CO gas sensing from ultrathin nano-composite conducting polymer film

Sensors and Actuators B 106 (2005) 750–757

CO gas sensing from ultrathin nano-composite conducting polymer film ¨ Manoj Kumar Ram∗ , Ozlem Yavuz, Vitawat Lahsangah, Matt Aldissi Fractal Systems Inc., 200 9th Avenue North, Suite 100, Safety Harbor, FL 34695, USA Received 4 May 2004; received in revised form 15 September 2004; accepted 23 September 2004 Available online 28 October 2004

Abstract New approaches are needed to solve environmental measurement problems associated with mobile sources for the measurements of CO, NOx , aromatic hydrocarbons HC and particulates. There is a need to develop fast, rapid, cost effective, low power, and non-intrusive rugged sensors that can be easily installed. In order to be useful as a sensor, the device and technique should be able to detect the emissions from at relevant low concentrations. Our effort has focused on the use of highly organized ultrathin conducting polymer/metal oxide (SnO2 and/or TiO2 ) films for sensing of CO gas for the first time. The supramolecular approach has been utilized to fabricate films of conducting materials via in situ layer-by-layer (LBL) self-assembly technique. We have used UV–vis spectroscopy, atomic force microscopy (AFM), conductivity and AC impedance measurements to quantify the different characteristics of the ultrathin nanocomposite polymer films to ensure reproducibility, stability and reliability in CO sensing. © 2004 Elsevier B.V. All rights reserved. Keywords: Gas sensor; CO; Conducting polymers; Nanocomposites; LBL; In situ self-assembly; SnO2 and TiO2

1. Introduction Much of the welfare of modern societies relies on the combustion of fossil fuels. To a greater or lesser extent all processes for energy are associated with the production of toxic by-products such as CO, NOx and aromatic hydrocarbon [1]. A significant part of CO and NOx emission originates from exhaust of motor vehicles, due to their increasing number in each year. The interaction of CO and NOx with sunlight tends to produce O3, which, due to its strongly oxidizing behaviour is believed to be harmful to plants and to the respiratory system of human beings [2]. In order to be useful as an air-monitoring system, a sensor, device and/or technique should be able to detect the gases (CO, NOx , and CO2 ) at environmentally relevant concentrations [3,4]. Metal oxide-based gas sensors are known to be widely used for combustion products. Their poor selectivity is typically bypassed partially by forming arrays of sensors distinguished by their cross-sensitivity [5]. Disadvantages include base∗

Corresponding author. Tel.: +1 727 723 1077; fax: +1 727 723 3007. E-mail address: [email protected] (M.K. Ram).

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.09.027

line drifts upon interaction with poisoning species such as SO2 and NO2 and the dual response of oxides used in the automotive field, particularly SnO2 , to oxidizing (NO2 ) or reducing (CO) gases [6,7]. Doping with metals in SnO2 was shown to significantly increase sensitivity [8,9]. Such gas sensors, however, still lack selectivity and sensitivity at ambient humidity, which increases its conductivity. Conducting polymers (polypyrrole, polythiophene and polyaniline) have shown very promising results for application in gas sensors and are currently used in electronic nose systems [10]. There has also been considerable interest in the use of conducting polymers, particularly, in the form of thin films or blends or composite, as sensors for air-borne volatiles (example: alcohols, ethers, NH3 , NO2 and CO) [10]. The use of polythiophene has shown the detection of ppb of hydrazine gases [11]. Monkman et al. measured NOx and CO at low concentrations using Langmuir–Blodgett films of polyaniline [12]. Electrochemically synthesized conducting polymers, such as polypyrrole and poly-3-methylthiophene doped with copper and palladium detected the CO gas accurately [13]. Polyaniline is unique among the class of electrically conducting polymers in that their electrical properties can reversibly be con-

