Glass Transition Temperature (Tg )

At the macroscopic scale, the glass transition temperature, Tg, represents the temperature above which a material changes from a stiff glass into a viscous fluid or a rubbery material. Besides polymers, which are the most common materials with a glass transition temperature, also various amorphous solids, organic liquids, alloys, or inorganic glasses may exhibit a glass transition. At the molecular scale, the T g of a polymer is the temperature above which large segmental motions of the polymer chains become possible within the time scale of the experiment. The glass transition temperature depends on the molecular architecture. Substituents that restrict the backbone rotation of a very simple polymer, such as polyethylene, will increase its T g, and the presence of polar groups will have an even stronger effect (Table 1). Besides the substituents on a flexible polymer backbone, the chemical structure of the backbone itself obviously has a major impact on the glass...