Considerations for Measuring Glass Transition Temperature
By Engaged Expert
John MoylanJohn Moylan is a senior expert in the testing of polymer based composite materials for the global Aerospace sector.
Glass transition temperature, or Tg is an important characteristic of polymeric materials. It marks the point at which a material changes from a glassy to a rubbery state, and influences many other material properties.
Glass transition temperature is characteristic of the type of polymer chain, the amount of crosslinking and chain length or molecular weight. Since most materials have a distribution of molecular weights and cross-linking, the Tg is not a discrete temperature, but rather a temperature range. A single temperature is generally reported for convenience, but the material will actually change properties over a range of temperatures.
Because of its close relation to strength and flexibility, it is critical to understand and apply the concepts of glass transition temperature during the product development lifecycle to ensure polymeric materials, components, and assemblies will function appropriately in the end use environment.
Is there an ideal Glass Transition Temperature (Tg)?
Determining an ideal glass transition temperature depends on the desired physical properties of the material itself, along with the application and environment in which the product or material will operate.
When materials like rubbers and elastomers need to retain flexibility, they should be used and stored above the glass transition temperature (otherwise, it runs the risk of becoming brittle). However, materials such as rigid plastics and composites should be used below the glass transition temperature. If used above that point, the heat will cause them to have lower modulus and a higher coefficient of thermal expansion. For composites, this causes lower mechanical resin based properties such as compression, shear and interlaminar tension. For electronic materials, printed wiring boards may be thermally stressed during assembly or use. This results in mechanical stresses in plated through holes, vias, leads and solder joints. For optimal reliability, a high Tg is important.
External factors, like humidity or moisture level, may also affect glass transition temperature. Because moisture tends to diffuse slowly through a material, it may act as a plasticizer and causes the material to reach an