Volume 59, Issue 2, March 2015
Index of content:
The dynamic motion of single bubbles with unique shapes rising freely in hydrophobically modified alkali-soluble emulsion polymer solutions59(2015); http://dx.doi.org/10.1122/1.4904911View Description Hide Description
The motion of single bubbles rising through hydrophobically modified alkali-soluble emulsion polymer (HASE) solutions is experimentally examined. In this study, we prepare 1.0, 1.1, and 1.6 wt. % HASE solutions and the bubble rise motion is explored depending on the value of the suspending viscosity. Consequently, in a HASE solution with high-viscosity, we find threadlike thin and long branches formed at the trailing edge of the bubble. For moderate-viscosity, we observe bubbles with long and large trailing skirts. These are different from those for Newtonian fluid systems. In the case of low-viscosity, many thread-shaped tails are formed along the bottom edge of the bubble. We observe that threadlike tails irregularly or regularly (unidirectionally) rotate depending on the bubble size along the bottom edge of the bubble. In addition, the bubble rise motion is considered based on modified physical parameters and the terminal bubble rise velocity. In our experimental conditions, it is reported that a jump discontinuity of the terminal bubble rise velocity has not occurred.
Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow59(2015); http://dx.doi.org/10.1122/1.4905714View Description Hide Description
Blends of 75 wt. % amorphous polylactide (PLA) with 25 wt. % poly[(butylene succinate)-co-adipate] (PBSA) and poly[(butylene adipate)-co-terephthalate] were separately prepared using an internal batch mixer. The morphology and viscoelastic properties of these two blends were analyzed and compared. Annealing did not cause any pronounced morphology changes nor a subsequent modification of the viscoelastic behavior for both blends. However, applying a shear for 20 min at a rate of 0.05 s−1 induced significant droplet coalescence in both blends, although the changes in the viscoelastic response were more prominent in the PLA/PBSA blend. It was also shown that applying a shear for 10 and 20 min at a rate of 0.2 s−1 caused a slight droplet coalescence and minor changes in the viscoelastic behavior of both blends. Moreover, the Palierne model was used to calculate the interfacial tensions between the blend components. It was also utilized to estimate the droplet size after applying annealing and shearing.
A constitutive model for entangled polymers incorporating binary entanglement pair dynamics and a configuration dependent friction coefficient59(2015); http://dx.doi.org/10.1122/1.4905921View Description Hide Description
Following recent work [e.g., J. Park et al., J. Rheol. 56, 1057–1082 (2012); T. Yaoita et al., Macromolecules 45, 2773–2782 (2012); and G. Ianniruberto et al., Macromolecules 45, 8058–8066 (2012)], we introduce the idea of a configuration dependent friction coefficient (CDFC) based on the relative orientation of Kuhn bonds of the test and surrounding matrix chains. We incorporate CDFC into the “toy” model of Mead et al. [Macromolecules 31, 7895–7914 (1998)] in a manner akin to Yaoita et al. [Nihon Reoroji Gakkaishi 42, 207–213 (2014)]. Additionally, we incorporate entanglement dynamics (ED) of discrete entanglement pairs into the new Mead–Banerjee–Park (MBP) model in a way similar to Ianniruberto and Marrucci [J. Rheol. 58, 89–102 (2014)]. The MBP model predicts a deformation dependent entanglement microstructure which is physically reflected in a reduced modulus that heals slowly following cessation of deformation. Incorporating ED into the model allows “shear modification” to be qualitatively captured. The MBP model is tested against experimental data in steady and transient extensional and shear flows. The MBP model captures the monotonic thinning of the extensional flow curve of entangled monodisperse polystyrene (PS) melts [A. Bach et al., Macromolecules 36, 5174–5179 (2003)] while simultaneously predicting the extension hardening found in PS semidilute solutions where CDFC is diluted out [P. K. Bhattacharjee et al., Macromolecules 35, 10131–10148 (2002)]. The simulation results also show that the rheological properties in nonlinear extensional flows of PS melts are sensitive to CDFC but not to convective constraint release (CCR) while those for shear flows are influenced more by CCR. The monodisperse MBP toy model is generalized to arbitrary polydispersity.
