There are some drawbacks and advantages of DSC and DMA,

DMA is more sensitive than DSC but for DMA we usually use relatively much bigger sample than DSC, so it is very important to use low heating rates so your sample has enough time to reach thermal equilibrium with the chamber environment, that's why for Tg, it is highly recommended to use 2 C/min. This becomes more critical when you compare neat resin with reinforced composites, as fillers usually have different conductivity compare to polymer matrix. This can result in observing a shift in Tg of your sample! Also, the thermocouples in the machine are usually (in the best case) on the surface of the sample and the surface which does not necessarily represent the temperature of thw bulk (whole ) of the sample. (If you use nanoparticles, it will become even much complex as in nanocomposites, interface scattering of phonons during heat transfer reduces thermal conductivity!!!).

Regarding which temperature should be used, different standards present three different temperatures:

1. Onset of the drop in storage modulus(E')

2. peak in loss modulus (E")

3. peak in tan delta

The first value represents (in my view point) much properly the Tg as Tg is basically the temperature in which short range relaxations (some people consider 50 C-C bonds for the length of the regions) can occur which reduce the resistance against deformation. If you do DMA in LVE region, you work in fairly small deformations(

Hello dear, Tg can be determined easily by DMA, because it can be identified when occur a decreasing on storage modulus value. Furthermore, Tg can be observed better by DMA than DSC, because the deflection on baseline on Tg is bigger than DCp measured by DSC. The change on modulus is the order of 10 a 1000, while the change on DCp is the order of 10-30 %. Therefore the sensibility of DMA is bigger than DSC, and the determination is better.

Could you present your data?

Thank you, Prof. Edwin. Thus, it is better to measure Tg from the Storage Modulus curve.

Prof. Gyorgy, I haven't completed the test. I wanted to clarify my doubts regarding the question I posted before commencing the Experiments.

OK, I would suggest to do both DSC and DMA, evaluate it and then you can decide for yourself the clarity of the transitions. You can also compare several experimental data available in the literature.

I agree that DMA is a good option to measure Tg. Unfortunately, Tg, being a second order transition is rate dependent. Different DMA parameters, like G', loss tangent, etc. will react differently. To me, the most common and typically easiest to detect is the peak in tan delta, but the method chosen really should be related to what you are looking for. If use temperature is above Tg, you are looking for stiffening of the polymer matrix and tan delta peak does a good job. If, on the other hand, your polymer use temperature is below Tg, G" maximum or onset of G' drop can be better choices. In any case, make sure you use the same methodology to compare material. Set a given strain value and frequency. If you do not do this comparisons will be meaningless.

Thank you Prof. Rafael and Prof. Gyorgy. Now I have a better insight about which to choose and how to proceed with the tests.

There are some drawbacks and advantages of DSC and DMA,

DMA is more sensitive than DSC but for DMA we usually use relatively much bigger sample than DSC, so it is very important to use low heating rates so your sample has enough time to reach thermal equilibrium with the chamber environment, that's why for Tg, it is highly recommended to use 2 C/min. This becomes more critical when you compare neat resin with reinforced composites, as fillers usually have different conductivity compare to polymer matrix. This can result in observing a shift in Tg of your sample! Also, the thermocouples in the machine are usually (in the best case) on the surface of the sample and the surface which does not necessarily represent the temperature of thw bulk (whole ) of the sample. (If you use nanoparticles, it will become even much complex as in nanocomposites, interface scattering of phonons during heat transfer reduces thermal conductivity!!!).

Regarding which temperature should be used, different standards present three different temperatures:

1. Onset of the drop in storage modulus(E')

2. peak in loss modulus (E")

3. peak in tan delta

The first value represents (in my view point) much properly the Tg as Tg is basically the temperature in which short range relaxations (some people consider 50 C-C bonds for the length of the regions) can occur which reduce the resistance against deformation. If you do DMA in LVE region, you work in fairly small deformations(

All the above answers give you an idea about this main relaxation. There are excellent books giving more details about. In this relaxation process the elastic and viscose molecular dynamics are very complex! Since tan delta is the ratio of loss to storage modulus (tanδ=Ε΄΄/Ε΄), gives you a better value relating the energies participating to the relaxation. Some people use the maximum in E΄΄.This value is smaller from that estimated from tanδ peak about 10 to 15 0C. In conclusion use tanδ, as most of the people do and good luck.

Dear all, thanks for your valuable inputs. It has given me many valuable information.

Tan δ is widely used to measure/represent Tg of polymers.

One simple way is when you do DMA, temperature ramp from low to high temperature, when you see G' decrease exponentially, that temperature is Tg or Tm.

One extra comment, it is useful to carry out the run using two frequencies (such as 1 and 10Hz). A glass transition event is dependant on the timescale of the experiment, effectively it requires a finite time for chain motion to occur, and as a consequence the temprature determined for the Tg (for example peak in Tan Delta) will move to a higher temprature at the higher frequency. A melting event will not be frequency dependant.

It has been suggested to use a ramp rate of 2degC/min, which I would agree with for thermoplastic polymers, but in my experience for some thermoset polymers a slightly faster ramp rate of 3degC has been preferable to reduce any possible effects of post cure during the instrument run.

There is an excellent book titled "Thermal Analysis of Plastics. Theory and Practice" published by Carl Hanser Verlag in 2004 containing a chapter on DMA that explains these questions in more depth. The chapter is available in public domain.

Let me quote two fragments from the book giving some important clues:

"Rieger believes there are several arguments in favor of defining the glass transition temperature in terms of the maximum loss modulus G" instead of the maximum loss factor tan delta. Because G" is a measure of dissipated energy, he contends that it makes sense to define the temperature at maximum loss as the transition temperature. A further argument is that the temperature of maximum G" remains the same, regardless of whether a pure material or a blend is being considered. ASTM D 4065-2001 also recommends that the evaluation be based on the temperature of maximum loss modulus."

"Generally, the temperature of G" max at a frequency of 1 Hz corresponds to the midpoint temperature Tmg (half step height) observed in DSC. A large number of experiments on 40 representatives of different classes of polymer have borne out this observation."

Since you are investigating the effect of reinforcement on a polymer matrix, I expect that you are going to demonstrate the enhancement of mechanical strength and stiffening of the composite and its effect displayed through change in Tg in contrast to the pristine polymer. In that case, the damping or stiffening is best conveyed through the tan delta value from DMA.