Pom-pom-like constitutive equations for comb polymers

In analogy with the pom-pom model, we introduce a simple model for comb polymers with multiple side-arms attached to a linear backbone by considering a set of coupled equations describing the stretch in the individual interbranch backbone segments. The stretch equations predict a sudden onset of backbone stretch as the flow rate is increased. Drag-strain coupling smooths this transition to some extent. For a series of well characterized polyisoprene and polystyrene combs, we find good agreement with the experimentally determined transient stress growth coefficients in uniaxial extension

Morphological effects of spatial and temporal gradients of shear in a faithful human right coronary artery cell culture model

Dysfunction of the vascular endothelium can initiate atherosclerosis. Mechanical forces, particularly wall shear stress (WSS) are believed to cause endothelial dysfunction. Present in vitro cell culture models are often simplified and thus, ignore the wall shear stress spatial gradients inherent in complex geometries. The aim of this project was to study endothelial cell response in an anatomically correct right coronary artery model (RCA) under more physiologically realistic flow conditions.Human Abdominal Aortic Endothelial Cells (HAAECs) were seeded in the lumen of a pre-treated faithful RCA and a straight tubular model. The cells were subjected to steady or non-reversing oscillatory flow (Re=196, alpha=1.82) at a mean physiological flow rate of 20 dynes/cm2 for 8, 12 and 24 hours of flow. The results show that under all flow conditions, the cells became progressively more elongated and aligned. Moreover, differences in endothelial morphology in the inner (myocardial) and outer (pericardial) walls were seen in the inlet region. The morphologic adaptation to steady and oscillatory flow was similar. The results suggest that spatial, not temporal gradients in shear in the inlet region are responsible for the differential endothelial cell response

Branch-point motion in architecturally complex polymers: Estimation of hopping parameters from computer simulations and experiments

Relaxation of branched polymers under tube-based models involves a parameter p2 characterizing the hop size of relaxed side arms. Depending on assumptions made in rheological models (e.g., about the relevant tube diameter for branch-point hops), p2 had been set to values varying from 1 to 1/60 in the literature. From large-scale molecular dynamics simulations of melts of entangled branched polymers of different architectures, and from experimental rheological data on a set of well-characterized comb polymers with many (∼30) side arms, we estimate the values of p2 under different assumptions in the hierarchical relaxation scheme. Both the simulations and the experiments show that including the backbone friction and considering hopping in the dilated tube provides the most consistent set of hopping parameters in different architectures. © 2014 American Chemical Society.This work has been supported by the projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, Marie Curie Network, European Union), MAT2012-31088 (Spain), and IT654-13 (GV, Spain).Peer Reviewe