Volume 51, Issue 4, July 2007
Index of content:
51(2007); http://dx.doi.org/10.1122/1.2736424View Description Hide Description
Carbon nanotubes have exhibited unusually large changes in selective physical and mechanical properties when added to polymers or polymer composites in small quantities. To understand their rheological behavior and processibility, we mixed multiwalled carbon nanotubes(MWNT) in epoxy and created suspensions of different dispersion qualities, MWNT aspect ratios, concentrations, suspensionnetwork structures, and MWNT orientation states. Their rheological properties were measured with a cone and plate rheometer. It was found that as MWNT dispersion quality improved or their network connections, the aspect ratio or concentration increased, the MWNTs interactions became stronger as indicated by a higher storage modulus , complex viscosity, and steady shear viscosity. It was found that suspensions which contained a mixture of separated MWNT along with small MWNT aggregates exhibited that was independent of frequency suggesting solidlike behavior. This frequency sweep method could be used to characterize the network structure state for well dispersed nanotubesuspensions. The steady shear viscosity was found to be more sensitive to MWNT separation than and . All suspensions exhibited shear thinning behavior and nanotubes showed statistically significant alignment in the direction of the shear flow. Aligned MWNTsuspensions not only displayed lower than randomly orientated MWNTsuspensions, but also had lower and values. Transmission electron microscope (TEM) images of cured suspensions and a recently developed capillary method was used to validate the MWNTssuspension state and its microstructure.
51(2007); http://dx.doi.org/10.1122/1.2736405View Description Hide Description
The complicated nonlinear dynamics in film blowing process has been investigated focusing on the multiplicity, bifurcation, and stability in the dynamic solutions of the system using both the theoretical model simulation and experiments. A number of interesting findings have been revealed about the dynamics of the process including a fundamentally different behavior of the system with a maximum of three steady states for the nonisothermal operations in contrast to the isothermal approximation where only two steady states were predicted. These differences have been identified as stemming from the fact that multiple values of a bubble radius at the freezeline height can give the same value of the air pressure inside the bubble depending upon the process conditions and the values of the bifurcation parameters of the system. The stability of the three steady states also displays many different patterns dictated by the process conditions, including the hysteresis in the bifurcation diagrams. These stability results of the system are quite reminiscent of those in the well-known CSTR (continuous stirred tank reactor) cases found in the literature of chemical reaction engineering. The theoretical models of the film blowing system developed by this study produce quite accurate predictions of the bubble and film shape results both in the steady state and the transient solutions as compared with experiments. It is considered that the findings of this study can be readily utilized to enhance the stabilization and optimization of the film blowing operation.
51(2007); http://dx.doi.org/10.1122/1.2736413View Description Hide Description
In this study we present a systematic investigation of the highly nonlinear creep behavior of thermo-reversible gels composed of octadecyl coated silica particles suspended in decalin. These suspensions display a gelation transition below a volume fraction dependent critical temperature. The mechanical response of the resulting gels is characterized by a time for the elastic modulus to recover after preshear that can take several hours. Once steady state is reached, upon application of a constant stress, , the compliance of the gel falls into two regimes. Below a critical stress, , the strain produced in the gel increases slowly with time where the rate of increase decreases with time. Above , at short times, the strain response is nearly identical to that observed when . However, at a stress dependent characteristic time, , the gel yields under the shear stress and begins to flow similar to a liquid leading to a rapid increase in the strain by several orders in magnitude. decreases with increasing stress and above a certain stress falls below the measurable time windows and the gel appears to flow at the instant that the stress is applied. is also found to be a strong function of volume fraction and temperature. We develop a simplified model built on the hypothesis that the phenomenon is the result of a competition between the rate of stress-induced bond-breakage events and the rate at which these broken bonds are reformed. Below the critical stress, bond-reformation rates can match the rate at which bonds are broken thereby retaining connectivity within the gel network to support the applied stress and permitting a slow increase in compliance with time. However, above the critical stress, the bond-breakage rates overwhelm the rate at which the gel can heal itself thereby resulting in the eventual degradation of the gel structure and the generation of liquidlike behavior.
