STUDY OF THE CHAIN CONFORMATION OF THERMOTROPIC NEMATIC MAIN-CHAIN POLYESTERS

The conformation of main chain mesomorphic polyesters is studied by sm all angle neutron scattering (SANS) in the isotropic and in the nemati c phases, by using mixtures of deuterated and undeuterated polymers. P articular attention is given to neglect the transesterification effect s occurring mainly at high temperature for these LC polymers. In the i sotropic phase, despite the presence of long rigid mesogenic groups, t he LC polyester chains have a Gaussian conformation shown by the varia tion of the radius of gyration as a function of the molecular weight. This result is confirmed from the scattering variation in the intermed iate range of the scattering vector. In the nematic phase, the SANS da ta are well fitted to a model of cylinder, in which the main chain pol ymer is confined. In the unoriented phase, the measurements in the int ermediate range give the values of the radii of cylinders: they lie in between 10 angstrom and 19 angstrom depending on the degree of polyme rization of chains. In the oriented nematic phase, the scattering patt erns are highly anisotropic: they correspond to very long, thin and we ll-oriented cylinders. We have calculated the fully extended chain len gths using for the monomer length that measured in situ by X-ray diffr action. Then the comparison of this length with the measured height of the cylinders gives the existence of hairpins and their number per ch ain. For the short chain, the conformation is almost completely elonga ted in the nematic direction, whereas hairpin defects appear in longer chains. Their number decreases slightly with decreasing temperature. The orientational fluctuations of cylinders relatively to the nematic director are weak as shown from the high values of their order paramet er (P2 > 0.9). These results are discussed for two spacer lengths as a function of the molecular weight and of the temperature.