I calculated the degree of crystallinity for polyethylene using the ΔHm and ΔHc value that I got from DSC curve as following: % Crystallinity = ([ΔHm – ΔHc] / ΔHm0 )*100 . But because in my case the ΔHc is bigger than ΔHm, the Crystallinity% is negative at the end. How come the ΔHc is bigger than ΔHm?
Dear Negar Ghasdian
A negative value of PE crystallinity means that you made an error in your approach.
Although I am not sure what the symbols in your equation exactly mean, I would say that you use an incorrect equation. The crystalliny in % should be calculated by dividing the measured melting enthalpy by the theoretical value for purely crystalline PE (cca 290 J/g) and multiplying by 100.
Secondly, the evaluation of crystallinity from DSC requires precise baseline treatment. I cannot help here since I never did the evaluation myself, but it should not be a problem to find a specialized paper on this topic. However, this can´t cause the crystallinity to be negative, but incorrect baseline correction could lead to crystallinity > 100%.
Regards
Josef
Dear Josef
Thank you for your reply. I found this equation on Perkin-Elmer website, and the symbols are ΔHm: melthing enthalpy, ΔHc: crystallization enthalpy and ΔHm0: reference value for melting enthalpy of a 100% crystalline polymer. The reason we should subtract the ΔHc from ΔHm is because we need the enthalpy given by crystalline part of polymer before melting it. looking at my DSC curve, it is apparent the crystallization peak is bigger than melting peak, so it is not a baseline problem.
Negar,
The example from Perkin Elmer is for poly(ethylene terephthalate)/PET. PET cold crystallizes between its glass transition temperature and its melting point. That is the ΔHc in the % crystallinity equation.
Poly(ethylene) does not does cold crystallize. However, as PE is cooled from the melt it crystallizes. This is ΔHc is not the one referred to in the Perkin Elmer file. Use the % Crystallinity = (ΔHm / ΔHm0 )*100 equation that Josef mentioned above.
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