Hierarchical structure of phase-separated segmented polyurethane elastomers and its effect on properties

Polyurethanes were prepared from 4,4'-methylenebis(phenyl isocyanate), 1,4-butanediol and poly(1,4-butanediol adipate) polyester polyol. The hydroxyl functional group ratio of polyol/total diol was kept constant at 0.4, while the ratio of the isocyanate and hydroxyl groups (NCO/OH ratio) changed between 0.90 and 1.15. The polymers were prepared by one-step bulk polymerization in an internal mixer. They were characterized using a number of methods including Fourier transform infrared spectroscopy, wide-and small-angle X-ray scattering, differential scanning calorimetry, dynamic mechanical analysis, light transmittance and tensile testing. Changing the stoichiometry modifies the molecular weight in accordance with the laws of stepwise polymerization, and the relative concentration of end groups also changes at the same time. Competitive interactions among various groups including chain-end functional groups lead to the formation of slightly ordered phases of sub-nanometre size at both ends of the composition range. These structures assemble into larger units at the 10 nm level, which associate further to even larger entities of micrometre size causing scattering of light and a decreased transparency of the samples. The order of the primary units, together with the number and size of assemblies at both higher levels, decrease as the composition approaches equimolar stoichiometry. The amount of less ordered amorphous phase has a maximum in this range. The stiffness of the polymers is determined by the amount of this phase, while ultimate properties are influenced also by molecular weight and the number of physical crosslinks formed. (C) 2010 Society of Chemical Industry