Nanostructure-Controlled Shape Memory Effect in Polyurethanes

Different hard segment content (HSC) polyurethanes (PUs) have been synthesized using linear (hexamethylene diisocyanate; HMDI) and alicyclic (ichorone diisocyanate; IPDI) diisocyanates to understand the effect on structure-property relationship and how it affects the shape memory behavior of PUs. Structural details have been elucidated through NMR and other spectroscopic techniques including the quantification of HSC. The nature of interactions between the polymer chains and their extent has been revealed. Thermal and mechanical properties as a function of the chemical structure and HSC indicate faster degradation in higher HSC. Layer-by-layer self-assembly through extensive hydrogen bonding has been established through XRD, small-angle neutron scattering, AFM, and optical images by capturing nanometer scale to micron scale inhomogeneities. The greater interaction in the HMDI system as compared to IPDI PUs leads to the crystallization of the hard segment. The differential shape memory effect has been reported with retracting force in the HMDI system with increasing HSC helps to recover greater percentage of the permanent shape; on the contrary, a decreasing shape recovery value is obtained in the IPDI system. Calorimetric measurement shows that the crystallinity of the soft segment decreases in both the systems which results in decreasing shape fixity efficiency with increasing HSC.