Volume 52, Issue 3, May 2008
Index of content:
Universal scaling characteristics of stress overshoot in startup shear of entangled polymer solutions52(2008); http://dx.doi.org/10.1122/1.2899147View Description Hide Description
We have carried out conventional rheological measurements to explore the well-known stress overshoot behavior in startup shear of eight entangled polybutadiene solutions. In the elastic deformation regime with , we have identified universal scaling features associated with the stress maximum for samples of different levels of entanglements per chain ranging from 27, 40 to 64. Specifically, at the moment of the peak stress where the applied strain is : (a) varies linearly with , (b) scales with as . The combination of (a) and (b) yields a striking “prediction” that . Remarkably, these scaling laws form master curves when the peak stress, strain rate, and peak time are all normalized with the crossover modulus, reciprocal Rouse time, and Rouse time , respectively. The dependence of on and on is weaker in the crossover regime with . Equally noteworthy is the emergence of a super-master curve for the normalized stress as a function of the normalized strain at various applied rates in these solutions. The solution with only 13 entanglements per chain exhibits behavior deviating appreciably from the well entangled systems. Strain recovery experiments revealed irreversible deformation (i.e., flow) when the sample is sheared beyond the stress maximum (for ) or when sheared with for a period longer than the Rouse relaxation time.
52(2008); http://dx.doi.org/10.1122/1.2892039View Description Hide Description
Several linear ethene∕-olefin copolymers with butene, hexene, octene, dodecene, octadecene, and hexacosene as comonomers were characterized in linear-viscoelastic shear flow. Creep and creep recovery measurements were used to obtain the zero shear-rateviscosity and the steady-state elastic recovery compliance . The correlation between the zero shear-rateviscosity and the weight average molar mass established for metallocene catalyzed high density polyethylene () was found to be obeyed by all samples containing comonomer contents up to . For the linear steady-state elastic compliance an increase with growing molar mass or comonomer content was observed.
Departure from linear velocity profile at the surface of polystyrene melts during shear in cone-plate geometry52(2008); http://dx.doi.org/10.1122/1.2896110View Description Hide Description
Polymer melts show a strongly bulged rim after loading to the rheometer gap. If such a rim is sheared at surface particle tracking shows for a monodisperse and a polydisperse polystyrene (PS) that the surface velocity profile is non-linear from the beginning. For cone angles 0.1 and recipes are provided how to shape the rim closer to spherical, before the test is started. The recipes are for reptation times around and have to be time-temperature shifted for longer ones. By increasing the temperature and squeezing the sample to more than the cone-truncation gap, the melt surface tension and the wetting of the tools are used to shape the rim. The efficiency is quantified by measuring the time evolution of the rim curvature prior to shear, the surface velocity profile during shear, and the viscosity and first normal stress difference. The surface velocity profile strongly depends on pre-treatment, but viscosity and first normal stress difference are quite insensitive to it. For PS the shear rate dependence in the range has been investigated. Even for the best pre-treatment, a strong departure from the linear surface velocity profile is observed at strains around 10.
52(2008); http://dx.doi.org/10.1122/1.2897609View Description Hide Description
Good knowledge of snow rheology is useful for the mitigation of avalanches. However, experiments with snow are difficult and the few available data provide only a partial knowledge of snowflows. In this study we investigated the rheological behavior of a dense flow of dry snow, which often occurs in real avalanches. To this end, we carried out systematic small-scale in situflows down a flume with natural snow. Over three winters, we performed approximately 100 experiments with various slopes and flow discharges and we characterized them by measuring the velocity profile and basal stress. This data set, unique in its extent, allows us to identify various generic characteristics of dense flow of dry snow, which are found to differ from common fluids. We point out that snowflows develop as a very viscous upper thick layer over a much less viscous thin layer. We interpret this heterogeneity as a consequence of a shear-induced evolution of the snow microstructure that gives rise to different materials between the lower part made of single snow grains and the upper layer made of large aggregates. Finally, we show that a single constitutive law that describes dense flow of cohesionless grains can represent the behavior of each layer assuming different grain sizes. Beside its practical importance, this study on snowflow provides new insights into the rheological behavior of similar materials: the wide variety of cohesive granular materials such as humid sand, powders, and bituminous suspensions.
