Structure and thermomechanical properties of polyurethane block copolymers with shape memory effect

Shape memory polyurethane (PU) block copolymers composed of 4,4 ' -methylenebis-(phenylisocyanate), poly(tetramethylene glycol), and 1,4-butanediol as a chain extender were synthesized by a two-step process. FT-IR spectra showed that carbonyl peak appearing at 1700 cm(-1) increased with higher hard segment content, whereas another carbonyl peak at 1730 cm(-1) decreased. It suggests that hard segments get more aggregated to form domains in the PU block copolymer as hard segment content increases. Such domain formation has a significant influence on the mechanical and thermomechanical properties of PU, such as maximum stress, tensile modulus, and elongation at break. Especially, maximum stress, tensile modulus, and elongation at break increased significantly at 30 wt % of hard segment content, and the highest loss tangent was observed at the same composition. Heat of crystallization as measured by differential scanning calorimetry is also dependent on the hard segment content. Finally, 80-95% of shape recovery was obtained at 30-45 wt % of hard segment content, and the control of hard segment content in PU block copolymers is very important in determining their physical properties.