Within a piece of polyethylene plastic, what primarily holds neighboring polymer chains to one another?

Core Concept

Polyethylene is built entirely from saturated C-H bonds, so no permanent dipoles, ion sites, or N-O-F donors exist along the backbone. Dispersion forces act everywhere, however, and each chain exposes thousands of contiguous electrons pressed close to its neighbors over long segments. These summed instantaneous-dipole attractions give the bulk plastic its coherence and strength. Covalent bonds run only along each individual strand and do not connect chains.