Differences in behavior between dilute and concentrated polymer solutions in elongational flow

In this paper, the different behavior of dilute and concentrated solutions (or melts) during an elongational flow is discussed. Some experimental results obtained with a concentrated polyacrylamide solution in water are presented together with a brief review of previous results relative to dilute solutions and melts. An interpretation of the data is attempted on the basis of well known concepts of macromolecular physics together with assumptions concerning the destruction and reformation of entanglements during the flow of the polymer.     

Rotational diffusion coefficient of rod-like polymer with a slight flexibility in semidilute and concentrated solutions

Summary A confined stiff chain model is suggested for the prediction of the rotational diffusion coefficient of a rigid rodlike polymer with a slight flexibility above the region of dilute solution (c1/L³). It shows a fairly good agreement with the experimental data of various polymers. Among them, PBLG and PBT with more rigidity are more consistent with the model when the log-jamming effect is considered. The predicted rotational diffusivity shows approximately the inverse seventh-power of length, which is less than 9 of Doi-Edwards tube model, but larger than the experimental value 5.7 of M-virus, while it shows the inverse power of concentration is a little larger than the value 2 of tube model except for the rodlike virus M-13.     

Free volume treatment of concentrated polymer solutions

Very different concentration dependences of the viscosity of polymer solutions are predicted by the free volume treatments of Fujita and Kishimoto and of Kelly and Bueche. This latter is conveniently extended, and it is shown that it can describe a given set of experimental data over a concentration range much larger than the Fujita-Kishimoto equations.     

Vapor-liquid equilibrium calculations for concentrated polymer solutions

The free-volume theory of polymer solutions initiated by Prigogine and developed by Flory and Patterson is reduced to practice. This theory, which represents a substantial improvement over the older theory of Flory and Huggins, facilitates calculation of solvent activities in polymer solutions from a minimum of experimental binary data. However, it is necessary to characterize each pure component with three molecular parameters which can be obtained from PVT data; such data, unfortunately, are often unavailable. Nevertheless, using reasonable approximations based on Bondi's correlations as needed, molecular parameters are given for 22 solvents and for 16 polymers. Binary parameters are given for 20 systems. Illustrative examples are presented.     

Viscometric behavior of concentrated cellulose acetate solutions

The viscometric behavior of concentrated cellulose acetate solutions in a variety of solvent and mixed solvent systems has been studied. Rapid increases in solution viscosity with increase in cellulose acetate concentration are apparently related to polymer nonsphericity and the tendency of molecules to experience extensive nonhydrodynamic interactions. By employing a well-defined polymer solution theory in conjunction with an empirically determined calibration curve, accurate predictions of concentrated cellulose acetate solution viscosities are possible.     

Prospects and problems in processing highly concentrated polymer solutions

Conclusions -- For processes of fibre-spinning by the wet method, a general relationship has been found with respect to the proportion of regeneratable solvent from the precipitation and plasticizing baths and from final wash water, which permits one to expertly evaluate a technological process.-- It has been shown that the processing of highly concentrated polymer solutions is facilitated when one reduces the activation energy for viscous flow, when one brings the polymer into solution in the liquid-crystalline state, when one uses surface-active substances, high-speed break-up devices, filter media with a high contaminant capacity, with preliminary clean-up of low-viscosity polymer solutions, when one reduces the free gas in the solution under pressure, and when one meters the spinning solution at an elevated pressure through the metering pump.Interbranch Scientific-Technical Conference (April 1–3, 1991, Mytishchi).Translated from Khimicheskie Volokna, No. 5, pp. 4–7, September–October, 1991.     

Predicting melt flow instability from a criterion based on the behavior of polymer solutions

A criterion, based on the behavior of polymer solutions, is developed and applied for the prediction of the onset of flow anomalies observed at the capillary entrance for polymer melts. It is shown that a direct correspondence exists between the flow anomalies observed for polymer solutions and polymer melts. The onset of these anomalies can be correlated with a critical Weissenberg number which is consistent with the equality of the shear wave velocity and friction velocity. This critical condition can be employed to derive expressions useful for predicting the critical recoverable shear and critical shear stress for melt fracture.     

