Polymers are large molecules composed of many repeated smaller chemical units linked together in long chains. These structures give rise to unique material properties, and one of the most important is the Glass Transition Temperature (Tg). The Tg is a specific temperature range marking a reversible transition where the material changes from a hard, brittle, glass-like state to a softer, more flexible, rubber-like state. This thermal property dictates the operational limits and suitable applications for plastics, fibers, and rubber materials, determining whether the finished product will be rigid or pliable at its intended service temperature.
Defining the Glass Transition State
The glass transition is rooted in the movement of the polymer’s molecular chains. Below the glass transition temperature, the material exists in the glassy state, where the polymer chains are essentially frozen in place, only able to vibrate slightly. This restricted movement results in a material that is hard, rigid, and often brittle, much like a typical glass. There is insufficient thermal energy to allow for large-scale rotation or conformational changes within the molecular backbone.
As the polymer absorbs heat and its temperature approaches the Tg, the thermal energy increases, allowing specific segments of the molecular chains to gain mobility. This onset of localized molecular motion is responsible for the dramatic change in the material’s physical properties. The chains start to move and slide past one another, requiring space between molecules known as free volume.
Once the temperature rises above the Tg, the polymer enters the rubbery state, characterized by significant segmental movement and increased free volume. In this state, the material is soft, pliable, and capable of large, reversible deformations, similar to a rubber band. The glass transition is not a true phase change, like melting or boiling, because the polymer remains an amorphous solid both above and below the Tg. Instead, the transition is classified as a kinetic event, occurring over a temperature range and dependent on the rate of heating or cooling.
How Glass Transition Temperature is Measured
Scientists rely on specialized thermal analysis techniques to pinpoint the temperature range where the glass transition occurs. One common method is Differential Scanning Calorimetry (DSC), which measures the difference in heat flow between a polymer sample and an inert reference material as they are heated. At the Tg, the polymer’s heat capacity undergoes a noticeable stepwise change. This shift in the heat flow curve is recorded by the instrument, allowing for the determination of the glass transition temperature.
Another method is Dynamic Mechanical Analysis (DMA), which focuses on the material’s mechanical response to oscillating stress. The DMA instrument applies a small, cyclical force to the sample while varying the temperature. As the polymer passes through its Tg, the large-scale molecular movement causes the material’s stiffness, or storage modulus, to drop dramatically. This sharp loss of mechanical rigidity provides an accurate measure of the temperature at which the material loses its glassy characteristics.
Factors That Influence a Polymer’s Tg
The chemical structure and composition of a polymer are the primary determinants of its glass transition temperature.
Backbone Stiffness
The stiffness of the polymer backbone is a major factor. Rigid structures, such as aromatic rings found in polystyrene, significantly hinder chain rotation and result in a higher Tg, typically around 100°C. Conversely, a highly flexible backbone, like the silicon-oxygen chain in silicone rubber, allows for easy rotation and movement, leading to a very low Tg, often well below room temperature.
Chain Length and Cross-linking
The overall length of the polymer chains (molecular weight) also impacts the Tg. Longer chains tend to have fewer chain ends per unit volume, which restricts the available free volume and increases the Tg. Furthermore, introducing cross-links—covalent bonds connecting adjacent polymer chains—restricts mobility severely, causing a substantial rise in the glass transition temperature.
Plasticizers
The addition of small molecules called plasticizers works to reduce the Tg. Plasticizers insert themselves between the polymer chains, increasing the free volume and allowing the chains to move more easily at lower temperatures.
Practical Uses Dictated by Tg
The glass transition temperature is essential for material selection, as a polymer’s application is determined by whether it operates below or above its Tg.
Products that need to be rigid and dimensionally stable, such as plastic water bottles made from polyethylene terephthalate (PET) or polyvinyl chloride (PVC) pipes, are used in their glassy state, well below their Tg. For example, PET has a Tg of about 70°C. This allows it to maintain its hard, rigid structure at room temperature, though it will soften significantly if exposed to hot water.
Conversely, materials intended for flexible, elastic performance, like rubber tires or elastic tubing, must be utilized in their rubbery state, far above their Tg. The rubber used in tires, polyisoprene, has a Tg far below 0°C, ensuring the material remains soft and flexible even in cold conditions. If a flexible material is cooled below its Tg, it becomes brittle and cracks upon deformation, which is why some food packaging becomes fragile in a freezer. Engineers use the Tg to select materials that provide the desired mechanical properties across the service temperature range of the final product.