Investigation of the sensor properties of some polymers and their organic/inorganic hybrid films

A water soluble conductive polymer, poly(sulfonic diphenylaniline) (EPSDA) was synthesized by using electrochemical polymerization and characterized. The organic-inorganic hybrid nanocomposites containing EPSDA and nano-oxides (nano ZnO and Al2O3) were prepared to obtain humidity sensors. (3-mercaptopropyl) trimethoxysilane (MPTMS) was added into the composites to adjust the hydrophilicity and stability of the sensing layer. The humidity sensing properties of prepared thin film sensors were investigated by LCR meter and QCM. In the measurements by LCR meter, the organic/inorganic hybrid films composed of EPSDA-MPTMS and nanocomposites of EPSDA-MPTMS-nanoZnO or nanoAl2O3 were prepared and they were covered onto the interdigited electrodes. The electrical properties such as impedance, reactance and resistance of these thin film sensors were measured at various relative humidities in the frequency range of 100-1 kHz and under 0.2 V of potential bias. Humidity sensing properties such as relative humidity-impedance characteristics, response and recovery times, hysteresis, repeatability/reversibility and sensing capabilities against varied solvent vapors of the thin films were investigated. The sensing characteristics of the sensors were developed by the addition of nanomaterials. The nanocomposites with 50 wt% of ZnO-EPSDA-MPTMS (S7) and 50 wt% Al2O3-EPSDA-MPTMS (S10) showed better humidity sensing properties than pure EPSDA-MPTMS at 1 kHz alternating current frequency. The sensor coded as S7 exhibited quick response (response time, 90 s and recovery time, 60 s), good linearity (R2=0.9832), good repeatability in the humidity range of 10-95% RH and low hysteresis. Similarly, the sensor coded as S10 exhibited fast response/recovery time (65 s/ 55 s), good repeatability, good linearity with high correlation (R2=0.9949) and low hysteresis in the humidity range of 10-95% RH. In the study of some solvent vapors sensing properties of the developed impedimetric sensors, depending on the structure of the sensors and vapor concentration, the magnitude of the impedance changes correlated with the polarity of the exposed solvents. Compare with the humidity response, the prepared humidity sensors show lower response to other polar organic solvents, but insignificant response to non-polar solvents. When the complex impedance plots of the sensors in the different relative humidities were investigated, in general, it was seen that the shapes of the curves changed from a semicircle to a line with increasing RH, indicating different sensing mechanisms at low and high RH. In the QCM measurements, firstly, the thin films composed of EPSDA and MPTMS were coated onto the quartz crystal electrode to prepare humidity sensor. The frequency shifts and the change of resistance due to the adsorption and desorption of water vapor on the surface of the modified QCM electrodes were simultaneously measured. The sensor which was composed of EPSDA and MPTMS (QE1) shows good sensing properties such as fast response/recovery time (70 s/44 s), good repeatability, good linearity with high correlation (R2=0.9911) and low hysteresis. Secondly, the EPSDA-MPTMS-nano Al2O3 nanocomposites with different weight percents of nano Al2O3 were coated onto the QCM electrodes. The nanocomposite with 50 wt% Al2O3-EPSDA-MPTMS (QE4) showed excellent sensing properties, such as quick response (response time, 65 s and recovery time, 45 s), good linearity with high correlation (R2=0.9914) over the large humidity range (9.7?93% RH) and small hysteresis. When the effect of some solvent vapors on sensors responses were investigated by QCM method, it was determined that the responses of the developed sensors against organic solvent vapors were insignificant. It was be able to obtain high humidity selectivity by QCM method. Additionally, experimental results showed that the developed sensors have high stability and sensitivity and the responses were stable within one month. Keywords: Humidity sensor, conductive polymer, poly (sulfonic diphenyl aniline), impedance, quartz crystal microbalance .