Molecular Simulation of Permeation Behavior of H₂O in PBT Polyether Polyurethane Elastomer

Based on the method of Grand Canonical Monte Carlo and molecular dynamic simulation, the adsorption, diffusion and permeation behavior of H₂O in PBT polyether polyurethane (PU_PBT) elastomer were simulated. The results show that the heat of adsorption of H₂O on PU_PBT at 298, 318, 338, 358 K is 41.15, 40.23, 36.84, 34.16 kJ/mol, respectively, in the fugacity range of 0~1000 kPa. The adsorption equilibrium has been reached when the temperature is 298 K. With the increase of temperature, the adsorption capacity of PU_PBT toward H₂O is declined. The adsorption of H₂O on PU_PBT is not a uniform adsorption, H₂O molecules are adsorbed to the lower potential energy region near the center of the holes in the polymer. When the pressure is 101 kPa, the free volume fractions of H₂O/PU_PBT are 14.37%, 15.55%, 17.00% and 17.85%, and the diffusion coefficients of H₂O into PU_PBT are 1.488×10⁻⁶, 1.999×10⁻⁶, 4.086×10⁻⁶ and 4.462×10⁻⁶ cm²/s, respectively, at the temperature of 298, 318, 338, 358 K. And the diffusion of H₂O into PU_PBT is not a uniform diffusion, but a jump-motion diffusion in free volumes. The solubility coefficient of H₂O molecules in PU_PBT is the major factor that affects the permeability coefficient of the system. With the increase of temperature, the permeability coefficient of H₂O into PU_PBT decreases gradually.