Morphology of polyparaphenylene powders and pellets

Semiconducting organic polymers have recently gained new interest since the synthesis of high quality films of polyacetylene by Ito, Shirakawa and Ikeda 1. Observations that its conductivity can be changed many orders of magnitude by doping with acceptors and donors, led to much study 2. Some authors have reported on other organic semiconductors, namely, polyparaphenylene 3, polypyrrole 4 and poly(paraphenylenevinylene) 5 which showed similar properties. In our search for stable conducting materials, we have been investigating polyparaphenylene and we wish to report on the morphology of powders and pellets, showing some interesting features. Polyparaphenylene was synthesized by the method described by Kovacic et al. 6. Extensive purifications were performed on the crude polymer according to the known sequence 6. I.r. and X-ray data have confirmed the nature of the product. Analysis performed on the purified materials gave the following contents: Non-annealed 2/0, AI _ 0.4~o, Cu -~ 0.1 ~o; Annealed for 24 h product:Cl~ oJ ,,~ o at 400°C: C1 ~-0.8~o, A120.2~, Cu-~0.01~o. Three different series of powders were examined with a binocular microscope, namely, non-annealed powder, powder annealed for 24 h at 400°C in an oven under nitrogen flow, powder annealed for 7 days under the same conditions. All the samples showed the same morphology, an example of which is given in Figure 1 where the powder annealed for 7 days is represented. Pellets were made from the three series of powders under pressure (800-850 MPa) at room temperature. The surface of the pellets, observed with the microscope, demonstrated a somewhat different behaviour according to the heat treatment on the mother powder. The nonannealed powder resulted in pellets with a smooth shiny surface, but showed some darker rectangular domains randomly oriented. The 24 h annealed powder gave pellets with a rather rough surface as compared with the previous ones and also prominent rods which tended to appear. In the last series (7 days annealed powder) the pellet surface was quite rough with rods sticking out from the plane without any preferential orientation. The compacity of these pellets remained low, about half of what usually resulted even after a few hours under pressure. Their gross density was ~0.7, instead of 1.23 for the pellets