摘要:

Simultaneous fits of the Kaye‐BKZ and Wagner equations to shear and uniaxial extensional flow data are not the most critical tests of these equations, because their memory functions depend on two strain invariants, and this dependence can be varied independently for shear and for uniaxial extension to obtain a fit for each. In planar deformations, however, the memory function depends on only one invariant; this dependence can be measured in single‐step shear experiments, as is reported here for a branched polyethylene, IUPAC X, using a sliding plate rheometer. Predictions are then made for three different planar deformation histories: start‐up of steady simple shear and steady planar extension, and exponentially growing shear, all tests in which the memory function depends on only one invariant. The predictions in steady shear and exponential shear are in rough agreement with the data. The theory for planar extension, however, greatly underpredicts the experimental strain hardening of IUPAC X, which has been reported to be similar to the strain hardening usually seen in uniaxial extension for LDPE. Thus, the Kaye‐BKZ and Wagner single‐integral equations cannot simultaneously describe both strain softening in shear and extreme strain hardening in planar extension using a damping function obtained from one of these flows.

ISSN:0148-6055    

DOI:10.1122/1.550028

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

The isothermal and nonisothermal linear viscoelastic creep compliance was measured for an amorphous styrene‐butadiene random copolymer. The compliance master curve was constructed from the linear viscoelastic creep isotherms via time‐temperature superposition. The polymer was annealed at a constant temperature, and then the nonisothermal creep response was measured as the specimen was heated at a constant rate through the glass‐to‐rubber transition. At a specific annealing temperature, a single nonisothermal master curve was obtained for all heating rates using the temperature reduced timetT*,where thetT*integral was determined from thelog αTshift factor. At annealing temperatures well above and well belowTg,the nonisothermal master curve was identical to the master curve constructed by time‐temperature superposition; however, at annealing temperatures nearTgthe nonisothermal master curves were significantly different. This difference between isothermal and nonisothermal behavior is a consequence of nonequilibrium effects in the glass‐to‐rubber transition. The implication of the experimental observations are discussed.

ISSN:0148-6055    

DOI:10.1122/1.550055

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

The elongational viscosity of low‐molecular weight melts is not easy to measure because of the low stretching forces involved. We turn therefore to the method of gravity spinning, in which an isothermal filament is allowed to elongate under its own weight. By this means one can obtain forces of the order of tens of dynes, several orders of magnitude lower than previous work on melts. Although the experiment is easy to perform in principle (the only on‐line measurement that needs to be made is the diameter variation with length), there are difficulties in practice having to do with establishing a steady spinline with a zero end force. The nonconstant deformation along the spinline requires use of a viscoelastic model in the data analysis; therefore, two kinds of differential constitutive models were used. Three low molecular weight polypropylenes, used in the manufacture of melt‐blown fabrics, were studied at 180°C and 200°C. The basic check, that the elongational viscosity be three times the shear viscosity at low deformation rates, was achieved over a substantial part of the spinline for the thinnest of the materials. Further conclusions are not independent of the rheological model. We could not distinguish between the two differential models that were selected and chose the simpler “convected Maxwell model” for a detailed study. If a single time constant (rather than a spectrum) is used, its value needs to be larger in elongation than in shear. The time constant in elongation can be correlated in a reasonable way with molecular weight.

ISSN:0148-6055    

DOI:10.1122/1.550029

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

Sketches and diagrams are presented to demonstrate a new phenomenon of nonhomogeneous sedimentation in liquid‐solid systems with a small density difference. Low Reynolds number sedimentation in moderately shear‐thinning viscoelastic fluids results in a striking pattern of rising liquid pockets, which resemble bubbling (aggregative) fluidization within a progressively compacting bed. Inhomogeneities in particle concentration become increasingly apparent with increasing shear‐thinning, until ultimately particles could not be dispersed at all in some fluids. The nature of the inhomogeneities is described and the conditions under which they become apparent are qualitatively discussed.

ISSN:0148-6055    

DOI:10.1122/1.550030

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

In many cases it may be beneficial to measure the overall textural properties of an object by a single number rather than focusing on particular parameters in a rheological model. Intuitively, the concept of “firmness” may serve this purpose. Firmness may be used to classify fruits and vegetables by maturity, foods by textural properties, foamed plastics by their cushioning properties, etc. The definition of firmness has hitherto not been rigorously given. Using a previously developed general rheological model, which is suitable for quantifying the behavior of solids made of linear and nonlinear viscoelastic materials, theoretical and experimental groundwork is laid, whereby an object may be subjected nondestructively to a series of minute deformations by vibration excitation, in conjunction with computerized determination of various indexes for automatic classification by firmness. Development of several alternative firmness indexes is described and their relative utility for automatically classifying viscoelastic bodies, such as fruits and vegetables by firmness, is analyzed and compared. A review of model firmness indexes, by which only two‐way classifications are possible, is followed by the introduction of monotonic firmness indexes, where resonance modes are effectively eliminated, when classification into any number of firmness grades with superior sensitivity is possible. The validity and generality of the approach is demonstrated by firmness classification tests on tomatoes and oranges.

ISSN:0148-6055    

DOI:10.1122/1.550031

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

The Mullins effect refers to the dissipation in cross‐linked rubber of mechanical energy in apparent excess of that due to linear relaxation processes or irreversible structural changes. Although it reflects nonaffine network motion in response to an applied deformation, contrary to previous assertions Mullins softening is found to be independent of the extent to which the junctions of the network are affinely displaced at elastic equilibrium. Rubbers of widely varying network structure were observed to exhibit comparable degrees of Mullins softening. Physically this softening arises from equilibration of local imbalances in the force along the network chains and from contraction of the primitive path of network chain ends. These mechanisms, which transpire too rapidly to be included in the linear relaxation spectrum, can be modeled empirically by the introduction of irreversibility into the damping function in the constitutive equation.

ISSN:0148-6055    

DOI:10.1122/1.550032

出版商:The Society of Rheology    

年代:1989

数据来源: AIP

摘要:

A simple experiment using the falling ball technique was used to demonstrate unambiguously that a 2500 wppm aqueous Carbopol 960 solution has a yield stress. A series of photographs taken over a period of several months revealed that a nylon ball with a density of 1.156 g/cm3showed no measurable movement when placed well within a Carbopol solution having a density of 1.008 g/cm3.

ISSN:0148-6055    

DOI:10.1122/1.550006

出版商:The Society of Rheology    

年代:1989

数据来源: AIP