Time- and shear history-dependence of the rheological properties of a water-soluble extract from the fronds of the black tree fern, Cyathea medullaris59(2015); http://dx.doi.org/10.1122/1.4905006View Description Hide Description
A shear-thickening polysaccharide extracted from the New Zealand black tree fern was characterized for its time-dependent rheology and the effects of shear-history. The material exhibited antithixotropic behavior at shear rates between 4 and 10 s−1. The parallel growth of normal stresses with time at these shear rates indicated the development of an elastic network under shear. The formation of association structures is proposed to take place via cooperative zipping of stretched chains where associative groups are exposed only on shearing. Hysteresis effects were also observed at shear rates between 4 and 10 s−1. This was probably caused by rapid structure formation at these shear rates, which cannot be easily disrupted at subsequent lower shear rates (<10 s−1). Preshearing at high shear rates (∼1000 s−1) fully stretched out and aligned the polysaccharide chains, exposing all sites for intermolecular association at appropriate lower shear rates.
Simultaneous morphological and rheological measurements on polypropylene: Effect of crystallinity on viscoelastic parameters59(2015); http://dx.doi.org/10.1122/1.4906121View Description Hide Description
A rotational rheometer equipped with an optical module is adopted to characterize the evolution of rheological parameters and at the same time the morphology development during isothermal crystallization. This allowed the determination of the effect of crystallinity on the viscoelastic functions of an isotactic polypropylene. A linear multimode Maxwell model was then applied to obtain the modulus and relaxation time of each mode as a function of the measured crystallinity degree. It was found that at low crystallinity levels, the increase of moduli with crystallinity is about the same for all the modes whereas, when crystallinity degree rises, the increase of moduli is higher for the modes characterized by higher relaxation times. Concerning the relaxation times, it was found that the increase due to crystallinity is about the same for all the modes and reaches a factor of about 10 when relative crystallinity degree is close to 10%. The effect of crystallinity on complex viscosity was also assessed by determining a shift factor due to crystallinity. It was observed that the data collected at the lowest frequencies need higher shift factors with respect to the other ones. This was ascribed to the fact that more than one shift factor is needed to describe the effect on moduli, and low frequencies stir modes with higher relaxation times, which undergo a moduli increase larger than modes with shorter relaxation times.
Unexpected maximum in the concentration dependency of the relaxation frequency of Brownian stress in a colloidal suspension59(2015); http://dx.doi.org/10.1122/1.4906227View Description Hide Description
Concentrated and semidiluted sheared suspensions of silica nanoparticles in Diglycidyl Ether of Bisphenol A have recently been shown to exhibit a low-frequency relaxation process of the shear moduli measured by oscillatory rheology. This process, which is slower than the structural α-process of the matrix, was interpreted as Brownian stress relaxation resulting from strain-induced perturbations of the isotropic filler distribution. In this paper, we extend the rheological investigation of the low-frequency anomaly to ultra-diluted DGEBA/silica suspensions. We illustrate that the Brownian relaxation process depends in a complex manner on the filler volume concentration: For very dilute systems, the relaxation frequency increases with the concentration, whereas for semidilute or concentrated systems, the opposite behavior can be observed. This nonmonotonic dependency of the relaxation frequency leads to a maximum of the relaxation frequency at a volume concentration around 0.133. It can no longer be modeled by Peclet frequencies, since the classical Peclet frequencies depend only on a single concentration dependent physical quantity, viz., the suspension viscosity. A modified Peclet frequency depending on the suspension viscosity and the average surface-to-surface distance between the fillers as a structural, concentration dependent length scale allows for an accurate description of the Brownian relaxation for all concentrations.