Generally the melting point increases with an increase in crystallinity.

In semicrystalline polymers there at least three different, more or less independently variable quantities which describe the polymer. One is the degree of crystallinity which can be calculated from the crystalline/amorphous areas of the X-ray diffractogram or form the integral melting heat determined by DCS. The second is the long period or folding length (and its distribution) which can be determined from the shape of the DSC curve or from SAXS measurements. The third is spherulite size (and its distribution) which can be determined from polarization optical microscopy or light scattering. Of these the second is related to the observed minimum of the DSC curves which is sometimes called the melting point. In fact we have a distribution of metling points: the lonher the folding length, the higher the corresponding melting point (the closer to the ideal isngle crystal melting point). It is true that longer folding length usually involves higher crystallinity, but not necessarily. Thus it is better to speak of a relation between folidng length and melting point than between crystallinity and melting point - at least in my opinion.

For high crystallinity polymeric material, its  need more energy to vibrate the polymer chain and break the polymer chain compared to low crystallinity material. 

Certain polymers (PMMA, PC) can not really undergo crystalisation because of their inherent structure and grouped commonly as amorphous polymers. However the chemical structure of other polymers like PE, PP support crystalisation and in such cases crstalisation depends on experimental conditions, like cooling rate, time (in case of isothermal experiments). These polymers however cannot undergo complete crysztallisation to achieve 100% crystallinity, when cooled from respective melt. Hence some part is amorphous, which starts flowing at Tg (glass transition) and crystaline part melts at Tm (melting tempearature). Now coming to answer of your exact question: The crystalline region of solid polymer consist orderly arranged polymer chains. Now depending on our cooling rate, or time (for Isothermal experiments) , we not only get different % crystallinity but also different degree of long range order- commonly described by thickness of spherulite structure. Usually lower the cooling rate or longer the cooling time at any specific temperature (Isothermal Expt.), polymer chains get more time to arrange in crystaline order and to form bigger crystalline structure, which results in different thickness of spherulites. Now it is easy to imagine that, higher the thickness of spherulite higher temperature it needs to melt. Hence in DSC heating curve, there is no sharp melting but broad region of melting, which means at different melting temperatues different size of spherulites melt to show fluid characteristics. Hence it is more correct to say that size of polymer arrangement (i.e. for example, crystallite size) affects melting point rather than actual % crystallinity .

then we can say that no crystallinity no melting point,  just there is physical and thermal alteration in the structure ??? Also there is no point applying DSC on samples????

we have to take 13C-NMR and FT-IR (CH asymmetric and simetric vibrations)

Right??

Dear Bayram on amorphous samples with no crystallinity you can observe only Tg (glass transition) on DSC curves (a step in the cp curve), perhaps enthalpy relaxation after annealing. Relaxation spectroscopy (dielectric, mechanical and broadband NMR) you can observe further sub-Tg relaxation modes not detected by DSC. FTIR  provides information on local (molecular) vibrations. C13-NMR provides information of chemical composition.

Dear Bayram,

any polymer consists of molecular chains which are more or less disordered. If you have effects between the chains which decrease this disorder, the free movements of the chains are hindered and the melting points increases.

This ordering effects can be influenced by van der Waals Forces as well by voluminous side groups which hinders free movement. In the sum all these effects, depending on the polymer type, can cause amorphous and crystalline areas in the polymer itself.

The more crystallinity and the more ordering you have the higher the melting point will be. Additional increase of the melting point you will get if you are able to realize  within this structure a kind of chemical crosslinking.

From theoretical viewpoint the polymer chemistry has endless possibilities to realize polymers with high melting points concerning these principle rules. Up to now not everything which might be possible is not yet realized.

With best regards

Michael

Hi Gyorgy

I have a follow-up question about how the crystal size can affect the melting temperature of a polymer. I have a DSC curve of a XLPE sample, it is a sample piece from nuclear power plant used cable insulation. It shows three different endothermic peaks with their peak maximum at about 100 oC, 110 oC and 120 oC., respectively. I am having difficulty in defining what do such three peaks represent. With 10 oC difference between the 110 oC and 120 oC peaks, is it possible that the XLPE has some very small crystals that melt at 110 oC, and then some large crystals that melt at 120 oC? Thank you. My email is [email protected]. I would very much appreciate it if I could send you my DSC curves for some discussion. 

Is any relationship between the long period and crystallinity of polymers  ?

 Hi Shuaishuai

The observed peaks may be due to fractionated crystallization. The material must have undergone long term heating from the nuclear plant. This heating may have induced annealing dir the heating period, leading to the formation of crystals of different sizes and shapes. Normally, PE exhibit a melting temperature range of 110 - 130 oC, depending on  the nature, composition and crystallinity of the material. Therefore, observing different peaks in this range of temperature for the same material may be linked to the formation of fractionated crystallization. I will send you a similar result to your email.

Thanks

Stephen, please send to my email address: [email protected]

Thank you.

I have a question as well. I have a couple of DSC measurements of the first melt peak of homo-PP that have been injection moulded to plates and then stored. The same day as injection moulding one DSC was performed, one day after, 3 days and one week. What is of interest is that I expected an after-crystallisation that normally take place in PP, but in DSC I observed the opposite. The integral of the melting point/curve decreased instead of increasing over time. Why might this be? Parallel with this measurements tensile tests have been performed and they show an increased strength over time. (and less elongation before break)

how the degree of crystallization affect on strength?

any polymer consists of molecular chains which are more or less disordered. If you have effects between the chains which decrease this disorder, the free movements of the chains are hindered and the melting points increases.

Hi Mohammed Arif P

Personally, yes, they do correlate. As I know, the long period is associated with the thickness. If the polymer has a long period, it means that the polymer has high crystallinity. However, the %crystallinity also depends on the crystallization rate.

Responding to this discussion, and particularly Gyorgy's comments - with a question, I am afraid:

if we start from thermodynamic point of view, DSC has to be the base reference: given sufficient sensitivity, heat flow analysis should provide all intricate detail of transformations and transitions; but in practice sensitivity has evident limits due to weak effects, small volume fraction of relevant phases, etc.

So - to what extent can DSC be combined with other methods?

SAXS is an obvious possibility, but what about FTIR, Raman etc.

Are there commercial solutions offering combined kit?