Determination of the Intrinsic Viscosity by Viscosimetric Measurements

According to the lUPAC nomenclature, the intrinsic viscosity should be named limiting viscosity number but this denomination is not widely used yet. As shown in Eq. (4.7), another condition for the determination of the intrinsic viscosity is that the shear rate has to be y- 0. The influence of the shear rate on the determination of the intrinsic viscosity is discussed in Chap. 5. 

The intrinsic viscosity [q] has the same unit, [ml g ], as the reduced viscosity rjred a better understanding, the intrinsic viscosity can be considered as a measure for the volume demand of the single polymer coil in ideally diluted solution. The intrinsic viscosity is proportional to the reciprocal density of the polymer coil in solution according to Eqs. (4.6) and (4.7). 

The intrinsic viscosity [q] represents the most relevant variable to describe the viscous behavior of a polymer solution and most viscosimetric measurements have the aim of its determination. Therefore, in the upcoming chapters the focus will be on a detailed description of the determination of the intrinsic viscosity, the influence parameters on the intrinsic viscosity and the establishment of structure-property-relationships with the intrinsic viscosity. 

The intrinsic viscosity can be determined experimentally with the first two terms in Eq. (4.1). In a real polymer solvent system the state of an ideally dilute solution is never reached, even very dilute solutions have a finite amount of polymer. The second term in Eq. (4.1) captures these interactions between the single polymer coils in a not ideally dilute solution. When Eqs. (4.3)-(4.7) are inserted in Eq. (4.1), a linear dependency of the reduced viscosity / ed from the concentration c is obtained  

For the execution of the experiment, the specific viscosities for a dilution series of the polymer have to be determined as described in Chap. 3. The intrinsic viscosity [q] is determined from the/-axis intercept of a plot of q p/c (or /j ed) vs the concentration c and an extrapolation of the linear fit of the data to c- 0, as can be seen in Figs. 4.2 and 4.3. 

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For the determination of the intrinsic viscosity [q] of the particular polymer-solvent system, the specific viscosity /jsp can be evaluated from the relative viscosity r j. via Eq. (2.11). With the specific viscosity and known concentrations of the solutions, the evaluation can be directly started according to Eq. (4.9) in Determination of the intrinsic viscosity by viscosimetric measurements in the next chapter. With the determined intrinsic viscosity [q], the critical concentration of the viscosimetry can be evaluated according Eq. (7.7). 

Single point measurements are commonly used to determine the intrinsic viscosity via the so-called Fikentscher K-value. This method is, as shown in Determination of the intrinsic viscosity by viscosimetric measurements in Chap. 4, only applicable under very special conditions and should not be used anymore. Generally, single point measurements only serve for the determination of a viscosity value. 

Viscosimetric determinations. The Newtonian intrinsic viscosity of the xanthan molecule was determined by measuring the viscosities of several dilute polymer solutions with a Contraves Low-Shear viscometer. Extrapolation at zero polymer concentration of the reduced specific viscosity gave the value of the intrinsic viscosity, and the Huggins constant was calculated from the slope of the curve. 

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