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trolled by changing the oxidation state of main chain and by the protonation of amine nitrogen chain [14]. Electrical and electrochemical characterization of Langmuir–Blodgett (LB) films of polyanilines have shown good reproducibility in sensing characteristics [15]. Ram and co-workers has shown the use of LB films of poly(ortho-anisidine) for the estimation of protonic acid (HCl and H2 SO4 ) in water at a sensitivity less than 0.1 ppm [16]. The sensing, ageing and mechanical characteristics of the conducting polymer films have been improved by composite fabrication [17,18] similar to the composite inorganic materials which detects the toxic exhaust gases with better accuracy [19,20]. We made an attempt to fabricate CO gas sensor based on highly organized ultrathin films of polyaniline nanocomposite ultrathin films based on our earlier experince. Our approach focuses on the fabrication, characterization and application of polyaniline–SnO2 and polyaniline–TiO2 nanocomposite film for CO gas sensing application. The films were characterized by conductivity, impedance (EIS), UV–vis, cyclic voltammetry (CV), and atomic force microscopy (AFM) techniques. The effect of temperature and SO2 gas sensing on films properties are also discussed at length.

2. Experimental 2.1. Substrate activation and PSS/PANI films The substrate preparation for ultra thin film deposition is an important task for obtaining the desired number of layers. To obtain number of layers, hydrophilic and protonated substrates were prepared using the work of Ram et al. [21,22]. The different thicknesses of in situ self-assembled polyaniline–composite were fabricated. The first layer of the activated surface is deposited using a sulfonated polystyrene (PSS, Mw = 70,000) solution for 15 min, prepared by using 2 mg ml−1 of PSS in water, which provided the charges necessary to adsorb the first layer of the polycation. The PSS and PANI films were also deposited from our earlier work [20]. The active solution for polyaniline (PANI) contained SnO2 solution, 0.026 monomer, 0.012-ammonium perdisulphate and 0.026 M para-toluenesulfonic acid (p-TSA). The active solution was stirred for 15 min after the addition of the

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monomer and then filtered, then used for the deposition of the polyaniline–SnO2 nanocomposite films. The indium–tin oxide (ITO) coated glass and interdigitated electrodes were also treated separately for deposition of the films. These were first washed with methanol/chloroform, and later treated with aqueous ammonia for 5 min to create the hydrophilic surface. The positively charged surface on ITO coated glass plate or interdigitated electrodes was created similarly to the glass treatment. Such substrates were preserved in deionized water before the deposition of in situ self-assembled LBL films. The substrates kept in deionized water can be used for a period of 2–3 days. 2.2. Fabrication of conducting polymer SnO2 composite film The SnO2 particles were prepared using the standard procedure. SnCl4 was made soluble in 1 M HCl then added to 400 ml of deionized water. Aqueous ammonia was added drop-wise to this solution to obtain a dispersion of fine SnO2 particles. It was centrifuged to remove the excess of ammonia and unreacted SnCl4 solution. The Scheme 1 shows the schematic of nanocomposite deposition of PANI–SnO2 films. The pH of the dispersion medium was adjusted by addition of water, followed by the appropriate amounts of aniline monomer and (NH4 )S2 O8 oxidant to start the polymerization of aniline. The polyaniline was synthesized by oxidative polymerization by the use of oxidant as (NH4 )S2 O8 and aniline monomer in HCl media. The PSS treated glass plate, interdigitated electrodes or ITO coated glass plate were introduced in the resulting solution to fabricate the film as a function of time. 2.3. Fabrication of conducting polymer TiO2 composite films The colloidal TiO2 was prepared as using tetra-2-propoxy titanate obtained by dissolution of TiCl4 in 2-propanol. A 0.5 ml of this 8.65% solution in 2-propanol was injected into 5 ml of a 0.05 M aqueous HClO4 solution [23]. The resulting solution was then bubbled with nitrogen for a few hours until a transparent solution was obtained. This TiO2 sol was stabilized by mixing with 5 ml of 0.2 wt.% aqueous

Scheme 1. The polyaniline–nanocomposite film fabrication by in situ self-assembled process.

