Viscosity

Polymers solutions can be fluid or thick. Measuring their resistance to flow is one of the most important physical analysis method in polymer science. However there are several different methods to measure viscosity, which tell you about different material properties.

We can divide viscosity into two types of measurement…

Dynamic Viscosity

Dynamic viscosity measurements record the resistance of a material to an external force. A brookfield test or a rheometry measurement is often standard procedure for analysing gels, fluids or adhesives.

Kinematic Viscosity

Kinematic viscosity is the analysis of how gravity affects a substance’s flow. Here we assume there is no external forces or pressures acting on the material and it is flowing purely due to gravity.

Often the relationship between Dynamic and Kinematic viscosity is density dependant, and so two materials with the same dynamic viscosity could have completely different kinematic properties.

Deciding on an analytical technique to measure viscosity often depends on whether the fluid acts as a newtonian, or non-newtonian material (i.e. does their viscosity change with shear). Many polymer solutions are non-newtonian, and so specific viscosities depend on how much shear they are under at any point in time.

However some other polymers are used in dilute solution as an additive part of a complex mixture of materials. Many cosmetics or gels contain several different polymer additives to control their touch or feel on the skin.

So one key characterisation parameter for many polymers is their ‘intrinsic’ viscosity – a measure of a solute’s contribution to the total viscosity of a solution.