the glass-transition temperature

A plastic ruler on a hot summer day bends easily and feels leathery; the same ruler pulled from a freezer is hard and snappy. That change of character happens around a temperature called the glass-transition temperature, written Tg. Below Tg the amorphous parts of a polymer are frozen into a hard, brittle, glass-like solid; above Tg those same parts loosen up and become soft, rubbery, and flexible. It is the single most important temperature for knowing how a plastic will feel and behave in use.

Unlike ice turning to water, the glass transition is not a sharp melting point but a gradual change spread over a range of temperature. What happens physically is that, as you warm through Tg, the amorphous chains gain enough thermal energy for their segments to start wriggling and sliding past one another; below Tg that motion is frozen out. This gives real numbers you can design with: polystyrene has a Tg around 100 degrees C (so it is a rigid glassy solid at room temperature), PVC around 80 degrees C (so unplasticised PVC pipe is stiff), and natural rubber around minus 70 degrees C (so it is soft and rubbery at room temperature — well above its Tg). Adding a plasticiser lowers Tg, which is exactly how rigid PVC is turned into flexible vinyl. Tg is commonly measured by differential scanning calorimetry.

It matters because Tg sets the usable temperature window: a rubber must be used above its Tg or it goes hard and brittle, while a rigid structural plastic must be used below its Tg or it goes soft. This is not academic — the Space Shuttle Challenger was lost because cold weather stiffened an O-ring toward its glass-transition range, so it could not spring back and seal. One honest clarification: Tg applies only to the amorphous fraction of a polymer; a fully crystalline material melts (has a Tm) instead. And Tg is not a true thermodynamic phase transition — the value you measure depends slightly on how fast you cool or heat, so quoted numbers are approximate.