Why You Should Consider Frequency When Testing Rubbers and Polymers with Dynamic Mechanical Analysis (DMA)

Written by Travis Parkman, Product Specialist Mechanical EIT

Vibrations on a component can have adverse effects on the overall performance of a system, this can include its mechanical, thermal, or electrical performance. However, this effect is often neglected. One of the greatest culprits, our assumption that materials act the same at different frequencies, which is not the case. This is especially true for viscoelastic materials.

EFFECT OF FREQUENCY ON GLASS TRANSITION TEMPERATURE

To highlight this effect, I conducted a simple experiment where I measured the glass transition temperature (T_g) of polymethyl methacrylate (PMMA).

Figure 1.  PMMA, also termed Plexiglass, has great biocompatibility, making it vital to the medical industry.  It is used as ‘bone cement’ to fill in the gaps between implants and bones.  It also has applications in eye contacts and dentures [1].

The glass transition temperature is described as the temperature where the polymer chains start to relax and move. This will cause a rigid material to become more flexible as it begins to enter its rubbery state. Depending on the application, this can be beneficial or have negative implications.

The T_g can be measured through differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) and DMA. Each method has its own advantage depending on the application and material. DMA has the advantage of being one of the most sensitive thermal techniques for measuring T_g for any material.

The glass transition temperature of a viscoelastic material can be identified by the peak in tan δ vs. temperature plot. This peak signifies when the material dissipates the most amount of energy. The benefit of using DMA is that we can measure the state change at different frequencies. For this experiment, I ran the tests at three different frequencies, 5, 10 and 100 Hz. The results from this experiment are shown in Figure 2.

Figure 2. tan δ vs. temperature for different temperatures

The results show that the material properties are affected by the loading frequency. In Figure 2a, we can see that the glass transition increased in temperature with the frequency and in Figure 2b, we see an increase in the elastic modulus as well.

This effect can be explained by the viscous behavior in viscoelastic materials. When a load is applied to the material, it needs time to relax to the initial deformation. As the frequency increases, the allowable time window to recover decreases; this will stiffen the material. Therefore, more energy (higher temperature) is required to relax the material from a glassy state to a rubbery state.

Table 1 shows that the frequency effect on the Tg is non-linear and have a larger effect at lower frequencies.

Table 1. Glass transition temperatures of PMMA sample

APPLICATIONS WITH DIFFERENT FREQUENCY RANGES

As Table 1 shows, that the effect of frequency can be large. Measuring the T_g with a DMA allows us to identify this effect and allows us to predict the materials behavior in operation where it will encounter certain frequencies. Table 2 shows typical frequency values and ranges associated with different applications.

Table 2. Expected frequencies for different applications.

THERMAL ANALYSIS LABS MEASURING CAPABILITIES

Selecting the appropriate testing parameters to model your current application is often the greatest hurdle in setting up a DMA experiment. Ways to narrow down your parameter’s is to do a self-assessment, literature search, or use great resources like our Thermal Analysis Labs.

Currently, we provide direct measurement from 1 – 200 Hz and can model high frequency behavior above 1 MHz using the Williams-Landel-Ferry (WLF) Equation. If you have questions on current DMA testing approaches for your application or want to inquire about our testing capabilities, contact us at info@thermalanalysislabs.com.

Works Referenced

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 [1] Wasserman, S. (2019, September 25). What is PMMA and how is it used in the medical world. ANSYS Blog. https://www.ansys.com/blog/what-is-pmma-how-it-is-used-healthcare

[2] Jones, D. S. (1999). Dynamic mechanical analysis of polymeric systems of pharmaceutical and biomedical significance. Int. J. Pharm., 179, 167-178.

[3] Baurier, H. (2008). Dynamic mechanical analysis to improve tire performance. Tire Technology International, 116-119.

[4] International Electrotechnical Commission (2010). Secondary Lithium-Ion Cells for the Propulsion of Electric Vehicles – Part 2: Reliability and Abuse Testing, International Standard IEC 62660-2.

[5] Ganguly,A., Saha, W., Bhowmick, A., & Chattopadhyay, S. (2008). Augmenting the performance of acrylonitrile-butadiene-styrene plastics for low-noise dynamic applications. J. App. Polym. Sci, 109, 1467-1745.

[6] Reinhold, K., Kalle, S., & Paju, J. (2014). Exposure to high or low frequency noise at workplaces: differences between assessment, health complaints and implementation of adequate personal protective equipment. Agronomy Research, 12 (3), 895-906.

[7] Song, B. & Nelson, K. (2015). Dynamic characterization of frequency response of chock mitigation of a polymethylene diisocayanate (PMDI) based rigid polyurethane foam. Latin American Journal of Solids and Structures, 12. https://doi.org/10.1590/1679-78251585