摘要:

Modeling of the response of solidlike polymers is often difficult, not only due to the highly nonlinear behavior of the materials but also because of the difficulty of obtaining relevant material data in the laboratory. Here, we examine the possibility of using concepts from finite elasticity theory to describe the isochronal single-step stress relaxation response for a polymer glass (polycarbonate) far below its glass transition. Torque and normal force measurements from torsional stress relaxation experiments are used to obtain isochoric values for the derivativesW1andW2of the strain energy density function in terms of the deformation invariants at specific time values (isochrones). The values ofW1andW2are then used to determine isochronal values of the Valanis–Landel [Valanis, K. C. and R. F. Landel “The Strain–Energy Function of a Hyperelastic Material in Terms of the Extension Ratios,” J. Appl. Phys.38, 2997–3002 (1967).] (VL) function derivativesw′(λ)and to predict the tension and compression responses for different deformations λ below yield. It is found that, for the conditions examined, the experimentally obtained tension and compression responses are well described within the VL framework, despite the fact that polycarbonate is a compressible material. This success suggests that the set of experiments required to describe the nonlinear behavior of glassy materials may be smaller than previously thought. Also, volumetric measurements in the uniaxial deformations indicate a densification of the glass at large deformations and long relaxation times, which is consistent with concepts in the literature that invoke mechanically accelerated aging to describe mechanical and structural interactions in the physical aging of glassy polymers.  

ISSN:0148-6055    

DOI:10.1122/1.550843

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The biaxiality of the steady state solutions and their stability to out-of-plane disturbances in shear flows of spatially homogeneous liquid crystal polymers using two approximate models, BMAB-Doi and BMAB-HL1, which are derived from the kinetic theory developed by Bhave et al. (1993) (BMAB) using the Doi and the first Hinch–Leal closure approximation, respectively, are studied. By casting the models in a novel biaxial representation of the orientation tensor with two built-in order parameters and a triad of directors, we show explicitly that the steady states of the BMAB models exhibit biaxial symmetry except for some uniaxial degeneracy at isolated Peclet numbers and polymer concentration values. Moreover, we obtain all the steady states in which two directors are confined to the shearing plane and analyze their stability with respect to both in-plane and out-of-plane disturbances. We find that (1) flow-aligning family is the unique stable solution family in the BMAB-Doi model, where two order parameters are of opposite signs; (2) the flow-aligning family in the BMAB-HL1 model is stable only in a finite range of polymer concentration0  10,which grows with respect to the Peclet number; (3) The loss-of-stability in the flow-aligning family atN=10is caused by a one-dimensional director rotational instability pertinent to the existence of the maximum allowable degree of orientation with respect to the flow-aligning major director,56,and is coincident with the change-of-sign behavior of the first normal stress difference and the smaller order parameter as well.

ISSN:0148-6055    

DOI:10.1122/1.550816

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Collagen gels are used extensively for studying cell–matrix mechanical interactions and for making tissue equivalents, where these interactions lead to bulk deformation of the sparse network of long, highly entangled collagen fibrils and syneresis of the interstitial aqueous solution. We have used the confined compression test in conjunction with a biphasic theory to characterize collagen gel mechanics. A finite element method model based on our biphasic theory was used to analyze the experimental results. The results are qualitatively consistent with a viscoelastic collagen network, an inviscid interstitial solution, and significant frictional drag. UsingDASOPT, a differential-algebraic equation solver coupled with an optimizing algorithm, the aggregate modulus for the collagen gel was estimated as 6.32 Pa, its viscosity as6.6×104 Pa s,and its interphase drag coefficient as6.4×109 Pa s m−2in long-time (5 h) creep. Analysis of short-time (2 min) constant strain rate tests gave a much higher modulus (318.3 Pa), indicating processes that generate high resistance at short time but relax too quickly to be significant on a longer time scale. This indication of a relaxation spectrum in compression is consistent with that characterized in shear based on creep and dynamic testing. While Maxwell fluid behavior of the collagen network is exhibited in shear as in compression, the modulus measured in shear was larger. This is hypothesized to be due to microstructural properties of the network. Furthermore, parameter estimates based on the constant strain rate data were used to predict accurately the stress response to sinusoidal strain up to 15% strain, defining the linear viscoelastic limit in compression.

ISSN:0148-6055    

DOI:10.1122/1.550817

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The absence of a model to predict near wall viscosity of complex suspensions instigated an investigation for a new method to determine the wall shear stress. If the inner wall of a flow model is covered with a highly flexible gel layer, the local wall shear stress will deform this gel layer. Through the known properties of the gel layer, the measured deformation can be transformed to the wall shear stress. To measure the deformation of the gel layer, speckle pattern interferometry was applied. The performance of the developed speckle apparatus was evaluated using a well-controlled benchmark experiment, and a resolution of 50 nm was achieved. The deformation of a gel layer was measured in a two-dimensional rectangular duct, using a Newtonian and a non-Newtonian measuring fluid. The wall shear stresses were measured as a function of the flow rate and compared to theoretical predictions, and the results demonstrated the potential of the method.

