A great strength of the broad class of materials known as "polyethylene" is their versatility. These materials can be tuned to an enormous degree through changing the polymer microstructure, and in particular the branching structure. Different choices of catalyst, comonomer and type of reactor produce quite different materials. With respect to the melt processing properties, both the molecular weight distribution and the presence and distribution of long chain branching are critical, strongly influencing the melt viscosity and non-linear flow behaviour such as extension hardening. To understand, control and predict how reactor conditions affect material processing requires two separate modelling tasks: (1) prediction of the statistical distribution of molecular weight and branching in polymers emerging from the reactor, and (2) prediction of the melt rheology given the molecular weight and branching distribution. This presentation will review recent efforts in both of these tasks, in particular the development of computational rheology methods based on the "tube model" for entangled polymers - such as our "BoB" rheology predictor (http://sourceforge.net/projects/bob-rheology/). We also briefly highlight some current challenges, and work which addresses these, in particular:<br/><br/>(i) Linear rheology of polydisperse linear polymers: accurate prediction of this requires fuller understanding of the interplay between different relaxation mechanisms of constraint release and contour length fluctuations.<br/>(ii) How far will chain strands in a branched polymer stretch? A topological number known as "priority" must be revised to include flow rate dependence and collective behaviour of multiple branch-points.<br/>(iii) What happens after maximal stretch? Recent experiments from the Hassager group have indicated an overshoot in extensional true stress. One possible explanation is a sudden onset of "entanglement stripping."

Polyethylene: Predictions from reactor to rheology

251st National Meeting (2016)

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