Polymeric materials exhibit complex dynamics and rheology and, in many cases, show hierarchical relaxation over many different timescales. This in turn affects the processing and properties of the final materials. Such fluids include branched low density polyethylene, which is a fundamental material that appears in plastics of all forms. The processing of these materials, and the relation to properties, has frustrated and encouraged industry for many years: a simple recurring problem is instability in extrusion that leads to imperfect plastic parts, and with costly results. The ability to predict and control this behaviour as a function of molecular chemistry has attracted a long history of collaboration between academia and industry.
Central to the understanding of polymer melt flow is the concept of "entanglements" - the simple idea that since long macromolecules are unable to pass through one another, their dynamics are severely restricted in the melt state. This simple concept has fuelled over 40 years of theoretical and computational research into polymer melt flow, with many successes. The central framework for this has been the "tube model", which derives from the notion that since chains cannot cross, the chains are restricted in their dynamics to a tube-like region of space: movement along the tube is allowed, but movement perpendicular to it prohibited. The general success of the tube model has, for many years, been able to disguise a problem, that no-one is really able to define (in a fundamental mathematical way) what an entanglement is! Disagreements between different exponents of the tube model often arise, and come down to different understandings of the nature of entanglements. It is also clear that some quantitative failings and lack of complete generality of the tube model can be traced to the same source. There is therefore an urgent need to set the science of entangled polymer dynamics on a more secure footing: at present this appears to require a hierarchy of modelling, and a rigorous attention to details of the coarse-graining procedure: from atomistic and coarse-grained molecular dynamics, through simplified single-chain models known as "sliplink" models, before moving on to tube models.
The central aim of this workshop is to bring together experts in all three levels of modelling with a view towards discussing the current state in each area and proposing methods for mapping between models.
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