Effect of the Molecular Weight on the Whole Conformation of a Liquid Crystalline Comb-like Polymer in Its Melt

The whole conformation of a comb-like polymethacrylate (PMA-CH 3 ) has been determined by small-angle neutron scattering (SANS) in the isotropic and nematic phases. Its molecular weight dependence is obtained from six well-defined M values. In the isotropic phase, the persistence length is found to be l p = 10 ± 1.5 Å, and the relation between the radius of gyration, R g, and M or N, the number of monomers, is R g = 0.143 M 1/2 = 2.85 N 1/2 . Therefore, there is practically no difference between the conformation of the PMA-CH 3 backbone and that of the polystyrene. This result is striking since it appears that the backbone conformation of a polymer with a hanging group of 23 Å length every 2.5 Å along the chain is quite the same as that of the standard of linear polymers. In the unoriented nematic phase, the same values of l p and R g are found. This means that the l p value remains unchanged in the nematic phase. In the oriented nematic phase, the values of the inertial radii of the chain, R ∥ and R ⊥, in the direction parallel and perpendicular to the nematic director have been measured as a function of M: R ⊥ = 0.12 M 0.48±0.03 and R ∥ = 0.015 M 0.63±0.05 . These results are confirmed by the dependence of the form factor on the scattering vector in the intermediate range. They are discussed in term of smectic fluctuations.