Structure–property relationship of shape memory polyurethane cross-linked by a polyethyleneglycol spacer between polyurethane chains

Shape memory polyurethane (SMPU) cross-linked by a polyethyleneglycol (PEG) spacer at its side was compared with a linear SMPU and the one randomly cross-linked by glycerol. The SMPU was composed of 4,4′-methylenebis(phenylisocyanate) (MDI), polytetramethyleneglycol-2000 (PTMG-2000), 1,4-butanediol (BD), glycerol, and PEG-200 as a spacer. PEG-200 connected the glycerol hydroxyl groups of two polyurethane chains via the connecting agent, MDI. T_m of the soft segment increased with the increase of cross-linking content. Cross-link density measured by the swelling experiment increased in proportion to the glycerol content. A surprising increase in maximum stress compared to the linear SMPU was attained without any sacrifice in strain. Remarkably, the stress–strain curve revealed that the PEG cross-linked SMPU exhibited a similar behavior and superior tensile mechanical properties to natural rubber. Storage modulus also increased and loss tangent became broad in distribution over the temperature range after PEG cross-linking. Shape recovery rate went up to 96.8%, and shape recovery speed was three times faster than that of linear SMPU.