The molecular basis of CO2 interaction with polymers containing fluorinated groups:: computational chemistry of model compounds and molecular simulation of poly [bis(2,2,2-trifluoroethoxy)phosphazene]

Ab initio molecular orbital calculations of CO2 and model compounds have been used to identify the nature of specific interactions between CO2 and the fluorinated substituent groups of polymers such as poly(trifluoropropyl methyl siloxane) and poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) that exhibits high CO2 permeability and perm selectivity. Second-order Moller-Plesset (MP2) perturbation calculations (6-311++G** basis set) were used to obtain energies, dipole and quadrupole moments, and polarizabilities of CO2, three alkanes (CH4, CH3CH3, and CH3CH2CH3), and three fluoroalkanes (CF4, CH3CF3, and CH3CH2CF3 Results of energy calculations of three CO2-alkane and three CO2-fluoroalkane dimers indicate that CO2 forms a favorable quadrupole-dipole interaction with the fluoroalkyl groups. The maximum quadrupole-dipole interaction energy obtained was -11.5 kJ mol(-1) for CO2-CF3CH2CH3. This value is less than interaction energies typical for hydrogen bonding but greater than the London dispersion values reported for the interaction Of CO2 with the carbonyl group of poly(methyl methacrylate) (PMMA). Electrostatic potential distributions indicate a small redistribution of electron density to the fluorine atoms of the trifluoroalkanes and to the oxygen atoms of CO2 in the CO2-CF3CH3 and CO2-CF3CH2CH3 dimers. Sorption and molecular association Of CO2 with PTFEP has been investigated by molecular simulation of an amorphous cell using the COMPASS molecular mechanics force field. CO2 Sorption isotherms obtained by Grand Canonical Monte Carlo (GCMC) simulation indicate an upward deviation from the linear relationship between log S and the Lennard-Jones potential well depth parameter, epsilon/k, in agreement with reported permeability data. Pair-correlation analysis obtained from molecular dynamics simulation show strong correlation Of CO2 with the trifluoromethyl group of PTFEP in agreement with the MP2 results showing an association Of CO2 with both CH3CF3 and CH3CH2CF3. (C) 2003 Elsevier Science Ltd. All rights reserved.