Volume 58, Issue 3, May 2014
Index of content:
58(2014); http://dx.doi.org/10.1122/1.4866049View Description Hide Description
Rheology has achieved a strong position for the characterization of polymeric materials during the last 40 years. Dynamic-mechanical measurements are widely used for this purpose. On several examples, this paper demonstrates the potential of creep-recovery whose application has still been rather limited. In many cases, dynamic-mechanical experiments suffer from the fact that for several reasons, the angular frequencies applied are not chosen low enough to reach the terminal regime for which relationships between rheological quantities and molecular parameters have been established. In creep and a subsequent recovery, the time scales can be extended into the stationary regime in the linear range of deformation, and therefore, creep recovery is an efficient method to directly determine the zero-shear viscosity η 0 and the linear steady-state recoverable compliance . For a polymer melt with long relaxation times, it is shown how time-dependent creep data converted into dynamic-mechanical quantities can be used to extend the frequency scale to the terminal regime. The power of and its temperature dependence is demonstrated for the analysis of the branching structure of a polymer. Furthermore, from such kind of measurements, interesting insights into the interactions between particles and matrix molecules in filled polymeric materials were obtained. As shown in elongational experiments, the steady state of deformation at a constant stress is reached at shorter times than at the corresponding constant strain rate. The experimental consequences are discussed. Another interesting aspect of creep is that a constant stress implies a constant capillary number. The advantage of this experimental condition for investigations of the droplet deformation in polymer blends is demonstrated.
Strain hardening of molten thermoplastic polymers reinforced with poly(tetrafluoroethylene) nanofibers58(2014); http://dx.doi.org/10.1122/1.4867389View Description Hide Description
The influence of poly(tetrafluoroethylene) (PTFE) nanofibers on the extensional viscosity of various molten thermoplastic polymers, including isotactic polypropylene (iPP), high density polyethylene (HDPE), low density polyethylene (LDPE), and atactic polystyrene (PS), has been investigated. It has been shown that PTFE nanofibers, generated in situ by shearing of crystalline PTFE inclusions during compounding with another molten polymer, formed an entangled network, which in turn drastically changed the rheological behavior of polymers studied. The entangled network of PTFE nanofibers induced the strain hardening effect in the nanocomposites based on iPPs, HDPE, and PS, which do not show the strain hardening themselves. Moreover, the strain hardening in the nanocomposite with LDPE was enhanced in comparison to neat LDPE. The higher the content of PTFE nanofibers and the larger the strain rates applied, the more pronounced the strain hardening occurred. Additionally, the presence of PTFE nanofibers significantly improved the melt strength of studied thermoplastic polymers.
58(2014); http://dx.doi.org/10.1122/1.4866296View Description Hide Description
We undertake here a systematic study of the rheology of blood in steady-state shear flows. As blood is a complex fluid, the first question that we try to answer is whether, even in steady-state shear flows, we can model it as a rheologically simple fluid, i.e., we can describe its behavior through a constitutive model that involves only local kinematic quantities. Having answered that question positively, we then probe as to which non-Newtonian model best fits available shear stress vs shear-rate literature data. We show that under physiological conditions blood is typically viscoplastic, i.e., it exhibits a yield stress that acts as a minimum threshold for flow. We further show that the Casson model emerges naturally as the best approximation, at least for low and moderate shear-rates. We then develop systematically a parametric dependence of the rheological parameters entering the Casson model on key physiological quantities, such as the red blood cell volume fraction (hematocrit). For the yield stress, we base our description on its critical, percolation-originated nature. Thus, we first determine onset conditions, i.e., the critical threshold value that the hematocrit has to have in order for yield stress to appear. It is shown that this is a function of the concentration of a key red blood cell binding protein, fibrinogen. Then, we establish a parametric dependence as a function of the fibrinogen and the square of the difference of the hematocrit from its critical onset value. Similarly, we provide an expression for the Casson viscosity, in terms of the hematocrit and the temperature. A successful validation of the proposed formula is performed against additional experimental literature data. The proposed expression is anticipated to be useful not only for steady-state blood flow modeling but also as providing the starting point for transient shear, or more general flow modeling.