Macromolecules increase the viscosity of a solution already at very low concentrations.
The increase of the viscosity is not only dependent on the molecular weight but also
dependent on the shape of the macromolecules. The shape and conformation of a random
coil polymer depends on the solvent as well as on the temperature. In order to use
viscometry measurements as a relative method to determine the molecular weight it
is necessary to perform the measurements at constant temperature in the same solvent.
In general, the viscosity of a polymer solution is denoted as n , the viscosity of the solvent n, and the polymer concentration c. In the literature the following viscosity numbers are defined.
The intrinsic viscosity [n] ist defined as
[n] is the limit of the reduced viscosity at infinite dilution. The extrapolation leads in general to a linear dependence and the intrinsic viscosity [n] and can be obtained graphically by extrapolation to concentration c=0
Abb.1: A standard Ubbelohde capillary viscometer.
Mark-Houwink-equation relates the intrinsic viscometry with the molecular mass
The log-log plot of [n] versus the molecular weight leads to a straight line with
the slope a and the intercept log K. For many polymer/solvent systems the a- and
K-values are tabulated in the literature (Polymer Handbook).
For viscosity measurements a Ubbelohde viscometer is available.
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