| 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. |
