摘要:

The shear flow behavior of a unidomain nematic solution of poly(n‐hexyl isocyanate) (PHIC) subjected to an electric field was studied experimentally in a parallel plate rotational rheometer. At a shear rate of 0.4 s‐1, the Miesowicz viscosity ηc(with the director oriented along the velocity gradient) was found to be 35 times greater than the steady shear viscosity in the absence of an electric field. The mechanism for this electrorheological (ER) effect is the orientation of the permanent dipole moment of the PHIC molecules; the ER effect is an order of magnitude larger than that for low molecular weight liquid crystals. Director tumbling is postulated to occur at a shear rate of 0.4 s−1for the PHIC solution if the applied dc electric field is lower than approximately 0.4 MV/m. At higher electric fields, flow alignment is regained. In this case, the transient stress undershoot is suppressed. Steady state viscosity vs electric field data were fit to Carlsson and Skarp’s two‐dimensional approximation of the Ericksen, Leslie, and Parodi theory, yielding values for the Leslie coefficients α1, α2, and α3that compare favorably with those predicted by Doi theory.

ISSN:0148-6055    

DOI:10.1122/1.550302

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

The effects of fluid rheology on the pressure drop in flow transverse to periodic arrays of circular cylinders (porosity=0.7) have been evaluated experimentally with several polyisobutylene solutions, at low Reynolds numbers. Care was taken to avoid degradation of the polymer during the permeation experiments. Specifically, the connection between extensional viscosity behavior and the flow resistance through the arrays has been evaluated experimentally with three dilute solutions of different molecular weight polyisobutylenes in polybutene at the same concentration. Fiber spinning of these solutions indicates that the apparent Trouton ratio of polymer contributions, at higher Deborah numbers, is proportional to the molecular weight. At Deborah numbers between 2 and 5, the (fRe) product attains a different asymptotic limit in each case. The high Deborah number limit of (fRe) is proportional to molecular weight, similar to the apparent Trouton ratios obtained from fiber spinning. In contrast, thefRe data for a strongly shear thinning solution of polyisobutylene in decalin do not show an upper limit even at a Deborah number of 8, based on a strain rate dependent relaxation time. Fluctuations in the downstream pressure are observed for the Boger liquids above the onset Deborah number, indicating the presence of an elastic instability. Hence, the assumption of a steady flow field appears to be untenable for computing the viscoelastic flow resistance in such media.

ISSN:0148-6055    

DOI:10.1122/1.550303

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

When shear oscillations are performed with melts of polystyrene (PS)/poly(methylmethacrylate) (PMMA) blends the total deformation has a large elastic portion at low frequencies. This finding is caused by the interfacial tension acting between the two phases of the blends. A simple model allows one to describe the influence of the interfacial tension on the storage and the loss moduli in a broad frequency range for all mixing ratios of the blend components. The weighted relaxation spectra τH(τ) of such blends show an additional peak with a characteristic relaxation time that is correlated with the interfacial tension. From this characteristic relaxation time the interfacial tension α between the melts of PS and PMMA can be determined. The resulting α=(1.9±0.3)×10−3N/m at 170 °C is in good agreement with the value obtained from recovery following melt elongation of the blends.

ISSN:0148-6055    

DOI:10.1122/1.550304

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Computer simulations were carried out to investigate the shear resistance of electrorheological fluids, which are suspensions of electrically neutral particles in an insulating solvent, under ac and pulse electric fields. The shear stress of the system under a constant shear flow is found to pass through a minimum as the frequency of the electric field is increased. Two types of aggregation due to the induced dipoles are observed: at low frequencies the aggregation follows the applied field, while at high frequencies the particle clusters are unable to completely break up. This mechanism gives results which agree qualitatively with rheological experiments.

ISSN:0148-6055    

DOI:10.1122/1.550305

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Helical flow of a polymer melt is analyzed using nonlinear viscoelastic constitutive theory with and without inclusion of the shear stress components, τ13, arising due to an interaction of two shear flows. The problem of helical flow is approximated by considering flow between two parallel plates with one plate moving at constant velocity orthogonally to the main pressure‐driven flow. The governing equations involved in the theory are derived and then solved along with the equation of motion using numerical techniques. The gapwise distributions of shear rates, shear and normal stresses, viscosity, and velocities are obtained, as well as torque and pressure‐throughput relationships. The predicted data for pressure‐throughput relationships are compared to results obtained from rotational extrusion experiments carried out for polystyrene and polypropylene melts exhibiting, respectively, high and low sensitivity of viscosity to temperature. The isothermal solution with or without inclusion of τ13was found to be unable to quantitatively predict pressure drops for helical flow especially at low flow rates and high rotational velocities. For the case of τ13=0, a simplified approach to nonisothermal flow, based on adiabatic temperature rise due to viscous heating, is presented. The nonisothermal approach with τ13=0 is found to correctly describe experimental pressure‐flow rate data for both polymers. The inability of the theory with τ13≠0 to describe the experimental data was possibly due to a significant overprediction by the theory of the second difference of the normal stresses.

ISSN:0148-6055    

DOI:10.1122/1.550307

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550308

出版商:The Society of Rheology    

年代:1992

数据来源: AIP