From conformational analysis, we have determined the two lowest energy crystal structures for the hard segments of 4,4'-diphenylmethane diisocyanate/1,4-butanediol (MDI/BDO)-based polyurethane elastomer. Both crystal form, (1), there is strong hydrogen bonding between linear chains with a density of 1.30 g/cm(3), while in the other form (2), van der Waals bonding gives rise to a double helix structure with a density of 1.22 g/cm(3) and a formation energy 1.6 kJ/mol higher than form (1). The double helix crystal has a unit cell length of 18.8 Angstrom which is about half the 34.7 Angstrom unit cell length of the hydrogen-bonded crystal. The X-ray diffraction predicted for each crystal is presented and compared with experiment.