THE CRYSTALLINE-STRUCTURE OF POLY(3-OCTYLTHIOPHENE) AT HIGH-PRESSURE

The crystalline structure of poly(3-octylthiophene) as a function of pressure up to 3 GPa (30 kbar) has been studied by energy dispersive x-ray diffraction. The distance between neighbouring thiophene chains in the direction normal to the thiophene rings, bi, has been found to decrease with increasing pressure. This is believed to increase the planarity of the polymer main chains and explains why the thermochromic transition is inhibited under pressure. The crystallographic repetition distance a in the direction along the octyl side chains is found to be constant during increment of pressure. This shows how the alkyl side chains in cooperation act as rigid interchain spacers even if, independently, they are flexible and rather disordered. A lack of change in the interchain charge complex transfer integral can be understood from the pressure behaviour of a. The measurements indicate that the crystalline fraction of this paracrystalline polymer increases with pressure, and that the size of the crystalline domains increases slightly.