Dynamical Mechanical Analysis (DMA)
What is it?
Dynamical Mechanical Analyzers apply a sinusoidal stress or strain to a sample, allowing a quick determination of moduli and damping as a function of temperature. DMA is extremely sensitive to changes in the material and is able to detect small structural transitions in a sample, some of which are not be detectable in the DSC. Many other tests are possible, from dynamic methods varying temperature, frequency, strain, or stress to static force tests (for example, stress-strain, creep recovery, and stress relaxation.) A wide range of fixtures or jigs are available allow samples tested in a variety of shapes and forms. These include tension, flexural, compressive, and shear.
We offer the following methods* with a variety of DMA instruments available:
*Not all possible specific testing methods may be listed here
Testing using this method allows designers to see how a material responds when a load is applied and then removed. When a part is put under load in the field, it may distort over time and cause overall failure. This gives an idea of when the material will need to be replaced, or if it is even acceptable for use. Creep testing applies a load and tracks the deformation; the recovery test removes this load and tracks how quickly and how much the material recovers from the load.
Curing and Photo/UV DMA
When a material cures and sets, one can often see a drastic shift in the modulus value. Running DMA on a curing material can allow the determination of the gelation and vitrification points as well as the modulus and Tg of the cured material. Monitoring the sample throughout the cure process gives us an understanding of the cure profile. shows when a material has set enough, and to what extent it can be used. One example is superglue, while the initial set has occurred it is not fully cured until sometime later. Use of curing studies and can improve production times; if there is a quality problem in cured materials it can also determine the time that the material needs to adequately finish the cure. Depending on the application, the testing can be run using a temperature scan or an isothermal study.
In addition, we offer photo-DMA testing for photo-curing applications. While we are able to perform photo-calorimetry , photo-DMA compliments that data by measuring modulus changes while photo-curing. As with photo-DSC, a second scan allows the measure of the cured Tg.
UV DMA can also be used to study the degradation of materials under UV exposure.
As mentioned above, viscoelastic materials have a frequency dependence that has striking implications for the materials performance. Using isothermal holds, Frequency scans allow the measurement of dynamic modulus, complex viscosity, and damping (tan delta) over a range of 100 frequencies across many decades.
Time Temperature Superposition (TTS) is often used to analysis this data. Using the WLF model, TTS allows us add frequency data together and create a mastercurve, which predicts behavior at frequency outside the measurement range of the instrument. One can also predict performance at longer times and different frequencies than is feasible to measure. Several methods can be used to both shift the curves and to test the shift for validity.
Moisture sensitive materials are relatively common and not fully understanding how variations in humidity change a product can lead to serious problems in use. Phase transitions, stiffness, swelling, and shrinking can all occur under humidity changes. In addition to testing samples under a set humidity, it is also run vary the humidity during the run to investigate changing of conditions.
When a product is going to be exposed to liquids or fluids in use, how the material will interact with the solvent and whether they decay over time needs to be considered. Simply immersing the product in those materials is often inadequate as it does not consider the material will simultaneously see stress/strains applied to it. Immersion testing allows the traditional DMA tests to be performed in the fluid to see material changes either isothermally or while heating.
Often materials are exposed to elevated temperatures for long periods of time. Isothermal runs can be used to track changes under these conditions.
A relatively specialized test, stress relaxation can be looked at as the inverse of creep-recovery. Instead of apply a force, the material is deformed as quickly as possible and held while the changes in the applied force necessary to hold the deformation is tracked. A polymeric material will show a decay in the force needed to hold the new position as the material relaxes into the new shape.
DMA instrumentation is able to perform Stress-Strain tests within the limits of its force motors. For delicate samples, this allows the measurement of traditional tests like Young’s Modulus that would normally not be possible in standard Universal tests.
Temperature Scan with single frequency/strain
Modulus values generally change with temperature, meaning that while a product may meet the design specs at room temperature, at a higher temperature it can fail. In addition, DMA is one of the best ways to obtain Tg for highly cross-linked or filled materials where the transition may be weak. DMAs are extremely sensitive to transitions and are able to detect more subtle, such as alpha and beta transitions, which effect material performance in the solid state.
Temperature Scans with multiple frequencies or strains
Many materials are exposed to a variety of frequencies and temperatures when the end product is in use. When this potential issue is not taken into account, unexpected failures are likely to occur. A temperature scan under multiple frequencies can reveal how materials change when both frequency and temperature are changed. For example, the glass transition temperature of a polymer can shift to higher temperatures as frequency increases. See frequency scans segment in for more information.
Strain level can also change polymers, particularly above the Tg. Similarly to the frequency-temperature scan discussed above, temperature-strain scans can be run.