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

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 (nonmesomorphic) 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.