ISSN:0148-6055    

DOI:10.1122/1.550818

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

In this article the three-dimensional behavior of constitutive models containing fractional order time derivatives in their strain and stress operators is investigated. Assuming isotropic viscoelastic behavior, it is shown that when the material isincompressible, then the one-dimensional constitutive law calibrated either from shear or elongation tests can be directly extended in three dimensions, and the order of fractional differentiation is the same in all deformation patterns. When the material isviscoelastically compressible, the constitutive law in elongation involves additional orders of fractional differentiation that do not appear in the constitutive law in shear. In the special case where the material iselastically compressible, the constitutive laws during elongation and shear are different; however the order of fractional differentiation remains the same. It is shown that for anelastically compressiblematerial, thefour-parameter fractional solid—the rubbery, transition, and glassy model, which has been used extensively to approximate theelongationbehavior of various polymers, can be constructed from thethree-parameter fractional Kelvin—the rubbery transition model inshearand theelastic bulk modulusof the material. Some of the analytical results obtained herein with operational calculus are in agreement with experimental observations reported in the literature. Results on the viscoelastic Poisson behavior of materials described with the fractional solid model are presented and it is shown that at early times the Poisson function reaches negative values.

ISSN:0148-6055    

DOI:10.1122/1.550823

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The multiaxial linear viscoelastic behavior of a soda–lime–silica glass has been investigated in the transition range using shear and uniaxial creep-recovery experiments. The shear and uniaxial viscoelastic constants and retardation functions have been precisely measured. The no-stress and the no-temperature dependence of the viscoelastic constants show, respectively, linear viscoelastic and simple thermorheological behaviors of the glass. The deviatoric part of the viscoelastic behavior is first investigated from the shear experimental data. A generalized Maxwell model with one set of parameters is shown to correctly simulate the shear creep-recovery experiments. Using the deviatoric part of the model, the spheric part of the viscoelastic behavior is deduced from the uniaxial experimental data. In particular, we show experimental measurements of the bulk equilibrium modulus and the temperature dependence of the hydrostatic pressure viscoelastic behavior for a soda–lime–silica glass. A second set of parameters of the generalized Maxwell model is determined for the hydrostatic pressure viscoelastic behavior.

ISSN:0148-6055    

DOI:10.1122/1.550824

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends (PI:PDMS=1:9by weight) having viscosity ratiosηd/ηm=0.155,0.826, and 4.02, whereηdandηmare the viscosities of the droplet and matrix, were investigated by directly measuring the droplet size distributions with a video microscope. Steady state measurements were performed by sequentially stepping up the shear rateγ̇(step-up) and step decrease (step-down) of the shear rate. Dynamic frequency sweep measurements were also performed immediately following cessation of steady shear flow. In the step-up measurements for blends withηd/ηm=0.155and 0.826, stresses were proportional toγ̇and the droplet sizes were well regulated by the flow and inversely proportional toγ̇forγ̇⩾0.553 s−1,in good agreement with the Doi–Ohta model [J. Chem. Phys.95, 1242–1248 (1991)]. The results of step-down measurements were consistent with dynamic moduli measurements, because the droplet sizes were almost unchanged during these measurements. The results are in accord with Palierne’s model [Rheol. Acta29, 204–214 (1990)]. For theηd/ηm=4.02blend, however, the droplet sizes were insensitive toγ̇,resulting in worse agreement between the experimental and theoretical results than for the other blends.

ISSN:0148-6055    

DOI:10.1122/1.550871

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The effect of fiber orientation on the rheological properties of short glass fiber-reinforced composites was investigated by dynamic oscillatory shearing with parallel plate fixtures. As an oscillatory shear amplitude and frequency applied to fiber-reinforced composites increased, more fibers in the composites were aligned in the flow direction, thus the complex viscosity gradually decreased. This phenomenon was confirmed by observing the fiber orientations using optical photographs. The complex viscosity depended upon the strain amplitude, and pre-oscillatory shearing frequency, and shearing time. Experimental results for fiber orientations and complex viscosity were compared with predictions available at the present time. The predictions of the dependence of fiber orientation upon strain amplitudes and fiber volume fractions are in qualitative agreement with experimental data. However, there were effects of the magnitude of frequency and oscillatory shearing time on fiber orientation, thus complex viscosity could not be predicted successfully although these effects were clearly demonstrated by the experiment.

ISSN:0148-6055    

DOI:10.1122/1.550825

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Results of Rheotens experiments on a commercial liquid crystalline polymer melt are reported. In a nonisothermal extension of extruded filaments under the action of constant tensile force, we find a maximum melt tension at break or melt fracture stress of aboutσB=7MPa, rather independent of extrusion speed or melt temperature.

ISSN:0148-6055    

DOI:10.1122/1.550826

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The spinning flow of liquid crystalline polymers is numerically calculated using a modified Doi model. The flow dealt with is an axisymmetric spinning flow for the examination of velocity and molecular orientation developments inside a filament and is solved with a finite difference method. The velocity profile varies from a convex profile to a uniform one through a concave one that is peculiar to the liquid crystalline polymers. The distribution of the molecular orientation near the nozzle exit is found to cause the generation of the concave profile. Furthermore, the results of the simulation suggest the requirement of a long spinline for the attainment of a uniform profile of the molecular orientation inside a fiber.

ISSN:0148-6055    

DOI:10.1122/1.550827

出版商:The Society of Rheology    

年代:1997

数据来源: AIP