A method for making a thermoplastic material includes: (a) introducing into an extruder an elastomer composition including an ethylenically unsaturated elastomer polymer, a first free radical initiator, and a second free radical initiator; (b) in a first zone of the extruder, heating and partially crosslinking the elastomer composition at a first crosslinking temperature to form a thermoplastic, partially crosslinked elastomer composition; (c) introducing into the extruder in a second zone downstream of the first zone a thermoplastic polymer composition; (d) mixing and heating the thermoplastic polymer composition and the thermoplastic, partially crosslinked elastomer composition to a second crosslinking temperature higher than the first crosslinking temperature, wherein the thermoplastic polymer composition is liquid at the second crosslinking temperature; and (e) mixing and further crosslinking the elastomer composition to form a thermoplastic material having a dispersed phase of the crosslinked elastomer composition in the thermoplastic polymer composition.