Measurement of Polymer Viscosity

The following section will now focus on experimental methods for determining viscosity and how the viscosity function relates to analyzing single-screw extrusion processes. 

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Methods for the Measurements of Polymer Viscosity Under High-Pressure CO2... 

The polymers dissolve in l,l,l,3,3,3-hexafluoro-2-propanol [920-66-1/, hot phenols, and /V, /V- dim ethyl form am i de [68-12-2] near its boiling point. The excellent solvent resistance notwithstanding, solvents suitable for measurement of intrinsic viscosity, useflil for estimation of molecular weight, are known (13,15). 

Molecular Weight. Measurement of intrinsic viscosity in water is the most commonly used method to determine the molecular weight of poly(ethylene oxide) resins. However, there are several problems associated with these measurements (86,87). The dissolved polymer is susceptible to oxidative and shear degradation, which is accelerated by filtration or dialysis. If the solution is purified by centrifiigation, precipitation of the highest molecular weight polymers can occur and the presence of residual catalyst by-products, which remain as dispersed, insoluble soHds, further compHcates purification. 

Dilute Polymer Solutions. The measurement of dilute solution viscosities of polymers is widely used for polymer characterization. Very low concentrations reduce intermolecular interactions and allow measurement of polymer—solvent interactions. These measurements ate usually made in capillary viscometers, some of which have provisions for direct dilution of the polymer solution. The key viscosity parameter for polymer characterization is the limiting viscosity number or intrinsic viscosity, [Tj]. It is calculated by extrapolation of the viscosity number (reduced viscosity) or the logarithmic viscosity number (inherent viscosity) to zero concentration. 

Extensional viscosity that results purely from shear deformation seems to be of less interest, but has been measured (108). The theology of several different polymer melts in terms of shear viscosity and uniaxial and biaxial extensional viscosity has been compared (231). Additional information on the measurement of extensional viscosity are also available (105,238—240). 

Moonej Viscosity. This is a measurement of the viscosity of the polymer that is commonly used ia the mbber iadustry. Mooney viscosity values typically range from 25 to 100. Mooney viscosity generally relates to polymer molecular weight, with the lower Mooney viscosity polymers providing improved flow and processiag characteristics and the higher Mooney NBRs providing improved physical properties. 

When the full distribution is needed, it is measured by size-exclusion chromatography (also called gel permeation chromatography). This is a solution technique that requires dissolution of the polymer in a reasonable solvent such as tetrahydrofuran or tetrachlorlobenzene. For polymers that require exotic solvents or solution temperatures above about 150°C, a simple measurement of solution viscosity can be a useful surrogate for the actual molecular weight. The viscosity of the pure polymer (i.e., a polymer melt viscosity) can also be used. Such simplified techniques are often satisfactory for routine quality control, particularly for condensation polymers such as PET that vary in average molecular weight but usually have a polydispersity of 2. 

Figure 5 shows three different types of capillary viscometers often used for viscosity measurements of polymer solutions. The disadvantage of the Oswald viscometer is that the viscometer has to be charged with the solution to a precise level and fine adjustments need to be made at the temperature of measurement. The Ubbelohde viscometer, also frequently referred to as the suspended level viscometer, is particularly useful when a series of different polymer concentrations is to be measured. The filling volume needs not to be adjusted precisely. The largest dilution ratio obtainable is limited only by the ratio of the volume of bulb B to that of the volume between the bottom of bulb B and the top of bulb C. For the compact version (Figure 5(c)) smaller sample volume is needed. There are also capillary viscometers available that can be coupled with liquid... 

Viscosity measurement or rheology. Viscosity is an important property of many fluids, particularly consumer products. Pancake syrup that does not appear thick or a runny shampoo has little consumer appeal. Viscosity is also an important measurement for liquid polymers. It is a measure of polymer chain length and branching. On the other hand, viscosity does not have the sensitivity to detect small amounts of impurities in pure substance. 

The usefulness of viscosity as a measure of polymer Molecular weight was recognised in the early work of Staudinger (1930). Solution viscosity is a measure of the size or extension in space of polymer molecules. It is empirically related to Molecular weight for linear polymers the simplicity of the measurement and the usefulness of the viscosity-Molecular weight correlation are so great that viscosity measurement constitutes an extremely valuable and simple tool for the molecular characterisation of polymer. 

Fig Capillary viscometers commonly used for measurement of polymer solution viscosities (i) Ostwald-Fenske and (ii) Ubbelohde. 

Accurate measurements of fluid viscosity are important in many industries for such diverse uses as monitoring syrup manufacture or studying polymer structures such as polymer branching, chain conformation, solvent interactions or polymer molecular weight (MW). Historically, the drop-time type glass capillaries, such as the Ubbelohde or Cannon and Fenske types, have been widely used to measure fluid viscosity. However, this traditional method is tedius and labor intensive, and lacks the desired speed and sensitivity to... 

The experimental determination of polymer intrinsic viscosity is done through the measurement of polymer solution viscosity. The connotation of intrinsic viscosity [hi/ however, is very different from the usual sense of fluid viscosity. Intrinsic viscosity, or sometimes called the limiting viscosity number, carries a far more reaching significance of providing the size and MW information of the polymer molecule. Unlike the fluid viscosity, vdiich is commonly reported in the poise or centipoise units, the [h] value is reported in the dimension of inverse concentration xinits of dl/g, for exanple. The value of [hi for a linear polymer in a specific solvent is related to the polymer molecular weight (M) through the Mark-Houwink equation ... 

We have omitted a great deal of detail in this discussion of polymer viscosity. The interested reader will find some of the missing information supplied in Flory (1953). In particular, we have omitted all numerical coefficients, which limits us to ratios as far as computational capability is concerned. Numerical coefficients are available for Equation (92), for example, and this allows coil dimensions to be evaluated from viscosity measurements. A general conclusion that unifies all of this section is that any factor that causes a polymer chain to be more extended in space —whether by coil unfolding or swelling by solvent —tends to increase [77]. This is exactly what we expect in terms of the purely qualitative picture provided by Figure 4.8. Example 4.6 illustrates this for some actual polymers. 

Experimental error entered the data, of course, through the manual measurement of the viscosities of the four very dilute fractions. Another error in the viscosities was introduced by contamination of each fraction by some of the fraction preceding it. Not all of the liquid in the measuring syphon could be removed as the syphon emptied thus a small portion of the fraction was retained and added to the incoming fraction. Furthermore, the previous procedure required exact weights in each fraction, whereas now only relative weights are necessary. The relationship developed here can be extended to polymers other than cellulose. To do this, the values of K and a have to be determined for the particular polymer, dissolved in the desired GPC solvent which was used to establish the calibration curve. 

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