Synthesis and characterization of polymer precursors bearing 2,2-dicyanovinyl groups and their curing to thermally stable and electrically conductive resins

Abstract

4-Carboxybenzaldehyde was condensed with malononitrile to afford 1-(2,2-dicyanovinyl)-4-benzoic acid, which was converted to the corresponding acid chloride. The latter reacted with half the molar amount of an aromatic diamine or bisphenol to yield cyano-substituted polymer precursors. Crosslinked polymers were obtained by curing at 300°C for 40–60 h, which were stable up to 333–400°C in N 2 or air and afforded anaerobic char yield of 35–60% at 800°C. Prior to pyrolysis all polymer precursors are of amorphous structure and behave, at room temperature, as insulators. A dramatic decrease of the electrical resistivity is observed with increasing pyrolysis temperature. All resulting conductive materials exhibited crystalline structure revealed by their X-ray diffraction profiles. The temperature dependence of the electrical resistivity in the range −173 to 327°C (100 to 600 K) of all pyrolysed materials suggests that they have semiconducting properties. The variable-range hopping model as well as a thermally activated conduction model were applied in order to interpret the experimental data. The observed electrical conductivity seems to be thermally activated and may be associated with intermolecular and intramolecular hopping conduction processes. The activation energies for the intermolecular process range from 0.265 to 0.012 eV and for the intramolecular process from 0.380 to 0.047 eV, depending on the pyrolysis temperature. The electrical behaviour of all conductive materials can be easily controlled, from insulating to semiconducting, by controlling both the pyrolysis temperature and duration. The highest electrical conductivity of 20 S cm −1 has been obtained for two samples pyrolysed at 800°C for 40 h.