Volume 46, Issue 4, July 2002
Index of content:
Flow improvement of waxy oils mediated by self-aggregating partially crystallizable diblock copolymers46(2002); http://dx.doi.org/10.1122/1.1485280View Description Hide Description
Precipitation and gelation of long chain paraffins from oil presents a challenge to the recovery and processing of waxy crude oils and fuel stocks. Diblock polymers consisting of a crystallizable polyethylene (PE) block and an amorphous poly(ethylenepropylene) (PEP) block self-assemble in oil to form expansive plate-like aggregates, consisting of a PE core within a PEP brush layer. In the presence of crystallizable paraffins the crystalline PE core can promote nucleation of solubilized long chain paraffins or may cocrystallize with the paraffin phase with the soluble PEP brush providing steric stabilization of the wax crystals. We examine the effect of PE-b-PEP additives of varying PEP brush length (5 and 11 K) on the yield stresses of straight chain paraffin gels (ranging in length from 24 to 36 carbons) in decane. PE-b-PEP addition at levels as low as 500 ppm produce reductions of wax gelyield stresses by factors of 3000. At higher PE-b-PEP addition levels gels can be formed with higher yield values than solutions without polymer. The location of the minimum in the yield stress with respect to polymer addition depends on the molecular weight of the paraffin and the PEP brush length. Microscopic crystal dimensions and mobility correlate with the observed rheological results. Potential underlying mechanisms for the observed efficiencies are discussed.
46(2002); http://dx.doi.org/10.1122/1.1485278View Description Hide Description
In this paper, the theoretical interrelation between the ratio of dynamic moduli and the number and distribution of relaxation times required to fit the generalized Maxwellmodel to the data was investigated. Theorems were derived for making interval estimation of the relaxation times of the generalized Maxwellmodel from the ratio of linear combinations of the dynamic moduli at different frequencies. According to these theorems, given dynamic moduli and at two frequencies a and b or three frequencies a, b, and one must select at least one relaxation time in the relevant interval for the model to fit the data precisely. As a result, from a set of dynamic data and one can determine a series of from which the minimum number and distribution (interval estimation of of relaxation times of the model can be estimated. The approach was applied to polystyrene data reported in the literature. The results were discussed and compared with those of relevant work, especially the nonlinear regression model-fitting procedure.
Investigations on the temperature dependence of the die entrance flow of various long-chain branched polyethylenes using laser-Doppler velocimetry46(2002); http://dx.doi.org/10.1122/1.1479352View Description Hide Description
Laser-Doppler velocimetry is used to visualize the entrance flowpattern of polyethylene melts undergoing an abrupt 14:1 contraction flow through a slit die. Due to the high accuracy of laser-Doppler velocimetry, a quantitative determination of the velocity vectors in the corners of the contraction region is obtained. Experimental data of this kind together with a thorough rheological characterization are intended to be a base for numerical simulations. Three low-density polyethylenes were investigated. The entrance flow patterns were measured at different output rates and temperatures. For the three polyethylene melts large secondary flow regions were found in the corners of the die. Very significant differences in vortex size as a function of temperature point out that temperature has a significant effect on secondary flow. Furthermore, it is found that changes of the output rate influence secondary flow, too. For the three polyethylenes of different rheological properties it could be shown that within the accuracy of the measurements the vortex sizes come to lie on one curve if plotted as a function of the Weissenberg number. This curve exhibits a maximum that can qualitatively be related to the strain-hardening behavior of the samples.
Using creeping squeeze flow to obtain low-frequency linear viscoelastic properties: Low-shear rate measurements on polydimethylsiloxane46(2002); http://dx.doi.org/10.1122/1.1487368View Description Hide Description
The problem of measuring the linear viscoelastic (LVE) properties of polymers at low frequencies is a long-standing one, especially for fractional melt index polyolefins with broad molecular weight distributions and small time–temperature shift factors. A squeeze–flow apparatus utilizing Newton’s interference rings was devised that could measure minute displacements less than the wavelength of the light used. Maximum transient stresses achieved with the prototype ranged from 0.3 to 70.3 kPa. Its simplicity allows several samples to be run simultaneously under vacuum, minimizing degradation and cost, both critical considerations for long experiments. This paper covers the design of the prototype and results for polydimethylsiloxane at room temperature.