51(2007); http://dx.doi.org/10.1122/1.2721616View Description Hide Description
We have investigated the intentional temporary degradation and the subsequent recovery of the drag-reducing properties of surfactantsolutions. Degradation was achieved by exposing the fluid to mechanical stresses after which it showed significantly reduced drag reduction capability. The recovery time varies from a fraction of second to several minutes, depending on the surfactant concentration, the counter-ion concentration and the temperature. The recovery process generally showed a linear increase of the drag reduction level with time, with the rate of recovery being essentially independent of the flow velocity and the initial level of degradation, though the recovery at rest is considerably slower than the recovery under turbulent flow conditions. The recovery time decreases sharply with increasing concentration and temperature, and with increasing counter-ion concentration for the cationic surfactant. The mechanical degradation apparently affects not only the shear-induced structure but also the rod-like micelles themselves.
51(2007); http://dx.doi.org/10.1122/1.2742391View Description Hide Description
We consider the effects of adding a PIB-PDMS diblock copolymer as a compatibilizer in model blends composed of polyisobutylene (PIB) and polydimethylsiloxane(PDMS). The ratio of PIB to PDMS was varied from 20:80 to 80:20 and, hence, these blends are dubbed “concentrated” blends. Most experiments were conducted on blends containing 0.01 or compatibilizer. All blends had a droplet-matrix morphology, with the minor phase being the drop phase; thus, phase inversion occured when PIB and PDMS were in a 50:50 ratio. Previously, we have studied effects of compatibilizer in “dilute” blends with PIB and PDMS in a 10:90 ratio. Much of the rheological behavior of the concentrated blends studied here is found to be qualitatively similar to that of dilute blends: compatibilizer increases the terminal complex viscosity, the terminal relaxation time, steady shear viscosity, and the ultimate recovery upon cessation of shear. However, there are two noteworthy differences. The first is that in blends in which PIB forms the continuous phase, the compatibilizer can suppress coalescence of the PDMS drops. Therefore, rheological properties that depend on drop size, e.g., relaxation time or ultimate recovery, are correspondingly affected. Second, the compatibilizer increases the viscosity, especially the terminal complex viscosity, of the concentrated blends far more than it does in dilute blends. This can be interpreted in terms of a partial immobilization of drop surfaces by the compatibilizer. Indeed, in blends with PIB as the continuous phase, the viscosity is only slightly lower than of a suspension of rigid spheres, suggesting that the compatibilizer immobilizes the interface almost completely.
Filament stretching and capillary breakup extensional rheometry measurements of viscoelastic wormlike micelle solutions51(2007); http://dx.doi.org/10.1122/1.2718974View Description Hide Description
A filament stretching extensional rheometer and capillary breakup extensional rheometer are used to measure the extensional rheology of a series of wormlike micellesolutions experiencing a uniaxial elongational flow. The experiments are performed using a series of wormlike micellesolutions of both cetylpyridinium chloride and sodium salicylate (NaSal) in an aqueous sodium chloride solution and cetyltrimethylammonium bromide and NaSal in de-ionized water. The linear viscoelasticity of all the wormlike micellesolutions is well described by a Maxwell model with just one or two relaxation times while the steady shear measurements all demonstrate characteristics of shear banding at large shear rates. In transient homogeneous uniaxial extension imposed by a filament stretching rheometer, each of the wormlike micellesolutions demonstrate significant strain hardening. At large extension rates, the wormlike micellesolution filaments are all found to fail through a dramatic rupture near the axial midplane at a constant stress independent of imposed extension rate. The result is an extensional viscosity that decays linearly with increasing extension rate. This filament failure likely stems from the local scission of individual wormlike micelle chains. For the more concentrated solutions, as the imposed extension rate is reduced, a critical extension rate is found below which the filament does not rupture, but instead elastocapillary pinch off is recovered and the elastic tensile stresses achieved in the fluid filament grow far beyond the value observed at rupture. This dramatic upturn in the elastic tensile stress and the extensional viscosity at low extension rates is not intuitively expected and is most likely a result of structural changes to the entangled wormlike micellesolution. Strain hardening is also observed in capillary breakup rheometry experiments, however, when the results of filament stretching and capillary breakup rheometrymeasurements at nominally the same extension rate are superimposed, the results do not agree; the extensional viscosity measurements from filament stretching are in some instances more than an order of magnitude larger. This result calls into question the use of capillary breakup rheometry for quantitatively measuring the extensional viscosity of wormlike micellesolutions.
