Studies on the nature of order in a rigid random copolymer

The transition from the nematic melt to the solid state in a rigid rod‐like copolymer was found to be characterized by unusual thermodynamic and kinetic properties. The transition occurred with negligible volume changes. The enthalpy and the entropy of the transition were found to be lower by at least one order of magnitude in comparison to the values usually observed for a crystal to nematic transition. The ordered solid state displays a glass transition and its structure is characterized by high symmetry, i.e., an orthorhombic one. Dielectric relaxation studies, investigations on the plastic flow properties of the solid state and studies on the dynamical mechanical properties revealed a high degree of rotational freedom already at room temperature in the solid state. These experimental findings led us to the conclusion that the solid state displays a three‐dimensional positional long range order but is rotationally disordered with respect to rotations about the long axes of the rod‐like molecules. The structure is similar to those of plastic crystals known for spherical molecules such as cyclopentane, or to smectic E modifications known for rod‐like molecules. Also the properties reported above for the rod‐like copolymers resemble those reported in the literature for plastic crystals.