Brownian dynamics simulations of isolated polymer molecules in shear flow near adsorbing and nonadsorbing surfaces46(2002); http://dx.doi.org/10.1122/1.1485279View Description Hide Description
We use Brownian dynamics methods to simulate the interactions of polymers with nonadsorbing and adsorbing solid surfaces at equilibrium and during flow under very dilute conditions. Near a nonadsorbing surface, under uniform shearing flow, we find there is a depletion layer near the wall whose thickness decreases with increasing Weissenberg number, because of the compression of the chain in the shear gradient direction due to finite extensibility, in qualitative agreement with a scaling law derived by Hur et al. (2000). We also demonstrate how the presence of the wall interferes with the molecular tumbling in shear flow. For an irreversibly adsorbing wall, the adsorbed molecule is more stretched than an equivalent unadsorbed molecule in the bulk at the same shear rate. This results from a propensity for the “beads” representing the polymerconformation to affix to the surface sequentially starting at one end, due in part to the tendency in a random-walk polymer for a free end to lie at the periphery, and in part to an increased likelihood of initial contact between polymer and wall during fluctuations that expand the coil in the direction perpendicular to the wall.
Steady-shear rheology of block copolymer melts and concentrated solutions: Defect-mediated flow at low stresses in body-centered-cubic systems46(2002); http://dx.doi.org/10.1122/1.1475979View Description Hide Description
We investigate the low-deformation-rate behavior of sphere-forming block copolymers in the microphase-separated state by both creep and dynamic viscoelasticmeasurements. Polystyrene-polyisoprene and polystyrene-poly(ethylene-alt-propylene) diblock, triblock, and starblock systems of varying molecular weights and compositions, all of which form a polycrystalline body-centered-cubic (bcc) mesophase, are studied as melts and as concentrated solutions in a matrix-selective solvent. At low stresses, the bcc lattice is preserved, and steady-shear measurements reveal a Newtonian viscosity behavior. Following models for high-temperature creep in metals and ceramics, and incorporating the thermodynamic barrier for endblock pullout from spherical microdomains, we have been able to collapse the zero-shear viscosity versus temperature data for diblock melts of a given composition onto a master curve. By analogy with the behavior of polycrystalline metals and ceramics, we anticipated a substantial effect of thermal history on viscosity, through its impact on the grain size. However, only a modest thermal history effect was observed.
Development of wavy texture in startup flows of liquid crystalline polymer solution through a slit cell46(2002); http://dx.doi.org/10.1122/1.1485276View Description Hide Description
The development of wavy texture in startup flows of liquid crystalline polymers through a slit cell is examined to clarify the mechanism of this phenomenon, using a liquid crystalline solution of 50 w % hydroxypropylcellulose. The startup flow at a constant flow rate is generated by a piston moving at a constant speed. It takes a comparatively long time until the wavy texture appears, and the induction time decreases with increasing apparent shear rate. However, it is found that the apparent shear strain required for the onset of wavy texture is independent of the apparent shear rate except at low apparent shear rates. For the slit with 0.52 mm thickness the wide wavy texture is observed at the initial stage of the emergence of wavy texture, and after that the wavelength of the wide wavy texture approximately halves regardless of the apparent shear rate. Furthermore, it is suggested that a twisted structure of molecular orientation exists in the induction period. The results obtained in the present experiments imply that the long-range order elasticity contributes to the emergence of wavy texture.
