Influence of Dangling Chains on Molecular Dynamics of Polyurethanes

The effect of dangling chains on phase-separated microstructure and molecular dynamics for polyurethanes (PUs) was investigated. PUs with different dangling chain lengths and polar groups were prepared through changing the types of diol extender. The molecular dynamics was studied by a combination of dynamic mechanical analysis (DMA) and broadband dielectric relaxation spectroscopy (BDRS). Four relaxations (processes), namely, a secondary relaxation (beta), the soft phase segmental, relaxation (alpha), the I process associated with hydrogen bond, and Maxwell-Wagner-Sillars (MWS) interfacial polarization process caused by charge accumulation at hard/soft phase interfaces, were detected. The I process occurred in temperatures lower than that of MWS process but higher than alpha relaxation in general. The beta relaxation remains unaffected with changing dangling chain lengths or polar groups. However, the glass transition temperature (T-g) of the soft phase shifts to lower temperature, and the segmental motion becomes faster with increasing dangling chain length, while the introduction of a polar ester group into the dangling chains makes it slow down, corresponding to a higher T-g, and results in a higher fragility. On the other hand, there is an absence of I process, and the MWS process shifts to higher frequencies when longer dangling chain is introduced. In the case of increasing the hard segment content, the I process reappears and the MWS process slows down. It is suggested that these results are related to the H-bond interactions within hard segments and the micromorphologies of PUs.