As the question title suggests, I am wondering about the peculiarities of DTA curve of glasses (and particularly inorganic glasses) that require fast quenching rates, e.g. TeO2 glass.
More precisely, is it common for these particular glasses that glass transition temperature is less obvious, i.e. relatively concealed compared to other glass compositions?
yes, you may not see the glass transition event if the heating rate is faster in DTA/DSc for a very fast cooled glass.
If you heat the glass near Tg for some time, structural relaxation may occur, then you may probably see the glass transition event. Or if you use a very slow heating rate while doing DSC/DTA, you may probably see the Tg. When you are using slow heating rate, you are actually giving much time to your glass for structural relaxation . This does not happen when you use a faster heating rate. that is the reason one may miss the glass transtion event.
This may be the case of semi-crystalline polymers when the Tc is only observed on cooling run.
TeO2 alone can't form glass. In combination of other glass modifiers, Tg (Glass transition Temp.) of TeO2 glass may vary depending on glass composition. You can see one of our recent papers on Al2O3 inclusion effects on Thermal properties for more details. Yes compared to other traditional glass formers Tg of this glass is less. But usually lies in the range ~ 300-450 oC, again it is composition dependent.
yes, you may not see the glass transition event if the heating rate is faster in DTA/DSc for a very fast cooled glass.
If you heat the glass near Tg for some time, structural relaxation may occur, then you may probably see the glass transition event. Or if you use a very slow heating rate while doing DSC/DTA, you may probably see the Tg. When you are using slow heating rate, you are actually giving much time to your glass for structural relaxation . This does not happen when you use a faster heating rate. that is the reason one may miss the glass transtion event.
I'm afraid that the glass transition is being confused with crystalline melting points. Quenching will greatly reduce any crystallization tendencies of any polymer, leaving only the Tg to be detected. If you're not seeing a Tg, it may be well outside the range in which you're looking. Commonly, one differentiates between Tg and Tc by the sharpness of the transition (latter is sharp; former is broad).
The measured Tg is usually dependent on the heating rate, which is why most people settle on an intermediate heating rate to give a balance of quick answers with minimum distortion in the measured Tg. With organic polymers, we typically use heating rates of 15 deg C per minute, which commonly elevates the Tg 15-30 deg C, depending on other factors. The reason for the elevation is actually an artifact of rapid heating rate -- one "overshoots" the actual transition by some amount before the process of translational motion registers.
Do not overlook the effects of degradation ad moisture on your measurements, and on the possibility that your additives are functioning as fillers (insoluble) rather than polymer modifiers (insoluble).
Annealing processes implied in other answers are not normally a problem below the Tg of a polymer, because Tg is the onset of translational motion, which is required for any reorganization such as crystallization.
In general Tg is the first deviation from the linearity on the DSC/DTA curve. Actually, in most of the cases, you first have a hump (with different size, according to the quantity) caused by moisture, whose maximum is at around 100 degrees on the DTA, and then the Tg appears as small peak or linearity bend. There is a problem when the Tg coincides to the moisture hump, then you need to keep your probe and the chamber dry, but I think that this problem is not present for oxide glasses, since they are stable at higher temperatures than the polymers and the chalcogenide ones.
As for the shape of the DTA curve of unstable glasses, which require quenching to form - in general it has small deviation, which is the Tg; one or more exothermal peaks, corresponding to Tcr and one or more endothermal effects ot Tm. The stable glasses have Tg and wide, but small by intensity, exothermal and endothermal humps.
The question is surprising. But the answers may be still more surprising! The reason for this surprise is that this question makes a classical point of glass science, I have been teaching this point for nearly 20 years at Rennes University. My advice would be to read the various papers published by C. T. Moynihan in the late 70's or 80's. It is also important to outline that the DSC scan emphasizes the change in Cp at glass transition. Consequently, glass transition appearsas a drop in base line. This change is proportonnal to the difference between the Cp of the solid phase and the CP of the liquid. It is not an endothermic effect. It is inappropriate to speak about a "peak of glass transition". When an endothermic peak is observed, it results from delay in relaxation due tu the difference between the cooling rate and the heating rate.
The shape of a DTA (or DSC) curve depends on the relative cooling rate (at glass formation) and heating rate (on the DTA measurement). Ideally, DTA heating rate should the same as the cooling rate used for the solidifiaction of the glass melt. For quenched or hyperquenched glasses, this is not possible. Therefore the shape of the DTA curve undergoes some modification, because the time allowed for structural relaxation is different on cooling and on reheating.
In the case of hyperquenched glasses, structural relaxation occurs below the glass transition temperature that corresponds to the effective cooling rate applied. This effect leads to the deviation of the base line. As a result glass transition is less clealy seen, especially if the Cp difference between liquid ad solid state is small.
In summary, DSC scan must be understood taking into account
1) The cooling rate used for glass synthesis and the resulting fictive temperature Tf.
2) The heating rate of the DTA:DSC set-up
3) The annealing rate
4) The Cp difference between galss forming liquid ans solid glass
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials Chemistry