Origin of shear-induced phase transitions in melts of liquid-crystal polymers

Laurence Noirez (1)

1 LLB - UMR 12 - Laboratoire Léon Brillouin

Flow induced mechanical properties are often coupled with instabilities, spurt effects, or induced phase transitions. Recent studies have revealed that side-chain liquid crystal polymers exhibit typically shear-induced phases inside the isotropic ͑nonmesomorphic͒ liquid state. We present an experimental approach which brings a new understanding for nonlinear flow behaviors. The strategy consists in comparing the critical times issued from the flow behavior of a liquid-crystal polymer to the equilibrium orientational-order relaxation time was characterized. We demonstrate that shear-induced phases do not originate from a flow coupling to conventional orientational order parameter fluctuations. It does not also correspond to a direct coupling with the viscoelastic terminal time, leading to the conclusion that an additional relaxation process takes place with time scales longer than the terminal time. The identification of a low-frequency elastic plateau by viscoelastic measurements corroborates this conclusion.

Domaines: Matière Molle [cond-mat.soft]

Laurence Noirez. Origin of shear-induced phase transitions in melts of liquid-crystal polymers. Physical Review E , 2004, ⟨10.1103/PhysRevE.72.051701⟩. ⟨hal-01361939⟩