Chemical Features of Convected Constraint Release in Polymer Melts

The concept of entanglements, as proposed by de Gennes and quantified by Doi and Edwards, pervades studies of polymer melt dynamics. Entanglements provide mechanical strength once a polymer has solidified, and give rise to the non-Newtonian effects such as die swell, spurt, and normal stresses that often plague polymer processing. The fused filament fabrication method of additive manufacturing, in particular, can produce weak parts because of poorly entangled interfaces between printed filaments. It is now known from simulations that the degree of entanglement can change dramatically under strong flows. I will discuss some of the observed phenomenology from simulations, the circumstantial evidence from experiments; and new theories to describe these effects quantitatively. Here, we compare predictions of a recently developed constitutive equation for disentanglement to united-atom polyethylene and Kremer-Grest molecular dynamics simulations in steady-state shear and extensional flow. Quantitative agreement is obtained in measurements of re-entanglement following cessation steady-state shear flow, which confirm the model prediction of re-entanglement on the Rouse time. We find that entanglement kinetics are independent of molecular weight and thus the number of entanglements, but depend the number of Kuhn segments per entanglement strand Ne. Hence, melts with stiffer molecules will disentangle faster than those with more flexible chains. We interpret this in terms of the number of retraction events required to effectively remove an entanglement.