Chemistry
The Relationship Between Glass Transition Temperature and Polymer Chain Mobility
Quick fact
The glass transition temperature is not a true phase transition like melting, but a kinetic transition: it's the temperature at which the cooperative movement of polymer chain segments becomes possible on the timescale of the experiment. For many common polymers, this temperature is far below room temperature—for example, natural rubber's Tg is about -70°C, which is why it stays flexible in the cold.
Why this is interesting
You know how a plastic bottle becomes stiff when you put it in the freezer, and a rubber band gets brittle when it's cold? That's not just because of temperature—it's a dramatic change in the ability of the polymer's chains to move. What exactly happens at the molecular level that causes this transformation?