Thin Solid Films 441 (2003) 56–62 0040-6090/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00864-2 Polyaniline–poly(2-acrylamido-2-methyl-1-propanosulfonic acid) composite thin films: structure and properties Liliana Hechavarrıa, Hailin Hu*, Marina E. Rincon ´ ´ Centro de Investigacion en Enegıa, UNAM, Av. Xochicalco SyN, Temixco, Morelos 62580, Mexico ´ ´ Received 1 December 2002; received in revised form 22 May 2003; accepted 23 May 2003 Abstract Polyaniline (PANI)–poly(2-acrylamido-2-methyl-1-propanosulfonic acid)(PAMPS) composite thin films are obtained by a chemical bath deposition method on conductive glass substrates. The incorporation of the polyacid in PANI, demonstrated by atomic force microscopy, ultraviolet–visible spectroscopy and Fourier transform infrared spectroscopy studies, causes changes in the electronic structure of PANI and gives a different microstructure when compared to the unmodified PANI film. The lower optical transition band gap as well as an oriented fibril microstructure of the composite film give rise to a decrease in the redox potentials of PANI, favoring a lower optical switch voltage for the PANI–PAMPS composite film-based solid electrochromic devices.  2003 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Optical properties; Electrochemical properties; Atomic force microscopy 1. Introduction Conducting polymers have been extensively studied for their promising applications in gas sensors, batteries, light emission diodes, solar cells, electrochromic devices (ECDs), etc. Since they are insoluble in most common solvents, important efforts have been directed to improve the solubility and, therefore, processability of these macromolecules. Reports on the improved solubility by the addition of lateral groups on the main chains of the polymers or by the formation of the conductive polymer in the presence of a soluble polyelectrolyte, are encour- aging in terms of the film processability. In the case of polyaniline (PANI) a water solution of a polyacid-doped PANI can be obtained w1,2x. SEM micrographs of the composite material show long fibrils with approximately 50 nm in diameter and 1–5 mm in length of PANI– poly(acrylic acid) (PAA) colloidal suspensions w1x. Additionally it was found that these fibrils were very uniform in shape, and that the dopingyundoping of the colloid in solution gave substantially the same color change as solid PANI films on substrates. *Corresponding author. Tel.: q52-55-5622-9747; fax: q52-55- 5622-9742. E-mail address: hzh@cie.unam.mx (H. Hu). In our previous work w3x it was shown that PANI– PAA thin solid films could be directly deposited on plastic substrates from an aniline solution bath. The advantage of using one step film formation is obvious from the process point of view. Moreover, a solid polyacid-doped PANI thin film is interesting from the materials science point of view since it is a composite material containing two different phases. And this com- posite film gives different macroscopic properties com- pared with the PANI thin film doped by small molecules such as HCl. Based on these ideas and considering that PANI–PAA showed a poor adhesion on glass substrates, we found that poly(2-acrylamido-2-methyl-1-propano- sulfonic acid)(PAMPS) is a better candidate for the application purposes. The purpose of this work is to study the morphology and electrochemical properties of the composite PANI–PAMPS films and to discuss the improvement of the ECDs-based on this composite material. 2. Experimental PANI thin films were prepared with a solution method described elsewhere w3x. Briefly, doubly-distilled aniline monomer (J.T. Baker) was dropped into a stirred 2 M