Viscometry

Viscometry is a classical method for polymer characterisation. Mostly, the target property is intrinsic viscosity [eta]. By means of scale relationships, intrinsic viscosity is connected to molecular mass; thus, viscometry represents a feasible method for molecular mass determination – if the scaling factors are given for the particular solute/solvent system.

Intrinsic viscosity gives a measure for the volume demand of a particle or molecule. In hydrodynamic calculations, [eta] can be used to describe the frictional properties of the particle. In combination with other parameters, [eta] gives the particle shape and hydration.

In colloidal science, viscometry is a simple method providing a deal of information. It is however neccessary to obtain results in high precision, as the volume demand of colloids (and thus the physical effect measured) is much smaller than, for example, of macromolecules, commonly swollen with huge amouts of solvent. Very sensitive equipment must be used for particles only a few nanometers in diameter.

In principle, two basic techniques for measurements of [eta] are available. Most commonly, the "Ubbelohde viscometer" is applied, where the solution gravitationally flows through a vertical capillary, opposed by capillary forces. The time needed for a specific volume of solution to flow through the capillary is measured. This technique becomes very insensitive for small viscosities that barely deviate from the viscosity of the pure solvent. Furthermore, capillary forces demand for certain corrections and, in general, time measurements are somewhat limited in precision.

The alternative setup is a capillary viscometer. This instrument measures a difference in pressure at either side of a capillary, through which the solution is pressed with arbitrary inlet pressure. According to the Hagen and Poisseuille Laws, a viscosity can be calculated from the pressure decay over the capillary. In addition to the much higher precision of a pressure measurement, a basic advantage of this technique is that it is not limited to Earth's gravity field. The inlet pressure can vary, so that the variable shear rate gives access to phenomena of thixotropy and shear dilution. A further advantage lies in the considerably reduced sample demand of these instruments.

At Nanolytics, viscometry measurement is carried out in a combined viscosity/ refractive index detector, giving us the opportunity to monitor mass balances during the measurement. The sensitivity of this instrument is extremely high. We can determine the intrinsic viscosity of a component with as little as 100 µl of dilute solution.