Our last article discussed how one could use a Rheometer with an oven to detect thermal degradation of a polymer. Here is an outline of the process and results of the testing we did.
We selected very narrowly distributed polybutadienes for our testing. Samples with a diameter of 25 mm were cut from a sheet of approximately 2 mm thickness. While the oven was preheating to 190 °C in its parking position, the sample was put into the 25 mm parallel plate geometry of the rheometer at room temperature and the gap was closed with a defined axial force to have the same starting conditions for different samples. With these preparations it was possible to run frequency sweeps in a very reproducible way.
The resulting curves were practically identical and the PI values calculated differed only about 0.3 %. These measurements were performed under laboratory conditions, so the whole procedure before starting the measurements, i.e. sample loading, closing of the geometry, was the same in all cases.
In working environments where this is not possible, it is especially important to know whether the polymer regarded is stable e.g. against oxidation at elevated temperatures. Otherwise different results can simply be based on different times it took to start the measurements. This possible degradation of a polymeric sample can be detected using certain rheometers. For that purpose a different polymer sample was prepared and loaded into the rheometer at 190 °C as described above.
After equilibration, 15 identical frequency sweeps were done over a time span of 60 min. From earlier measurements we knew roughly, where the crossover could be observed. Therefore, to save time the job was programmed to collect only 6 data points between 10 rad/s and 100 rad/s, which led to less than 1 min pure measuring time for a single frequency sweep. Due to the capability of the rheometer’s measuring and evaluation software to create and run complex testing procedures, the whole test ran completely without any input from the operator’s side once the sample had been loaded.
As shown in the below chart, the G’ and G“ curves shift to lower values the longer the sample is exposed to the 190 °C inside the oven.
Subsequently the crossover shifts to lower moduli and higher frequencies (below), indicating a decrease of the average molecular weight of the polymeric material tested and a broadening of its molecular weight distribution.
Expressed in terms of the polydispersity-index, we saw an increase from 3.9 to 4.2. For usual technical polymers with a broader distribution any kind of degradation would initially affect the higher molecular weights and lead to a narrower distribution. Since the polybutadienes used for this study had a very narrow distribution, degradation led to an increase of smaller molecular weights and thus broadened the molecular weight distribution.
The data evaluation from this test clearly showed that indeed a thermal degradation of the polymer had occurred.
It was confirmed that using the normal force controlled sample loading for polymer samples of an unknown thickness, the rheometer equipped with the oven 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 could be solely attributed to a thermal degradation of the polymer sample tested. Preheating the oven in its parking position and selecting a suitable frequency range minimized the time needed for the measurement, which enabled a very cost-effective, and speedy, polymer analysis.
To get more details, including schematics and instruments used, read the application note: Automatic Detection of the Thermal Degradation of a Polymer.