Question

5) (16 pts) The T of two bulk polyethylene samples, both of M. = 120,000, varies by more than 80 "C! Fully explain how you could have two polymer samples with with the same chemical formula and Mn, but with such wildly different Te!

Answer 1

Glass Transition Temperature: it is the temperature at which amorphous polymer start to turn into viscous liquid and rubbery structure.

To understand this correctly first we need to think of factors affecting Tg of a polymer:

1. Molecular weight : increase in molecular weight increases Tg.

2. Molecular structure : insertion of bulky groups or side chain increases Tg of polymer due to reduced mobility.

3. Chemical cross-linking : increase in cross-linking reduces mobility and result in increased Tg.

4. Polar groups : presence of polar groups leads to higher intermolecular forces and thus increased inter-chain attraction and cohesive force result in increased Tg.

So, two sample having same compostion and same molecular weight can have different Tg if their molecular structure and chemical cross-linking is different which results in variation of their mobility and free volume. Structure of a polymer determines how molecules can orient and moves in the solid state therefore structure of polymer greatly influence properties like Tg., density, crystallinity and melting point.