Applications and Physical Properties
PANI is the most widely used conductive polymer due to its: good environmental and air stability; water affinity; slight hydrophilicity; bulk synthesis in high quantities; spontaneous reactivity with various chemical species. These properties have led to construction of very efficient sensors, gas analytes and liquid analytes . Many PANI sensors involve spin-coated films of uniform composition that can react readily with reducing or oxidising agents, and protonating or deprotonating acids/bases.
All of these classes of materials are expected to lead to spontaneous alterations in the conductivity and optical properties of the polymer, resulting in sensing calibration curves that enable calculation of the analyst concentration. The mathematical relationship that shows the response of resistive sensors is AR/Ro, where ‘Ro’ and ‘R’ is the electrical resistance before and after contact between the polymer and the agent that will be detected, respectively, and AR = R - Ro. The protonated, emeraldine salt form of PANI is conductive, so the emeraldine base formed may be used for acid detection based on an increase in conductivity after exposure to acids. This mechanism involves the movement of charge carriers along macromolecules in a reversible process.
Many studies have compared the performance of PPy and PANI nanostructures as sensors. As humidity sensors, PANI nanorods exhibit negative response values (R < Ro) at low humidity, whereas PPy nanorods of similar size and shape show a positive response (R > R0) . The sensitivity of a sensor is defined as the slope of the response versus concentration curve and both polymers demonstrate linear behaviour with sensitivity values of -0.364 for PANI and 0.168 for PPy.
This change in conductivity values is brought about by the formation of ‘polarons’ (‘radical cations’) . Polarons and bi-polarons are charge carriers in emeraldine salt that move along the polymer chain upon application of electric field, and are responsible for the electrical conductivity of the polymer, as evidenced by electron paramagnetic resonance studies . All of these acid-base reactions are completely reversible (which is essential for a chemical sensor). I am focusing my discussion on the emeraldine salt (which is the conductive form) and its insulating base form, but PANI can exist in several oxidation states. These oxidation states are the fully reduced leucoemeraldine form, the half-oxidised emeraldine form, and the fully oxidised pernigraniline form. These oxidation states can be controlled by altering and tuning the synthetic conditions or from post-synthesis oxidation-reduction reactions. For example, hydrazine can alter PANI from the emeraldine form to the leucoemeraldine form and thus lead to the development of polymeric sensors for the detection of hydrazine .
Due to the high conductivity of the emeraldine salt of PANI, most studies have been devoted to polymer sensors. Usually, the polymer is exposed to basic molecules such as ammonia gas [29, 30]. Less research activity has been focused on use of the insulating emeraldine base form of PANI, which is used as a sensing material for acidic gases such as hydrochloric acid. Detection of acid is required in many fields and applications, such as rocket launches. In the latter, hydrochloric acid is formed in the exhaust plumes of rocket motors after decomposition of ammonium perchlorate (which is used as a rocket propellant).
This work was initiated and pioneered by the Kaner research team.
They were the first to develop PANI nanofibres that can be used as sensor materials for detection of acids, bases, reducing agents, organic vapours, and alcohols [26-28]. To demonstrate the importance of fibrillar morphologies, thin films of nanofibres and of standard PANI were compared. This comparison was used to evaluate the response time and sensitivity of conventional PANI with those of nanofibres in the same oxidation state. Electronic transport and electrical resistance were monitored as a function of time while polymers were exposed to various gases. Examples of the analytes used were hydrochloric acid, ammonia, hydrazine, chloroform and methanol, and each analyte induced a distinctive response upon polymer contact: protonation- deprotonation, reduction, swelling, and conformational alignment, respectively.
According to their work, the importance of a fibrillar morphology was demonstrated through the response time. They showed that the response was significantly better and optimised for thin films composed of PANI nanofibres compared with the response time observed for spherical conventional particles produced by a common one-phase polymerisation process. The source of this phenomenon was the high surface area and porous structure. The porous structure is the key parameter that enables easy and rapid diffusion of gases through the film. The small diameter of nanofibres and their narrow size distribution leads to rapid diffusion of dopants within the polymer chains. All of these factors explain why nanofibres provide a faster and most significant response to dopant exposure compared with films. Systematic studies on film thickness showed no difference in the response magnitude or time.
PANI and its composites have been studied widely as sensors for ammonia detection because, through exposure to basic ammonia vapours, deprotonation occurs which leads to a decrease in conductivity [30a]. More detailed studies on these systems revealed a relationship between film thickness and the response. As the film thickness is increased, the sensor appears to be less effective in terms of sensitivity and time of response [28b]. Furthermore, a larger amount of base is required for complete conversion of the polymer.
This is the reason for the slow response time in previous works on ammonia sensors. That is, fewer dopants are required to reach the percolation threshold for conductivity than those that must be removed to convert the polymer to the insulating form.
Any small amount of water would be detrimental because the product of the reaction between ammonia and hydrochloric acid (ammonium chloride) is not neutral. If it comes into contact with water, it will be hydrolysed back to reform hydrochloric acid, which subsequently re-dopes the polymer chain, thereby diminishing the effectiveness of the sensor. Hence, the response of a base sensor is slower than that of an acid sensor. Porosity, surface area, and film morphology explain why conventional polymers respond less efficiently to base de-doping than nanofibres.
More niche composite materials can be used as sensors. Some recent conductive composite materials are based on the dispersion of PANI in cellulose paper for the synthesis of highly efficient sensors. The sensors work by tracking the pH changes of the paper upon exposure to acid or bases. The sheets of pressed cellulose fibres used by Souza and co-workers were obtained from Eucalyptus grandis and Eucalyptus europhylla . These cellulose sheets were modified with PANI particles synthesised through chloroform, and were subsequently dispersed and used as a colorimetric sensor .
Examples of other types of molecules that can be detected beyond acid-base detection and estimation of concentration are nitrogen oxides. It has been shown that the resistivity of a polymer increases constantly upon exposure to nitrogen oxide gas . With regard to the selectivity of the whole process, PPy nanofibres have shown an excellent response to ammonia, but showed no response to nitrogen oxides even at <100 ppm, which is a crucial selectivity difference compared with PANI .