Interactions between polyaniline and methanol vapour C.K. Tan, D.J. Blackwood * Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 117600, Singapore Received 4 October 1999; received in revised form 25 May 2000; accepted 30 June 2000 Abstract UV±VIS spectroscopy, electron spin resonance spectroscopy (ESR) and conductivity measurements have been used to characterise the interactions between methanol vapour and polyaniline. The data indicates that the methanol hydrogen bonds to two locations on the emeraldine base such that it is able to form a bridge between the polymer chains causing twisting. This prevents the few remaining polarons from moving beyond a few monomer units, effectively localising them, giving rise to a decrease in conductivity and an increase in absorption at ca. 1.6eV. However, in the case of the emeraldine salt protonation of the quinoid moieties' nitrogen restricts the number of potential hydrogen bonding sites, preventing the methanol from forming a bridge between two polymer chains. At the same time, the high conductivity of the emeraldine salt allows the charge that is transfer to the polyaniline, as a result of the hydrogen bonding to the methanol, to be distributed along the conjugated chain in the form of polarons and bipolarons. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Sensors; Methanol; UV±VIS spectroscopy; ESR 1. Introduction In recent years, attention has been given to the use of conducting polymers in chemical sensors, as the active layer for the detection of organic vapours. The sensitive para- meters in these sensors are either changes in the work function [1±3], the conductivity [4±6] or the optical absorp- tion coef®cient [7] of the polymers. However, there is still no satisfactory explanation of the mechanism of interaction between organic chemical vapours and conducting poly- mers. The charge on the polymer chain is believed to be mobile, moving along the conjugation of the p backbone of the polymer. The conductivity of the polymer is dependent on the formation of charge-transfer complexes, as for example (P  X  ), where the P  denotes the polymer and X  the doping ion. The counter ion is known to affect the con- ductivity by changing the morphology of the polymer- dopant complex, the concentration of chemical defects or the oxidation state of the polymer [8,9]. In the case of polyaniline, the defects take the form of either polarons or bipolarons [11,12], depending on the extent of doping. The aim of this paper is to characterise the interactions between methanol vapour and polyaniline using UV±VIS optical spectroscopy, electron spin resonance spectroscopy (ESR) and four point probe conductivity measurements. Polyaniline was chosen due to its ease of synthesis and environment stability [10]. It can exist in many forms, depending on the extent and method of doping [11,12], however, this paper will only be concerned with the blue emeraldine base and the green emeraldine salt (speci®cally the chloride salt), the latter being the most conductive form. These two forms of polyaniline are readily inter-converted by washing with either distilled water or hydrochloric acid. In the past few years, there has also been widespread research into the possible utilisation of electrochemically and chemically grown polyaniline in sensor and biosensor applications [13±16]. An example of a biosensor that has been developed is an enzyme microsensor for glucose [14]. This was fabricated by depositing a glucose oxidase- entrapped polyaniline ®lm on top of platinum ®bre, by electrochemical oxidative polymerisation of aniline in a pH 7 buffer solution in the presence of glucose oxidase. This shows that the microsensor was able to amperometri- cally determine glucose in the concentration range of 10 4 ± 10 3 . In electronic band structure, the mobile discontinuities in polyanilines manifest themselves as mid-band gap states, as shown by the valence effective Hamiltonian calculations performed by Bredas et al. [12,17,18]. In the case of a p-type-doped conducting polymer, such as polyaniline's emeraldine salt, the polaron state contains one electron Sensors and Actuators B 71 (2000) 184±191 * Corresponding author. Tel.: 65-874-6289. E-mail address: masdjb@nus.edu.sg (D.J. Blackwood). 0925-4005/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0925-4005(00)00615-8