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Table 1 Resistance measured for in situ self-assembled PANI–SnO2 protonated and unpropotaned substrates Protonated substrate

Unpropotaned substrate

Polymerization time (min)

Resistance (k
)

Polymerization time (min)

Resistance (k
)

15 30 45 60 90

62.9 20.3 40.7 16.5 10.0

15 30 45 60 90

100 20.7 40.7 47.2 28.6 Fig. 1. UV–vis spectra of in situ self-assembled films as shown in figure.

solution of polyvinyl alcohol. The TiO2 -conducting polymer hybrids were fabricated by in situ self-assembly technique. PANI–TiO2 films were fabricated on a polystyrene sulfonate treated substrates. The chemical oxidative polymerization of aniline in the presence of colloidal TiO2 was carried out in HCl acid medium at pH 2 and using 0.5 M ammonium persulfate drop-by-drop addition as oxidizing agent. The pH of the reaction mixture remained constant throughout the reaction. The multilayered films were deposited by treating the PANI–TiO2 film in PSS solution for 5 min. Such reaction continued for 2 h and so multilayered structures of PANI–TiO2 and films were deposited on the substrates (glass, ITO coated glass and interdigitated electrodes).

3. Results and discussion 3.1. The effect of surface treatment The PSS was deposited on the positively charged and hydrophilic surface and later, polyaniline–SnO2 was deposited over such PSS films. The conductivity of the self-assembled polyaniline (PSS/PANI–SnO2 ) films was measured using two-probe technique. The treated surface (protonated one) shows better conductivity than the hydrophilic surfaces in the absence of protonation indicating that the protonated surface coated better PSS layer than the unpropotaned surface leading to better coating of PANI–SnO2 particle. This effect is demonstrated in the results summarized in Table 1. 3.2. Optical characterizations Optical spectroscopy of conducting polymers is a wellknown technique for characterization of the conducting states corresponding the absorption bands of inter- and/or intra-gap states. The UV–vis absorption spectra of polyaniline/PSS are shown in Fig. 1 (curve 1). It shows the bands at 340, 425 and 860 nm are obtained. Fig. 1 (curve 2) depicts the UV–vis spectra of PANI–SnO2 composite self-assembled film deposited on a PSS/glass substrate. Since PSS does not absorb in the spectral region of concern, the absorption is due to PANI–SnO2 composite materials only. A typical absorption spectrum of PANI–SnO2 has three distinct absorption bands: 320, 425 and 900 nm. The 320 nm band is attributed

to the ␲ ␲* transition. The 425 nm band is due to the polaron state. The wide peak at 900 nm is due to the dopant. Fig. 1 (curve 3) shows the optical spectra of self-assembled PSS/PANI–TiO2 film. It reveals two sharp absorption bands at 340 and 800–850 nm for the film made at a pH of 2.8 using HCl. The 340 nm band is attributed to ␲ ␲* transition and the 425 and 800–850 nm bands are due to the protonation of polyaniline (polaron and bipolaron), indicative of the conducting state. Fig. 2 shows the UV–vis absorbance as a function of bilayers. The increase in the number of PSS/PANI bilayers (1–25 bilayers) results in a gradual increase of absorption in the optical spectra. Similarly, this result and those obtained in the case of the above polymers suggest an excellent uniformity in the deposition process such uniformity is required for reproducibility in the gas measurements. The uniformity of PSS/PANI bilayers films was maintained after undoping the films using aqueous ammonia (figure not shown). The UV–vis absorbance increases gradually with the increase in the number of bilayers of PSS/PANI–TiO2 for PSS/PANI–SnO2 LBL films as a function of time, which reveals the uniformity in deposition (figure not shown). 3.2.1. The annealing effect on the films Fig. 3a shows the UV–vis spectrum as a function of annealing temperature. The increase of temperature shows the change in the structure of polyaniline from emeraldine salt to emeraldine base structure. We have also passed CO on PANI–TiO2 films, which show the characteristics bands of emeraldine base at around 330 and 620 nm (figure not

Fig. 2. UV–vis spectra of PSS/PANI films vs. number of bilayers (1–25).

