On the structure of polyurethane hard segments based on MDI and butanediol-1,4: X-ray diffraction analysis of oriented elastomers and of single crystals of a model compound

Single-crystal x-ray diffraction analysis is performed on a model compound (bisurethane of diphenylmethane-4–monoisocyanate and butanediol-1,4) for the thermoplastic polyurethane (TPU) hard segment formed from diphenylmethane-4,4′-diisocyanate and butanediol-1,4. The resulting structure is compared to structure models of corresponding TPU hard segments, especially to the structure proposed by Blackwell and Ross. Our results confirm this structure model showing a planar zigzag of the (CH₂)₄ group and planar hydrogen bonding between the urethane groups of adjacent molecules. X-ray diffraction analyses of polymeric TPU hard segments and of TPU elastomers with noncrystallizing soft segments lead to a revision of the dimensions of the proposed lattice cell, resulting in a more plausible value of 1.3226g/cm³ for the crystal density.