Falling coaxial cylinder viscometer for polymer solutions

A simple, inexpensive glass viscometer can be constructed to time the rate at which a rod falls into a liquid held in a concentric closed-end glass cylinder. This equipment can be used to measure absolute values of apparent viscosities of Newtonian and non-Newtonian fluids. Calibration is not required. The technique is an adaptation of a method used previously with polymer melts at elevated temperatures. The present article describes the modifications needed for lower-viscosity fluids, such as paints, and validates the flow analysis with results of study of a characterized Newtonian fluid.     

Flow analysis and end corrections in falling coaxial cylinder viscometry

The assumptions involved in the analysis of flow of polymer melts in a closed end falling coaxial cylinder viscometer^(1–3) are examined and it is shown that true values of shear rate and shear stress at the wall of the moving cylinder can be obtained by assuming classical Poiseuille flow. Flow curves obtained with a variety of apparatus geometries, driving loads and polymers are consistent with those of other measurement techniques. This apparatus, which is a simple modification of a capillary extrusion rheometer, provides measurements of apparent viscosity at much lower shear rates than can be obtained with the former viscometers. A quantitative measurement of end correction effects is provided by the present analysis. End effects, which are formally similar to those in capillary extrusion, were measured for several polystyrenes and polyethylenes.     

Bead-spring polymer melts

This analysis of polymer rheology uses conditional probability distributions to describe the phase space dynamics of all macromolecules in a polymer melt. The result is a viscoplastic constitutive equation for the polymer stress. Using conditional probability distributions makes the use of a large number of bead-spring chains in the modeling system possible, but precludes evaluating the intermolecular contribution to the total stress. Both the kinetic and intramolecular contributions are evaluated for a system composed of an arbitrary number of bead-spring chains that interact with one another using molecular dispersion forces. The analysis predicts that the kinetic contribution is isotropic and the intramolecular contribution is viscoplastic. The intermolecular contribution is assumed negligible in comparison to the intramolecular contribution because it results from physical bonds among chains, while the intramolecular contribution results from chemical bonds within a chain. © 1993 John Wiley & Sons, Inc.     

Miscibility studies of polymer blends by viscometry methods

The intrinsic viscosities of blends of poly(vinyl chloride)/poly(ethylene-co-vinyl acetate) (PVC/EVA), poly(vinyl chloride)/poly(styrene-co-acrylonitrile) (PVC/SAN), and poly(ethylene-co-vinyl acetate)/poly(styrene-co-acrylonitrile) (EVA/SAN) have been studied in cyclohexanone as a function of blend composition. In order to predict the compatibility of polymer pairs in solution, the interaction parameter term, Δb, obtained from the modified Krigbaum and Wall theory, and the difference in the intrinsic viscosities of the polymer mixtures and the weight average intrinsic viscosities of the two polymer solutions taken separately are used. © 1994 John Wiley & Sons, Inc.     

Transitions in polymer melts

It is shown that the liquid-liquid transition T_ll in polymer melts need not be a true polymer transition but can be an artifact of the torsional braid analysis technique. The shift in T_ll with molecular weight then is due to the variation of viscosity with molecular weight.     

Rheology of rubber-modified polymer melts

The influence of rubber particles on rheological properties of styrene-acrylonitrile, and poly(vinyl chloride) was investigated. The temperature dependence of the viscosity functions was found to be independent of the kind of rubber and its concentration. The extrudate swell becomes smaller with increasing particle concentration, the entrance pressure loss remaining nearly unaffected. The strong viscosity increase at small shear rates is more pronounced the higher the concentration and the smaller the size of the particles. It can be interpreted by the assumption of a yield stress caused by the fillers. This yield stress is independent of temperature within the accuracy of the measurements and comes out distinctly higher in elongation than in shear. An analytical expression of the dependence of the viscosity on particle concentration cannot be given. It is demonstrated how sensitively an agglomeration of particles is reflected in elongation and recoverable strain. Based on the morphology shown by electron micrographs of acrylonitrile-butadiene-styrene and acrylonitrile-styreneacrylic ester copolymers, two different models for the occurrence of a yield stress are discussed qualitatively.     

Contact-angle development of polymer melts

In powder coatings, the quality of the final film is often associated with its smoothness. This is affected by the wetting and levelling process which occurs during film formation and is mainly determined by surface tension (driving force) and viscosity (resistance).

