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

A series of shape-memorized crosslinked ester-type polyurethanes (PUs), bas ed on different compositions of 4,4'-diphenyl methane diisocyanate (MDI), p oly(butylene adipate) glycol (PBAG) with different molecular weight (MW) an d trimethylol propane (TMP), were synthesized. The morphology of samples wa s investigated by using DSC, WAXD, and dynamic mechanical analysis (DMA). I t 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 dec reasing the length of the soft segment (MW of PBAG), the glass transition t emperature of studied samples increased. But the range of transition broade ned and the modulus ratio E'((Tg - 20 degrees C))/E'((Tg + 20 degrees C)) a lso decreased. The shape-memory behavior was studied by the bending test me thod adopted from the shape-memory alloy. The sample with high T-g showed n ot 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 str ucture, the deformed samples completely recovered their original shape and rendered shape-memory behavior under the complex deformation. The shape-mem orized crosslinked ester-type PUs can be applied at different operating tem peratures. A mechanical viscoelastic model is discussed for the shape-memor y behavior of PUs, and the modified Bonart's viscoelastic model properly de scribes the mechanism of the shape memory of PUs. (C) 1999 John Wiley & Son s, Inc.