The structure of MDI/BDO‐based polyurethanes: Diffraction studies on model compounds and oriented thin films

Abstract

Abstract Polyurethanes based on MDI/BDO are known to crystallize in three distinct forms. Two of these crystal structures, type II and type III, have been characterized by electron and x‐ray diffraction experiments on model compounds and oriented thin films of a 77% hard‐segment polyurethane elastomer. The model compounds were of the form BDO(MDI/BDO) n with n = 1, 2, 3. Results from the model compounds indicate that the MDI/BDO repeat crystallizes in the type‐II structure, which has a contracted conformation with a monomer repeat length of 1.70 nm. MDI/BDO‐based polyurethanes also exhibit the type‐II structure under quiescent crystallization conditions. Studies of oriented thin films of an MDI/BDO‐based polyurethane indicate that a change in crystal form occurs on stretching to the fully extended monomer conformation. This crystal form is termed type III and has a monomer repeat length of 1.92 nm.