摘要:

The origin of the nonlinearity observed in Bagley plots of thermotropic copolyesters has been investigated experimentally. With the aid of a two‐piston slit rheometer the dependence of the viscosity on the hydrostatic pressure has been determined directly. This dependence only accounts for part of the nonlinearity of the Bagley plots. Using slit dies with either a convergent or a divergent entrance, concave as well as convex pressure profiles are obtained. This can only be explained by a change in the molecular orientation induced by the flow in the entrance zone. The presence of wall effects has been evaluated by measuring the pressure profiles in slits of different heights; it does not explain the nonlinearity of the Bagley plots. The origin of the nonlinearity of Bagley plots must be due to both pressure and molecular reorientation effects. The nature and importance of these reorientation effects are shown to depend on the entrance geometry.

ISSN:0148-6055    

DOI:10.1122/1.550799

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Aqueous suspensions of starch granules, heated to above their gelatinization temperature, can cool to give rubbery gels. The current physical model of these systems attributes gelation to the association of amylose (linear polysaccharide), extracted from the granules into the continuous aqueous phase. The granules themselves are pictured as randomly dispersed in the matrix, reinforcing the gel in the manner that carbon black reinforces rubber. The characteristic increase in a composite’s rigidity with the volume fraction of the reinforcing particles is studied in the present work, using an artificial starch gel, as a means of testing this physical picture. Granules freed of amylose are combined with purified amylose solution to give gels with particle volume fractions covering a wide range. A new technique is presented for preparing the gels at space filling granule volume fractions. The variation of the reinforcement is in reasonable agreement with that predicted by Nielsen’s modification of Kerner’s equation. Granules swollen at 85 °C are found to be less rigid than those swollen at 70 °C and their reinforcing attributes are correspondingly less. The weak dependence of the loss tangent on the granule volume fraction also agrees with prior experience in reinforced systems. The picture which emerges is thus entirely consistent with the current physical model of these systems.

ISSN:0148-6055    

DOI:10.1122/1.550739

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A lyotropic solution of hydroxypropylcellulose inm‐cresol has been studied in a nonhomogeneous shear (plane Poiseuille) flow, and in a set of flows with mixed shear and extension (slit contractions). The average molecular orientation is measured using flow birefringence, while laser‐Doppler velocimetry is used to characterize extensional kinematics in the slit‐contraction flows. In slit flow, we observe very similar behavior to recently published observations in another model lyotropic, PBG inm‐cresol [B. D. Bedford and W. R. Burghardt, J. Rheol.38, 1657 (1994)]. Steady flow at low rates gives way to an instability characterized by large scale structural heterogeneities and time‐dependent birefringence at higher rates. Throughout the entire flow rate range, however, average birefringence measured in slit flow may be quantitatively predicted from simple shear flow data, assuming that the locally averaged rheological and structural properties in the slit flow are equivalent to those occurring in simple shear flow at identical stress levels. In slit‐contraction flows we have observed substantial enhancements in molecular orientation in the vicinity of the contraction, resulting from extensional velocity gradients. Orientation is studied in a variety of geometries in which contraction ratio and shape are varied to change the relative balance of shear and extension. We attribute the increased alignment to a transition from tumbling to flow‐aligning dynamics due to the presence of extension in the contraction region.

ISSN:0148-6055    

DOI:10.1122/1.550793

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The results of an experimental study into the coupling of mass and momentum transfer for dilute polymer solutions are presented to test the numerous theoretical investigations of this effect. Solutions of high molecular weight, nearly monodisperse, linear polystyrene in oligomeric polystyrene were sheared in the cone‐and‐plate geometry. After long times under shear, the concentration of the high molecular weight species was measured as a function of radial position using a gel permeation chromatography based technique. An increase in polymer concentration of over 200% near the apex of the cone and a depletion of polymer near the edge was measured indicating a migration of polymer as a result of shearing the sample. The time evolution of the nonuniform concentration profile was found to be very slow and a strong function of both the polymer molecular weight and the applied shear rate. The experimental results are compared to theoretical predictions of a dumbbell based model and measured values of the physical parameters. While the method of solution of the highly coupled problem invokes a number of simplifying assumptions, the comparison to experimental results suggests that the theory substantially underestimates the rate of migration.

ISSN:0148-6055    

DOI:10.1122/1.550740

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The melt rheology of two zinc alkali phosphate glasses was studied with oscillatory flow experiments to accelerate efforts to melt process them with organic polymers. The glasses differed markedly in chemical durability and melt viscosity. Melt viscosities were time independent at temperatures near their glass transition temperature. The ω dependence of the complex viscosity η* conformed excellently to predictions, η*(ω)=η0/(1+iωτ), for Hookean dumbbells. Fit parameters η0and τ were used to estimate a melt viscosity–average molecular weightMusing a model reported by Ferry and co‐workers [J. Applied Phys.26, 359 (1955)]. Arrhenius activation energies ΔH‡for viscous flow were consistent with those traditionally found for inorganic glasses. At higher temperatures, the melt viscosity of both glasses increased monotonically with time. The viscosity increase was exponential at long times, and the transition time to the exponential dependence was found to be strongly dependent on the shear strain. The steep viscosity rise is expected to cause significant processing challenges. Sample crystallization is thought to be responsible for this behavior.

ISSN:0148-6055    

DOI:10.1122/1.550741

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The double‐reptation ansatz for stress relaxation in broadly polydisperse melts of linear chains relates the molecular weight distribution φ(n) and the dynamic viscosity η(ω), and has been used to infer φ(n) from rheological data. I show that the microscopic theory of constraint release implies the double‐reptation model is valid for practical distributions φ(n). I also provide a heuristic theoretical argument for the Cox–Merz empirical rule that the shapes of the functions η*(ω) and η(γ̇) are similar, which allows the shear‐thinning curve to be predicted.

ISSN:0148-6055    

DOI:10.1122/1.550742

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A hypothesis is presented regarding the dynamics of lubricating layers in shipping polymer melts and solutions. It is proposed that such polymer solutions and melts produce an asymmetric wavy extrudate in which the steep of the wave advances. (AIP)

ISSN:0148-6055    

DOI:10.1122/1.550743

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550744

出版商:The Society of Rheology    

年代:1996

数据来源: AIP