Dear colleague Abdalwali Japir,

Have a nice time and good day.

Peace be up on you.

The values of Tg and Tm are functions of heating rate so you can use different heating rates from low  to high values.

Also the value of Tg may occurs through a small range   of temperature so be careful.

I wish this is useful and help you.

Yours

A. El-Denglawey

Dear colleague Abdalwali Japir,

Have a nice time and good day.

Peace be up on you.

The values of Tg and Tm are functions of heating rate so you can use different heating rates from low  to high values.

Also the value of Tg may occurs through a small range   of temperature so be careful.

I wish this is useful and help you.

Yours

A. El-Denglawey

Hello, if your sample did not show Tg, it means that this sample is amorphous, in the case of the sample that exhibited a Tm and Tg this sample is semicrystalline because it has ordered (Tm) and disordered (Tg) arranges.  In this type of sample, sometimes the crystallinity is due mainly to hydrogen bond.

Thanks a lot.

Reasons why you did not track Tg nor Tm may be as follows:

1)Impurities coming from the synthesis and plasticizing the polyesters. Tg is shifted to much lower temperatures. That may also be why both material are not crystalline.

2) Tg is very unclear due to the broad molecular weight distribution of the polyesters.

Hello, DSC is not a reliable instrument for Tg and you can use DMTA or NMR for this purpose. if with this instrument your polymer did not exhibit Tg, then yor sample is amorphous.

1. In fact, some polymers having a high degree of crystallinity, does not have a Tg - they melt without using the highly elastic state (see pictures).

2. It is possible in the case of your first polymer Tg is at negative temperatures?

If lowering the heating rate doesn't improve the heat flow curve, then following are the issues you might need to resolve. 1) Tg shifted to lower temperature. 2) Thermocouple has lower sensitivity (smaller peaks merged or don't appear). To handle such issues, make sure the lower range of thermocouple (S/B-type) has a sub-ambient temperature (e.g: -50 deg. C) and has higher sensitivity. The data acquisition periods are frequent (e.g: 0.1 second per data point acquisition). The heat flow calibration and error correction are performed using standard certified reference materials with well-defined transitions and heat of fusion (e.g: materials with smectic - nematic liquid-crystal transition).       

Tg = temperature range where movement of chains of polymers occurs. in thermoplastic materials Tm is much more pronounced than Tg so you cant see it in thermosetic (crosslinked) polymers you can always see the Tg since it does not melt.

sometimes, sample preparation and size (film or powder) of the sample matters. Less size and a large amount of sample, in this case, it may helpful.

There are no reasons for polymer to have no glass transition unless it is fully crystalline (as Alex has already noted). However, Tg sometimes can be difficult to detect. DMTA is much more sensitive to Tg than DSC (as Mojtaba has already correctly mentioned). In order to make glass transition more prominent in DSC experiments, one can pretreat the sample by quenching it from the melt state, thus trying to freeze its molten (fully amorphous) structure. For the quenching, one should heat the sample ~30-50C above its melting temperature, to keep it isothermally at this temperature for a few minutes in order to fully melt all crystallites and erase the thermal history, and than to cool it below its expected Tg at high cooling rate. The cooling rate should be, at least, higher than 20C/min, preferably 50-100C/min, which can be tricky for cooling from +50C down to ~0C (I assume, for this you should keep the service temperature at  -50C). After the quenching, the final heating scan should be done with high heating rate (20-50C/min).

In some cases heavily crosslinked structures may also produce almost no glass transition. I remember to have seen epoxy-silane based hybrid resins in combination with normal epoxy, crossliked with amine at varying compositions. At very high epoxy-silane content the DMA Tg almost disappeared.

Gyorgy, since glass transition in polymers is associated with cooperative segmental mobility, disruption of the segmental mobility by very dense cross-linking can principally cause the transition to disappear. Alternatively, if X-link density distribution becomes very nonuniform, the transition could becomes hardly detectable even by DMTA. Haven't you examined this X-linked material via dilatometry? Didn't it show the slope change?

A Tg taken, for instance, as the step of the specific heat is ill-defined. The true definition of Tg is often neglected. This is the temperature marking the crossover from the metastable equilibrium liquid and the nonequilibrium glass. Once in such state, glasses evolve toward equilibrium. Hence, to seek for the Tg, I recommend performing annealing experiments at different temperatures. If the thermodynamics of the system evolves with time, the temperature is below Tg, else it is above

@ Dr Schsuter: it was not my own research, I read it in a conference proceeding many years ago so I cannot recover in my memory all details. Probably  you are right that  there was some minor transition, but the decrease of the (mechanical) relaxation strength was conspicuous. According to molecular theories about 50 C long segments are needed in polymers for a well defined Tg. This is obviously not true for molecular glasses, but there the mobility is not limited by chemical bonds above Tg. thank you for your note

Hy Dear colleague Abdalwali Japir!

I apologize for the error in Figure 2 - this is the corrected version.

Dear Abdul If you look to the values of Tm in both cases you will find a big change. So there are two basic reasons as others stated above. One is crystalinity if 2nd has high degree of crystalinity  then Tg will not appear and second reason is sensitivity may be if you will slow the heating rate and change the scale of heat flow may you will be able to detect Tg. One other method is during experiment take samples of reaction mixtures and quench them then run DSC scan on those samples you will find on what time or at what extent of polymerization Tg disappears and you will be able to find the answer.

Regards

thanks a lot 

Dear Mr Japir a good usefull tip that can make more pronunced the Tg (dCp) is to use a bigger heating rate (10 to 30 °C/min) in the second or third heating curve in dsc.

Dear Mr Japir

You have to use DMA test method to better evaluation of Tg.