Molecular transitions and dynamics at polymer/wall interfaces: Origins of flow instabilities and wall slip

This article reviews recent results on capillary melt flow anomalies. Long standing controversies and debates in this field are illustrated by summari zing previous results and clarified with an extensive discussion of the mos t recent results. Explicit molecular mechanisms for flow instabilities are presented in contrast to a background of 40 years' continuous and far rangi ng research. New experiments show that the widely observed extrusion anomal ies (including oscillating flow, discontinuous flow transition and sharkski n) of linear polyethylenes (LPE) originate from interfacial molecular trans itions, which may or may not be stable depending the specific flow conditio ns. A global flow instability (commonly known as oscillating capillary flow ) evidently arises from a time-dependent oscillation of the global hydrodyn amic boundary condition (HBC) between no-slip and slip limits at the capill ary 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 tra nsition involving interfacial unbound chains that are entangled with the ad sorbed chains. In other words,local and global stress oscillations result i n the observed macroscopic sharkskin-like and bamboo-like extrudate distort ions respectively. A second molecular mechanism for wall slip is also clear ly identified, involving stress-induced chain desorption off low surface en ergy walls. An organic coating of capillary die walls produces massive chai n desorption and a large magnitude wall slip at rather low stresses, wherea s bare metallic and inorganic surfaces (e.g., steel, aluminum, and glass) u sually retain sufficient chain adsorption and prevent catastrophic slip up to the critical stress for the coil-stretch transition. The intricate inter facial flow instabilities exhibited by LPE are also shared by other highly entangled melts such as polybutadienes. In contrast, monodisperse melts wit h high critical entanglement molecular weight (M-e) such as polystyrene of M-w=10(6) show massive wall slip on low energy surfaces but no measurable i nterfacial stick-slip transition before reaching the plateau around 0.2 MPa . Tasks for future work include (i) direct molecular probe of melt chain ad sorption and desorption processes at a melt/wall interface as a function of the surface condition, (ii) new theoretical studies of chain dynamics in a n entangling melt/wall interfacial region as well as in bulk at high stress es, (iii) test of universality of the established physical laws governing m elt/wall interfacial behavior and flow for all polymers, and (iv) developme nt of tractable experimental and theoretical methods to study boundary disc ontinuities and stress singularities.