Structure and mechanical properties of shape memory polyurethane based on hyperbranched polyesters

Hyperbranched shape memory polyurethane (HB-SMPU) were prepared from 4,4'-diphenylmethane diisocyanate, poly (butylenes adipate) glycol, and hyperbranched polyester as chain extender via a two-step process. The morphology of the HB-SMPU films was investigated by the use of wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM), DSC and dynamic mechanical analysis (DMA). It was found that the PBAG soft segments in HB-SMPU with 15-35 wt.% of hard segments were in a crystalline state, however, the PBAG segments were in an amorphous state in the HB-SMPU with 40 wt.% hard segments. The increase of the hard segment content resulted in the decrease of the melt transition temperature of the HB-SMPU. The HB-SMPU with 25 wt.% of hard segment content possessed highest loss tangent. Heat of crystallization was dependent on the content of hard segment in the HB-SMPU. 96-98 wt.% shape recovery could be obtained for the HB-SMPU with 15-35 wt.% of hard segment. The content of hard segments in HB-SMPU was very important in determining their physical properties.