Explaining the Glass Transition Temperature

There are many kinds of polymers, including plastics, rubber, elastomers and latexes, but they all have one thing in common — they are composed of long chains of molecules organized into complex arrangements. In crystalline polymers, the chains arrange themselves in an orderly fashion, often with one molecular chain locked against another. In other polymers, classified as amorphous, the long chains of molecules are randomly packed together with no discernible order.

As an interesting side note, glass is an amorphous material, but it’s not a polymer — it’s a non-polymeric amorphous solid. Still, the behavior of glass can be useful in understanding how polymers work. It’s hard and brittle at room temperature, yet it’s soft and pliable when it gets heated. As we’ll discuss next, amorphous polymers behave in a similar manner.

When crystalline polymers are heated and reach a certain temperature, the orderly arrangement of their long chain structure transitions to a random and disorganized arrangement. The solid polymer literally transitions and melts into a liquid state. The temperature at which this change occurs is the melting point (Tm). 

Amorphous polymers do not melt when heated. Instead, as heating continues, the polymer chains become mobile, causing the material to become softer and less rigid, or vice versa. The point of this change is known as the glass transition temperature (Tg). The Tg of a specific polymer may be listed as a single temperature, but this number is an average value calculated from a range of temperatures. As with melting, glass transition is signified by a change in heat capacity — i.e., it absorbs heat, but rather than getting hotter, the material goes through this change of state. It’s important to note that some polymers can have a portion of their chains that are crystalline and a portion that are amorphous. Such polymers would have both a melting point and a glass transition temperature.

The most common way to measure the Tg of amorphous polymers is differential scanning calorimetry, or DSC. In DSC, the polymer sample is placed in the calorimeter, which measures quantities of absorbed or emitted heat in the sample as the temperature increases. As the changes to these polymers are gradual, by convention, the Tg is reported as the midpoint of this transition.

Many important commercial polymers, like those made by RUICO, fall into the amorphous category. The glass transition temperature of these polymers typically ranges from -80°C to +100°C. The Tg can be manipulated by varying the polymer composition. For example, in a styrene-butadiene latex emulsion, the styrene has a Tg of +100°C while the Tg of butadiene is -85°C. However, changing the ratio of styrene to butadiene will cause a shift in the Tg, as well as a shift in the end product’s performance.