It comes form the definition. Tan delta is E"/E" which does not have the same maximum as E" by itself. While E" describes the lost energy, tan delta describes the ratio of lost and stored energies. Sometimes the tan delta curves can plotted easier than the E" curves (especially if they are plotted on a logarithmic scale) - may be they are preferred by some researchers. Relaxation transition temperatures are not exact material constants (they depend on frequency and temperatures), shift factors and activation energies, as well as limiting moduli are more important. Tan delta and E" values can be both very useful if more material compositions are e.g. compared under identical conditions. Then it is meaningful to compare the peak temperatures of the relaxation strengths. if you are interested in Tg the best is to compare values obtained by DSC, TMA, DMA, dielectric spectroscopy etc.

It comes form the definition. Tan delta is E"/E" which does not have the same maximum as E" by itself. While E" describes the lost energy, tan delta describes the ratio of lost and stored energies. Sometimes the tan delta curves can plotted easier than the E" curves (especially if they are plotted on a logarithmic scale) - may be they are preferred by some researchers. Relaxation transition temperatures are not exact material constants (they depend on frequency and temperatures), shift factors and activation energies, as well as limiting moduli are more important. Tan delta and E" values can be both very useful if more material compositions are e.g. compared under identical conditions. Then it is meaningful to compare the peak temperatures of the relaxation strengths. if you are interested in Tg the best is to compare values obtained by DSC, TMA, DMA, dielectric spectroscopy etc.

Thank you for your valuable information. The logic behind what you mentioned i understood, but still now the chemistry behind that not getting. From both Tan d and E", we could able to calculate Tg ( the peak maximum will give ). While both are different means, did the material have different Tg ? From the 2 results which one can be consider as material Tg. Because for a single polymer / compatible polymer blend, will give only one Tg. Kindly clarify me with your valuable opinion.

Please don't forget that the experimental glass-transition temperature Tg is not a real well-defined critical temperature of a thermodynamic phase transition. It depends on the experimental technique employed and even on the velocity or frequency used for a given technique. Glass transition is a very complex and largely debated issue, and no widely accepted theory exists yet.

Still, what can be said is that the experimental Tg is simply the crossover temperature when the average relaxation time tau_rel(T) of the supercooled liquid equals the typical time scale of the experiment tau_exp(T), e.g., the inverse of the measuring frequency for mechanical or dielectric experiments: tau_rel(Tg)=tau_exp(Tg).

When T>Tg, tau_rel(Tg)

Thank you sir. As a student i know very little bit, i have to go further more. Once again thank you sir for motivate me.

It has been a pleasure, Raji Vijay.

If you want to learn and get deeper in the physics of glasses, you could find useful this link to my old MSc course on glasses:

http://www.uam.es/personal_pdi/ciencias/mramos/Docencia_archivos/Vidrios.html

At the beginning of the first chapter, you will see a useful list of recommended bibliography.

And Happy New Year 2015!

Thank you sir