摘要:

From constant strain-rate experiments in extensional flows, nonlinear strain measures and effective damping functions are derived for a linear high-density polyethylene melt. Experimental results are compared with predictions of two molecular theories, the Doi–Edwards model and the molecular stress function approach of Wagner and Schaeffer. Orientation (but not stretch) of macromolecules at small and large strains is well represented by the Doi–Edwards orientation tensor. Stretch of macromolecules is found to be isotropic as predicted by the molecular stress function model, which is in excellent agreement with the data. For the latter model, a novel strain energy function is presented.

ISSN:0148-6055    

DOI:10.1122/1.550939

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Barium titanate/insulating oil suspensions were investigated to determine the dielectric polarization mechanisms that govern their electrorheological (ER) behavior. The dynamic yield stress of 19.3 vol %suspensions exhibited Maxwell–Wagner-like frequency dependence, with the dynamic yield stress increasing significantly with ac electric field frequency, as expected for suspensions composed of particles with large dielectric constants. The dynamic yield stress at a given frequency was proportional toEm, whereEis the applied electric field strength and the exponentmincreased with frequency. For electric field strengths of 2 kV/mm at 1 kHz, dynamic yield stresses were approximately 500 Pa. Rheological experiments in which particle surface and oil conductivities were varied suggest that the dielectric relaxation of these suspensions is controlled by the particles’ bulk conductivity. The dielectric relaxation depended strongly on field strength, becoming broader with increasing field strength. Harmonic analysis of the current passing through the suspensions verified that the nonlinear contribution to the apparent suspension conductivity increased withEand decreased with frequency. The current harmonics for barium titanate/dry air suspensions were similar to those of the oil based suspensions, suggesting that nonlinear conduction may arise from field-enhanced dissociation of surface groups, as opposed to field-enhanced dissociation of ion pairs within the continuous phase.

ISSN:0148-6055    

DOI:10.1122/1.550946

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The electrorheological (ER) phenomenon is widely attributed to the chaining of micron-sized polarizable particles when subjected to an external electric field. It has been hypothesized that the strength of the particle–particle interactions determines the rheological properties of ER fluids. On the basis of an electrostatic polarization model, we proposed that by controlling the geometry of the particles, the dielectric properties of ER fluid can be enhanced, resulting in increased strength of the particle–particle interactions. In this work, we have developed systems featuring anisotropic particles and conducted a systematic study of the role of particle geometry in the response of ER fluids. Our findings appear to be in agreement with the electrostatic polarization model.

ISSN:0148-6055    

DOI:10.1122/1.550944

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Experimental measurements of birefringence and velocity gradient components are reported for steady mixed type flows of a0.076 g/cm3solution of2.89×106 MWpolystyrene in a mixed toluene/oligomer solvent. The flow field is produced in a co-rotating two-roll mill with a series of different ratios of the gap width to roller radius chosen so that the flow type at the stagnation point for a Newtonian fluid would range from0.0196⩽λ⩽0.20,where‖E‖/‖Ω‖=(1+λ)/(1−λ).Additional data are also reported, for comparison purposes, for a similar polystyrene solution in a simple Couette flow. Finally, the stress-optical relationship is used to obtain a generalized extensional viscosity as a function of strain rate. This viscosity shows a range of strain rate thinning as predicted by reptation theory, followed at a critical Weissenberg number of 0(1) based on the Rouse relaxation time by the initial stages of a region of strain rate thickening, as predicted by the Marrucci–Grizzuti extension of reptation theory that allows for stretching of the primitive chain. Detailed comparisons with birefringence predictions from this model, using measured flow data as input, show good qualitative agreement but a number of quantitative differences. Most notable are: the fact that the model predicts an orientation angle that is rotated further from the principal strain rate direction than what is observed experimentally, especially for the smallest λ values; the lack of a distinct plateau region prior to the onset of chain stretch; and a rate of increase of steady state chain stretching with Weissenberg number that is significantly weaker in the experiments than what is predicted via the model.

ISSN:0148-6055    

DOI:10.1122/1.550957

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Polymer processes which employ elongational flows have been successfully used for shaping ceramics by using crowded (50–60 vol %) suspensions of ceramic particles, but the full realization of their potential rests on an understanding of the extensional flow behavior of filled polymer systems. A uniaxial rheometer was constructed to evaluate the extensional flow properties of crowded suspensions based on polyisobutylene. The shear viscosity was evaluated for comparison using a Weissenberg rheometer and correlated with published literature values for a similar molecular weight polyisobutylene. Trouton’s Law was verified for the unfilled polymer. The shear and elongational viscosities of filled polyisobutylene, containing up to 42 vol % and 45 vol % ceramic, respectively, were measured. The dependence of shear and elongational relative viscosities on powder volume fraction were compared. In uniaxial flow, Trouton’s Law was obeyed for crowded suspensions containing up to 42 vol % ceramic powder.

ISSN:0148-6055    

DOI:10.1122/1.550947

出版商:The Society of Rheology    

年代:1998

数据来源: AIP