摘要:

Shear‐thinning viscosities have been investigated for dilute solutions (c

ISSN:0148-6055    

DOI:10.1122/1.550550

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The fundamental problems in two immiscible polymer blends, such as deformation, break‐up, and coalescence of the dispersed phase, were considered. Instead of formulating a single droplet problem, it was assumed that there is a kind of structure of the interfaces, in which the interfacial area (Q) and its anisotropy (qij) are equilibrated due to the competition between flow and interfacial tension. Relaxation mechanisms of the interfaces in heterogeneous systems were phenomenologically considered so that a more general constitutive equation was proposed, which can be used not only for arbitrary volume fractions but also for arbitrary flow fields. Also, the effect of simple shear flow on the morphology of polystyrene (PS)/linear low‐density polyethylene (LLDPE) blends was experimentally investigated. Whereas most works along these lines have been done visually in a flow cell, our samples were quenched after steady shear and their resulting structures were analyzed by scanning electron microscopy. In order to achieve a better understanding of morphological effects on polymer blending processes, the semiphenomenological expressions describing the interface contributions of immiscible polymer blends were formulated and compared with dynamic shear experiments of PS/LLDPE blends.

ISSN:0148-6055    

DOI:10.1122/1.550551

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Carefully prepared aqueous solutions of drag‐reducing polymers have been characterized by means of steady viscometric and molecular weight measurements. Solutions of anionic polyacrylamide (Cyanatrol‐750), 0.5–50 ppm, and nonionic poly (ethylene oxide) (Polyox WSR‐303), 100–1000 ppm, were investigated. Steady shear viscosity measurements, with a Rheometrics Stress Rheometer and a Contraves LS30 rheometer, reveal substantial shear thinning for Cyanatrol solutions above 2 ppm and Polyox solutions above 500 ppm, for the shear‐rate range investigated. Zero‐shear viscosity was accurately determined for all solutions. Intrinsic viscosity, hydrodynamic radius of gyration, and overlap concentration were also estimated. The Carreau and Carreau–Yasuda models successfully fit all shear viscosity data, and allow estimates of elasticity. Reduced variables were used for generalizing the shear viscosity data. Aging tests show that both polymers are stable. The molecular weight and polydispersity of Cyanatrol were measured using the technique of band sedimentation/low angle laser light scattering. The drag‐reducing capability of these polymers has been confirmed by pipe flow experiments.

ISSN:0148-6055    

DOI:10.1122/1.550605

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

This study examines the surface defects known as sharkskin and the physical mechanisms that cause its appearance. Authors of previous papers who studied different polyethylenes attribute the occurrence of sharkskin, either to the initiation of slip at the fluid–wall interface or to the existence of local tensile stresses at the die exit. To test the slip hypothesis, the present authors studied the flow of a linear low density polyethylene (LLDPE) through capillaries of different geometries. The results obtained show clearly that experimental methods for determining slip velocities do not give conclusive evidence of the existence of slip at the wall for the flow of the LLDPE used, under conditions that would normally give rise to sharkskin. The observations obtained using silicone fluids of different molecular weights and an LLDPE suggest that this phenomenon results from the cracking of the fluid at the die exit, due to the high tensile stresses in that region. Such an explanation is supported by flow birefringence measurements and by numerical modeling results published in the literature.

