Influence of dangling chains on the microphase separation and damping properties of polyurethane

Understanding how dangling fragments of polyurethanes (PU) affect its microphase separation structure and damping properties can provide insights for designing desired materials at the molecular scale. By varying the types of diol extenders (such as 1,2-ethyleneglycol, 1,2-propanediol, and 1,2-butanediol), PU samples with different dangling residues were successfully synthesized. Using atomic force microscopy and small-angle x-ray scattering, we confirmed that the introduction of dangling chains disrupts microphase separation and demonstrated a correlation between the degree of suppressed microphase separation and the size of the dangling residues. Fourier transform infrared spectroscopy and molecular dynamics simulations confirmed that dangling chains reduce the hydrogen bonding index while increasing the phase compatibility between soft and hard segments. Differential scanning calorimetry (DSC) measurements revealed an increased glass transition temperature (T_g), indicating hindered movement of backbone segments due to dangling chains. Moreover, lengthening the dangling chains further decreased T_g. Dynamic mechanical analysis (DMA) demonstrated improved damping properties with the introduction of dangling chains, although increasing chain length led to deteriorating damping properties, consistent with observations from DSC. These findings suggest that the variations in macroscopic properties of PU induced by dangling chains are linked to hydrogen bonding interactions and micromorphology at the molecular level.