The effect of hard segment ordering in copolyurethane elastomers obtained by using simultaneously two types of diisocyanates

A number of polyurethanes are synthesized, based on the novel diisocyanate 4,4′-dibenzyl diisocyanate (DBDI), the traditional 4,4′- methylene bis(phenyl isocyanate) (MDI), or combinations of these. Two equivalent series are prepared, one with hydroxyterminated poly(ethylene adipate) and one with poly(tetrahydrofuran) macrodiol. Mechanical measurements and X-ray crystallinity data are shown for the materials as prepared and after annealing. DBDI has a variable geometry, which allows crystallinity to develop, and leads to an increase in mechanical properties (tensile stress, ultimate strength stress, tear strength, hardness and strain energy): whereas the residual elongation is also dramatically increased. The general improvement in properties with DBDI is retained when both diisocyanates are included, especially when reacted together in a random fashion, rather than sequentially, in the prepolymer stage. Under these conditions, the residual elongation returns to that of a more conventional polyurethane, eliminating the major inconvenience derived from the high permanent deformability and high crystallinity which is undesirable for an elastomer. Scanning electron microscopy reveals coarse texture at the 10 μm level, and a finer texture at the 100 nm level which may be associated with the hard and soft polyurethane segments.