92168.pdf

Rheology 251

Electroviscous effects

When dispersions containing charged particles are sheared, extra
energy is required to overcome the interaction between ions in the
double layers around the particles and the electrical charge on the
particle surfaces, thus leading to an increased viscosity.
For charged flexible chains, in addition to the above effect (which
is usually small), the nature of the double layer influences the chain
configuration. At low ionic strengths the double-layer repulsions
between the various parts of the flexible chain have a relatively long
range and tend to give the chain an extended configuration, whereas
at high ionic strengths the range of the double-layer interactions is
less, which permits a more coiled configuration. Therefore, the
viscosity decreases with increasing ionic strength, often in a marked
fashion.

Polymer relative molecular masses from viscosity measurements

Viscosity measurements cannot be used to distinguish between
particles of different size but of the same shape and degree of
solvation. However, if the shape and/or solvation factor alters with
particle size, viscosity measurements can be used for determining
particle sizes.
If a polymer molecule in solution behaves as a random coil, its
average end-to-end distance is proportional to the square root of its
extended chain length (see page 25) - i.e. proportional to M?^5 , where
MT is the relative molecular mass. The average solvated volume of
the polymer molecule is, therefore, proportional to A/J^5 and, since
the unsolvated volume is proportional to Afr, the average solvation
factor is proportional to A/J^5 /A/r (i.e. M?^5 ). The intrinsic viscosity of
a polymer solution is, in turn, proportional to the average solvation
factor of the polymer coils - i.e.

foj = KM?-^5

where K is a proportipnality constant.
For most linear high polymers in solution the chains are somewhat
more extended than random, and the relation between intrinsic