The Glass Transition Temperature (Tg) is the temperature at which an amorphous material, particularly a polymer or glass-forming substance, changes from a hard, rigid and glassy state to a softer, more flexible or rubbery state. It is an important property of materials because it determines their mechanical behaviour, thermal stability, processing conditions and practical applications. The Glass Transition Temperature is not a fixed structural constant like the melting point of a pure crystalline substance; rather, it depends on the molecular structure and mobility of the material. Two important factors that affect the Glass Transition Temperature are molecular flexibility and intermolecular forces. Molecular flexibility has a strong influence on Tg. The flexibility of a material depends mainly on the nature of its molecular backbone, the presence of rotatable bonds and the size and structure of side groups. When molecular chains are highly flexible, they can rotate and move more easily. This movement requires comparatively less thermal energy. Consequently, the material undergoes the glass transition at a lower temperature. For example, polymers containing flexible aliphatic chains generally have lower Tg values because their molecular segments can undergo rotational motion relatively easily. On the other hand, a rigid molecular structure generally produces a higher Glass Transition Temperature. Rigid structures restrict the movement of molecular segments, so a greater amount of thermal energy is required before significant molecular motion can occur. Aromatic rings incorporated into a polymer backbone often increase rigidity and therefore raise Tg. For instance, polymers containing benzene rings in their main chains tend to have higher Tg values than structurally similar