The conformation of a main-chain liquid crystalline polyester in its oriented nematic phase has been determined by small-angle neutron scattering. The data are fitted by a model of rigid cylinder with orientational fluctuations. For a low degree of polymerization (~9) the chain is almost completely elongated in the direction of the nematic field. For a polymer 3 times longer, the existence of two hairpins is shown at high temperature; this number decreases with decreasing temperature.PACS numbers: 61.30.Eb, 61.12.Ex Liquid crystalline polymers (LCPs) have generated much interest in the recent years both because of the challenge of understanding such systems and because of their important technological applications in high modulus fibers, nonlinear optics, synthetic membranes, and electronic devices. A main-chain LCP is a linear link of repetitive units that may, but often does not, possess intrinsic mesogenic character. The liquid crystalline character of the main-chain LCP is a property of the chain backbone. For a semiflexible main-chain LCP in its nematic phase, the tendency to follow long-range orientational order of the nematic phase enters into competition with the random conformation necessary to maximize the polymer entropy. Depending upon the chain molecular weight M, two possible chain conformations have been predicted by de Gennes [1]: For low M values the chain adopts a linear conformation essentially parallel to the nematic director whereas for high M values it can present hairpin defects where the chain executes a contour reversal (180°) with respect to the director. The number of hairpins is an exponential function of the temperature. Statics and dynamics of hairpins in wormlike chains have been developed [2] by Warner and co-workers and a precise temperature dependence of the chain size has been predicted: The chain size increases with decreasing temperature [3]. From a practical point of view the existence of hairpins can play an important role in the rheology and in the nonlinear dielectric and optical properties of mainchain LCPs. We report, for the first time, on the experimental observation of the hairpin conformation by smallangle neutron scattering (SANS) associated with wideangle x-ray scattering (WAXS). The crucial problem is to determine the dimensions of one chain in the direction parallel and perpendicular to the nematic director. It has been resolved by SANS using for the chain a cylinder model. WAXS has been used to measure the monomer length and the monomer orientational order of the polymer in the nematic phase. The existence of the hairpins is deduced from the comparison of the height of the cylinder to the total length of the chain.SANS allows the determination of the conformation of one chain from a melt of 50% deuterated (D) chains dispersed in protonated (H) ones [4]. Here the difficulties lie in finding soluble nematic polymers (in order to obtain the H and D chain mixture from a solution) with a nematic phase in a low-temperature range (r<200°C). A rather good candidate is t...

The phenomenon of transesterification of a mesomorphic polyester is investigated in the isotropic and the nematic phase using small-angle neutron scattering (SANS). In the isotropic phase, the kinetic experiment results are in good agreement with the theoretical predictions of Benoit et al. for the determination of rate constant and activation energy of the transesterification process. In the nematic phase, SANS in the intermediate scattering vector range is not sensitive to the transesterification. This is perhaps due to the cylindrical shape adopted by the polymer chain. The results are discussed as a function of the molecular weight for two different spacers.