homogeneously held on the surface by chemical bond. In addition, FT-IR has been used to prove that grafting sites, methymethacrylate group of TSM, on the modified surface were not reacted themselves during the surface reaction. We observed that carbonly streching band, characteristic IR band of TSM, appeared at 1690 □-1 from FT-IR spectra. If the methymethacrylate group of TSM was reacted themselves, the carbonly peak should be shown about 1700 □-1. In deed, the carbonly peak at 1713 □-1 was observed after adding initiator and heating of the surface modified silica particles. Therefore, based on above experimental results, we can confirm that TSM are homogeneously self-assembled on silica surface as a monolayer and proceed the PPY grafting reaction on the surface.

PPY grafting on the modified silica surface. For the comparison of conducting properties of between PPY grafted modified silica (PMS) and PPY deposited bare silica (PDS), we investigated the percolation of both them. Fig. 1 displays the results of percolation studies7 for PMS and PDS particles respectively. Although the conductivity-initial pyrrole concentration relationship for PMS shows an inflection point at around 30 mM of initial pyrrole concentration, the transition is much broader and more gradual than that observed for either the PDS or theoretically predicted curve for a two-phase mixture of both a conductor (PPY powders) and an insulator (silica).7 When the two pure components possess greatly different conductivites, the percolation curve predicts a discontinuous change in conductivity, because of percolation effects, at a volume fraction of one-third. This clearly is not observed in the PMS but in the PDS, a clear percolation threshold is observed. In the PMS case, the lack of abrupt percolation behaviour may be attributed to the lack of an effective insulating phase in the system as a result of homogeneous PPY grafting on the surface. Also, the conductivity of both PMS and PDS shows an order of difference using 4 probe measurement. The PMS shows 1.3 s/□ electrical conductivity but the PDS is 0.18 s/□. The thickness of the PMS is thinner (60 nm) than that of PDS (130 nm). Furthermore, adhesion of PPY films to surface of glass plate was found to be drastically increased when a monolayer of TSM was present. Shown in Fig. 2 is a photography of both PPY-deposited glass plate (A) and PPY-grafted glass plate (B) after attempted removal the PPY films with scotch tape. The morphology of both PPY-grafted glass and PPY-deposited plates shows a typical granular type PPY films from AFM.

Reference

(1) Meyer, W. H. Adv. Mater. 1993. 5, 254.

(2) (a) Cawdery, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commum.          1988, 1189 and references there in.

(b) Mehrotra, V.; Keddie, J. L.; Miller, J, M.; Giannelis, E. P. J. Non-Cryst.        Solids 1991, 77, 155.

(3) Roberts, S.; Ondery, G. Chem. Eng. 1996, 103, No 7. page 44-45.

(4) (a) Skothein, T.; Peterson, L. G.; Inganas, O.; Landstrom, I. J. Electrochem. Soc.      1981, 128, 1625.

(b) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104,      2031.

(5) Miasik, J. J.;  Hooper, A.; Tofield, B. C.  J. Chem. Soc. Faraday Trans.1,          1986, 82, 1117.

(6) Wasserman, S. R.; Biebuyck, H.; Whiteside, G. M. J. Mater. Res. 1989,             4(4), 886.

(7) Conem, M. H.; Jortner, J. J. Phys. Rev. Lett. 1973, 30, 696.

Scheme 1. A method for the preparation of PPY-grafted silica.

A (PDS)         B (PMS)

Fig. 1. Contact conductivity of PMS          Fig. 2. Photograph of an PPY films

(open squares) and (filled squares)            remaining after attemped

as a function of initial pyrrole               removal with scotch tape.

concentration