[DTU student #12]

I have been searching for quite some time about the glass transition phenomenon, but there is a question which I have found several different answers to, and hope to clarify for my understanding.
Which materials will show a glass transition? I have found answers that says that all amorphous materials show glass transitions, and others that says only glasses show glass transitions. Which is correct?

I think all amorphous materials should show glass transitions, since I tried synthesizing a MOF crystal, which had a glass transition during the first DSC upscan (I assume it had an amorphous region).

I also cant seem to find an answer to what really is the difference between a glassy structure and an amorphous one? I know the glass is also amorphous so what is the difference? Perhaps the degree of short range order is larger for glasses? However, amorphous solids should not have purely random structures either, so they must also posses some form of local ordering.

Any help is greatly appreciated

[Corribus]

I would call any amorphous solid a glass when the material is in a glassy state. But "glass" also has a common meaning to refer to amorphous silicates, so its use can be confusing and is therefore better off avoided for non-silicate amorphous solids. Note that amorphous solids are not appropriately called "glasses" if they are in a rubbery state - so amorphous solid is the more general term. Also note that "glass" only refers to the noncrystalline portion of the solid, so semicrystalline materials can have glassy regions but not be appropriately called "glasses" when referring to the bulk material.

[DTU student #12]

Thank you for your reply

I am asking because I synthesized a metal organic framework, and analyzed it with DSC. It had the characteristics of a glass transition during the first upscan, but not the second, which I found confusing.

Would it be possible that the "magnitude" of a glass transition is so small that it becomes difficult to observe? The DSC data is not the best quality since only small quantities could be analyzed (it expanded quite a lot, lifting the lid of the crucibles).

Would I be right in saying that all amorphous materials should have a glass transition? And it should only be referred to as glass, if the bulk material is vitrified? As such, the glass transition observed only shows that the crystal had amorphous regions.

I must admit I still find the two terms, amorphous solid and glass quite confusing, since they seem to be used interchangeably

Best regards

[Corribus]

It is common for first and second heats in heat-cool-heat mode to appear differently because the first heat-cool cycle erases the thermal history of the material. The glass transition can be very difficult to observe in DSC; you might try DMA instead if it is compatible with your sample. If the glass transition is showing up on the first heat but not the second, you might try adjusting the cooling rate, which can affect crystallinity; remember DSC measure the average properties of the sample, so any crystalline regions will contribute to the data. Maybe upon cooling, your sample becomes more crystalline, and therefore doesn't manifest a strong glass transition. In either case, the first heat is more representative of the physical state of your system as synthesized, so if that's what you want information on, the second heat doesn't matter anyway.