Viscosity, K-value, molecular weight

The viscosity of solutions of concentrations up to 10% hardly changes between 2o°C and 4o°C. The temperature only affects the viscosity of solutions of higher 

The average molecular weight of povidone and copovidone is expressed in terms of the K-value, in accordance with the Pharmacopoeias that apply in Europe and the USA [13]. It is calculated from the relative viscosity in water. The same methods can be applied to copovidone, and they give K-values between the limits given in Section 4.2.I.2. They are based on the relative viscosity of a 1% solution in water at 25°C. The relationship between the K-value and the relative viscosity is shown in Fig. 91. The curve was obtained using the method of determination and calculation described in Section 2.3.2.1. 

The average molecular weight of a polymer can be viewed and measured in three different ways as indicated in Table 156. 

As these methods of determining the average molecular weight are relatively time-consuming, that of copovidone is now expressed in terms of the K-value, in accordance with the European and U.S. Pharmacopoeia methods for povidone. 

The number-average of the molecular weight, Mn is determined by methods that measure the number of molecules. Values of copovidone recently determined by this method lie between 15000 and 20000. 

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