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What does a Loss Modulus Curve MeasureHelpful Member! 

mta02hm (Materials) (OP)
1 Mar 07 4:59
Hi all,

I've just been handed a data summary from a DMA test. I'm trying to find help interpreting a "loss modulus" curve.  
I know that the Tg can be taken from the peak in the Loss Modulus but I want to understand what is happening in the material to cause this peak?

Thanks in advance for your help!
Helen
Helpful Member!  Demon3 (Materials)
1 Mar 07 6:49
Polymers are viscoelastic, i.e. part elastic (bounce back into shape)and part viscous (flowing).

A Google search for "loss modulus definition" gave me this link that describes it very well.

http://www.polyacs.org/nomcl/pmse.mechterm.html

"For dynamic experiments, the favorite today for many purposes, a sign wave is imposed on the sample. The wave may be of constant or variable frequency. Then, the modulus is split into two quantities, a storage modulus, E’, a measure of the energy stored during a cycle, and the loss modulus, E’’, a measure of the energy lost. The quantity E’’ has a maximum at the glass transition of the polymer, where it softens from a glassy to a rubbery polymer. The quantity tand is the ratio of E’’ to E’."


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

mta02hm (Materials) (OP)
6 Mar 07 4:57
So the Loss Modulus is a measure of a materails ability to dissipate heat, but what is happening in the material as it goes through the Tg transition? The graph I have shows the loss modulus increasing to a peak around -30C and then dropping back down. Is the energy "lost" to internal motions? Does the peak exsist due to some kind of strain hardening affect, so when the material is at the Tg temperature no more energy can be taken in so the loss modulus drops again?

I'm trying to understand what is happening to the structure of the polymer as it goes through the Tg, any help would be very much appreciated!
Demon3 (Materials)
6 Mar 07 5:13
You can think of it as a viscosity. The loss modulus is related to the amount of energy lost due to viscous flow.

The Tg is where the polymer chains undergo crankshaft motions with about 15 units participating (if I remember correctly).

Also the Tg is not a well defined, constant parameter, it depends very much on testing speed so if you were to rerun your DMTA at a higher frequency you would get a lower Tg and if the DMTA were run at a lower frequency then the Tg would be higher.

It is rather important to realise that polymer properties are time/frequency dependent (as described by the famous WLF equation). For example if you were to use your material for sound/vibration damping you would find the damping factor (tan delta) changes with temperature and frequency so the damping would be uneven for different frequencies.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

trose (Aerospace)
14 Jun 07 17:02
My interpretation of the loss modulus (i.e. viscosity) peak is that there is an initial rise due to chain lengthening that cause the viscosity to rise.  As the material becomes cross linked or highly crytalline there is a rise in storage modulus.  This reaction consumes molecules that otherwise would flow and contribute to the total energy loss due to flow.  The less material that can flow the lower the loss measurement.  Ultimately, when all reactions are complete and there is no more reduction in material that can flow, loss modulus will cease changing.

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