If your sample is thermally degraded, then obviously the melting temperature and the crystallinity is different than before. But in the present case, the melting temperature is meaningless, as the material is no more the same. So which material are you talking about ? The resultant, degraded, material can be whatever, depending on the severity of the thermal treatment.

Juste take the sample of sucrose, having its own melting temperature and crystallinity. If you heat it too much, it will be degraded progressively into caramel, but in reality there is a number of other compounds wich are formed. You will see that the overall melting temperature is increasing with the severity of the treatment, until it tends to infinity when you get a char, which can not be melted anymore. But you won't be measuring the melting temperature of the same product as before. Same remark about crystallinity.

So in my opinion, the changes you observe are just a way to follow the degradation, but can not be generalised to all systems. It depends on the starting materials.

Alain

The answer is upon the polymer you are dealing with. Usually, a degraded polymer has a lower Tm given the chains are shorter. Crystallinity will increase or decrease, depending on the original MW and how far the degradation has gone. I suggest to look for papers dealing with degradation process of similar polymers. Good Luck!

Exact, thermal degradation of polymers may have different main effects, depending on the polymer: statistical chain breaks (e.g. for PE as all C-C chemical bonds are identical), elimination reactions (e.g. for PVC as C-Cl is in the weakest bond, thus leaving HCl), depolymerisation reactions (e.g. for POM as C-O is the weakest bond), or crosslinking (e.g. for aromatic polymers). For a given polymer, the mechanisms can be even different if oxygen (air) is present because of thermo-oxidation. Therefore, there is no single answer to this question.

Alain

Dear Nowsheen,

Certain degraded samples can show a decrease in melting temperature and an increase in crystallinity. Without any information about your product, it seems to me that your product degrades chemically inducing new entities of lower molecular weight leading to an organized structure explaining a decrease in the melting temperature with an increase in crystallinity.

Further analytical methods can be used to confirm the hypothesis.

Good Luck

Thanks to you all for your valuable suggestions. I will start by going through the paper you suggested Johan Sánchez.

It is likely that amorphous region bonds are selectively degraded during thermal treatment relative to those bonds within a crystallite. The absolute change in MW is probably less important than the change in crystallite size (likely larger) and concentration (likely higher). The point about degradation via elimination of small molecules is quite relevant, since that result will differ considerably from simple oxidation of amorphous region species. [e.g. PVC vs. PE or PP] Please tell us what polymer you are studying so we can be more specific in our answers.