I have a storage modulus vs. temperature plot which shows a little hump just before the decay in the data. It is quite obvious that it is some sort of transition, however I am pretty sure that the phase is pure so this transition can't be related to a secondary Tg.
Please provide more details on the composition. You say "decay": does it mean simply decreasing E' or chemical decomposition? Is your polymer thermoplastic or thermoset? Semicrystalline or amorphous? In semicrystalline polymers the alpha (gass) transition occurs before the "decay" which is attributed to premelting. Larger bet transitions can be observed in polymers with mobile side groups (as e.g. acrylates). If the data are confidential, you may send it to my email address [email protected] - otherwise I cannot help much more.
I added an answer before but it did not get through. Please provide more details on your polymer: thermoset or themoplastic, amorphous or semicrystalline etc.
In semicrystalline polymers the highest temperature transition usually elongs to premelting phenomena and the next is the glass transition. Beta transition is very well defined in acryaltes and in other polymers with mobile side greoups. The assignment of the transition is easier if both mecahnicla and dielectric spectroscopy are used.
Dr. Banhegyi, thanks for your answer. The polymer is amorphous commercial epoxy resin, a thermoset resin with nano acrylonitrile butadiene rubber fillers ( elastomeric fillers). The weight ratio is 100:10 epoxy : nano rubber. Decay means the sharp decrease in E'. I did not observe the same trend in different weight ratios of the same formulations.
Please have a look at the 10NBR-NP/ R2 formulation's plot, the little hump just before the decay is quite obvious. No such trend is observed in different weight ratios of the same formulation.
Dear Dr Ozdemir, Is it not possible that it is a post-curing effect? What was the curing condition? If the test temperature exceeds the curing temperature, the curing reaction may go further and the crosslink density may increase - this may increase the E' value as well. Perhaps in the case of the 15 an 20 NBR samples the mobility is higher thus the curing reaction may go to completion at the original curing temperature. The 5 NBR sample aslo exhibits a gradual increase of E' in the pre-Tg region. If I were you I would make DSC tests at the same heating rate on each sample. If there is post-curing, there would be some exotherm on the DSC at or around the glass transition. Could you also present the E" curves? You may also run the DMA curves twice. This bump may disappear in the second run - in this case it is a post-curing effect.
I have seen such a bump before the glass transition in the case of polymer blends produced by solvent casting, where there was some minor amount of solvent which evaporated in the first heating run. In the second run the blend porved to be phase separated. (This research is briefly mentioned in http://www.dtic.mil/dtic/tr/fulltext/u2/a229744.pdf).
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