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at this temperature, SnO2 nanoparticles undergo an ordering process. 3.3. Atomic force microscopy (AFM)

Fig. 3. (a) UV–vis spectra of PANI–TiO2 films as a function of annealed temperature. (b) UV–vis absorption of PANI–SnO2 before and after annealing.

shown). The intra-gap energy states are typically affected when one of the conducting polymers is exposed to oxidizing or reducing species. The gases being used in this effort have a doping effect on the polyaniline polymers, and therefore, optical spectroscopy is a good tool for understanding the processes that are taking place when exposed to those gases. Fig. 3b shows the UV–vis spectrum of PANI–SnO2 selfassembled films treated with NH4 OH and annealed at different temperature from 100 to 800 ◦ C. Unlike the surface topography or morphology of the film, the optical absorption spectra (except for the absorption intensity) seem unchanged which reflects the stability of the electronic structure of the material. The PANI–SnO2 film deposited on glass (10 min deposition) was initially treated with aqueous ammonia to yield the emeraldine base then annealed at various temperatures up to 800 ◦ C for 2 h at each temperature (Fig. 3b). Based on the optical spectra of the material at the different stages, the material exhibits an excellent stability up to 200 ◦ C. Curve 1 shows characteristic absorption bands of polyaniline that are red shifted (320, 385 and 850 nm) due to the interaction with the oxide nanoparticles. Treatment with ammonia yields characteristic bands of the emeraldine base (curve 2). The film treated at 100–500 ◦ C (curves 3–5) show a diminished 600 nm band (n–␲* ) due to the loss of water and restructuring. The film annealed at 800 ◦ C (curve 6) shows a change in absorption characteristics due to the degradation of polyaniline being characteristic of the oxide only. Also

The surface morphology of the film was investigated by an atomic force microscope (AFM), which was a Quesant instrument working in tapping mode in air at a constant contact force. We wanted to show the characteristics of the film at different temperature and its stability. The uniformity in the deposition process and surface topography of the films were observed by AFM studies. Each picture is representative of various samples, since similar images were found in four different regions of the samples. Fig. 4a shows the granular structure of PANI–SnO2 as studied previously by Ram et al. [21]. The surface morphology of self-assembled films treated at different temperatures was examined using AFM technique because it is important that the film morphology remains intact at elevated temperatures. Fig. 4b shows the AFM of PANI–SnO2 annealed at 100 ◦ C for 20 min. The granular structure of the film is maintained till 250 ◦ C. However, a partial loss of the granular structure is observed at the higher temperature, probably due to restructuring of the film upon removal of intrinsic water and particle fusing. The annealed PANI–SnO2 particles are aggregated into clusters in the range of several 100 nm. Because of the poor contrast in the micrograph, it is difficult to measure the size of the primary spherical particles accurately. As with the optical spectroscopy annealing results in removal of traces of moisture and some restructuring of the material as is shown at 500 ◦ C (Fig. 4c). The PANI–SnO2 annealed at 800 ◦ C shows the ordered structure particle of simply SnO2 . The definitive size particles can be seen in Fig. 4d. The similar morphology has been observed for polyaniline–TiO2 as a function of annealing temperature. 3.4. Cyclic voltammograms Fig. 5 shows the cyclic voltammograms of polyaniline–TiO2 composite film in 0.1 M HCl medium. It shows two main redox couples at 0.14 and 0.63 V for polyaniline–TiO2 composite film. The first peak in the cyclic voltammetry of polyaniline–TiO2 composite film is due to the surface electron transfer at 0.15 V, which has almost identical characteristics as that of polyaniline. However, it is slightly more anodic than the peak 1 of CV of polyaniline, and this behaviour suggests that the incorporation of TiO2 in polyaniline does not retard its conductivity indicating the formation of a conducting composite of polyaniline and TiO2 . Interestingly, the peak 2, which is due to quinoid structure in polyaniline has a higher current than polyaniline 0.48 V versus SCE. However, the peak 3, which is due to protonation at 0.64 V in polyaniline–TiO2 composite. The CO gas-treated films show better conductivity and electrochemical activity than those prior to CO gas exposure.

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Fig. 4. AFM image of self-assembled PANI–SnO2 (a) as made (b) annealed at 100 ◦ C (c) annealed at 500 ◦ C and (d) annealed at 800 ◦ C.