聚合物浓溶液气液平衡估算的基团贡献法

评价了用聚合物浓溶液气液平衡估算的９种基团贡献法的预测模型,即ＡＳＯＧ,ＵＮＩＱＵＡＣ,ＵＮＩＦＡＣ,ＵＮＩＦＡＣ－ＦＶ,Ｆ－Ｈ,Ｅｌｂｒｏ－ＦＶ,ＧＫ－ＦＶ,ＧＣ－ＦｌｏｒｙＥｏＳ和ＧＣＬＦＥｏＳ的预测能力,建议使用ＵＮＩＦＡＣ－ＦＶ,Ｅｌｂｒｏ－ＦＶ,ＧＫ－ＦＶ,ＧＣ－ＦｌｏｒｙＥｏＳ（有待简化）４个模型来预测聚合物浓溶液的气液平衡。并展望了其今后的发展趋势。     

Capillary flow instability of ethylene polymer melts

The capillary flow instability resulting in extrudate distortion has been studied for ethylene polymer melts using a molecular structure approach. It is found that the instability initiates at a critical value of elastic strain energy independent of (average) molecular weight for linear polyethylene. Once the flow breaks down, a slip interface within the melt is formed near the capillary wall, causing an abrupt increase in volumetric throughput. The velocity gradient within the melt remains continuous through the instability, however. Low molecular weight species present in the molecular weight distribution of linear polyethylene tend to suppress slip. Blends of linear and branched polyethylene exhibit instability behavior characteristic of both components throughout the entire range of composition. Results are discussed in terms of specific molecular mechanisms.     

Elastic behavior of concentrated solutions of acrylonitrile copolymers

The elastic behavior of concentrated solution of acrylonitrile copolymer was investigated by the capillary end correction method. The results were as follows. (1) The shear stress is proportional to recoverable shear strain in accordance with Hooke's law below critical concentration; above a critical concentration, however, the shear modulus depends on shear stress. (2) The log–log plots of zero shear modulus against polymer concentration and molecular weight fall on two straight lines with different slopes. The intersection of lines is considered to be the onset of elastically deformable entanglement network. We denote this inflection point as (C_c)_e or (M_c)_e. (3) The log–log plot of viscosity against polymer concentration does not show a change of slope at the critical concentration (C_c)_e. (4) By the application of the kinetic theory of rubberlike elasticity to the pseudo-network structure of concentrated polymer solution, in the range of C_c < C < (C_c)_e or M_c < M < (M_c)_e, the number of chain entanglement per molecule is kept one; moreover, in the range of C > (C_c)_e, or M > (M_c)_e, the number of chain entanglement increases to three.     

A rheometer for concentrated polymer solutions containing volatile solvents

A new rheometer cell and integrated transducer, which is capable of performing both steady state and dynamic measurements in a controlled environment and at elevated temperatures and pressures, is described here. This cell mounts onto either the Rheometrics Mechanical Spectrometer or the Rheometrics System Four rheometer and utilizes the wide range of shear rates and frequencies available on these instruments. The unique capabilities of the rheometer and cell permit measurement of the rheological properties of concentrated polymer, solutions containing volatile solvents at temperatures above the normal boiling point of the solvent. The steady shear and dynamic properties of several polystyrene/ethylbenzene solutions having polymer concentrations greater than 50 wt percent are presented to demonstrate the performance of the rheometer.     

Determination of graessley's relaxation time in concentrated polymer solutions and melts

The method consists of a comparison between experimental data and Graessley's theoretical function of the dependence of viscosity of concentrated polymer solutions and melts on the velocity gradient in a linearized plot using a three-parameter empirical equation. The procedure is objective and allows the use of a computer.     

Expansion-contraction behavior of dilute polymer solutions

Expansion-contraction behavior of capillary jets of dilute polymer solutions has been studied by using small diameter capillary tubes. The test liquids were water, glycerine solution and dilute solutions of polyethylene oxide and guar gum. It has been found that polyethylene oxide molecules degraded considerably at high shear rates; however the molecular degradation had little effect on the observed expansion-contraction behavior of capillary jets. On the other hand, the solvent viscosity had an appreciable effect on the issuing jets and on the final magnitude of the evaluated normal stresses     

