Final answer:
Polyethylene has a lower glass transition temperature than polypropylene due to differences in their chemical structures. Polypropylene's higher Tg is a result of steric hindrance from the methyl groups.
Step-by-step explanation:
The difference in glass transition temperature between polyethylene and polypropylene can be attributed to differences in their chemical structures. Polyethylene is composed of long chains of -CH₂- units, while polypropylene has similar chains but with a methyl group (CH₃) attached to every other carbon. The presence of the methyl groups in polypropylene disrupts the packing of the polymer chains, making it less dense and more flexible than polyethylene.
As a result, polypropylene has a higher glass transition temperature than polyethylene. The higher Tg of polypropylene indicates that it requires more thermal energy to overcome the intermolecular forces and allow the polymer chains to slide past each other.
This is because the methyl groups create steric hindrance, hindering the movement of the chains and increasing the energy required for them to slide and rotate.
In summary, polyethylene and polypropylene have similar chemical structures, but the presence of methyl groups in polypropylene leads to a higher glass transition temperature due to increased steric hindrance and reduced chain packing efficiency.