Nonlinear shear and extensional rheology of long-chain randomly branched polybutadiene

Abstract

We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Startup of steady shear experiments is used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching.

Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers

“…Although many interesting observations have been made concerning the effects of architectural complexity on the rheological behavior of polymers, important details concerning the exact role of macromolecular structure on various rheological phenomena are still unknown. An inexhaustive list of examples includes the strong qualitative change of the linear viscoelastic response of branched polyethylenes as compared to their linear counterparts, due to long chain branching [2]; strong extensional hardening of branched polyolefins in startup uniaxial extensional flow which increased with branching content [3,4]; multiple stress overshoots in startup shear flow for highly branched styrene-butadiene rubbers [5,6]; failure of the Cox-Merz rule (comparing the dynamic oscillatory complex viscosity with steady-state shear viscosity) with the steady viscosity being above the complex one for highly branched polymers [7]; weakened shear-thinning behavior with increased branching levels in steady-state shear flow for polybutadienes [8]. Unraveling the molecular origins of these phenomena requires an in-depth knowledge of the structure-property relationships, and hence the need for well-defined macromolecular architectures, with the eventual aim to develop predictive tools.…”

Biopolymer Blends Based on Poly (lactic acid): Shear and Elongation Rheology/Structure/Blowing Process Relationships

“…Indeed, some gripping forces can happen depending of branch density and length. The literature suggests that the strain-hardening is exhibited when the rate of the deformation considerably exceeds the rate of molecular relaxation [30]. The longest relaxation time was obtained by analyzing the linear viscoelastic data in terms of its discrete relaxation spectrum, as presented in our previous paper [11].…”

Rheological Properties of Casting Solutions for Starch Edible Films Production

“…As follows from the figure, the viscosity of CS solutions decreases more sharply as the shear rate increases. The difference in the curve's shape can be explained by different configuration of the initial macromolecules [12]: linear for SA and branched for CS. There are two types of starch macromo- The comparison of experimental and theoretical evaluated solutions viscosity is shown in Table 3.…”