X-ray investigation of the structure of polyurethane elastomers based on 1,5-naphthalene diisocyanate

The structure of the crystalline hard domains of polyurethane elastomers based on 1,5-naphthalene diisocyanate (NDI) has been investigated by X-ray methods. The specimens were pressed films prepared from NDI and 1,4-butanediol (chain extender), with poly(oxytetramethylene) as the soft segments. Small angle X-ray data show that the hard domains are lamellae oriented preferentially with their planes parallel to the film surface. The corresponding wide angle data show that the hard domains are crystalline, and that the degree of crystallinity can be more than 60%. The hard segment chains are relatively extended and are oriented perpendicular to the film surface: they probably form a staggered array in which they are linked in sheets by hydrogen bonds. Molecular models suggest that the hydrogen bonds are shorter, and hence stronger, than those formed in the hard domains of equivalent elastomers based on diphenylmethane diisocyanate (MDI) or dicyclohexylmethane diisocyanate (HMDI). In addition, the stacking of the naphthylene units is unencumbered by the presence of the bridge methylene groups in the latter systems, and these structural features probably account for the higher melting temperature of the hard domains.