Discussion
Under which combination of conditions does the glass transition of a polymer increase?
Answer: D
The glass transition rises with an increase in chain length and decrease in plasticizer content.
The glass transition temperature is set by how easily chain segments can move. Anything that adds free volume or lubricates the chains lowers it; anything that removes free volume or restricts motion raises it.
- Longer chains (higher molecular weight) have fewer chain ends per unit volume. Chain ends carry extra free volume, so fewer of them raises Tg.
- Plasticizers are small molecules that sit between chains and increase free volume, so less plasticizer raises Tg.
Every other option includes a change that lowers Tg: more plasticizer, less cross-linking, more branching (which adds chain ends) or weaker inter-chain interactions.
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