Molecular physics of a polymer engineering instability: Experiments and computation

Abstract

Entangled polymer melts exhibit a variety of flow instabilities that limit production rates in industrial applications. We present both experimental and computational findings, using flow of monodisperse linear polystyrenes in a contraction-expansion geometry, which illustrate the formation and development of one such flow instability. This viscoelastic disturbance is first observed at the slit outlet and subsequently produces large-scale fluid motions upstream. A numerical linear stability study using the molecular structure based Rolie-Poly model confirms the instability and identifies important parameters within the model, which gives physical insight into the underlying mechanism. Chain stretch was found to play a critical role in the instability mechanism, which partially explains the effectiveness of introducing a low-molecular weight tail into a polymer blend to increase its processability. There are numerous types of experimentally observed instability in polymer melt flow; a recent review [1] highlights three forms observed in extrusion that occur at increasing rates of flow. The first two are "sharkskin" instabilities, which develop due to free surface effects, and "stick-spurt" or "stick-slip" instabilities, which result from material compression and stick-slip at the wall. While the mechanisms underlying the formation of these first two are relatively well understood [2,3], the third class, termed "volume instability", is less so [4]. This instability in converging flows for extrusion and injection moulding has been known for many years, and an empiricism has been developed. But as yet there is no understanding of the underlying physics of the problem, and the inherent connection between the viscoelastic instability and the molecular polymer dynamics. Understanding the underlying physical process would greatly enhance industry's ability to define efficient processing conditions. This letter outlines recent work in which we created an idealised model flow, related to the engineering flows in that the essential elements are present, but simplified so that the experimental variables are well-controlled and so the whole flow field can be modelled. We have use molecularly well-characterised materials (building on previous work for monodisperse materials under tightly controlled flow conditions [5,6]) so that the connection between viscoelastic properties and molecular structure can be maintained without empirical fitting. Through these careful *

Analytic solutions of the Rolie Poly model in time-dependent shear

“…The original intended use for this model was for computationally demanding calculations such as complex flow geometries. Indeed the model's computational simplicity has been exploited in numerical studies of polymer processing [4][5][6][7][8][9], flow instabilities [10,11], and shear banding [12][13][14][15][16]. However, the relative simplicity of the Rolie-Poly model also makes it amenable to analytic calculation.…”

An experimental evaluation of the formation of an instability in monodisperse and polydisperse polystyrenes

“…The application of stretch to the molecules within extensional flow at the slit outlet, after their orientation within the shear flow near the walls of the slit, seems to be a possible trigger for the initial instability. Numerical studies based on the linear stability analysis of the ROLIE-POLY model [36] found a similar instability for CE2, the onset of which was observed by either increasing molecular weight of the material (represented by R / d ) or reducing the amount of constraint release. Based on these findings, one possible reason for the failure of either DOW1569 or DOW1568 to go unstable under the flow conditions investigated would be the low constraints placed by individual molecules on adjacent chains.…”

“…It is therefore plausible that the mechanism is the same in all three cases. Any predictive tools used to model the monodisperse case (see for example [36]) should accurately capture the instability mechanism present in both the monodisperse and polydisperse polystyrene, allowing a direct insight into instabilities which occur in industrially relevant flows.…”

Finite volume simulations of behavior for polystyrene in a cross-slot flow based on Rolie-Poly model

“…Lord et al (2010) further investigated the flow behavior of polydisperse polystyrene for threedimensional case in a similar way, and the multi-mode Rolie-Poly constitutive equation was used in simulation. Hassell et al (2008b) presented the experimental studies of flow instability of the monodisperse linear polystyrenes, and the formation and development of the instability phenomenon are observed. These experimental results were verified using the Rolie-Poly model, and the mechanism between viscoelastic instability and molecular polymer dynamics was discussed by the authors.…”