I have provided a DSC graph of my PLA in the attachments , the green line shows the second heating graph. 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 (
You have observed cold crystallization in your sample, so you have to add corresponding enthalpy in the formula. From your green curve it looks like almost all of your sample is crystallined before melting.
Does it mean that my sample has crystallized only to a very small amount?
HI. If you want to know the initial crystallinity of your sample, the cold crystallisation has to be eliminated from the melting enthalpy, because this crustallisation process was induced during the heating in your measurement and not was originally in your sample before. How fast are you runnig the DSC-Measurement?
Cooling rate as you surely know, determines very highly the crystallization. It depends strongly on crystallization kinetic of material. With a very high cooling rate you can reduce the crystallization dramatically. Is it your case? What kind of thermal history does your sample have?
But you mentioned a second heating. During the first you cooled the sample from the melt?. You can modify the crystallinity for example with a very low cooling program including a isothermal step at crystallization temperature range or in the contrary with a fast cooling program.
my sample was cooled from solution in room temperature, then 3 runs of DSC, first heating, cooling and 2nd heating
When you say your first answer is illogical I assume it means you get a positive number (or a very small negative number)? If that's the case, it's not necessarily illogical. Every DSC will have random measurement error due to baseline quality, choice of start and end measurement points, etc. that could affect the result. If you are measuring your third heat then it simply means that the material was completely amorphous after the cooling step and the "illogical result" is just machine error.
If you are measuring you first heating then you only need to measure the melting endotherm and use the regular crystallinity equation (without ΔHc).
Good luck.
by not logical I mean that the crystallinity is calculated 0.3% , while in the first heating curve, a big melting peak could be observed(red curve)
You calculated this from the third heat which is unrelated to your first heat. The sample is the same the processing conditions are not. Therefore the crystallinities will not be comparable and you must measure the 1st heat independently.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials Chemistry
- Polymer Chemistry