Study on the Shape Memory Mechanism of SMPUs and Development of High-performance SMPUs

The shape memory polyurethanes (SMPUs) drew much attention because of their extensive application potentials. Although a few investigations have been made, the relationships between structure and shape memory properties of SMPUs are less known. We synthesized series of SMPUs having either melting transition temperature (Tm) as switch temperature (Ttrans) (Tm-SMPUs) and series of SMPUs having glass transition temperature (T g) as Ttrans (Tg-SMPUs). We investigated their morphology with multiple techniques including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), wide angle x-ray diffraction (WAXD) and small angle x-ray scattering (SAXS). The shape memory properties of the polymers thus can be correlated to their morphological structure such We found that the polyurethanes having no or less hard domains can still exhibit significant shape memory effect under some conditions. When the SMPUs have 20--25% of volume fraction of hard phase they exhibit the highest shape recovery. With the increase of hard segment content (HSC), the volume fraction of hard domains rises. The presence of the higher fraction of hard phase can expand the shape recovery region, resulting in the decrease of shape recovery at lower recovery temperature. By changing the types of soft segments and chemical compositions, Tg can be readily located at a specific temperature but the resultant Tg-SMPUs have very different morphological structure and shape memory properties. The Tg-SMPUs can exhibit pronounced shape memory effect when deformation temperature Tdef is in a vicinity of Tg. With the decrease of Tdef the shape recovery and the recovery stress curves shift to lower temperature. Decreasing Tdef gives rise to higher deformation stress but can raise recovery stress significantly.;In order to improve the shape memory properties of the T m-SMPUs, a series of shape memory polymer polyurethane-ureas ( Tm-SMPUUs) chain extended with aromatic diamines were prepared. In comparison with Tm-SMPUs with identical HSC, the Tm-SMPUUs exhibit much higher recovery stress. It is possible to enhance the shape recovery and recovery stress without sacrificing much shape fixity by using rigid hard segments provided that the HSC is properly controlled. A series of non-aromatic T m-SMPUUs, which can show excellent ultraviolet light resistance, were synthesized with isophorone diisocynates (IPDI). The polyurethane-urea chain extended with IPDA exhibits over 95% of shape fixity and shape recovery as well as excellent resistance to stress relaxation. The maximum recovery stress of the polyurethane-urea is ∼4.2MPa which is comparable to those of the aromatic Tm-SMPUUs. A series of segmented polyurethanes having urethane chains as soft segments were prepared via a modified prepolymer copolymerization method. It was proved that Tg and shape memory properties of the Tg-SMPUs can be adjusted by employing the urethane chains as soft segments. It thus show a new way for developing high-performance Tg-SMPUs.