Glas transition temperature concerns glasses, hence the name. It concerns only amorphous materials. However, as soon as amorphous phase exists in a material, thereby called semicrystalline material, the glass transition can be observed. This is the case of most polymers, showing both melting peak (and nice XRD patterns) and Tg.

Alain

Glas transition temperature concerns glasses, hence the name. It concerns only amorphous materials. However, as soon as amorphous phase exists in a material, thereby called semicrystalline material, the glass transition can be observed. This is the case of most polymers, showing both melting peak (and nice XRD patterns) and Tg.

Alain

As mentioned by Alain, semicrystalline materials frequently occur in polymers. Low molecular glasses are usually amorphous. In inorganic glasses partial crystallization may also occur. There are also so-called condis crystals where at a certain temperature the long range order remains but the conformation disorder starts.

Dear Singh,

Your XRD is showing persistence of semi-amorphous (crystalline as well amorphous) phases. The sharp peaks tell about the well crystalline regions but the long range broad peak tells about the presence of amorphous phase. Therefore the base shift in the DSC graph is obvious.

I agree with both Prof. Celzard and Prof. Banhegyi.

Thank you very much Prof. Alain, Prof. Gyorgy Banhegyi and OMKAR SINGH KUSHWAHA

You may consider use of the internal standard method to quantify crystalline and amorphous phases; cf.; «Could anyone explain how the amorphous content of a material is analysed using the internal standard method by XRD?»: https://www.researchgate.net/post/Could_anyone_explain_how_the_amorphous_content_of_a_material_is_analyzed_using_the_internal_standard_method_by_XRD

Thanks Dr. Carlos

I am working on glassy semiconductors where I have come to this situation when the prepared sample shows peaks in  X- ray diffraction but show glass transition and crystallization peaks in DSC. This is the situation of partial crystallization. Sometimes, I deliberately create partial crystallization, by anealing for different times, in semiconducting glass near crystallization temperature to see the effect of partial crystallization on electrical and photo electrical properties.Hence, the situation of partial crystallization is not only in polymers but also may be in glasses also.

Thank you Dr. Kumar

There is no glass transition in pure crystalline materials. In your case the material consist of the mixture of crystalline and amorphous parts, and this is the reason why you observed sharp XRD  peaks we well as glass transition.

thank you Dr. Zhang and Dr. Wang

sometimes fully crystalline material, getting influenced by crystallization parameters, is not crystallized completely showing Tg. making it semicrystalline

Thank you Dr. Samui

Polymers frequently show both crystal melting and prior to that a Tg.  Annealing experiments will show how close the original sample is to equlibriium.  Drawing (pulling) will cause a secondary orientation in the amorphous phase which will also show up in DSC and DMA.  PET is a prime example of this as are some polyolefins.  

The DCS curve  has no reliable base line. The ripple on it cannot be attributed to Tg. Besides a Tg at 250  degrees C is too high for any amorphous polymer .. One has to have a better idea on the polymer  morphology before deciding on its Tg. One dimensional order can yield high Tg but the highest I saw till now is 125 degrees C.

I suggest that you obtain a better DSC trace to detect the Tg. Also hot stage microscopic observation will give you a better idea on Tg as the material will first become soft at TG temperature then become hard as time passes because  the segmental mobility will induce a better order if the material is crystalisable.

thanks for your suggestion Dr. Bedii but my derivative is not a polymer, it is a simple organic molecule with molecular weight of 1395. Similar organic molecules are reported in literature and such type of molecules usually possess very high Tg.

Thanks again

you can identify crystalline and amorphous phases by DRX with MAUD program. it's a free download program with tutorials and examples.

Thanks Hamouda Assia

Answer is yes, for eg. In normal systems such as water, depending on rate of cooling it can be even converted to glass, without getting into a crystalline state.

Your case is frequent not only in covalent polymers but also in coordination polymers. This is the case of lead(II) alkanoates. We have found three types of glasses in this series, regular glass, rotator glass and liquid crystal glass. It is interesting to know the nature of your Tg. It could be even different: condis glass or plastic crystal glass.

Respected Prof. Cheda

I did not expect this material to be crystalline at all as i am trying to synthesize amorphous materials and i designed the molecule so that it should be amorphous. The molecule is dendrimer type and has polyphenylene groups which are well known to produce highly amorphous materials with high Tg.

The other day I had not opened your pdf file with two plots. I suppose that the XRD pattern (probably powdwer-XRD) was registered at room temperature. It is clear that you have a crystalline order there. But the second plot is very confusing. Ordinate function is left and abscises you should write t/ºC. Is it a DSC thermogram? Which direction is ENDO, up or down? The effects seen there depends on the endo arrow. The one marked with a circle to be a glass transition, endo should be downwards. And the next effect would be then an exo-endo effect.

Moreover, if the glass transition exists, and you see Braag´s refflections, then you have a glass with partial order (one or two D orders if it is a nematic glass or a smectic one, and so). On the other hand, It would be much easier to If you are working with a covalent polymer, you may have at room temperature a low crystallinity polymer and you should calculate the % of crystallinity.

Respected Prof. Cheda

You are absolutely right, the powder XRD was recorded at room temperature and the second plot is DSC thermogram with ENDO down and encircled portion is the glass transition. I was quite confused when i got crystallanity and glass transition in same molecule. The electron diffraction pattern of nanoparticles of same material has also exhibited clear reflections (nanoparticles were prepared in THF/water mixture (10:90)).

At present i m working on solid state fluorescence behavior of different carbazole derivatives.

