Small‐angle X‐ray scattering studies of microdomain structure in segmented polyurethane elastomers

Abstract The small‐angle x‐ray scattering (SAXS) technique has been used to characterize the detailed microphase structure of two crosslinked segmented polyurethane elastomers. Both copolymers contain trifunctional polypropylene ether triols in the rubbery elastomeric block, but are synthesized with different hard segments: a symmetric 4,4′‐diphenylmethane diisocyanate (MDI) chain extended with butanediol (BD); and an 80/20 mixture of asymmetric 2,4‐toluene diisocyanate (TDI) and symmetric 2,6‐toluene diisocyanate (TDI) chain extended with ethylene glycol (EG). Calculations of SAXS invariants and determinations of deviations from Porod's law are used to examine the degree of phase segregation of the hard‐ and soft‐segment domains. Results show that the overall degree of phase separation is poorer in the asymmetric TDI/EG‐based copolymer than in the symmetric MDI/BD‐based copolymer. Determination of diffuse phase boundary thicknesses, however, reveals that the domain boundaries are sharper in the asymmetric TDI/EG system. The contrasting morphologies found in the two systems are interpreted in terms of differences in hard‐soft segment compatibility, diisocyanate symmetry, and diisocyanate length. Coupled with conformational considerations, this information is used to construct a new model for polyurethane hard‐segment microdomain structure. Important features of the model are that it takes into account the effects of hard‐segment sequence length distribution and allows for folding of the longer hard‐segment sequences back into the hard‐segment domain.