Fikentscher K-value

Fikentscher K value—0.5% solution in cyclohexanone or ethylene dichloride (used widely in Europe). 

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. 

Into the stirred reactor, maintained during the preparation at 70°C, while a slow stream of nitrogen bubbles through the equipment. Reactant 1 is fed in at a rate of 190ml/hr, while, simultaneously, reactant 2 is fed in at a rate of 80ml/hr. The overflow is collected as the product solution. After 42 hr the product solution has 87% of the monomer converted to polymer with a Fikentscher K value between 45 and 50 and no sign of gel formation. When the preparation is continued for 110 hr similar results are observed. 

To a solution of 30 gm of N-vinylpyrrolidone in 70 gm of distilled water is added 0.075 gm of 2,2 -azobis(isobutyronitrile) (AIBN). The solution is exposed to ultraviolet light with wavelength in the range of 310-410 nm. Polymerization takes place in 2-3 hr. Analysis of the solution shows a residual monomer content of only 1.7%. The Fikentscher K value of the product may range up to the unusually high value of 110. 

A procedure for the continuous polymerization of A-vinylpyrrolidone by chemical initiation has been described in Procedure 2-8. The same authors also claimed the polymerization of a 20 /o aqueous solution of the monomer, containing 0.1 7o of thioglycolic acid passed into a stirred reactor having a hold-up volume of 100 ml at a rate of 52ml/hr, while the reactor was irradiated with a Machlett OE G-50 X-ray tube, 50,000 volts at 50 ma. During a 12-hr period, conversion was 88% of a gel-free polymer having an average Fikentscher K value of 62.5 [60]. 

The values accepted are those of the Fikentscher k multiplied by 10,000. This system, which specifies the nitrocellulose and not its solutions, is the most scientific and consistent of all, but is limited to German practice only. For example, E510 and E9501.G. nitrocelluloses correspond to +-1 and 12.5 sec Hercules and X39-X140 and L125 Ardeer products respectively. 

Until recently the Fikentscher K was used as a characteristic value for widely-used polymers like poly(vinyl chloride) (PVC) or poly(styrene) (PS) in solution [43] ... 

The parameter K was assumed to be independent of the concentration but dependent of the molar mass. The determination of K was carried out with a single viscosity measurement. In reality the Fikentscher K depends on the concentration and shows only small changes with the molar mass for high molar mass polymers [44]. It should be used only in a small and known concentration and molar mass range, for which the K value was determined. In all other cases, the intrinsic viscosity should be used as a characteristic value for polymer solvent systems. 

In discussions of PVP, the viscosity measure called Fikentscher s K value has survived. This term is calculated from Eqs. (1) and (2) ... 

Table I is a guide to the relationships between Fikentscher s K value, intrinsic viscosity (calculated), and the weight-average molecular weight, M, calculated from Eq. (4) ...

The solubility of PVP in water-acetone systems varies considerably with the K value of the polymer. Addition of acetone to an aqueous PVP solution K value of 100) may be used to fractionate the polymer in order to study the molecular weight distribution. The phase relation of the PVP-acetone-water system is beyond the scope of the present chapter. Interested readers are referred to Fikentscher and Herrle [1] for a more detailed discussion of the topic. Along with this, ref. [4-8 and 50] should be reviewed for their discussion of molecular weight determinations, viscosity phenomena, and solubility characteristics. 

Viscosity determinations at the highest convenient concentration are of practical value, because in most of the applications of nitrocellulose, solutions of these concentrations are used, and there is usually little difference between the concentration in use and that used for the determination. All nitrocellulose solutions that are not very dilute show more or less abnormal viscosity, i.e. the rate of flow is not proportional to the applied stress or pressure. The ease of flow of a solution at one degree of stress cannot then be inferred from a viscosity determined at another. Nevertheless the measured viscosity of a comparatively concentrated solution is in general useful information. This advantage is sacrificed in what is the most rational of all viscosity systems, the German I.G. Method. Measurements are made in dilute solutions and the results are expressed in terms of the Eigenviscositat (k) of Fikentscher and Mark [99] a function which to a large extent is independent of concentration ... 

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