Can a Rheometer Detect Thermal Degradation of a Polymer?

Similar to any other material, the properties of polymeric materials are closely linked to their chemical nature. What makes polymers so different from low-molecular-weight substances is the huge influence of their molecular weight (MW) and their molecular weight distribution (MWD) on their macroscopic behaviour.

The molecular weight of a compound is the sum of the atomic weights of the atoms in the molecules that form these compounds. ASTM International Standards D5296 describes the properties as follows:

“The molecular weight (MW) and molecular weight distribution (MWD) are fundamental characteristics of a polymer sample. They are used for a wide variety of correlations for fundamental studies, processing, or product applications. For example, the observed MWD is compared to one predicted from assumed kinetics or mechanisms for a polymerization reaction. Differences between the values will allow alteration of theory or experimental design. Similarly, the strength, melt flow, and other properties of a polymer sample usually are dependent on MW and MWD. Determinations of MW and MWD are used for quality control of polymers.”

Without changing its chemical nature, we can e.g. “select” the rigidity or elasticity of a polymeric material just by varying the parameters of the polymerization process.

Thus, to make a polymer with the desired properties we have to be able to quickly determine the right moment to stop the polymerization process in order to always get the same MW and MWD. Also, these two parameters can be used to decide whether materials delivered meet the specifications or not.

One widely accepted method to determine if a polymer shows thermal degradation makes use of the fact that the crossover frequency determined with a frequency sweep depends on the MW of a polymer, whereas the crossover modulus is related to its MWD.

From the crossover modulus the polydispersity-index (PI) can easily be calculated:

PI = 100.000/Crossover Modulus  [Pa]

The measurements can be taken utilizing a rheometer with a precise and stable temperature control like an oven or the combination of an electrical heating with a second electrical heating from above. For the results described here, an oven has been used. In combination with the precise lift control a sensitive normal force sensor is a great tool to ensure the reproducible loading and axial relaxation of samples.  

Using the normal force controlled sample loading for polymer samples of an unknown thickness, the rheometer can show perfectly reproducible results from frequency sweep measurements. Having seen this great reproducibility any decrease of the crossover modulus and increase of the crossover frequency can be solely attributed to a thermal degradation of the polymer sample tested.

Preheating the oven in its parking position and the selection of a suitable frequency range minimizes the time needed for the measurement. In case the rheological properties of an oxygen-sensitive polymer have to be measured at higher temperatures over a longer time, the controlled test chamber can be flushed with nitrogen instead of air to exclude the damaging influence of the otherwise unavoidable oxygen.

We conducted a test on very narrowly distributed polybutadienes to see if degradation of a polymeric sample could be detected.  We will outline the process and results in our next article.  If you can’t wait, read the application note: Automatic Detection of the Thermal Degradation of a Polymer