Temperature dependency of water vapor permeability of shape memory polyurethane

Solution-cast films made in different temperatures of shape memory polyurethane have been investigated. Differential scanning calorimetry, tensile test, water vapor permeability with different temperature and humidity and the shape memory effect were carried out to characterize these polyurethanes. We can get some results as follow:||1. SMPU films cast at higher temperatures have higher crystalinity of soft segments and hard segments than the sample cast at low temperature. The difference of mechanical properties and shape memory effects due to the difference in casting temperatures is related to the increase of the overall and the segment crystallinity. It seems that moderate crystalinity offers better shape memory effect and tensile properties.||2. The films obtained are non-porous and the water vapor permeability of SMPU films depends on the solubility and diffuse transmission of water vapor in these non-porous films. The changes in the water vapor permeability of SMPU films with respect to the temperature follow an S-Shaped curve, and increases abruptly at Tm of the soft segment due primarily to the changes of the morphology of soft segments on increased temperature. Moderate crystalinity is more suitable for better water vapor permeability than very low and very high crystalinity.||3. The water vapor permeability dependency of the temperature and humidity contribute is the result of the change of diffusion and solubility with the surrounding air condition. The diffusion coefficient (D) are the function of temperature and show good fit the Arrhenius form but show different parameter values when above and below Tg or Tm.||4. The materials can keep the shape at lower temperature and recover the original shape under the heating process is attributed to the crystallinity of soft and hard segments, the degree of phase separation. Higher cast temperature is favorable for the formation of the crystal of in the soft segment and hard segment and therefore shows better shape memory effect