Molecular Transitions and Dynamics at Polymer / Wall Interfaces: Origins of Flow Instabilities and Wall Slip

Abstract

This article reviews recent results on capillary melt flow anomalies. Long standing controversies and debates in this field are illustrated by summarizing previous results and clarified with an extensive discussion of the most recent results. Explicit molecular mechanisms for flow instabilities are presented in contrast to a background of 40 years’ continuous and far ranging research. New experiments show that the widely observed extrusion anomalies (including oscillating flow, discontinuous flow transition and sharkskin) of linear polyethylenes (LPE) originate from interfacial molecular transitions, which may or may not be stable depending the specific flow conditions. A global flow instability (commonly known as oscillating capillary flow) evidently arises from a time-dependent oscillation of the global hydrodynamic boundary condition (HBC) between no-slip and slip limits at the capillary die wall. Other convincing observations show that sharkskin originates from a local instability of HBC at the die exit wall. The global and local interfacial instabilities both originate from a reversible coil-stretch transition involving interfacial unbound chains that are entangled with the adsorbed chains. In other words, local and global stress oscillations result in the observed macroscopic sharkskin-like and bamboo-like extrudate distortions respectively. A second molecular mechanism for wall slip is also clearly identified, involving stress-induced chain desorption off low surface energy walls. An organic coating of capillary die walls produces massive chain desorption and a large magnitude wall slip at rather low stresses, whereas bare metallic and inorganic surfaces (e.g., steel, aluminum, and glass) usually retain sufficient chain adsorption and prevent catastrophic slip up to the critical stress for the coil-stretch transition. The intricate interfacial flow instabilities exhibited by LPE are also shared by other highly entangled melts such as polybutadienes. In contrast, monodisperse melts with high critical entanglement molecular weight (M _e ) such as polystyrene of M _w =106 show massive wall slip on low energy surfaces but no measurable interfacial stick-slip transition before reaching the plateau around 0.2 MPa. Tasks for future work include (i) direct molecular probe of melt chain adsorption and desorption processes at a melt/wall interface as a function of the surface condition, (ii) new theoretical studies of chain dynamics in an entangling melt/wall interfacial region as well as in bulk at high stresses, (iii) test of universality of the established physical laws governing melt/wall interfacial behavior and flow for all polymers, and (iv) development of tractable experimental and theoretical methods to study boundary discontinuities and stress singularities.