What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?

Polyethylene is a polymer made from the polymerization of ethylene monomers. It can exist in different forms, primarily high-density polyethylene (HDPE) and low-density polyethylene (LDPE), which differ in their molecular structure. HDPE has a more linear structure with minimal branching, allowing for tighter packing of the polymer chains, while LDPE has a highly branched structure that creates more space between chains.

Density is defined as mass per unit volume and is influenced by how closely molecules are packed together. In HDPE, the linear arrangement of chains allows for a denser packing compared to the branched structure of LDPE, which results in lower packing efficiency and increased void spaces. This difference in molecular arrangement directly affects the overall density of the materials.

Crystallinity refers to the degree to which a polymer's molecular chains are ordered in a regular pattern. HDPE exhibits higher crystallinity due to its linear structure, leading to stronger intermolecular forces and a more compact arrangement. In contrast, the amorphous regions in LDPE, caused by its branching, contribute to lower crystallinity and, consequently, lower density.