51(2007); http://dx.doi.org/10.1122/1.2736399View Description Hide Description
A two-dimensional (2D) analysis of fiber spinning is presented based on a modified version of the two-phase model [Shrikhande et al., J. Appl. Polym. Sci.100, 3240–3254 (2006); Kohler et al., J. Macromol. Sci. Phys.44, 185–202 (2005)] that accounts for flow-enhanced crystallization (FEC). The modified model employs the extended pom-pom (XPP) constitutive equation for the amorphous phase and the rigid rod equation for the semicrystalline phase. Calculations are carried out for the high-speed spinning of nylons, polyethylene terephthalate (PET), and pure poly(L-lactic acid) (PLLA), as well as racemic mixtures of the latter (rPLA). Radial variations in temperature and degree of crystalline transformation and microstructure for the PLLA, rPLA, and higher speed PET examples show significant patterns in the skin and core regions of the fiber that reflect the interplay between FEC and thermal-induced crystallization (TIC). Insight is gained into the relationship of TIC and FEC in determining radial birefringence profiles. Likewise, similar considerations for the particular Nylon and lower speed PET examples indicate their less pronounced radial effects are the result of the same interplay.
Correlation between stresses and microstructure in concentrated suspensions of non-Brownian spheres subject to unsteady shear flows51(2007); http://dx.doi.org/10.1122/1.2724886View Description Hide Description
Stresses within unsteady simple shear flows of suspensions of non-Brownian spheres constrained to move in the velocity-gradient plane are calculated using Stokesian dynamics simulations. The unsteady flows considered include shear reversal and oscillatory flows of varying strain amplitude. The evolution of the stresses in time are reported along with the corresponding microstructural development for all flow conditions. For shear reversal, the shear stress rapidly decreases to a minimum before gradually returning to the steady state value reached in the previous direction, whereas the normal stress briefly changes sign upon reversal of shear before returning to the steady state value. For oscillatory shear flow, the shear stress increases with total strain before attaining a steady state that depends upon the applied strain amplitude, indicating irreversible behavior even at small strain amplitudes. The shear stresses show a nonmonotonic dependence on the applied strain amplitude that agrees with experimental results [Bricker and Butler, J. Rheol.50, 711–728 (2006)]. The steady state normal stresses also depend on the strain amplitude and may change signs at low strain amplitudes.
51(2007); http://dx.doi.org/10.1122/1.2723148View Description Hide Description
Dropdeformation in equiviscous polymer blends with dispersed phase volume fraction ranging from 0 to 10% has been investigated by video microscopy and image analysis in a parallel plate shear apparatus. Under steady external flow conditions the shape of individual drops, as measured by the three main axes, the orientation angle and the deformation parameter, showed marked time fluctuations around an average value depending on dispersed phase concentration and on drop capillary number. At small volume fractions the deformed shape was essentially coincident with the isolated drop case, whereas the higher the concentration, the larger the observed dropdeformation at a given capillary number. Such deviations from the isolated drop case can be attributed to hydrodynamic interactions, mainly due to flow-induced collisions among drops. The experimental results can be conveniently described by a mean field approximation whereby the forces causing dropdeformation are taken as proportional to blend viscosity rather than to continuous phase viscosity. By using this correction, i.e., by calculating the capillary number from blend viscosity, all the data collapsed to the isolated drop case, thus allowing one to exploit the small-deformation theoretical analyses available from the literature to predict the morphology of concentrated polymer blends.