Thermotropic rigid‐chain copolymers: X‐ray investigations of the structure

The structure of a rigid rodlike random copolyester, consisting mainly of p ‐hydroxybenzoic acid, hydroquinone, and carbonyl units, was investigated by means of wide‐angle x‐ray scattering. The lateral order of the chain molecules was found to be short‐range in the nematic melt, in close agreement with the spatial order in the melt of flexible chain molecules. The packing density was found to be surprisingly large in the anisotropic melt. The low‐temperature solid state consists of two phases, namely a frozen‐in nematic and a three‐dimensionally ordered crystal phase. Quenched samples were found to have a degree of crystallinity of about 25%, whereas well‐annealed samples may display a degree of crystallinity of up to 70%. The ordered phase apparently displays an orthorhombic unit cell. Its dimensions indicate that a cocrystallization of the comonomers has taken place. On the basis of the observations that the lattice dimensions change continuously as a function of annealing parameters and taking into account the fact that the transition enthalpy, entropy, and volume are smaller by at least one order of magnitude in comparison to those found for crystals of flexible chains, we arrived at the conclusion that the polymer displays a rotationally disordered crystalline state.