ISSN:0148-6055    

DOI:10.1122/1.550552

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550553

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550554

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The steady‐state viscosities η and first normal stress differencesN1as functions of the shear rate γ̇ are measured for highly concentrated and densely packed solutions and melts of hydroxypropylcellulose (HPC) and a polyester [OQO(OEt)10] made from the condensation of 1,10‐decane‐bis‐benzoyl chloride and ethoxy substituted hydroquinone. As the concentration of HPC is increased, systematic departures from the predictions of the Doi theory are found. First, beginning at a concentrationCof around 30% by weight, the zero‐shear viscosity begins to increase, rather than decrease, with increasing concentration. Then, at a concentration of 35%, the shear rate γ̇maxat whichN1reaches a local maximum begins to decrease, rather than increase, with concentration, the viscoelastic activation energy of the solution begins to exceed that of the solvent, and the shape of the curve ofN1versus shear rate begins to become temperature sensitive. At a concentration of 55% at room temperature, the region of negativeN1disappears, although a local minimum ofN1remains. Finally, for bulk samples of HPC at elevated temperatures and for 60% solutions and bulk samples of [OQO(OEt)10],N1is positive only, with no local minimum. We hypothesize that this progressive breakdown in the predictions of the Doi theory as concentration increases is caused by polymer–polymer hindrances to translational motion that at high concentration can only be relieved by increasing the temperature and thereby increasing the flexibility of the molecules.

ISSN:0148-6055    

DOI:10.1122/1.550555

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Molecular orientation in aqueous lyotropic solutions of hydroxypropylcellulose has been studied in steady and transient flows using the technique of flow birefringence. Birefringence is an increasing function of steady shear rate in the range from 0.01–100 s−1. Upon flow cessation, the orientation is seen to decrease towards a globally isotropic condition. It is hypothesized that this decrease in orientation may reflect a transition from a flow‐induced nematic back to a cholesteric phase. In this case, low orientation and ‘‘region I’’ shear thinning at low rates may be consequences of persistence of cholestericity in slow flows. The decrease in orientation appears to be well correlated with gradual increases in the complex modulus of the solution during relaxation. However, while birefringence reveals that the final state is optically isotropic for all previous shear rates, the long‐time value of the modulus depends strongly on previous shear rate. To further investigate structural differences in the relaxed state, shear stress and birefringence were measured upon flow resumption. If the relaxed state resulted from a high previous shear rate, a very large overshoot is observed in stress, while if the previous shear rate is low, a much weaker oscillatory pattern is observed. Birefringence results are qualitatively similar. Finally, birefringence and stress were studied upon flow reversal. Both exhibit damped oscillatory responses, but the qualitative shape of the profiles change as a function of shear rate within the linear regime. These results are compared and contrasted with the behavior of liquid‐crystalline solutions of poly(benzyl glutamate).

ISSN:0148-6055    

DOI:10.1122/1.550556

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A thorough rheological investigation into the region I flow of liquid‐crystalline hydroxypropylcellulose in water is reported. At low shear rates a region of shear thinning with a power‐law exponent 1/2 is observed over two decades in shear rate. Anomalous transient behavior is seen for applied stresses below the critical stress at the region I–II turnover such that the strain required to reach steady state increases as stress decreases. Recoil measurements show large recoverable strain and similar pseudostrain scaling in both regions I and II. The activation energy for the viscosity is also the same in regions I and II. This rheology combined withinsituflow‐small‐angle light scattering measurements of the defect texture size show the qualitative and quantitative accuracy of the defect texture stress balance previously proposed to explain region I flow.

ISSN:0148-6055    

DOI:10.1122/1.550557

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The rheological properties of polymeric liquid crystals are quite complicated and theories over the last decade and a half have made great progress in explaining this seemingly unique behavior. Most theories have considered only the elastic stress and neglected the viscous stress which can be measured by determining the amount of stress instantaneously lost on cessation of shear. The elastic stress is the stress that is left which subsequently decays with time after cessation. Data is presented that shows the viscous stress contribution determined from shear cessation experiments is present for both the biphasic and liquid‐crystalline mesophase of hydroxypropylcellulose‐water solutions. This system shows the three regions for the shear viscosity at higher concentrations. The viscous stress is the dominant contribution at high shear rates, well within region III, and is caused by viscous drag on the molecules. The viscous stress contribution is still present in the intermediate region II although to a lesser extent (approximately 20% of the total); however, as region I is approached it becomes more prominent. We explain this behavior as due to a phenomenon similar to superplasticity by viscous dissipation between domains or within defects.

ISSN:0148-6055    

DOI:10.1122/1.550558

出版商:The Society of Rheology    

年代:1994

数据来源: AIP