Polyurethane with an ionic liquid crosslinker: a new class of super shape memory-like polymers

This investigation reports on the synthesis of a new class of ionic liquid crosslinked shape-memory polyurethane (PU-IL) based on polycaprolactone diol and 4,4′-methylenebis(phenyl isocyanate) (MDI), which can overcome the major drawbacks of conventional shape-memory polyurethanes. In this case, the ionic liquid crosslinker in PU-IL exhibited not only a higher shape-recovery ratio (98%), but also showed excellent shape-fixity (98%). In the second cycle of the cyclic tensile test, PU-IL showed almost complete shape recovery while maintaining excellent shape fixity. The higher shape-fixity value in PU-IL was also supported by its higher crystallization and melting enthalpy, as evidenced by DSC analysis. The properties of this PU-IL were compared with those of conventional linear PU having 1,4 butanediol (BDO) as a chain extender as well as of non-ionic crosslinked PU using trimethylolpropane (TMP). Ionic liquid as a crosslinker reduced the glass transition temperature (T_g), whereas the non-ionic crosslinker increased the T_g. Interestingly, the soft segment crystallinity as well as melting enthalpy of PU-IL is higher than that of PU-BDO, whereas no melting or crystallization peak was observed in the DSC thermograms of PU-TMP. The DSC results were supported by DMA analysis. The higher T_g and the absence of a soft domain melting transition indicated complete intermixing of hard and soft phases in PU-TMP, but the ionic interaction in PU-IL restricted this. Inter-domain mixing in PU-TMP was also supported by the absence of a scattering peak in SAXS analysis. FT-IR spectroscopy showed stronger hydrogen bonding in PU-BDO followed by PU-TMP and PU-IL.