Crystals of highly soluble metal-organic polyhedra 18 (MOP 18) were synthesized and used as additives in Matrimid (R) 5218 to form mixed-matrix membranes. The MOP-18 linker was functionalized with long alkyl chains to help the crystals dissolve in Common organic solvents. Scanning electron microscopy images revealed that the MOP-18 molecules do not aggregate into large particles even at loadings as high as 80 wt%, and the absence of polymer veins or polymer rigidification at the MOP-18/polymer interface was attributed to the improved dispersion of the MOP-18 molecules in the polymer matrix. The increase in the modulus of the membranes from 1.4 GPa for the pure polymer to 2.4 GPa upon incorporation of MOP-18 was attributed to the strong affinity of the polymer chains for the alkyl chains of MOP-18. Gas permeation results showed that the membranes became more permeable as the temperature was increased from 35 to 70 degrees C and that CO2 plasticization, which occurred at 35 degrees C and 8 bar, was minimized when the membranes were heated to 70 degrees C owing to an increase in the plasticization pressure to 21 bar. At 70 degrees C. the H-2/CO2 selectivity remained constant when the transmembrane pressure was increased from 3 to 30 bar. The permeability and solubility data at high pressure and high temperature showed that the pore, core, and alkyl chains of MOP-18 introduced new sorption sites that significantly affected the gas transport properties of the membranes. The results also demonstrated that the MOP-18/polymer interface was mechanically stable at high pressure. (C) 2014 Elsevier B.V. All rights reserved.