Why do branched polyethylene chains have a lower density than straight chains?

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major industrial polymers Figure 1: The linear form of polyethylene, known as high-density polyethylene (HDPE).

Branched polyethylene chains have a lower density than straight, linear chains because the presence of branches prevents the polymer molecules from packing closely together in a dense, crystalline arrangement.12

In contrast, linear structures—such as high-density polyethylene (HDPE)—lack these branches, allowing the chains to align and pack tightly. This close packing results in a material with higher density, strength, and stiffness. Because the branches in low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) disrupt this efficient packing, the intermolecular forces between the chains are weaker, which contributes to the material's characteristic flexibility.12