46(2002); http://dx.doi.org/10.1122/1.1475978View Description Hide Description
The use of microcantilevers in rheological measurements of gases and liquids is demonstrated. Densities and viscosities of both gases and liquids, which can range over several orders of magnitude, are measured simultaneously using a single microcantilever. The microcantilever technique probes only minute volumes of fluid (<1 nL), and enables in situ and rapid rheological measurements. This is in direct contrast to established methods, such as “cone and plate” and Couette rheometry, which are restricted to measurements of liquid viscosity, require large sample volumes, and are incapable of in situmeasurements. The proposed technique also overcomes the restrictions of previous measurements that use microcantilevers, which are limited to liquid viscosity only, and require independent measurement of the liquid density. The technique presented here only requires knowledge of the cantilever geometry, its resonant frequency in vacuum, and its linear mass density. A simple yet robust calibration procedure is described to determine the latter two parameters, from a single measurement of the resonant frequency and quality factor of the cantilever in a reference fluid (such as air), if these parameters are unknown.
Nonlinear viscoelastic analysis of the torque, axial normal force and volume change measured simultaneously in the National Institute of Standards and Technology torsional dilatometer46(2002); http://dx.doi.org/10.1122/1.1475980View Description Hide Description
For compressible materials, the mechanical response to a torsional deformation (at sufficiently large levels of deformation) includes the nonlinear effects of a compressive normal force along the axis of the cylinder and a radial expansion of the cylinder, in addition to the expected torque response. The National Institute of Standards and Technology torsional dilatometer [Duran and McKenna, (1990)] has been used to measure simultaneously the torque, the axial normal force, and the volume change in response to a torsional deformation. In stress-relaxation experiments with an epoxy cylinder just below its glass transition temperature, the torque and normal force decay monotonically, but the volume change associated with the torsion shows an extended relaxation behavior with significant nonmonotonic decay at the lowest temperature investigated. The measurements are modeled with a series solution for torsion of an elastic, compressible material [Murnaghan (1951)]. The elastic solution is adapted for viscoelastic behavior by assuming that isochronal data can be treated as equilibrium elastic data, following a suggestion of Rivlin (1956).
Transient 1–2 plane small-angle x-ray scattering measurements of micellar orientation in aligning and tumbling nematic surfactant solutions46(2002); http://dx.doi.org/10.1122/1.1479353View Description Hide Description
A new x-ray shear cell capable of probing complex fluid structure in the flow-gradient (“1–2”) plane is used to study orientational dynamics of nematic surfactants in transient shear flows. Two surfactantsystems are studied, one of which (SDS/decanol) apparently exhibits shear aligning dynamics and the other of which (CPyCl/hexanol) exhibits director tumbling. Data on the SDS/decanol system exhibit what appear to be common characteristics of aligning nematics: constant orientation state in steady shear; a single undershoot of long duration in average orientation upon flow reversal; and no significant orientation change upon step increase or decrease in shear rate or upon flow cessation. These data are discussed in light of similar measurements on shear aligning thermotropic liquid-crystalline polymers, and polydomain model predictions of transient orientation for aligning nematics. Shear reversal data on CPyCl/hexanol show an oscillatory response typical of tumbling materials, but in better qualitative agreement with the Larson–Doi polydomain model than similar recent measurements on lyotropic polymer solutions.
Effect of annealing in the nematic phase on the transient shear response of thermotropic liquid-crystalline polymers46(2002); http://dx.doi.org/10.1122/1.1485275View Description Hide Description
The effect of isothermal annealing, prior to start-up of shearing flow, in the nematic region on transient shear responses was investigated using a glassy thermotropic liquid-crystallinepolymers (TLCP), PSHQ9, and a crystalline TLCP, PSHQ10. It was observed that the peak value of first normal stress difference of PSHQ9 remains constant, independent of the duration of annealing prior to start-up of shearing flow, whereas the of PSHQ10 increases steadily with increasing period of annealing in the nematic phase prior to start-up of shearing flow. On the other hand, it was observed that the peak value of shear stress remains constant in both PSHQ9 and PSHQ10, independent of the duration of annealing in the nematic phase prior to start-up of shearing flow. The observed dependence of for PSHQ10 on the duration of isothermal annealing in the nematic phase is found, via differential scanning calorimetry, to be attributable to the perfection (or growth) of the crystals in PSHQ10 during annealing prior to start-up of shearing flow. The experimental observations made from this study suggest that glassy TLCPs be used to obtain reproducible transient shear responses.
