The missing parameter in rheology: hidden solid-like correlations in viscous liquids, polymer melts and glass formers

摘要

BACKGROUND: Determination of dynamic relaxation consists of measuring the viscous and the elastic components of a material by generally applying a small (oscillatory) deformation. The shear stress is transmitted to the material via contact with a substrate. Dating at least back to Stokes, the no-slip boundary condition between the fluid and the substrate is supposed to be fulfilled during this measurement. We show that the viscoelastic parameters of fluids are usually not determined under no-slip boundary conditions and do not originate from the first linear regime. Viscous and viscoelastic fluids (entangled and unentangled polymers, glass formers) measured under no-slip conditions exhibit a fundamentally different response with a dominant terminal solid-like response.RESULTS: We show that the terminal behaviour of fluids such as liquid polymers or glass formers measured at the sub-millimetre scale and far above the glass transition is not viscous but solid-like. Instead of a viscoelastic behaviour scaling as ω and ω2 (ω is the frequency) for the viscous and the elastic moduli, respectively, the dynamic response is simplified;for low gap thickness, both viscous and elastic moduli are invariant with respect to the frequency (with the elastic modulus being larger than the viscous modulus) and enhanced by two to four orders of magnitude compared to the conventional viscoelastic response. Over a critical strain amplitude, the solid-like response decreases and is progressively replaced by the conventional viscoelastic behaviour. We discuss the implications of this observation and reconsider the assumptions inherent to a rheology measurement.CONCLUSION: The identification of so far neglected macroscopic elasticity in the fluidic state far above the glass transition temperature in entangled and unentangled polymers and glass formers shows that the liquid state is dominated by long range intermolecular interactions. This information is fundamental to understand and to foresee dynamic behaviour;it sheds further light on nonlinear phenomena such as large time scale relaxations, rheo-thinning, violation of the no-slip boundary condition and spectacular shear-induced instabilities (spurt effect, 'shark-skin' instabilities, gross melt fracture, etc.) that are unpredictable in the frame of the conventional viscoelastic approach. It also implies that the viscoelastic times (reptation, Rouse) in polymers are not the longest relaxation times of these materials.