Solid or Melt?
What is the definition of glass?
This was one of the exam questions on my final exam in the third year of university.
What is the definition of glass?
I knew that it was a tricky question and I had to write down something different than just a piece of chemical formula. The answer was probably hidden in the thermodynamic properties of the glass. I started to remember my lecture notes in the exam. Doing this was always a piece of cake for me since my visual memory was quite good. After visualizing the related pages in my mind, I quickly made a plan for how to answer the question. I carefully plotted the following graph and explained the thermodynamic mechanism of each step.
Once an ordinary liquid or melt cooling down, there is a decrease observed in its volume because crystallization takes place at the melting point of the liquid. When the temperature is kept decreasing, a further decrease is observed in the volume since the expansion coefficient of the crystal is lower than the liquid.
However, in case of no crystallization at Tm ( melting temperature), the volume keeps decreasing along the area which is called supercooled melt or liquid. As a result of this condition, a metastable thermodynamic equilibrium region exists.
As it can be seen from the figure, the curve which represents the metastable equilibrium follows a similar parallel behavior with the crystal curve. The reason behind following a parallel path by showing deviation is related to an increase in viscosity of the liquid during cooling. Reaching the equilibrium of the liquid takes place very slowly in meanwhile the viscosity becomes so high that continuous cooling cannot be enough to reach the equilibrium anymore. The best way to describe this continuous temperature change which is the indicator of viscosity, it to call as glass transition temperature, Tg.
Therefore, it can be concluded while below Tg value glass is considered as a solid material, above Tg it is a melt.
