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### Webinar: That’s How Dynamic Mechanical Analysis (DMA) Works

February 14, 2023 | Dr. Thomas Haenel

What kind of samples can we run in shear mode? Are materials such as PDMS (soft silicone) suitable for the shear mode frequency sweep? If so, what are the appropriate dimensions?

It depends on the sample’s dimensions and modulus. The shear test requires highest DMA forces. A soft silicone can be tested in larger dimensions than a stiff rubber. For example, a specimen with a thickness of 2 mm and a diameter of 10 mm with a modulus of 10 MPa and a deformation of 10 µm requires a force of approximately 8 N. With a modulus of 20 MPa, you need 15 N. If you have a NETZSCH DMA 242, you will find the DMA Calculator program in the Proteus® folder to calculate your specimen. For stiffer materials, you will need a NETZSCH Eplexor®.

What types of materials can I run in shear mode on the NETZSCH DMA? Are polymers such as PU and PDMS suited for shear mode (frequency sweep) testing on the DMA? If so, what are the appropriate dimensions?

It depends on the dimensions and modulus of the specimen. The shear test requires highest DMA forces. A soft silicone can be tested in larger dimensions than a stiff rubber. For example, a specimen with a thickness of 2 mm and a diameter of 10 mm with a modulus of 10 MPa and a deformation of 10 µm requires a force of approximately 8 N. With a modulus of 20 MPa, you need 15 N. If you have a NETZSCH DMA 242, you will find the DMA Calculator program in the Proteus® folder to calculate your specimen. For stiffer materials, you will need a NETZSCH Eplexor®.

Does the fact that the tan d curve is larger (in height) than the E'' curve mean anything?

No, tan d has its own scale on its own y-axis.

Is it possible to measure SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. sintering of small particles with DMA?

In principle, you can measure SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. sintering, i.e., shrinkage of the powder particles, in TMA mode. I would do this in compression mode, with a small preload in a crucible with a sapphire disc over the powder, and then you should be able to evaluate the delta L signal to see if the powder collapses or puffs up. Of course, this assumes that there is enough powder to get a sufficient delta L signal.

I have seen a not fully cured polymer matrix (epoxy) show two peaks in the E'' and T curve, but I could not explain why.

It's difficult to say what happened without knowing more about the material. For example, two peaks may occur if the material is undergoing a change in crystallization structure, e.g., beta to gamma conversion, or if it is a two-phase material, e.g., a polymer blend. If it only occurs on the first pass, but not on a second pass, it may also be that there was postcuring that preceded the DMA. This means that you saw the pre-crosslinked "E" first and then the fully crosslinked "E". As the DMA works at very low heating rates, this can happen. I've seen this before when measuring an adhesive, but with E'. The material reacts faster than the DMA measurement and this seems to increase the modulus again. In the second run, the modulus was higher but there were no two peaks in E'.

I am using a DMA 242 E, I am trying to measure a thermoplastic elastomer sample (COPE type) in tensile mode. At high frequencies (>33 Hz), I get a negative storage modulus. How is this possible?

A negative modulus is an indication that the amplitude is too low; in this case the DMA cannot distinguish between the stiffness of the sample from the system stiffness and mathematically meaningless values are obtained. The amplitude on the sample should be at least 3 µm, preferably a little higher. Please check this on your measurement in your Proteus®® analysis.

Can the static modulus be derived from dynamic tests? Is there an ASTM for this?

Due to the oscillating load, the storage modulus is often slightly larger than the Young's modulus. However, I am not aware of any ASTM standard that allows conversion from a dynamic load to a static load. As a rough approximation, the storage modulus can be considered as the Young's modulus.

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• #### Dr. Thomas Haenel

NETZSCH-Gerätebau GmbH Büro Rheinbach/Bonn