Role of droplet deformation on the transient rheological response of a polypropylene/liquid-crystal-polymer blend46(2002); http://dx.doi.org/10.1122/1.1487367View Description Hide Description
The transient rheological response and morphological development of 70/30 polypropylene–Rodrun 5000 liquid-crystalline emulsion-like blends have been analyzed. The droplet size distribution was broad. Our results indicate that the shear stress overshoot observed at γ=1.5 s.u. is not due to the liquid-crystalline nature of the droplets, but rather to their elongational deformation. However, quick retraction of the elongated droplets in the steady state is attributed to their nematic nature. As a consequence, the transients after resting for a short time resemble those of start-up experiments. Size polydispersity plays a significant role: shear stress undershoot and normal stress overshoot at γ=25 s.u. are only observed when droplets larger than 40 μm radius are present. Results of flow reversal experiments show that if the flow is reversed before the steady state is reached, then a delayed shear stress overshoot is produced. We relate this to the optically observed retraction of the elongated drops and subsequent stretching by the flow. On the other hand, the monotonic stress increase observed when the flow is reversed after the steady state is reached, is explained by the absence of retraction during flow reversal. The stability of the fibers (elongated droplets) during this particular flow reversal experiment is remarkable.
A thermomechanical framework for the glass transition phenomenon in certain polymers and its application to fiber spinning46(2002); http://dx.doi.org/10.1122/1.1485281View Description Hide Description
A thermodynamic framework is developed to describe a polymer melt undergoing glass transition that takes into account the fact that during such a process the underlying natural configurations (stress-free states) are continually evolving. Such a framework allows one to take into account changes in the symmetry of the material, if such changes take place. Moreover, the framework allows for a seamless transition of a polymeric melt to a mixture of a melt and an elastic solid to the final purely solid state. The efficacy of the model is tested by studying the fiber spinning problem for polyethylene terephthalate and the predictions agree well with the experimental results.
46(2002); http://dx.doi.org/10.1122/1.1485277View Description Hide Description
A microscope-mounted torsional shearing-flow cell is constructed and microscopic particle imagingvelocimetry is employed to directly visualize and map the velocity slip layer in a shearing flow of dilute micellar surfactant solutions of cetyltriammonium bromide/sodium salicylate. It is shown that the thickness of the wall-slip layer is about 100 μm at low shear rates, decreasing to around 50–60 μm at high shear rates. Surprisingly, we find that the wall slip layer emerges only near the upper rotating plate of the flow cell, and not the lower surface. The rheological properties of the wormy micellar solutions are also measured by using a stress-controlled rheometer and the results are compared to the visualization results of wall-slip velocity in the shearing-flow cell. The rheometer measurements are consistent with literature reports [Hu et al. (1998a,1998b)] in that there is a critical shear rate at which the apparent shear viscosity increases drastically. Above the critical point, the shear viscosity reaches a plateau and decreases at a higher shear rate where we observe that the flow becomes unstable. As the gap of parallel plate geometry or the cone angle of cone-plate geometry increases, the critical shear rate decreases, because the wall-slip layer forms in the flow cell even below the shear thickening transition. We extract the wall-slip velocities from the rheological data using a Mooney analysis and show, apparently for the first time, that the slip velocity from the Mooney analysis is consistent with that obtained by direct visualization experiments.
46(2002); http://dx.doi.org/10.1122/1.1487369View Description Hide Description
We consider fiber spinning for the upper-convected Maxwell fluid in the limit of a high Deborah number. We compare several choices of boundary conditions that may be imposed. In addition to the takeup speed and the upstream flow rate, we consider four different boundary conditions: the upstream velocity, upstream elastic stress, the force in the fiber, and the ratio of stress to the square of the velocity (the latter can be motivated by a limit of vanishing retardation time). We find that the effect of the boundary condition on stability is crucial; in one case we even find an instability even though the draw ratio is 1.