3.5. Gas detection via resistance and impedance measurements The gas sensing system consisted of a Parr pressure chamber equipped with the necessary valves to achieve vacuum, introduce gases, and perform in situ four-probe conductivity measurements. The Scheme 2 shows the gas sensor measurement used under this work. The sensor shown in Scheme 2 consists of interdigitated electrodes made from platinum on

Fig. 5. CV of PANI–TiO2 films as a functional of scan rate.

quartz plate. The two electrodes, which were interdigitated, were spaced to 50 ␮m between any two pairs. Each track in the interdigitated electrode was 50 ␮m in width and 40 nm in height. In fact, this paper finds the result of the conductivity change of the sensor films at room temperature. The film is connected to a digital multimeter with the change in resistance being recorded as gas pressure is varied. We have shown the humidity and temperature connection to our systems and we be exploited in future work. We have also used both the 4-probe gold electrodes deposited on alumina substrate in these experiments. The real-time resistance changes of the films were monitored with computer-interfaced electrometer upon exposure to CO gas. This paper presents the resistivity change of the film at room temperature. The change in resistance (R) values obtained for three different types of PANI–metal oxide polymers films are shown in Fig. 6(a and b). The R = (R0 − Rx ), where R0 is the initial resistance and Rx the resistance after each concentration of the gas. The maximum R is obtained for the 1000 ppm for the hybrid films. The CO gas acts as an oxidant and exposure results in a decrease in the resistance value of the polyaniline film. This is probably due to the fact that interaction between the polymer and SnO2 nanoparticles results in the reduced form

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Scheme 2. Block diagram of Fractal systems’ gas sensing and measurement setup.

of polyaniline, which becomes easily oxidizable. This paper presents the total change in the resistance (R) value of the sensor membrane for each concentration of the gas introduced at room temperature. The total change (R) of the resistance is calculated from the subtraction of resistance at no gas with each change of the resistance after introduction of the CO gas. It should be marked that the total change in the resistance value versus the concentration of CO is pre-

sented in the Fig. 6(a and b). There is a continuous change in the resistance value in PANI–SnO2 films indicating that CO does not saturate the films till 1000 ppm exposed for the film. The PANI–TiO2 shows the saturation after 800 ppm. It is important to note that reversibility of gas adsorption is easily taken place under vacuum at room temperature after few minutes. Similar thing happens with TiO2 (Fig. 6b). One could see the decrease in resistance trend as a function of CO concentration. 3.5.1. SO2 gas variation for nanocomposite films The SO2 is an important gas, which shows crosssensitivity with CO and NOx present in the systems. We inserted SO2 gas to PANI–SnO2 and PANI–TiO2 membrane films. Fig. 7(a and b) shows the results of 1000 ppm insertion of SO2 gas. There is a little change after insertion of 1000 ppm gas of SO2 gas in polyaniline–SnO2 film indicating that it is the higher concentration of SO2 gas can cause little change in sensing membrane. We can see the smaller change in resistance for PANI–TiO2 and PANI–SnO2 films for SiO2 gas whereas larger change in resistance has already achieved for CO gas as shown in Fig. 6(a and b). The stable resistance is always obtained for PANI–TiO2 containing membrane.

Fig. 6. (a) R (change resistance) vs. CO gas concentration for selfassembled PANI–SnO2 films. (b) R vs. CO gas concentration for selfassembled PANI–TiO2 films.

3.5.2. The sensitivity in the measurement We wanted to study the effect on sensor membrane at the introduction of 1 ppm of CO gas. The change in the resistance is observed on the PANI–SnO2 film as soon as CO was introduced in the measuring cylinder. The saturation in the resistance was also experienced after nearly 80 s. Later, it shows the straight line till 280 s of 1 ppm exposure of CO gas. It also

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ness and effect of temperature for sensing of CO gas. We will report in our successive paper about the CO sensing using our novel nanocomposite membrane in the presence of humidity, CO2 and NOx gases, respectively. The preliminary resistance change with the concentration of CO acceptance tests indicate that our sensors can be suitable for online detection and continuous monitoring of CO gas.

Acknowledgment This project was supported by the environmental protection agency (EPA).

References

Fig. 7. Response curve with SO2 introduction to: (a) PANI–SnO2 composite films and (b) PANI–TiO2 composite films.

