Mechanical, electrochemical and optical properties of poly(3,4-ethylenedioxythiophene) sulfated poly(beta-hydroxyethers) composite films

Poly(3,4-ethylenedioxythiophene)/sulfated poly(beta-hydroxyethers) (PEDT/S-PHE) composite films were prepared by electrochemical polymerization of 3,4-ethylenedioxythiophene (EDT) in the presence of S-PHE polyelectrolytes. The sulfate groups of S-PHE play the role of the counterion to the positive charges of the oxidized PEDT. Moreover, as a consequence of the good mechanical properties imparted by S-PHE, we succeeded in preparing free-standing PEDT composite films. All composites showed excellent mechanical properties, high conductivity and reversible electrochromic properties. In order to examine the influence of the S-PHE content on composite properties, a series of composites containing S-PHEs of differing molar ratios (MR) of sulfate groups to the monomer units of poly(beta-hydroxyethers) were synthesized. From the experimental results it was found that the conductivity of the composites is independent of MR value and, hence, PEDT content. In situ absorption spectra of the composite films suggested that composites with larger contents of PEDT possessed shorter pi-conjugation lengths. Electrochemical properties of the composites were measured by means of cyclic voltammetry and potential step chronocoulometry and results demonstrated that the rate of switching between the doped and undoped states of the composites occurs within 0.5 s. The absorption band of the composites in the visible region almost disappeared upon doping. All results indicate that the composites are interesting candidates as materials for electrochromic displays. (C) 1997 Elsevier Science Ltd.