Thursday, October 20, 2011: 1:24 PM
200 D (Minneapolis Convention Center)
We combine molecular simulations and ab initio calculations to investigate the effects of functional groups (cyano, trifluoromethyl, phenylsulfone and carboxyl) in polymers of intrinsic microporosity (PIMs) on the permeation and separation of CO2/N2. A robust equilibration protocol is proposed to construct model membranes with predicted densities very close to experimental data. The fractional free volumes (FFVs) in PIM-1 (with cyano), TFMPS-PIM (with both trifluoromethyl and phenylsulfone) and CX-PIM (with carboxyl) are 45.2%, 42.1% and 38.7%, respectively. Hydrogen bonding is observed to form among carboxyl groups, which contributes to the lowest FFV in CX-PIM. From wide angle X-ray diffractions, the estimated d-spacing distances agree well with available experimental results and the chain-to-chain distance in CX-PIM is the shortest among the three membranes. Ab initio calculations reveal that the interaction energies between the functional groups and CO2 decrease as carboxyl > phenylsulfone > cyano > trifluoromethyl; consistently, the simulated solubility coefficient of CO2 is the largest in CX-PIM. The simulated diffusion coefficient decreases with reducing FFV and correlates well with FFV. While the solubility selectivity of CO2/N2 increases in the order of PIM-1 < TFMPS-PIM < CX-PIM, the diffusivity selectivity remains nearly constant. The permselectivity follows the same trend as the solubility selectivity, which is experimentally observed. This computational study provides microscopic insight into the role of functional groups in gas permeation and suggests strong CO2-philic groups should be chosen to functionalize PIM membranes for high-efficiency CO2/N2 separation.