Shape‐memorized crosslinked ester‐type polyurethane and its mechanical viscoelastic model

A series of shape‐memorized crosslinked ester‐type polyurethanes (PUs), based on different compositions of 4,4′‐diphenyl methane diisocyanate (MDI), poly(butylene adipate) glycol (PBAG) with different molecular weight (MW) and trimethylol propane (TMP), were synthesized. The morphology of samples was investigated by using DSC, WAXD, and dynamic mechanical analysis (DMA). It was found that the morphology of the soft segment, which was PBAG with a different MW, was in an amorphous state and no crystalline domain was found. By increasing the crosslinked density (varying the content of TMP) or decreasing the length of the soft segment (MW of PBAG), the glass transition temperature of studied samples increased. But the range of transition broadened and the modulus ratio E ′   ( T   g−20°C) / E ′   ( T   g+20°C) also decreased. The shape‐memory behavior was studied by the bending test method adopted from the shape‐memory alloy. The sample with high T g showed not only a high recovered temperature ( T r ) but also a high recovered rate ( V r ) with a high modulus ratio. By introducing the chemical crosslinked structure, the deformed samples completely recovered their original shape and rendered shape‐memory behavior under the complex deformation. The shape‐memorized crosslinked ester‐type PUs can be applied at different operating temperatures. A mechanical viscoelastic model is discussed for the shape‐memory behavior of PUs, and the modified Bonart's viscoelastic model properly describes the mechanism of the shape memory of PUs. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1305–1319, 1999