Hi every one,
I wonder if the DSC heating rate affects the degree of crystallinity in semi-crystalline polymers?
thanks
There are two parts to this question. Let's assume that you are calculating your degree of crystallinity by enthalpy integration of your predominant endothermic peak in your heating curve.
First, independent of your material composition, the cooling rate selected during your DSC run can largely impact the degree of crystallinity. Crystallization is a time-dependent process; during a slow cooling, there is more time for molecular re-organization into crystals while the chains are still mobile, and you will typically see a higher degree of crystallinity. During a faster cooling, there is less time for the chains to organize into lamellae while the chains are still mobile, and this will reduce the degree of crystallinity. Cool fast enough (ie; quenching in liquid nitrogen), and you can often completely suppress the formation of crystals.
During heating, there is one additional factor which may affect your degree of crystallinity, which is cold-crystallization. This can be observed as a smaller exothermic peak which evolves above Tg (ie; after the chains become mobile again) but below the main melting peak. During cold crystallization, we typically observe the growth of small crystals that were nucleated during the cooling run, but did not have time to grow fully. Naturally, this will impact the observed crystallinity when the crystals fully melt at Tm. There are two ways to correct this and determine the degree of crystallinity that existed in your sample prior to the heating: 1) If you heat rapidly, you can suppress re-organization (cold crystallization) during the heating cycle or (2) You can integrate the exothermic cold crystallization peak from the heating curve, and subtract this value from your melting peak.
TLDR: The heating and cooling rate used during DSC can and will affect your degree of crystallinity. With proper experimental design, you can correct for this.
Dear Behzad Hashemi Soudmand, heating rate is the predominant factor in determining the degree of cristallisation as this latter is time dependant. My Regards
Generally if you heat crystallising materials too fast you some transformational events might not be revealed in your DSC analysis. I will suggest using 1degree/min or a heat and cool cycle.
There are two parts to this question. Let's assume that you are calculating your degree of crystallinity by enthalpy integration of your predominant endothermic peak in your heating curve.
First, independent of your material composition, the cooling rate selected during your DSC run can largely impact the degree of crystallinity. Crystallization is a time-dependent process; during a slow cooling, there is more time for molecular re-organization into crystals while the chains are still mobile, and you will typically see a higher degree of crystallinity. During a faster cooling, there is less time for the chains to organize into lamellae while the chains are still mobile, and this will reduce the degree of crystallinity. Cool fast enough (ie; quenching in liquid nitrogen), and you can often completely suppress the formation of crystals.
During heating, there is one additional factor which may affect your degree of crystallinity, which is cold-crystallization. This can be observed as a smaller exothermic peak which evolves above Tg (ie; after the chains become mobile again) but below the main melting peak. During cold crystallization, we typically observe the growth of small crystals that were nucleated during the cooling run, but did not have time to grow fully. Naturally, this will impact the observed crystallinity when the crystals fully melt at Tm. There are two ways to correct this and determine the degree of crystallinity that existed in your sample prior to the heating: 1) If you heat rapidly, you can suppress re-organization (cold crystallization) during the heating cycle or (2) You can integrate the exothermic cold crystallization peak from the heating curve, and subtract this value from your melting peak.
TLDR: The heating and cooling rate used during DSC can and will affect your degree of crystallinity. With proper experimental design, you can correct for this.
Thanks for your detailed answers. I have another question. Is this correct to declare that for a given cooling rate, different heating rates have minor effects on the degree of crystallinity?
In general, yes. But this depends highly on the structure of your polymer or blend. It's fair to say that the cooling and annealing history will have a much greater impact on the degree of crystallinity than your heating rate will.
Thanks a lot for dear Abdelkader BOUAZIZ , Adeyinka Aina , and Andrew Anstey answers.
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