Mechanically strong and resilient shape memory polyurethane with hexamethylene diisocyanate as mixing segment

Mechanical robustness and flexibility of shape memory polyurethane (SMPU) make them a prominent candidate in various field. However, the shape memory characteristics are hampered due to the lower breaking stress and strain originating from the slippage of hard segments during deformation and entropic elasticity of the segments. Herein, SMPU is synthesised by modification of Polycaprolactone diol (PCL) based soft segment by introducing a linear chain diisocyante, that is hexamethylene diisocyanate (HDI) as the mixing segment and rigid MDI (4,4′-methylene bis-phenyl diisocyanate) as the hard segment. The HDI based soft segment is expected to improve the chain flexibility, and MDI will retain the strength factor. The SMPU is characterised by chemical, structural and thermal analysis. The stress relaxation behaviour of the film was analysed w.r.t time and correlated with recovery studies using the Maxwell model. The thermomechanical conditions are optimised to attain higher shape fixity (SF) and shape recovery (SR) and the SMPU shows maximum SF (60.8%) and SR (97%) at 70°C temperature and 50% strain condition. Also, SMPU shows the tensile strength of 23.4 MPa with elongation at break of nearly 1270%. Thus, the combination of both diisocyanate and soft segments imparts strength and ductility to the SMPU.