Synthesis and characterization of a novel shape memory poly (urethane-urea) by using piperazine as a chain extender

Piperazine (PPZ) was used a chain extender to synthesize a series of novel poly (urethane-urea) (PUU) with poly (lactide-co-p-dioxanone) macrodiol (HO-P (LA-co-PDO)-OH) as a soft segment and hexamethylene diisocyanate (HDI) as a coupling agent. PPZ, as a cyclic secondary diamine is expected to reduce or even eliminate the crosslinking during the synthesis of PUU which is often happened when those primary diamines are used as chain extenders. Moreover, the cyclic structure of PPZ may be helpful to enhance the rigidity of the hard segments in the PUU. NMR was employed to verify the chemical structures of the PUU copolymers. The phenomenological observations on the reaction mixture and the solubility and melting state of the produced PUU confirmed that PPZ may significantly reduce the crosslinking compared to butanediamine (BDA) as a chain extender. Finally, the effects of the hard segment contents characterized by NCO/OH molar ratios on the Tg values of PUU were investigated by means of differential scanning calorimetry (DSC), and the results revealed that Tg values can be controlled from 37.32 to 43.50°C by increasing the NCO/OH ratios from 1.1 to 1.5 and the cyclic structure of PPZ increases the rigidity of the hard segments and thereby the Tg values. PUU had good shape memory properties: shape recovery ratio was more than 95%.