Viscoelastic properties of hydrogels (DMA)

[neejam]

Hi,

I am a Bachelor Student of biomedical engineering and I have a question regarding the complex compressive modulus. I am really a newbie in this field, hopefully somebody can help me.
I have measured four different hydrogel films to compare with the same conditions using a dynamic mechanical analyzer (DMA, measured in compression mode with a sinusoidal deformation of constant amplitude, ε=0.1%).
Now the results show that the materials that have the highest storage modulus also have the highest tan δ, which is a bit confusing to me as I though when measuring viscoelastic materials with a DMA it should always the other way round (so the ones with the highest E' should have the lowest tan δ). I am attaching E', E'' and tan delta graphs so you see what I mean.
Thank you!

https://dl.dropboxusercontent.com/u/38703467/storageModulus.gif
https://dl.dropboxusercontent.com/u/38703467/lossModulus.gif
https://dl.dropboxusercontent.com/u/38703467/tanDelta.gif

 

[Chestermiller]

The storage modulus is an absolute quantity, and tan δ, the ratio of the loss modulus to the storage modulus is a relative quantity. So it isn't really relevant to compare the storage modulus of a given material to its tan δ . The tan δ in the figures appears to be roughly decreasing with frequency, which is what would be expected.

Chet