Glass Transition, Crystallinity and Entanglement in Polymers
A polymer sample is not one thing: parts of it can be ordered, the rest frozen or rubbery depending on temperature. Chain stiffness, side groups and length above the entanglement threshold decide which, and that is what the material feels like.
What a learner can do afterwards
- Explains what changes at the glass transition and why it is not a first-order transition
- Predicts how a bulky side group or a polar backbone shifts the transition temperature
- Relates the entanglement molar mass to the jump in melt viscosity above it
- Reads a thermal trace and identifies glass transition, crystallisation and melting
1 · Read
You know how some plastics feel hard and glassy while others feel soft and bendy. In one polymer sample, ordered regions can sit beside messy ones. When you warm the messy regions past the glass transition, their chain segments start to wriggle and the sample turns rubbery. The change spreads over a range with no latent heat, so it is not a sharp first-order transition like melting.
When you compare two polymers, stiff chains need more heat before they wriggle. A bulky side group gets in the way of motion, so it pushes the transition temperature up. A polar backbone grips its neighbors harder, so it pushes the transition up too.
Short chains slide past each other, so their melt flows easily. Past a threshold length called the entanglement molar mass, the chains loop around each other like cooked noodles. From there, even a little extra length makes the melt much thicker.
When you read a thermal trace, find the step first. The glass transition shows as a step in the line. Crystallisation and melting each show as a peak, and both peaks come after the step on warming.
Stiffness, side groups and chain length set where a polymer turns rubbery, and a thermal trace shows each change in order.
2 · Watch
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