Constraint Release mechanisms for H-Polymers moving in Linear Matrices of varying molar masses

We investigate the influence of the environment on the relaxation dynamics of well-defined H-polymers diluted in a matrix of linear chains. The molar mass of the linear chain matrix is systematically varied and the relaxation dynamics of the H-polymer is probed by means of linear viscoelastic measurements, with the aim to understand its altered motion in different blends, compared to its pure melt state. Our results indicate that short unentangled linear chains accelerate the relaxation of both the branches and the backbone of the H-polymers by acting as an effective solvent. On the other hand, the relaxation of the H-polymer in an entangled matrix is slowed-down, with the degree of retardation depending on the entanglement number of the linear chains. We show that this retardation can be quantified by considering that the H-polymers are moving in a dilated tube at the rhythm of the motion of the linear matrix

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

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 branchpoint 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

A computational and experimental study of the linear and nonlinear response of a star polymer melt with a moderate number of unentangled arms

We present from simulations and experiments results on the linear and nonlinear rheology of a moderate functionality, low molecular weight unentangled polystyrene (PS) star melt. The PS samples were anionically synthesized and close to monodisperse while their moderate functionality ensures that they do not display a pronounced core effect. We employ a highly coarse-grained model known as Responsive Particle Dynamics where each star polymer is approximated as a point particle. The eliminated degrees of freedom are used in the definition of an appropriate free energy as well as describing the transient pair-wise potential between particles that accounts for the viscoelastic response. First we reproduce very satisfactorily the experimental moduli using simulation. We then consider the nonlinear response of the same polymer melts by implementing a start-up shear protocol for a wide range of shear rates. As in experiments, we observe the development of a stress overshoot with increasing shear rate followed by a steady-state shear stress. We also recover the shear-thinning nature of the melt, although we slightly overestimate the extent of shear-thinning with simulations. In addition, we study relaxations upon the removal of shear where we find encouraging agreement between experiments and simulations, a finding that corroborates our agreement for the linear rheolog

Uniaxial extensional rheology of well-characterized comb polymers

We present a detailed systematic investigation of the transient uniaxial extensional response of a series of well-characterized, anionically synthesized comb polystyrenes and polyisoprenes. The comb architecture consists of a linear chain backbone with multiple branches of equal molar mass, and represents an excellent model branched polymer. The linear viscoelastic response has been studied already in great detail. Our results indicate that the strain hardening becomes more important as the Hencky strain rate is increased. In general, the larger the number of entanglements of the segments between branches and/or of the branches, the stronger the strain hardening and the smaller the characteristic rate for its onset. The key molecular parameter appears to be the number of entanglements per branch. By varying it, one can tailor the amount and onset of strain hardening. This can be rationalized by accounting for the combined effect of backbone tube dilation and extra friction, brought about by the branches. In fact, we define an effective "stretch time" of the comb as the timescale for stretch relaxation along the dilated backbone tube when accounting for the large friction that comes from the branches and suggest that extension hardening occurs at rates equal to or greater than its inverse. The good comparison of this prediction to experimental data is a promising guide toward a universal framework for understanding the effects of branches on extensional rheology, and hence providing some insight into the behavior of long-chain branched polyolefins. © 2013 The Society of Rheology

Uniaxial extensional rheology of well-characterized comb polymers

We present a detailed systematic investigation of the transient uniaxial extensional response of a series of well-characterized, anionically synthesized comb polystyrenes and polyisoprenes. The comb architecture consists of a linear chain backbone with multiple branches of equal molar mass, and represents an excellent model branched polymer. The linear viscoelastic response has been studied already in great detail. Our results indicate that the strain hardening becomes more important as the Hencky strain rate is increased. In general, the larger the number of entanglements of the segments between branches and/or of the branches, the stronger the strain hardening and the smaller the characteristic rate for its onset. The key molecular parameter appears to be the number of entanglements per branch. By varying it, one can tailor the amount and onset of strain hardening. This can be rationalized by accounting for the combined effect of backbone tube dilation and extra friction, brought about by the branches. In fact, we define an effective "stretch time" of the comb as the timescale for stretch relaxation along the dilated backbone tube when accounting for the large friction that comes from the branches and suggest that extension hardening occurs at rates equal to or greater than its inverse. The good comparison of this prediction to experimental data is a promising guide toward a universal framework for understanding the effects of branches on extensional rheology, and hence providing some insight into the behavior of long-chain branched polyolefins. (C) 2013 The Society of Rheology. [http://dx.doi.org/10.1122/1.4789443]open114143sciescopu

Synthesis and Linear Viscoelasticity of Polystyrene Stars with a Polyketone Core

We report on a novel synthetic route to synthesize relatively large quantities of polystyrene (PS) star polymers with targeted arm functionality and molar mass and their rheological properties in the molten state. The synthetic route involves grafting styrene monomers onto a modified (aliphatic, alternating) polyketone backbone with a specific number of initiating grafting sites using controlled atom transfer radical polymerization (ATRP). Several polyketone precursors were used. This resulted in a large array of star polystyrenes with nonspherical cores and varying average arm length and number of arms. Their linear viscoelasticity was investigated and discussed in the context of the known response of anionically synthesized stars. Using a powerful characterization toolbox, including state-of-the-art interaction chromatography, rheometry, and tube modeling via the branch-on-branch (BoB) algorithm, we have assessed the viscoelasticity of these star polymers quantitatively. In particular, we have demonstrated a variability in molecular structure, which differs substantially from their anionically synthesized counterparts. Hence, whereas this new family of star polymers is not recommended for fundamental studies of polymer physics such as the molecular origin of relaxation mechanisms without prior extensive fractionation, they could be used in studies of mixtures as well as industrially relevant processing operations that require large amounts of polymeric stars. © 2015 American Chemical Society