Graft polymerization of norbornene onto polyurethane and the characterization of its effect on the shape recovery and low-temperature flexibility of polyurethane

Graft polymerization of norbornene monomers onto polyurethane (PU) was tested to reduce the intermolecular attractions between PUs using the rigid bicyclic ring structure of norbornene, to control the soft segment thermal properties, and to enhance the tensile, shape memory, and low-temperature flexibility properties compared with those of unmodified PU. The soft segment crystallization temperature (T_c) sharply decreased with the increase in norbornene, whereas the soft segment melting temperature (T_m) was not significantly changed by the norbornene content. However, the glass transition temperature (T_g) increased with the incorporation of norbornene. The tensile strength sharply increased with the increase in norbornene due to the chemical cross-linking between the grafted poly(norbornene), whereas a significant decrease in the maximum strain was not observed. The shape recovery at 10 °C sharply increased from 47% to approximately 90% after the graft polymerization of norbornene, and the shape retention at −25 °C decreased with the increase in norbornene. Finally, the graft polymerization of norbornene onto PU demonstrated enhanced low-temperature flexibility due to the reduced restrictions of rotational and translational mobility.