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ZnO:Polydiacetylene Films as Chromatic Sensors

A. Patlolla, J.L. Zunino III, D.P. Schmidt, Z. Iqbal, D.R. Skelton
U.S. Army, US

Keywords: sensors, thermal chromic, PDAs


Films of the conjugated polydiacetylenes (PDAs) can function as chromatic temperature, chemical and stress sensors, and can be prepared by spin-coating from monomer solutions followed by polymerization. The polymer backbone of PDAs is comprised of alternating double bonded (ene)-triply bonded (yne) groups which are responsible for intriguing temperature, stress and chemically-induced chromatic (blue, non-flourescent to red, fluorescent) transitions. PDAs thus have the potential for advanced applications as sensors, but many of these applications are to some degree limited by the irreversibility of the blue to red transition in many PDAs. We have found that ZnO uniquely forms a weak complex with acidic diacetylenes containing carboxylic groups: 10, 12-pentacosadiynoic acid (PCDA), 10, 12-docosadiynedioic acid (DCDA), and 10, 12-tricosadiynoic acid (TCDA), resulting in reversal of the chromatic blue to red transition. We have also observed that this reversibility is slowed down by mixing the PDA with an alloy of ZnO with ZrO2, thus allowing the mixed oxide-PDA films to function as elapsed time-temperature indicators. Nanocomposite ZnO:PDA and ZnO/ZrO2:PDA thin films have been prepared and their structural properties have been characterized by a variety of techniques including fourier transform infrared spectroscopy, Raman spectroscopy, differential thermal calorimetry (DSC) and extended x-ray absorption fine structure (EXAFS) measurements. The infrared and DSC measurements are particularly indicative of chemical interaction of ZnO and ZnO/ZrO2 with the end –COOH groups on the side chains. We have conducted testing and calibration of the thermal and stress sensing properties of the composite films using optical density measurements. It was also observed that the fluorescent red phase transforms to a non-flourescent blue phase in the presence of trace amounts of chemical vapors. Results on the sensing of trace amounts of vapors of tri-nitrotoluene (TNT) and dinitrotoluene (DNT) by optical densitometry and fluorescence, and efforts to enhance the chemical sensitivity of TNT detection by addition of single wall carbon nanotubes to the PDA, will also be discussed.
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