52(2008); http://dx.doi.org/10.1122/1.2901231View Description Hide Description
The failure of interfaces between a series of styrene-butadiene random linear monodisperse polymer melts was investigated with a contact mechanics method. A thin and a thick film of the same polymer were deposited on a metallic cylindrical probe and on a glass slide, respectively. The two films were put in contact for up to in a very confined geometry and the mechanical strength of interfaces formed in these conditions was then characterized by separating the probe from the glass slide. We observed different deformation micromechanisms of the layers depending on the time of contact, the debonding velocity, and the polymer used. We found that these different behaviors could be represented in a general deformation map as a function of two reduced parameters: the ratio between the contact time and the reptation time and the Deborah number. For values of , failure occurred by breakup of a viscoelastic fluid film in a confined geometry. However, for , two regimes were observed: for , failure occurred by interfacial crack propagation, while for , an elasticcavitation mechanism followed by melt fracture was observed. For these linear polymer melts.
52(2008); http://dx.doi.org/10.1122/1.2903546View Description Hide Description
The rheological behavior of a suspension of insulating particles dispersed in a slightly conducting liquid under the action of a DCelectric field is studied. The polarization of the particles induced by the field is shown to be responsible for a rotation of the particles (Quincke rotation) which, in turn, leads to a drastic decrease of the apparent viscosity of the suspension. The purpose of the paper is to provide a relation between the apparent viscosity of the suspension and the electric (E) field intensity. First, the steady-state solutions are searched for the angular velocity of a particle subjected to both DC E field and simple shear flow. Since the solutions are multivalued, their stability is studied using a linear stability analysis. Then the stable solution for the particle angular velocity is used to deduce the value of the apparent viscosity of the suspension. The predictions of the model are compared to experimental data which have been obtained on a suspension of polymethylmethacrylate particles dispersed in a low polar dielectric liquid. The agreement between experiments and theory is rather good even if the model overestimates the viscosity decrease induced by the field.
52(2008); http://dx.doi.org/10.1122/1.2885738View Description Hide Description
Measurements of the stress relaxation modulus of a thermoreversible colloidalgel are compared with the predictions of the soft glassy rheology (SGR) model of Sollich and co-workers [Sollich et al., Phys. Rev. Lett.78, 2020–2023 (1997); Sollich, Phys. Rev. E58, 738–759 (1998); Fielding et al., J. Rheol.44, 323–369 (2000)] in the linear and nonlinear regimes. The material is a 20% volume fraction suspension of organophilic silica dispersed in the organic solvent tetradecane that gels below the critical temperature of . We perform measurements in which prior deformation history and slip are carefully controlled. The linear stress relaxation modulus shows a significant dependence on waiting time, . SGR model predictions are applied to linear step strain measurements for a range of to assign a best fit estimate of the noise temperature, . We then test predictions of the SGR model for the nonlinear stress relaxation modulus, , based on the linear determination of the noise temperature. The measuredgel nonlinear relaxation modulus obeys time-strain separability. For strains greater than the yield strain, , the experimentally determined gel damping function, . The nonlinear stress relaxation response disagrees significantly with the SGR constitutive prediction. This discrepancy appears linked to the functional form of the strain-accelerated activated trap dynamics in the SGR model.
