Synthesis of chemically-crosslinked multi-arm star-shaped polyurethane with triple-shape memory effect

In this study, chemically-crosslinked multi-arm star-shaped polyurethanes (SPUs) were prepared using three, four, and six-arm polycaprolactone, hexamethylene diisocyanate, and 1, 4-butanediol. The hydrogen bonding indices of soft and hard segments were calculated using Fourier transform infrared spectra. The results indicated that the phase separation among hard and soft segments increased with the increment of PCL arm numbers. Moreover, the results of X-ray diffraction and differential scanning calorimetry showed that the crystallization ability of the three and four-arm SPUs were lower than that for six-arm SPU (6SPU), which is due to their higher crosslinking densities. In addition, the results of the mechanical studies showed that the crosslinking density and degree of crystallinity are the main effective parameters controlling the mechanical properties, by which 6SPU showed higher Young's modulus and lower elongation at break compared to other SPUs. Cyclic shape memory studies showed that 6SPU could fix approximately all the temporary shapes during three cycles and recover 100% of its original shape. Moreover, 6SPU could show triple-shape memory effect (TSME) by which it could fix two different temporary shapes. These results show that 6SPU has a high potential for practical applications due to its good mechanical properties, shape memory fatigue resistance, and TSME.