Modeling the structure of the hard domains in HMDI-based polyurethane elastomers

The structure of the hard domains of polyurethanes based on 4,4′-trans,trans-dicyclohexylmethane diisocyanate (HMDI), with 1,4-butandiol (BDO) as the chain extender, has been derived by X-ray diffraction and molecular modeling. X-ray diffraction patterns of drawn annealed films of HMDI/BDO/poly(tetramethylene adipate) elastomers contain crystalline hard domains: the unit cell is triclinic and contains two chains, each of which has two monomers repeating in c=37.5 Å. A number of chain conformations are compatible with this repeat, but these are reduced to two by the requirement that all the urethane groups should form hydrogen bonds. We have compared packing models for these two options in terms of the agreement between the simulated diffraction patterns for arrays of chain segments. The best agreement is obtained for a model in which the chain extender region has the gaucheplus-trans-gaucheminus conformation. Identical chains are linked in sheets by C=O···H−N hydrogen bonds along the α axis of the unit cell. The second chain is rotated by 180° about c relative to the first and positioned at 0,b/2. The crystallographic R-values for this model are 0.196 (observed data only) and 0.240 (observed plus unobserved data), which were much lower than those for the alternative models, and represent excellent agreement for a structure that is not fully refined. The hydrogen bonds have very similar geometry to those for diphenylmethane (MDI)/diol hard segments. Thus the higher melting point and other improved properties for HMDI-based polyurethanes are most likely due to stronger interactions between the stacked cyclohexane rings as compared to those between the phenylenes in MDI-based elastomers.