This is a fascinating subject and doesn't just apply to PLA, although it is particularly apparent in this polymer.  As the other contributors say, the quenched-in amorphous structure from cooling becomes sufficiently mobile above the glass transition for crystallisation to occur.  These crystals melt upon further heating.  My interest a few years ago, which I was unable to resolve, was whether the difference in latent heat between these peaks in the DSC could provide evidence of the crystallinity of the as-cooled material.  It may not, because the crystallisation process may be somehow different, or there may be reorganisation of the crystals on further heating, but some people use this to determine crystallinity without, so it seems, any good evidence for why it works.  I think that this would be a good project for further work, but am not in a position to do anything about it at present.

is a process where the polymer undergoes some small amount of crystallization while heating in the DSC.

see this:

http://www.perkinelmer.com/Content/applicationnotes/app_thermalcrystallinitythermoplastics.pdf

The exothermic peak occurs as a result of the polymer undergoing crystallization after having attained glass transition earlier. Later on, this crystalline state upon more heating, will eventually melt (190C). The areas of the peaks should give you the latent heats of crystallization and melting respectively. 

refer to http://pslc.ws/macrog/dsc.htm

I agree with the above answers, and I'd like to emphasize that the process of cold crystallization is so named, since the chains are partially aligned (arranged) not starting from the melt.

In fact, below Tg the chains are relatively rigid and can not move. Above Tg, with the heating energy, they possess the potentiality to be partly ordered and the cold crystallization takes place.

This is a fascinating subject and doesn't just apply to PLA, although it is particularly apparent in this polymer.  As the other contributors say, the quenched-in amorphous structure from cooling becomes sufficiently mobile above the glass transition for crystallisation to occur.  These crystals melt upon further heating.  My interest a few years ago, which I was unable to resolve, was whether the difference in latent heat between these peaks in the DSC could provide evidence of the crystallinity of the as-cooled material.  It may not, because the crystallisation process may be somehow different, or there may be reorganisation of the crystals on further heating, but some people use this to determine crystallinity without, so it seems, any good evidence for why it works.  I think that this would be a good project for further work, but am not in a position to do anything about it at present.

Dear Imran,

The cold crystallyzation occurs because your sample is mainly composed of amorphous (but crystallizable) material. Therefore, the exothermic cold crystallization achieved during the subsequent DSC heating scan indicates a chain reorganization (crystallization) during this heating because the initially amorphous structure is crystallizing. Then, at 190ºC, you would see the melting corresponding to both the initial crystalline fraction and these newly developed crystalline regions. To claculate your initial crystallinity degree (Xc), you should subtract the enthalpy of cold crystallization to the enthalpy of fusion and then divide it by 106 J/g.

Following paper would help you: Journal of Mechanical Behavior of Biomedical Materials 17 (2013) 242–251.

Best regards,

Erlantz.

Already perfect answers are given I agree with most. As Yomen Atassi mentioned above glass transition the chain segments getting some mobility try to reorganize. The orientation is reflected in DSC and is called cold crystallization. In silicones cold crystallization is also observed.

Well answered already above. One interesting addition. Even a treatment below Tg can affect cold crystallization later on. See examples in US Patent US 2012/0059132.

Hi every one....nice disccussion from all of you gauys....Edwin A. Murillo, Andreas Christel,Yomen Atassi AND OTHERS AS WELL,,, Any body can suggests me book on this topic particularly emphasizing and comprehensively disscussing cold crystallization. I try to find through google search  but got very small material  which is off course not enough to have a clear picture. I need to understand this process comprehensively as i need to understand  further  the impact or consequence of filler particles network behavior. Any help will be highly appreciated.

I think that you will find that such book does not exist.  It is clear from the discussion that while the general principles are known, there is still much to do to understand this process fully.  Good luck, and remember that the filler is unlikely to give any peaks in the DSC traces, although it will contribute to the mass of the material and may influence the crystallisation behaviour of the polymer.

Hi i would like to ask if you do see cold crystallization always for PLA in DSC?

I think so, but as there are several forms of PLA (e.g. PLLA, PDLLA etc) there may be some which crystallise more or less readily and may not show this.  I cannot remember.  Try it and see.  You can also see this phenomenon with other polymers, including PET.

Hi Robin, thank you so much. Yes you are right - i did not observe in my PLLA DSC curves any cold crystallization.

This phenomenon is observed in silicone polymers also. After glass transition there is orientation in some polymers, which results in crystallization.

Some good information on this extraordinary behaviour. What puzzles me is, for the PLA I've tested  I can see a Tg  followed by  cold crystallization while heating at a modest rate of 5C/min , usually occuring at 60C  and then the endotherm at ~ 160C but, do not see re-crystallisation during cooling from 190C down to ambient at 5C/min. Strange that thermal energy supplied during heating provides mobility for crystallisation but,  molecules do not organize and crystallize, with thermal mobility, during the cooling cycle.  Does the polymer need to transition below Tg in order to drive subsequent cold crystallisation on heating?  I have conducted the DSC on a heat, cool, heat , cool cycle and do not find recrystallisation during the cooling cycle. Associated with Gibbs energy, enthalpy + entropy balance  perhaps?

Hi Mike.  Good to hear from you again.  Yes, this is indeed intriguing behaviour.  Why should the polymer not crystallise at all on cooling, and only when it is reheated?  I don't know.  Maybe nucleation comes into it and this is where dropping below the glass transition comes in.  Have you tried cooling to below the expected crystallisation temperature but above the Tg and holding isothermally for quite some time?  This could give an answer as to whether it is essential to go below Tg.

Let me explain it in a general way, as per my experience. Supposing if a liquid is prone to crystallization, and one cools it fast enough to avoid crystallization and gets into a supercooled liquid state by surpassing crystallization zone (still some fraction may crystallize depending on the material tendency to crystallize and the applied cooling rate). Now, while subsequent controlled heating, the supercooled liquid will eventually crystallize when it again reaches the crystallization zone. However, some supercooled liquids may crystallize in single or many steps well before reaching the crystallization zone during heating, and this behavior, I suppose is defined as cold crystallization in literature. And please don't confuse with the metastable transitions and other behaviors that are inherent to the particular sample with the cold crystallization.

Cold crystallization occur above Tg when the polymer chain gain sufficient mobility to arrange themselves into ordered structure (i.e. crystalline structure) by chain folding. If the crystallization does not occur during the cooling cycle this may be due to not forming stable nuclei. The crystallization from melt require forming stable nuclei and growing of these nuclei.

Hi,

how cold crystallization is affecting mechanical properties of the polymer? is directly depending on the temperature you intend to use the polymer? thank you

Monica

Cold crystallization in genereal caused bèy the ordering of molecular chains in the crystalline PLA due to increase mobility during heating .

Hi all, please, its is also possible, that cold crystallization occurs AFTER melting point?

E.g. by PE at about 220 °C (melting at 112 °C) and by PP at about 235 °C (melting at 150 °C) .