Polyethylene consists of many random molecular chains with no particular orientation and no chemical bonds existing between chains. When heat is applied to such a material, the chains are free to slip and flow under relatively small outside force. Such a material is called a thermoplastic. If we are able to introduce cross-linking bonds between adjacent molecular chains, this adds form stability at higher temperatures. There will still be some loss of strength at elevated temperatures, but the crosslinked molecular chains are much more resistant to flow when stress is applied.