Study of ionic conductivity and microstructure of a cross-linked polyurethane acrylate electrolyte

A cross-linked polyurethane acrylate (PUA) was synthesized by end capping 4,4′-methylene bis(cyclohexyl isocyanate), H12MDI/poly-(ethylene glycol), PEG based prepolymer with hydroxy ethyl acrylate (HEA). Significant interactions of the Li+ ions with the soft and hard segments of the host polymer have been observed for the PUA complexed with lithium perchlorate (LiClO4) by means of differential scanning calorimetry (DSC), Fourier transform infra-red (FTIR) spectroscopy, 7Li magic angle spinning (MAS) NMR measurements and thermogravimetric analysis (TGA). The 7Li MAS NMR investigation of the PUA indicates the presence of at least three distinct Li+ sites at lower temperature, which merge to a single one at higher temperature in similar line with uncross-linked polyurethane. The results of TGA, DSC and FTIR spectroscopy support the formation of different types of complexes by the interaction of the Li+ ions with different coordination sites of PUA. No detectable interactions could be observed between Li+ ions and groups in HEA. The DSC data indicates the formation of transient cross-links with the ether oxygens of the soft segment and mixing of soft and hard phases induced by the Li+ ions. In addition, a Vogel–Tamman–Fulcher (VTF) like temperature dependence of ionic conductivity implies coupling of the ion movement with the segmental motion of the polymer chains in the cross-linked environment. Predominant formation of contact ion pairs of LiClO4 has been consistently observed through AC conductivity, DSC and NMR spectroscopic results. Swelling measurements of PUA with plasticizers reveal the improved dimensional stability for these cross-linked PUA in comparison with uncross-linked polyurethane.