Synthesis and shape memory effects of Si-O-Si cross-linked hybrid polyurethanes

A series of novel Si-O-Si cross-linked organic/inorganic hybrid polyurethanes (HYPUs) with shape memory effect were prepared from isophorone diisocyanate (IPDI), poly(ethylene oxide) (PEO), and a newly synthesized hybrid diol (HD) containing hydrolysable Si-OEt groups. After hydrolyzation and condensation of Si-OEt groups, the resulted films were characterized using wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and shape recovery test to get the insight into the relationship between shape memory behaviors and polymeric structures. The glass transition temperatures (Tg) and storage modulus increased with Si-O-Si cross-linking increasing in the hybrid polyurethanes. The hybrid polyurethanes can recover to their original shapes almost completely in less than 40 s in atmosphere and in less than 10 s in water, respectively, when heated at 25 °C above Tg. The shape memory mechanism is coming from the freezing at low temperature and activation at high temperature of micro-Brownian movement of amorphous molecular chains since the temperature ranges at which the sharpest changes of recovered curvature happened are found to be around their glass transition range. The high ratio of storage modulus below and above TgDMA accounted for the temporary shape fixing at low temperature. The samples with more Si-O-Si cross-linking have higher storage modulus at high temperature, resulting in faster shape recovery speed but lower temporary shape fixing.