Polyethylene vs Polypropylene Glass Transition Temperature ($T_g$)

The glass transition temperature ($T_g$) is a critical property indicating the transition of a polymer from a rigid, glassy state to a more flexible, rubbery state. This transition is closely related to the mobility of the polymer chains.

Molecular Structure Comparison

• Polyethylene (PE): The molecular structure is characterized by repeating units of $-\text{CH}_2-\text{CH}_2-$. This chain is relatively simple and lacks bulky side groups, contributing to its symmetry.
• Polypropylene (PP): The molecular structure includes a methyl ($-\text{CH}_3$) side group attached to every other carbon atom, represented as $-\text{CH}_2-\text{CH}(\text{CH}_3)-$. This bulky side group hinders chain rotation and packing.

Reason for Lower $T_g$ in Polyethylene

The glass transition temperature ($T_g$) is lower for polyethylene compared to polypropylene primarily because of polyethylene's molecular chain structure.

• The lack of bulky side groups in PE makes its chain structure more **symmetric**.
• This **symmetric** structure allows for easier rotation around the carbon-carbon bonds and more efficient packing of chains.
• Consequently, less thermal energy is required for the PE chains to achieve the segmental motion characteristic of the glass transition, resulting in a lower $T_g$.
• In contrast, the methyl groups in PP restrict chain mobility, necessitating a higher temperature for the transition.

Therefore, the symmetry of the polyethylene molecular chain is the key factor for its lower glass transition temperature relative to polypropylene.