What is the reason of observing a big melting peak in the first heating curve of a PLA sample in DSC, while the crystallinity is calculated almost zero according to the second heating graph ?
I have provided a DSC graph of my PLA in the attachments , the green line shows the second heating graph, the red line is the first heating graph and the black one is the cooling. The rate of heating and cooling is 10 ˚C/min. In different articles the crystalinity percentage of a polymer is calculated by the formula: {X%=(ΔHm+ΔHc)/ ΔHm°}
(where ΔHm is the melting enthalpy of the sample, ΔHc (
Hello dear I think, that this equation is not adequate to make the determination of crystallinity of PLA. Usually is employed this equation:
X=enthalphy of the sample/entalphy of the PLA 100% cristalline * 100
This equation is widely reported in the literature.
The formula you are suggesting is used for samples without recrystallization( cold crystallization). But as it can be seen in the graph, my sample has a recrystallization peak. If you have seen this equation being used for samples that have a recrystallization peak, please provide me with the source.
Thanks
Hedieh,
By comparing your 1st heat and 2nd heat you are comparing two different processing conditions for your material which is the source of your confusion. The first heat is the result of whatever you did to prepare your sample to be tested. It is clear that your sample crystallized during this process and those crystals melted in the 1st heat. Use X%=ΔHm/ ΔHm° to find that crystallinity.
The 2nd heat is a result of the cooling step that you performed which shows that the PLA did not have enough time to recrystallize and therefore you get a cold crystallization peak during the 2nd heat. Use X%=(ΔHm+ΔHc)/ ΔHm° for the 2nd heat.
Please see my answer to your previous question you posted for the rest of my explanation.
Good luck.
I also agree with Roddel Remy. Your PLA is originally crystalline which is why you see the first melting peak however your cooling rate is too fast / your polymer too slow at crystallising and so it doesnt recrystallise on the cool. On reheating, the polymer has enough energy and time to rearrange and therefore crystallisation occurs. Your crystallinity is calculating as zero as all the crystalline material formed on the second heat is melting out as you heat it to higher temperatures. To calculate the crystallinity of this material you need to use the first heating run as the second is an effect of your processing by DSC. Alternatively you can slow your cooling run down to see if you can get it to recrystallise on the cool so that you only get the melting transition on heating. As the two melt peaks are very similar in size it suggests that your material crystallises to the same extent on the reheat as was originally found in the material.
I also agree with Roddel Remy. If you have a chance to obtain WAXS patterns of both the original sample and the re heated one you will be able to see that both are crystalline. Original sample cannot show an exotherm because it is already crystalline!
Most of the answers above are OK, but you should have in mind, that the melting /crystallization enthalpy (heat) is a function of temperature. With other words the crystallization heat (at lower temperature) may be different from the melting heat (at higher temperature). Because of this reason the degree of crystallinity got with the equations presented above are -in principle- somewhat faulty. To get the precise degree of crystallinity of a sample you should determine the temperature-dependent enthalpy function of your sample by integrating the apparent heat capacity function and compare that with the enthalpy function of 100 percent crystalline material and amorphous material (available for many polymers from literature). The quotient of these two functions give you the crystallinity as a function of temperature. The procedure is in detail described in chapter 9.3.1 of our book "Calorimetry" from VCH-Wiley (see attachments).
You need keep in mind that crystallization is a dynamic process and sometimes it is slow depending on your MW and other interchain interactions. So if it is cooled down too fast and no crystallization can happen. All the structure is frozen before chains can move. When it is reheated and close to Tc, the chain can reorganize to crystalize. And further heating will get it melt again. To get best crystallization, you want to anneal your samples 5 to 10 degree C below Tc for some time (1h or longer depending on your sample).
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