Regards

See the XRD and DSC results seems to indicate that the material is semi crystalline in nature. 

Hi Mr.Singh, your TEM pattern shows the coexistence of both diffraction spots and very diffuse rings around the 000 reflection. This suggests to me that your material has coexistence of both glassy and crystalline phases.

I agree.  Semi crystalline polymers frequently show this sort of mixed phase.  Rapid quenching of normally crystalline organic and inorganic materials will also do the same.  Have you tried annealing to determine if additional crystallinity  develops?  A DTA or DSC of the material will show a lot more detail.

Thank u Mr Sankalp and Dr. Edward Barrall. DSC has already been attached. My material is an organic polyaromatic compound with molecular weight around 1400. No annealing experiment has been tried.

Regards

I agree with the previous comments on the semicrystalline character of materials. However, if you want to have a completely amorphous material you can quench it in liquid nitrogen from the melt. In this way, you will be able to clearly determine the Tg without any influence of the crystallinity. In reference to the structure with phenylene rings, take care, since sometimes you can detect pi-pi stacking, which forms nanostructures.

Thank u Prof. Angels Serra

Much of the comments made so far are in correct direction. If your sample is a polymer it is quite normal for crystallinity and amorphous phase co existence. However the small zigzag on  presumably the DSC trace you provided cannot be atributed to a Tg safely  mainly because the base line is poor. Such base line improperness is related to dirty pan or incorrectly filled pan (ie not adequate contact for heat conduction between sample and pan). Correction should give a proper base line which should run parallel to the x axis. If you need to clean the cell, heat it up to 500 oC and hold for 30 mitues before allowing to cool.

Thanks for your suggestion Dr. Bedii but rising dsc line can also be due to different heat capacity of sample and reference also. Is it correct?

Try empty pan as reference. It may cure the problem.

Thanks for your suggestion Dr. Bedii

I have been Reading several answers to your questions and I would like to pont out some things. One type of systems are regular polymers. I mean "covalent" polymers, or "macromolecules". Full crystallization in such a systems is  not frequent, even I would say almost imposible. These polymers are charactherized by a certains % of crystallinity. The amorphous part can be in solid state, that is, in a glass state. On heating the simple, the glass transition has to appear, increasing the heat capacity,  due to the glass have recovered tralational (usaully) energy, and a metastable liquid phase appears. Following heating, it may happen or not a crystallization process (conversión of the metastableliquidi into the cristal phase. On following hetaing, the crystalline part of the polymer melts.  The Tg transition appars obviouly allways bellow the melting point of the crystalline part of the sample. And the exothermic peak of crystallization too. glass state (without any order)

But other systems may present this phenomemum too in a different way. Other glass states, different to the amorphous glass state (without any order) may exist. E.g., plastic crystal glass or liquid crystal glass. 

I would like to support the broad explanation offered above on macromolecular TG issue. I have witnessed ocurrence of more than oneTg of the same polymer depending on the molecular morphology. The amorphous state gives the lovest Tg but, a Tg due to one dimensionally ordered segments very clearly exists at a much higher temperature.

Thanks for your kind attention Prof. Cheda

thanks Dr. Ts. Vassilev

Only in a mixed phases, crystalline and glass, peaks in X-ray diffraction and glass transition in DSC is observed. If you have only one phase, crystalline, then glass transition in DSC will not be observed. Similarly if you have only glassy phase then peaks in X- ray diffraction will not be observed. If X- ray diffraction peaks and glass transition both are observed then you do not have single phase system. You may call it partially glassy or partially crystalline.

I do not fully agree with  Dr A. Kumar comment,  for me it is  unnecessary simplification.  What about widening of X- ray diffraction peaks? 

Peak base broadening is mostly instrumental. Sometimes with random copolymers paek base broadening can also be observed due to species formed by different but akin sequences which is very often the case with oriented nematic polmers.   However I agree with Ts Vassilev that Dr Kumar's approach is over simplification as XRD may indicate existence of Tg as a halo but there are other reasons of halo appearing on XRD. 

I do not know whether thermal history is removed. If not do it by repeating the process. If syntzed may be not fully crystallized. The power fluctuation may also give such sudden shift of baseline.

it only exists in amophous compounds...

Crystalls melt only. They cannot exibit a Tg. Butif the material called crystalline is acytually a semi crystalline organic polymer than the amorphous component will exibit a Tg. I suspect the case being discussed is exactly this.  

All the answers I read here are referred to polymers, and I think all are wise answers. But, I have a question, is a polymer your organic sample? If not, there are other possibilities to explain your Tg simultaneously with a fusion from a solid (or other kind) phase. There are several types of glass states:

- amorphous (quenched from the isotropic liquid, without any order).This one is a very well known glass.It may pass to theisotripc liquid directly of crystalize and then melt.

- plastic crystal glass, a positional ordered solid, but frozen the orientational disorder of the plastic crystal (this material shows on heating a Tg followed by a fusion of the plastic crystal.

- liquid crystal glass, which is a phase with traslation and/or rotation freedoms frozen, This glass presents a Tg to liquid crystal (with orientational order) and later the clearing point. This liquid crystal glass present Bragg reflexions typical of LC mesophases

- Etc,,because there are two more glass states, condis glass and rotator glass.

Your Tg in your organic sample could not be of an amorphous glass, obviously. But it could be any of the other four glass transitions.

What I could say is that for this would be possible, your organic sample should behave as  mesogenic material. Its molecules sould have a geometric factor (rod-like, lath-like, disc-like), letting the molecules be oriented in the space.