A method for producing a high strength, flexible crosslinked polyethylene (PEX) material is provided wherein the base polymer is intermixed with a polymer modifier which affects the amorphous region of the polymer, leaving the crystalline portion of the base polymer virtually unaffected. Consequently, the amorphous region's density is lowered, resulting in a more flexible final product, while the crosslinked molecular structure and/or crystalline portion of the polymer, which controls the strength of the base polymer, remains strong. The polymer modifier can comprise a hydrocarbon fluid such as PAO, Group III basestocks, or a gas-to-liquid hydrocarbon. The plastic material has particular utility in the production of tubing, piping, and conduits for transporting gases, liquids, and the like, as well as wire and cable coatings such as jacketing and insulation.