I took under consideration density measurement (automatic pycnometer) but it did not give reasonable result probably because of voids presence. DSC is not right tool because of recrystallization during the heating. We did some TMDSC measurements and we saw some differences depending on samples preparation method, however we have problem with quantitative assessment and interpretation. Are there the best TMDSC scan parameters set up, to differentiate signal in terms of initial crystallnity level present in different samples? Could anyone share their experience with determination of Xc in PBT? Thanks.
Did you try to subtract the enthalpy of cold crystallization (its absolute value) from the melting peak? For PET samples this is a common procedure. You are essentially subtracting the portion of crystalline phase that is formed in situ from the total amount. What is left is the pre-existing crystalinity. That is: Xc = (Hmelting + Hcold_cryst)/H100%cryst
It is easy for PET as the cold crystallization take place at much lower temperature than melting. In PBT those process are almost simultaneous, so there are no separate peaks. There some discussion in literature about actual mechanism or recrystallization. Nevertheless I did not find statement saying how much of the final melting comes from melting of recrystallized crystals and how much comes from melting of “surplus” crystals which were grown along recrystallization. Are there any “surplus” crystals? Our doubts comes from the fact that, our samples manufactured using very different process parameters showed difference of Xc within experimental error. So, we think that Xc calculated by us, is a result of process taking place during heating in DSC rather than the amount of crystals present in the sample initially.
Im not sure if it will work for PBT but we have looked at crystallinity of other polymers through Infrared spectroscopy. In some polymers the carbonyl peak has two overlapping bands for amorphous and crystalline regions. These can then be deconvoluted and the peak heights compared to get the ratio of amorphous to crystalline domains. You need to record the IR spectra as you melt and crystallise a sample of PBT first to create a calibration curve. Refer to Kate Philipson's papers on PCL for more details.
Alternatively if you have access to a flash DSC then it may help. This heats the sample at very high rates (in excess of 1000 K/s) preventing recrystallisation.
Theoretically it is not necessary to integrate the peaks corresponding to cold crystallization and melting separately - net enthalpy change in the whole region is actually the Hmelting + Hcold_cryst term. However that's just theory and it may not work with your particular samples.
Quite right what Tibor Dubaj writes. The enthalpy difference between two different states is given by the simultaneous integration of all recorded DSC peaks between those two states. This is hardly theoretical.
It puzzles me in any case how the melting peak can be integrated separately if it is convoluted with the crystallization peak. If a crystallization and melting peak are convoluted the remaining peak may end up having zero enthalpy, and that simply means that the heats of the two processes cancel each other out, but on its own it doesn't tell you much about the melting enthalpy.
if you want to test crystallnity level in PBT samples in both deep and high supercooling annealing temperatures or fast and slow cooling rates, you must used FSC rather than traditional DSC. Two reasons for this. One you can not go some annealing temperature without crystals formation when you heat or cool sample to that temperatures. Another the recrystallization during the heat will give you some error which will be problem for determining absolute value.
FSC had two vision. Commercial vision is Flash DSC, which can cool and heat the polymer sample up to 5000 K/s. This is enough for PET but not for PBT.
Another is AG polymer Physics, Uni Rostock (Prof. Dr. C Schick) Vision, which can cool and heat polymer sample up to 1000000 K/s, which is enough to PBT.
Hope this is useful to you.
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