EVIDENCE OF A PHASE WITH 2-DIMENSIONAL POSITIONAL ORDER IN A SIDE-GROUP LIQUID-CRYSTALLINE POLYMER

X-ray scattering studies were performed to elucidate the structure and the character of the correlations in a mesophase with two-dimensional positional order formed in a side group polymer. X-ray diffraction pa tterns of oriented fibres display weak diffuse lines in the direction parallel to the orientation of the side groups and a regular pattern o f strong diffuse spots in the perpendicular direction, indicating posi tional order on the local scale. The side groups are close packed with in a two-dimensional (2D) centred orthorhombic lattice and are tilted with respect to their nearest neighbours. The positional order is anis otropic and is limited by a correlation length of about 30-40 Angstrom , that is approximately six to eight side groups are involved in the s hort range order. In the direction along the normal to the 2D plane, t he system shows no translational order. This phase displays the same u nit cell symmetry and direction of tilt as the layers of the K phase a nd is designated as TDK, where the first two letters stand for 'two-di mensional'. Unexpectedly, we found a regular pattern of weak diffuse s pots in the small angle scattering region. These indicate a short rang e modulated structure with the vector of modulation lying in the plane of the 2D order. A simple model of the packing of the side chains is proposed to explain the formation of modulated structure and the aniso tropic character of the positional correlations.