Philippe Martinoty

Viscoelasticity of suspensions of magnetic particles in a polymer: effect of confinement and external field.

We study the suspensions of magnetic particles, the precursor state of magnetic gels and elastomers. We use magnetic particles with a permanent magnetization which is high enough to overcome thermal energy and low enough to guarantee a long live time of the sample. These particles form a space-filling structure at very low volume fractions (approximately 0.5 vol %), which modifies the viscoelastic response of the matrix significantly. In confined geometry the particles form clusters of a size that depends on the sample thickness. Even small external fields induce a strong anisotropy in the mechanical and optical properties of the suspension. The action of the applied magnetic field induces a gel-like response in one direction but leaves the other directions liquidlike. The viscosity is a very sensitive mechanical test for the anisotropy of the material. Light scattering data confirm our mechanical results.

Conformation in solution of side-chain liquid crystal polymers as a function of the mesogen-graft amount

The conformation of side-chain liquid crystal polymers with different mesogen-graft amounts has been studied by small-angle neutron scattering in dilute solutions of toluene-d8 and THF-d8. It is shown that the radius of gyration increases by about twofold when the mesogen-graft amount increases from 0 to 100%, which suggests that the persistence length of the backbone increases by about 4-fold. Comparison with the results reported in the literature on the melt state suggests that the persistence length is not an intrinsic property of side chain liquid crystal polymers, but depends on inter-chain interactions.

Influence of the degree of polymerisation and of the architecture on the elastic properties of new polyurea elastomers

The elastic properties of new polyurea elastomers have been studied by varying the segmental molecular weight and the chemical nature of the polymer end groups. Three different types of elastomers were synthesized leading to three different types of response. The elastomers with a high degree of polymerisation and primary amines as terminal groups show two plateaus: at high temperature, the common permanent plateau related to the rubber behaviour of elastomeric systems and, at low temperature, a transient plateau associated with the hydrogen bonding of the urea motives occurring in the interfacial zone between the soft polyetheramine and the hard crosslinker domains. The elastomers with a low degree of polymerization and primary amines as terminal groups show that the transient plateau is masked by the glassy plateau because the hydrogen bonds occur in the same temperature range as the glass transition effects, except for very slow heating rates for which the transient network can be resolved. Lastly, the elastomers with no hydrogen bonding just show the common step in the elastic behaviour from the rubbery to the glassy state.