What is the glass transition temperature in polymers?

• A. The temperature at which melting occurs.
• B. The temperature at which a polymer begins to have changes towards being more rigid/brittle.
• C. The temperature at which crystallization occurs.
• D. The temperature at which decomposition begins.

Answer: (B)

The glass transition temperature is the temperature at which amorphous polymers switch from a hard, glassy state to a softer, more rubbery state as temperature rises. This happens because heating increases the mobility of the polymer chains; below this temperature the chains are essentially frozen in place, giving the material high stiffness and brittleness. Once you pass the glass transition, segmental motion becomes appreciable, free volume increases, and the material becomes more compliant, ductile, and able to deform elastically.

This is different from melting, which is the loss of crystalline order at the melting temperature. It’s also not about crystallization or decomposition, which are distinct processes. Tg is typically detected by techniques like DSC, where you see a step in heat capacity, or DMA, where the storage modulus drops and the damping increases near Tg. Factors such as a stiffer backbone or strong intermolecular interactions raise Tg, while plasticizers or moisture tend to lower it, reflecting how molecular mobility controls this transition.