Remember Indiana Jones and The Temple of Doom? Remember the snake pit? Good. You’re half way to understanding a fancy polymer chemistry concept.
Polymers are what we like to call Macromolecules, meaning simply that they are really big. To compare: if a water molecule were the size of a nickel, a polyethylene molecule would be 1/4 of a mile long. Back to Indiana Jones, so it’s a nice warm day in the snake pit and the snakes are wiggling happily to and fro, over and under, in a giant swirly spaghetti blob. Now you decide to drive your Land Rover (so British) through this snake room, and what do the snakes do? They slither out of the way, minimal dead snakes, maximum driving efficiency. Now what about a cold day in Egypt? (un-likely but necessary for this example) The snakes are moving very little now, stacked and entangled with each other. Here comes the Land Rover again, and things get messy this time around. (A shout out to Macrogalleria for this metaphor)
This is the exact (ish) way that polymers function, just think of all those snakes as long molecular chains. When they get cold, below their Glass Transition Temperature (Tg), they will break. Ever left a plastic bucket outside overnight in winter? Enough said. Ever put a rubber ball into liquid nitrogen? Do it.
Glass transition isn’t just for breaking stuff, it’s also very useful for imparting desirable properties to the polymer products in everyday life. A rubber band is only functional when flexible, and above its Tg. While those clear plastic wine glasses, the ones that always break, are below their Tg. The many states of semi-crytaline polymers, bringing variety and functionality to plastics all around you.