Solvent effect on the time constant of concentrated polymer solutions

The time constant, τ, signifying the onset of non-Newtonian behavior of polymer solutions is known to be a function of mass concentration, C, molecular weight, M, temperature, T, and zero shear viscosity, η₀. Williams' theory predicts that the time constant is also affected by the solvent character. To study the predicted effect, the time constants of poly(methyl methacrylate) solutions in chlorobenzene (good solvent) and m-xylene (poor solvent) were experimentally determined. It was found that the ratio of the time constant to the Rouse relaxation time was a function of the combined variable CM. For all values of CM, both the time constant and the ratio of the time constant to the Rouse relaxation time were larger in a poor solvent than in a good solvent. This behavior may be attributed to the relatively stronger attractive interpolymer molecular forces present in solutions in poor solvents. As the temperature is raised the poor solvent becomes better and the ratio of the time constant to the Rouse relaxation time is found to become independent of solvent character.     

Vortex flow of dilute polymer solutions

It has been observed that very d longchain polymers which are effective in turbulent drag reduction inhibit the formation of a vortex or air core as water drains from a tank. This paper considers the fluid mechanical velocity profile measurements have been performed. There appear to be at least two distinct mechanisms for the vortex inhibition—one involving the viscosity enhancement caused by polymer addition, and the other related to the viscoelastic properties of the polymer solutions. This second mechanism is shown to arise due to the generation of high normal stresses as the air core begins to form. The very close correlation between vortex inhibition and turbulent drag reduction suggests that normal stresses may also play an important role in this latter phenomenon.     

Molecular degradation of concentrated polystyrene solutions in a fast transient extensional flow

The extensional degradation of concentrated polymer solutions is studied. Extensional flows are prevalent in the polymer processing industry, but their effect on polymer degradation is often overlooked. Previous research into dilute solution extensional degradation proved that this type of flow is much more effective than shear flow in causing chain scission. This research extends these dilute solution studies into the concentrated regime, where intermolecular entanglements are expected to affect chain scission. A concentrated polystyrene solution is degraded in an opposed pistons device. This device cycles the solution across a sharp contraction, imposing a strong extensional flow. The flow field is modeled to determine the extension rate along the centerline, and the degradation is quantified by measuring the zero shear viscosity and the molecular weight distribution. The results show that degradation is more significant compared to dilute solutions. The zero shear viscosity drops 30% after a single pass across the orifice at high strain rates. As in a dilute solution, the extensional flow leads to preferential cleavage of the high molecular weight chains. All chains longer than a measured critical molecular weight are ruptured. Multiple passes across the orifice increase chain scission, although degradation is the most significant in the first pass through the high strain rate region.     

Laminar and turbulent entry flow of polymer solutions

Entry lengths and contraction losses have been determined for 0.01, 0.2 and 0.6% aqueous solutions of Separan AP-30. The Reynolds numbers ranged between 10² and 10⁵ in 1/2-in. and 2-in. tubes. At Reynolds numbers in excess of 2100, the entry lengths and contraction losses for the polymer solutions are significantly larger than those for Newtonian fluids. These effects are shown to be related to the transitional flow phenomena which occur with drag reducing polymer solutions. At low Reynolds number the contraction loss data show expected trends but are in quantitative disagreement with predictions based on recent studies in capillary tubes. These indicate that the analyses based on small tubes are less general than anticipated.     

Rheological behavior and fiber orientation in slit flow of fiber reinforced thermoplastics

The flow behavior and fiber orientation in slit flow of a short fiber reinforced thermoplastic composite melt are investigated. A slit die with adjustable gap and interchangeable entrance geometries was designed and built. The slit die is fed by a single screw extruder. The bulk viscosity is calculated from the axial pressure profiles measured using three flush mounted pressure transducers. The effect of entrance geometry and gap dimensions on the fiber orientation and bulk flow behavior is specifically considered. A skin-core composite fiber orientation is observed in the thickness direction. Fibers are oriented in the flow direction and parallel to the walls in the skin region irrespective of the entrance geometry. Different fiber orientation distributions in the core region can be realized by using different entrance geometries. However, the changes in the core fiber orientation are not fully reflected by the measured viscosities, due to highly oriented skin layer. Exit pressures obtained by extrapolation of linear pressure profiles are found to be all positive, but dependent on the die geometry and entrance conditions, even for the unfilled melts.