Friday, 24 July 2020

Design Properties for Engineers: Dynamic Mechanical Analysis (DMA) of High Performance Polymers

In this blog post, we have a closer look at the dynamic mechanical analysis (DMA), a thermo-analytical method that estimates the viscoelastic properties of a given material over the course of different temperatures. It steps away from a single point view toward a multipoint data view which is beneficial for polymer material selection tasks. 

DMA can be measured according ASTM D4065-94. Here, a fixed frequency of oscillation of 1 Hz and heating rate of 2°C/minute are defined. When performing the DMA it is important to cover the glass transition area for amorphous polymers as well as the melting point for semi-crystalline polymers. Results of a DMA are the storage or elastic modulus (E’), the loss or viscous modulus (E’’) and the tangent of the phase angle delta (E’/E’’). 

In the figure below, the storage modulus vs. temperature behavior of different high performance amorphous polymers is shown. They all show a significant drop in modulus in the glass transition region. Among the amorphous high performance polymers, PAI has with 275 °C the highest glass transition temperature. A continuous use temperature of 260°C is feasible. 

Storage modulus vs. temperature behavior of different high performance amorphous polymers 

In the next figure, DMA curves of semi-crystalline high performance polymers are shown. We can see a decrease in the modulus at the glass transition temperature. However, mechanical properties can be retained until the crystalline melting temperature.

PPS shows a glass transition of 88 °C and is fully melted at 270°C. From 80°C to 200°C the retained mechanical strength is still sufficient.

 DMA curves of semi-crystalline high performance polymers

Fluoropolymers have an excellent low temperature performance. This allows e.g. their usage as sealing gaskets at cryogenic temperatures where flexibility is needed. Storage modulus performance is high enough to provide the necessary mechanical performance, at low temperatures (down to -200°C) as well as at high temperatures (up to 250°C).

DMA curves of fluoropolymers

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