Especially for the surface tension the requirements are complex. On the one hand a lower surface tension facilitates the substrate wetting process, but if it is too low levelling is poor, resulting in wavy surfaces (“orange peel”). On the other hand, a higher surface tension promotes levelling, but if it is too high the wetting is poor, resulting in crater defects. Therefore, the surface tension of a powder coating has to be optimised carefully. It can be adjusted by addition of “levelling” or “surface flow” compounds to a formulation as well as changing the composition of the resin. Most of the current knowledge however is based on trial and error and a more systematic approach should provide better fundamental insight into the interrelation between the different parameters.

Since there is no generally accepted method of measuring wetting and levelling, a new method has been devised by which the actual melting and flow process can be monitored. The method provides information about the wetting and flow process of polymers by measuring the contact-angle of polymer melts as a function of temperature and gives significant and consistent results while data analysis provides quantitative information.

In this presentation, the method will be explained and examples will be given, showing the effects of structural parameters of coating binders on their wetting and flow behaviour.     

Elongational flow of polymer melts

A new technique is described whereby the rate of collapse of an air bubble within a molten polymer may be measured without the need for visual observation. The method involves use of a high speed recording dilatometer, From such data it is possible to measure an apparent elongational viscosity of the melt, and such measurements are presented for two polyethylenes (Tennite, a low density PE, and Plaskon, a high density PE), Limitations of the methods are discussed. This is one of a series of papers (1-3) documenting our development of a new experimental technique, and the corresponding mathematical modeling, whereby one may measure the elongational viscosity of polymeric viscoelastic fluids. Previous experimental work was confined to transparent fluids, since the technique depended on high-speed motion picture photography of the collapse of an air bubble within the fluid. In this paper we describe an attempt, largely successful, to develop a new experimental system which permits the study of molten polymers, including opaque fluids. Sample results are presented for both a low and a high density polyethylene.     

Thermal conductivity of polymer melts

Thermal conductivities were measured for melts of a series of polypropylene and a series of polyethylene samples that had been characterized by GPC. Results indicate that thermal conductivity depends upon the molecular weight distribution and degree of branching of the polymers. The results of this work can probably be used to predict the thermal conductivity of a commercially available polypropylene or polyethylene to within 25%.     

Thermal conductivity of polymer melts

A concentric cylinder conductivity cell with guard heaters was constructed and used to determine the thermal conductivity of polymer melts. Thermal conductivity was found to be a linear function of temperature for the melts studied, and the thermal conductivity decreased as the complexity of the polymer chain increased. The polymers studied were a linear polyethylene, branched polyethylene, polypropylene, polystyrene, nylon 6, and nylon 6,10. The measurements are precise to within ±6%.     

Rheotens-mastercurves and drawability of polymer melts

In a Rheotens experiment, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. For thermo-rheologically simple polymer melts, the existence of Rheotens-mastercurves is proven. Rheotens-mastercurves are invariant with respect to changes in melt temperature. Also, for polymer melts with different average molar masses, but similar molar mass distribution and branching structure, Rheotens-mastercurves are invariant to changes in the average molar mass. It is shown, by testing several polyethylenes with different molar mass distribution and different long-chain branching, that Rheotens-mastercurves allow a direct and quantitative assessment of the drawability of polymer melts under actual processing conditions, i.e. under the action of a constant tensile force and including the effects of the rheological prehistory in the extrusion die.     

Rheological behavior of two-phase polymer melts

Rheological behavior of dispersed two-phase polymer melts has been investigated by means of a capillary rheometer. The two-phase systems chosen for study were blends of two polymers: high-density polyethylene and polystyrene, which are incompatible in the molten state. In order to investigate the state of dispersion, photographs were taken of the microstructure of extrudate samples, showing that the polystyrene forms long fibers or droplets as a discrete phase dispersed in the polyethylene which forms a continuous phase. Measurements were made of the axial pressure distributions of the two-phase molten polymers flowing through circular tubes, which permit one, according to the theory advanced by Han, to determine viscous and elastic properties of the melts. Also measured was melt die swell ratio. It has been found from the two independent experimental techniques that there exist a maximum and/or minimum elastic property at a certain blending ratio, and that the elastic property decreases first with L/D ratio of a capillary and then levels off.     

Dispersing hairy nanoparticles in polymer melts

The dispersion of hairy nanoparticles in polymer melts of chemically identical chains was investigated as a function of both molecular weight and volume fraction. Here we provide conclusive evidence that the shape of the phase diagram is determined primarily by the ratio of the chain length of the polymer melt to the chain length of the polymeric shell structure (or hair) of the core/shell nanoparticles, and that the phase behavior of different hairy particles in various polymer melts can be superimposed into one universal graph. Other factors, including the hair density and the particle diameter, are not nearly as significant as the above-noted ratio in this phase separation. In addition, we show that there is a strong connection between the rheological dynamics of the particle-filled system and the thermodynamics of the phase separation behavior. The shear-induced nonlinearity in the particle-filled system appears to display features of a singularity near the phase transition point.     