Fig. 8. Response time of PANI–SnO2 membrane with 1 ppm CO gas.

reveals the reversibility in the sensor membrane as shown in Fig. 8. The detection of CO occurs instantaneously by the introduction of such gases in our engineered supramolecular organic–inorganic hybrid.

4. Conclusions We have fabricated conducting polymer–inorganic nano hybrid structures on glass, ITO coated glass plate and interdigitated electrode. The films treated at different temperature were characterized by various physical techniques. The gassensing properties with CO in nanocomposite materials have been studied. The polyaniline–metal oxide nanocomposite are sensitive to CO gas. The stability of the films with time and temperature were tested. Our measurements indicate that the nanocomposites supramolecular films are excellent candidates for recognition and detection of CO gas. This manuscript only presents the resistivity change of the film at room temperature. At present we are working on the optimization of films thick-

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Biographies Dr. Manoj Kumar Ram is a senior research scientist at Fractal Systems Inc. Dr. Ram earned his masters in physics (MSc) in 1986, masters of philosophy (MPhil) in physics, 1988, masters in physical sciences (MS) in 1992, and PhD in physical sciences (conducting polymers), 1995 at the National Physical Laboratory, New Delhi, India and BITS University, Pilani, India. Later, he served as a postdoctoral fellow in the Institute of Biophysics, Genoa, Italy, from 1995–1996. He worked as a scientific consultant for one year in the Elba foundation on a project involving

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sensor and conducting polymer. He worked as a scientific consultant in Polo Nazionale Bioelettronica, Italy, until March 2001. He taught graduate courses on polymer science and biosensors (immobilization techniques and detection) for the session 1999–2000 and 2000–2001 at the University of Genoa, Italy. He has participated in conducting polymers efforts for nearly 15 years that included synthesis, characterization and applications of conjugated polymers and synthesis of nanocomposite materials. He has published more than 60 papers and several book chapters and recently editing a book on Supramolecular Engineering of Conducting Materials under Transworld research Network. He is a member of the American Chemical, and American Physical Societies. ¨ Ozlem Yavuz joined Fractal Systems (January 2002) as a senior scientist. She obtained her PhD in physical chemistry ‘Carbazole Containing Polymers’ (12/99), MS in physical chemistry ‘Polymer–Metal–Protein Complexes’ (6/95), and BS in physical chemistry ‘Corrosion of Pyrite’ (2/93) all at Istanbul Technical University, Turkey. She had a postdoctoral fellowship at the Department of Chemistry, University of Central Florida (4/00–11/01), a visiting scientist at Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK (3/97–8/97 and 8/98–10/98), and a research assistant at Istanbul Technical University (1/94–4/00). She has published several papers in journal of international repute. Vitawat Lahsangah joined Fractal Systems Inc. in September of 2002 with a BSc in chemistry from the University of South Florida, Tampa. At present, he is actively involved in projects dealing with EMI shielding and gas sensing materials. Dr. Matt Aldissi is the president of Fractal Systems Inc. Dr. Aldissi received his BS in chemistry, MS in polymer chemistry and PhD in polymer science, all from the University of Montpellier, France. His prior two appointments were at Foster–Miller Inc. and Cape Cod Research as a senior staff scientist. He held the position of research fellow, then vice president for Advanced Technology at Champlain Cable Corp., Colchester, VT (1/1990–10/1993). Prior to that, he was a research staff member at Los Alamos National Laboratory (1/1983–1/1990), working on different aspects of conductive polymers for various applications including EMI shielding, batteries, photovoltaics, electrochromic, electroluminescence and nonlinear optical properties, ferroelectric polymers and biomaterials. Prior to Los Alamos, he held a postdoctoral position working on conductive polymers at the University of Pennsylvania, Philadelphia (10/1981–1/1983). He is the author of more than 100 publications and more than 25 patents and patent applications, and winner of two R&D 100 awards (1989 and 1990). Dr. Aldissi taught a graduate course on conducting polymers at the University of Vermont, Burlington, and is teaching short courses with Advanced Polymer Courses. He chaired a working group for the Department of Energy on the use of conducting polymers in surface transportation applications in 1992. He is a member of the American Chemical, Electrochemical and Materials Research Societies.