Are the various forms of polyethylene polymorphs?

Polyethylene (PE) plastics comprise high density PE, low density PE, cross-linked PE (PEX), linear PE, low density and high density linear PE, etc. Are these all considered polymorphs, or is there another name for them? (Polymorphism refers to crystal structure, so I'm thinking the various forms of polyethylene probably aren't polymorphs.)  
  
I guess a more general form of this question is: what is the name for the same basic chemical (polyethylene, in this case) that can form different compounds with different properties based on how the basic molecule branches (isomer doesn't quite seem to fit, either)?


