As you know, polymers behave very differently from metals. One aspect of that behavior involves the glass transition temperature. In this blog post, we are going to focus on the glass transition temperature of thermoplastics.
Before we talk about the glass transition temperature, Tg, lets take a moment to talk about the glass transition itself. It is a reversible transition that occurs when an amorphous material is either heated or cooled in a particular temperature range. As the material cools, it loses its flexibility and becomes brittle (hence the term glass); as it is heated, it becomes softer. This is what makes the glass transition important to thermoplastic polymers: above the glass transition region, a thermoplastic can be molded without breaking. This is one of the key properties that make thermoplastics so useful.
Glass Transition and Melting
Keep in mind that the glass transition is not the same as melting it is a totally different physical process. Glass transition does not involve a phase change like melting does, and it only occurs in amorphous polymers or the amorphous portion of a polymer. Only crystalline polymers can undergo a melting phase change.
Glass Transition Temperature
The Tg, referred to as the glass transition temperature, is what characterizes the glass transition. One of the definitions of Tg that is specific to polymers is as follows: it is the temperature at which molecular chains are able to slide past each other when a force is applied, hence the ability to mold and deform polymers without breaking them.
When a thermoplastic polymer reaches the Tg, either from above by cooing or from below by heating, four physical properties will change:
- the coefficient of thermal expansion (CTE)
- the specific heat capacity
- the dielectric constant
- the mechanical modulus
These property changes help to identify when the glass transition actually occurs.
Quantifying the Glass Transition
The glass transition itself appears as a step in the DSC curve for the material. It is typically measured using either the TMA, DMA, or DSC thermal analysis methods. The change in CTE can be measured using TMA while the change in mechanical modulus can be measured by DMA. A good rule of thumb for estimating the Tg is 2/3 of its melting point temperature in Kelvin. The Tg will always be lower than the melting point of the material.
The glass transition temperature, Tg, is the temperature at which polymer chains can slide past each other without breaking. When a polymer is cooled below its Tg, it gets very hard and brittle; when it is heated above its Tg, it becomes very soft and pliable. A good heuristic for estimating the Tg is that it will be about 2/3 the polymers melting point temperature.