Linear and nonlinear shear flow behavior of monodisperse polyisoprene melts with a large range of molecular weights52(2008); http://dx.doi.org/10.1122/1.2890780View Description Hide Description
Shear rheological data for a wide range of nearly monodisperse linear polyisoprene melts are reported herein using the results of linear and nonlinear measurements. The number of entanglements per chain ranges from about 0.5 to 230. In order to extend the range of available frequencies and shear rates the procedure of time-temperature superposition is improved and validated in both the linear and the nonlinear regime. In the linear flow regime a detailed comparison with the tube theory by Likhtman and McLeish [Macromolecules35, 6332–6343 (2002)] was performed. The overall agreement was found to be satisfactory but several minor disagreements, e.g., at the crossover regime and for the steady-state compliance, were observed and have been discussed. Until now, most nonlinear models for entangled polymers were compared with shear data on solutions. However, the equivalence between solutions and melts is not well established. In this paper the results of a series of nonlinear start-up shear experiments are compared with those predicted by the tube model [Graham et al., J. Rheol.47, 1117–1200 (2003)] and with several literature datasets on entangled solutions having similar numbers of entanglements. The melts were found to behave in a very similar manner to the previously studied solutions and agree well with the tube theory. Disagreements are observed when the shear rate exceeds the Rouse time of one entanglement and also for chains with very small number of entanglements. For the high molecular weight samples possible instabilities and edge fracture can occur at high strains and the results are discussed in light of these effects. The extensive dataset presented here could serve as a test for future theories and models of entangled melts.
Three-dimensional shear-driven dynamics of polydomain textures and disclination loops in liquid crystalline polymers52(2008); http://dx.doi.org/10.1122/1.2890779View Description Hide Description
We report the first numerical investigation of the coupling between flow and microstructure evolution in a full three-dimensional simulation of a model nematic liquid crystalline polymer (LCP), including both viscoelasticity and gradient elasticity, in a planar shear cell. We employ the molecular-based Doi–Marrucci–Greco theory to gain insight into the nature and dynamics of the complex, birefringent polydomain texture exhibited by LCPs and to provide a detailed description of the disclination loops that comprise the so-called worm texture. We solve the coupled equations of motion and evolution equation for the polymer configuration using a parallel implementation of a hybrid spectral/finite-difference algorithm. For an initial director orientation along the vorticity axis, the model predicts a streamwise-oriented striped texture in accordance with previous experimental and theoretical studies. Subsequent refinement of the texture is accompanied by the formation and development of one-half-strength, twist-type disclination loops, which reside between neighboring orientational domains within the polydomain texture. These observations provide the first numerical confirmation of the experimentally inferred loop structure reported by De’Nève et al. , Macromolecules28, 1541–1546 (1995). Though often depicted as separate phenomena in the literature, we find an explicit coupling between the disclination structure and polydomain texture.
Distinguishing between linear and branched wormlike micelle solutions using extensional rheology measurements52(2008); http://dx.doi.org/10.1122/1.2896120View Description Hide Description
A filament stretching extensional rheometer and capillary breakup extensional rheometer is used to measure the effect of branching on the extensional rheology of a series of wormlike micellesolutions. The experiments are performed using a series of linear and branched wormlike micellesolutions consisting of sodium oleate (NaOA) and octyl trimethyl ammonium bromide . The ratio of NaOA to is fixed at while the total surfactant concentration is varied. The shear rheology of wormlike micellesolutions is found to demonstrate a maximum in shear viscosity at followed by a sharp decrease in viscosity with increasing surfactant concentration. It has been shown through cryo-transmission electron microscopy imaging that the maximum in the shear viscosity for these fluids corresponds to the transition from linear entangled to branched micelles. The extensional rheology measurements for all of the wormlike micellesolutions below demonstrated some degree of strain hardening of the extensional viscosity, however, beyond little strain hardening is observed. The maximum value of the Trouton ratio is found to decay rapidly with increasing micelle concentration, starting from values of nearly at a concentration of and approaching an asymptote close to Newtonian limit of for concentrations of and above. These results are most likely due to the additional stress relief mechanisms available to branched micelles which appear to be extremely efficient in extensional flows. These stress relief mechanisms include the fast and fluid sliding of branch points along the length of the micelle and the increased occurrence of “ghost-like” crossing of micelles at entanglement points with increasing surfactant concentration. These observations demonstrate how sensitive the extensional rheology of wormlike micelles is to branching.