摘要:

Transient shear experiments show that a thermotropic liquid crystalline polymer (TLCP) in its nematic state behaves linear viscoelastically as long as the shear does not exceed a characteristic stress of 2×104Pa or a characteristic strain of approximately γ=2, whichever is reached first. The relaxation modes, as determined in dynamic mechanical experiments, are sufficient for describing the linear viscoelastic behavior in step strain and start‐up at constant rate. Beyond this linear region, the behavior deviates drastically from that of conventional homopolymers: Negative first normal stress differences are measured during steady shear at rates below 0.5 s−1; long relaxation modes are more affected in a large step strain experiment than short relaxation modes [thus no factorization ofG(t,γ) into separate time and strain functions]; no zero shear viscosity is detected in the accessible range; the normal stress relaxes faster than the shear stress; in a start‐up experiment at constant shear rate, the shear stress and the first normal stress difference both go through a maximum at approximately γ=2. The material, a random copolyester of 73 mol % 4‐hydroxybenzoic acid and 27 mol % 6‐hydroxy‐2‐naphthoic acid (Vectra A900 of Hoechst–Celanese), was shown to be stable at the experimental temperature of 290 °C for at least 1 h if first preheated to 320 °C.

ISSN:0148-6055    

DOI:10.1122/1.550171

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

A simplified three‐dimensional viscoelastic equation, based on the Green–Rivlin theory, was used to characterize the nonlinear creep behavior of two food products, i.e., apple and potato. Uniaxial and triaxial creep tests were conducted to evaluate the nonlinear behavior of apple and potato flesh. Four material property functions in the nonlinear constitutive equation were determined using experimental results. The nonlinear constitutive equation was then used to predict the load response of both apple and potato flesh under various loading conditions. The nonlinear constitutive model calculated values compared favorably with the experimental data. Results show that the nonlinearity of apple flesh under the uniaxial creep loading was significant especially for extended loading time periods. The initial load response has a pronounced effect on the nonlinearity of potato flesh under the uniaxial creep loading.

ISSN:0148-6055    

DOI:10.1122/1.550172

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

A complete constitutive theory that describes the nonlinear elastic behavior of foams and concentrated emulsions with two‐dimensional, polydisperse hexagonal structure is derived. The individual bubbles are irregular hexagons that can vary widely in size and shape. The straight thin films and threefold symmetric Plateau borders exhibited by perfectly ordered monodisperse systems are retained by polydisperse hexagonal structures. The constitutive theory is expressed as explicit relations between the Cauchy stress and the Cauchy–Green strain tensors, or in terms of a strain energy function. The important material characteristics are surface tension, number density of bubbles, and if the dispersed phase is gas, an equation of state. The specific spatial arrangement and size distribution of the bubbles does not affect the elastic response. The volume fraction of the phases only influences the stress through the dispersed phase pressure. Weaire, Kermode, and Fu have conjectured that the elastic response associated with monodisperse structure provides an upper bound for all two‐dimensional fluid–fluid networks; we extend this conjecture to polydisperse hexagonal structure, not just monodisperse structure.

ISSN:0148-6055    

DOI:10.1122/1.550173

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The viscoelastic behavior of commerical and model hydrophobically modified, ethoxylated urethane (HEUR) polymers is examined in aqueous solutions, alone and in the presence of sodium dodecylsulfate (SDS). An unmodified poly(oxyethylene) and a linear model HEUR thickener without internal hydrophobes are compared with three commercial HEUR thickeners, one of low molecular weight comparable with the model HEUR and two of higher molecular weight, comparable to the unmodified poly(oxyethylene). Shear thickening in the viscosity‐shear rate profile, storage and loss moduli, die swell, and dynamic uniaxial extensional viscosity behaviors are examined. There is general agreement among the elastic functions for the low molecular weight commercial HEUR which is primarily inelastic and for one of the higher molecular weight commercial HEUR thickeners, which exhibits a strongly elastic behavior. The addition of SDS to aqueous HEUR solutions increases solution viscosity; in one of the high MW HEUR thickeners this increase is associated with an increase in the elasticity of the solution. In the lower molecular weight HEUR, the increases in solution viscosity observed with SDS are not associated with a notable change in the elasticity of the solution. The difference in rheological response is interpreted not in terms of ‘‘real’’ molecular weight differences, but in terms of competition between intra‐ and interhydrophobe associations. Other differences are discussed and compared with the rheological response of a low molecular weight model HEUR thickener without internal hydrophobic linkages.

ISSN:0148-6055    

DOI:10.1122/1.550174

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

In the calculation of complex flows of non‐Newtonian fluids, fundamental difficulties with the existence of solutions and the convergence of numerical methods arise as a consequence of using one‐particle constitutive equations in which the stress (or extra‐stress) tensor in a fluid element is not affected by the stress and strain histories of neighboring material. For that reason, we formulate here a class of multiparticle constitutive equations based on the idea of polymer diffusion for which one expects that the above‐mentioned difficulties are avoided. The very general ideas are developed in the body tensor formulation of constitutive equations, and the modifications resulting for Maxwell, Oldroyd‐A/B, and K‐BKZ models are described explicitly. Application of the modified models in the numerical solution of non‐Newtonian flow problems is straightforward.

ISSN:0148-6055    

DOI:10.1122/1.550175

出版商:The Society of Rheology    

年代:1991

数据来源: AIP