PVT scaling parameters for polymer melts

Scaling parameters for two PVT equations of state are evaluated for 11 polymer melt systems using a nonlinear least square fitting algorithm that analyzes all of the experimental data simultaneously. Two different criteria are considered in this evaluation. In the first method, the fitting criterion is the difference between the calculated and experimental volume. In the second method, the criterion is the difference between the calculated and experimental pressure. In both cases, the differences between the scaling parameters obtained using the simultaneous fit procedure and those obtained using the earlier consecutive fit procedure are a few percent, which can have a significant effect in some calculations. In addition to being a more consistent method of evaluating scaling parameters, the simultaneous fit procedure leads to much better agreement between calculated and experimental values, in some cases by a factor of 2.     

Crystal nucleation in sheared polymer melts

A semiquantitative procedure has been developed for analyzing crystal nucleation in undercooled polymer melts that are undergoing flow. This analysis was applied to the specific ease of molten high density polyethylene experiencing low levels of shearing flow. A custom-made concentric cylinder viscometer, which could be operated by the Rheometrics mechanical spectrometer instrument, was used to make simultaneous measurements of transmitted torque and optical anisotropy in isothermal melts, The result of the analytical procedure developed here was molecular Size-dependence of chain distensions caused by prevailing shear. This distribution function was verified by testing against experimentally obtained values of birefringence. Total entropy reduction resulting from this distorted state was then calculated, and the corresponding increase in free energy was found to be at least enough to account for comparable crystal nucleation rates in flowing melts at higher temperatures and in quiescent melts at lower temperatures.     

Spinnability of melts of three-component polymer blends

It was found that incorporation of a third component — LDPE — in the starting two-component blend is an effective method of regulating the spinnability of melts of dispersions and decreasing the effect of superadditive jet spread. The decrease in the degree of jet spread with an increase in the concentration of LDPE is due to accumulation of undeformed particles of disperse phase, formed as a result of destruction of maximally deformed particles of LDPE in the inlet zone and capillary of the spinneret, in the dispersion coming out of the spinneret hole.State Academy of Light Industry of Ukraine, Kiev. Translated from Khimicheskie Volokna, No. 6, pp. 25–28, November–December, 1995.     

Cooling yarns spun from polymer melts

Conclusions -- The effect of the component air streams on yarn uniformity with respect to linear density has been established. This has afforded opportunity to find an optimum regime for yarn cooling in spinning from polymer melts. Analysis of the pulsations in air streams has made it possible to determine the basic sources for their origin and ways to reduce them.-- A manufacturing control of the yarn cooling regime using anemometers has been proposed.-- A way of intensifying the yarn cooling regime using an air jet on the yarn section after its initial hardening has been described.-- It is recommended to use the method of investigating the process of yarn spinning by analysis of the component air streams in preparing fibres from polymer melts on any type of spinning equipment.Translated from Khimicheskie Volokna, No. 4, pp. 26–30, July–August, 1990.     

Electrospinning of polymer melts: Phenomenological observations

Melt electrospinning is an alternative to solution electrospinning, however, melt electrospinning has typically resulted in fibers with diameters of tens of microns. In this paper we demonstrate that polypropylene fibers can be reduced from 35 ± 8 μm in diameter, to 840 ± 190 nm with a viscosity-reducing additive. Melt electrospun blends of poly(ethylene glycol)-block-poly(?-caprolactone) (PEG₄₇-b-PCL₉₅) and poly(?-caprolactone) (PCL) produced fibers with micron-scale diameters (2.0 ± 0.3 μm); this was lowered to 270 ± 100 nm by using the gap method of alignment for collection. The collected melt electrospun fibers often fused together where they touched, allowing the stabilization of relatively thick non-woven felts. The melt electrospun collection also included coiled circles and looped patterns of fibers approximately 150-250 μm in diameter. The polymer jet was visible between the collector and spinneret for particularly significant lengths, and underwent coiling and buckling instabilities close to the collector. The focused deposition of melt electrospun fibers was maintained when multiple jets were observed, with the collections from multiple jets separated by 3.8 ± 0.5 mm for a 5 cm collector gap. The frequent fusion points between melt electrospun fibers, and a reduction in diameter for the gap method of alignment, indicated that the melt electrospun fibers are still slightly molten at collection.