摘要:

Stress relaxation after rapid extensional strain was measured to obtain the extensional relaxation modulus. The research had the objectives of developing the lubricated squeezing technique for molten polymers, of applying the technique to two different polymers, and of testing the “separability hypothesis.” Equibiaxial extensional flow was generated with a Rheometrics RDS‐LA, using the lubricated squeezing technique. Thetimedependence of the relaxation modulus was found to be the same in extension as in shear, given by the relaxation modulus of linear viscoelasticity. Thestraindependence was markedly different than in shear. Separation of time and strain dependence into a product of two functions is suggested by the experimental results. This is a strong support for the separability hypothesis and simplifies the formulation of rheological constitutive equations. A polymer with linear macromolecules (PS) exhibited stronger strain dependence than a polymer with long chain branched macromolecules (LDPE). The parameters of an integral constitutive equation were determined in rapid strain experiments and the constitutive equation was tested experimentally with stress growth at start‐up of equibiaxial extension.

ISSN:0148-6055    

DOI:10.1122/1.549799

出版商:The Society of Rheology    

年代:1985

数据来源: AIP

摘要:

In this paper we modify the kinetic theory for suspensions of elastic dumbbells by introducing anisotropic hydrodynamic drag (expressed by a tensorial Stokes's law) and anisotropic Brownian motion (resulting from the introduction of a tensor into the Maxwellian velocity distribution). We show how our general formulation leads to Giesekus's 1982 constitutive equation, to the encapsulated dumbbell model of Bird and DeAguiar, and to several other constitutive equations. It is also shown how the Giesekus postulate for the form of the hydrodynamic drag tensor can be used to re‐interpret hydrodynamic interaction. Finally this development emphasizes the necessity for modifying the stress tensor expression when reptation is introduced into molecular theories.

ISSN:0148-6055    

DOI:10.1122/1.549800

出版商:The Society of Rheology    

年代:1985

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.549801

出版商:The Society of Rheology    

年代:1985

数据来源: AIP

摘要:

In an earlier paper capillary rheometer results were presented for two liquid crystalline copolyesters consisting of 60 mole % and 80 mole % p‐hydroxybenzoic acid (pHBA) and polyethyleneterephthalate (PET). In this paper obtained by means of cone‐and‐plate (CP) and parallel plate (PP) attachments of a Rheometrics Mechanical Spectrometer are compared with the capillary data. In particular, it is observed that over the range of conditions studied, viscosity data obtained in the CP and PP agree with those obtained by means of the capillary rheometer. Furthermore, there is no dependence of the viscosity on geometric or dimensional factors as is observed for low molecular weight liquid crystals under some conditions. Although the viscosity is still shear rate dependent for shear rates as low as5×10−2 sec−1,it is shown by stress relaxation experiments following the cessation of steady shear flow that no significant yield stresses exist. The primary normal stress difference(N1)for the 60 mole % pHBA/PET copolyester is always positive under steady shear conditions butN1for the 80 mole % pHBA/PET system can be negative for certain temperatures and shear rates. Calculated values of the die swell using Tanner's theory and cone‐and‐plate data for the 60 mole % pHBA/PET system were considerably higher than the measured values while the agreement between the theory and measured values was quite good in the case of the 80 mole % system.

ISSN:0148-6055    

DOI:10.1122/1.549830

出版商:The Society of Rheology    

年代:1985

数据来源: AIP

摘要:

A new cone‐plate rheometer has been constructed in order to measure torque‐induced sample volume changeΔVsas well as the usually measured total thrustFand torqueM.One aim is to useΔVsdata to assess the accuracy of time‐dependent valuesN1(t)of the first normal stress difference calculated fromFdata obtained in step shear and step shear rate experiments. The cone‐plate gap has an 8° angle and 25.4 mm radius. The total axial stiffness is 31.4 MN/m (whenFis 100 N);Fresolution is about 1 N.ΔVsresolution is about 0.001% at 130 °C; a silicone liquid is used as a dilatometer indicator. First applications made with a low‐density polyethylene sample (“IUPAC A”) at 130 °C subjected to a step shear rate of1 s−1for a duration of 14 s gave no detectable change in sample volume; it follows that, under these conditions, the maximum possible error (caused by sample volume changes) inN1values is only 1.3% in our Rheodilatometer and 4.2% in the BASF Weissenberg Rheogoniometer which was modified by Meissner. It also follows that the effects of any temperature rise (e.g., due to work done in shearing the sample) are negligible in the present measurements. This gives reason to question the reliability of certain torque data recently obtained by Y.‐H. Lin with a Rheometrics System Four rheometer.

ISSN:0148-6055    

DOI:10.1122/1.549831

出版商:The Society of Rheology    

年代:1985

数据来源: AIP

摘要:

The response of a rigid sphere to an extensional flow field near an advancing liquid‐gas interface in a pipe has been examined both from an experimental and theoretical standpoint. Experimental evidence is given to show that the particles are driven partway through the interface, and a theory is developed to explain this result. In the theory, two forces are considered to have an important effect on the particles. One of these forces is generated by an unbalanced drag flow which tends to push the particles through the interface, and the other is a capillary force generated by the curvature of the bumps which occurs when the particles project through the interface. Acting to counteract each other, these two forces ultimately cause the bumps to reach a maximum (equilibrium) height at some point in time. As predicted by the theory, all the data from this study are correlated by plotting the equilibrium height of the bumps as a function of the product of three dimensionless groups; the capillary number, the ratio of the particle and tube diameters, and the cosine of an angle which establishes the position of the particles on the advancing front relative to the axis of the tube.

ISSN:0148-6055    

DOI:10.1122/1.549802

出版商:The Society of Rheology    

年代:1985

数据来源: AIP