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The development of cost-effective technologies for the efficient capture and storage of CO₂ can help reduce the emission of greenhouse gases to the atmosphere and improve the efficiency of processes in the petrochemical industry that rely on these separations. The aim of this project is to develop computational procedures for optimizing the structure of polymers for use in adsorption-based separations, and hence to generate a polymer-based porous material with significantly higher selectivity and capacity for carbon dioxide adsorption than materials currently available.

Polymers of Intrinsic Microporosity (PIMs) are a novel class of porous polymer with potential applications in storage, separations, and purification.  Functionality in PIMs can be directly embedded in the material framework, avoiding the need for grafting amine groups and eliminating the presence of metal centers as in metallic organic frameworks. PIMs are disordered materials, but the chemistry of the framework is well controlled in comparison for example with activated carbons. They can have therefore some of the advantages associated with disordered materials, while maintaining control at a nanometer scale by knowing precisely the structure and chemistry of the monomers. The general structural scheme of PIMs is one of rigid backbone segments connected by non-linear or non-planar sites of contortion, which allows for a wide range of chemistries and resultant materials.

During the first year of this project we have concentrated on generating a realistic polymer structure. Development of realistic models of complex polymeric systems is a challenging task, because efficient packing at high densities requires sophisticated computational methods. As a first approximation we have generated the structure of PIM-1 under the assumption that the framework is rigid due to the sequence of connected aromatic rings. In fact, most of the interaction potentials developed and tested for the determination of thermodynamic and transport properties of analogous small molecules assume that aromatic rings are rigid; therefore, in perfect chains, the only sites in the framework where PIMs could be flexible are the diether (or diamine) ring and the spiro-center. A polymer chain was constructed using a biased growth algorithm where a polymer chain is obtained by adding repeat units one at a time; this is a suitable computational model, although in practice these polymers are not synthesized by chain growth but by a step polymerization of two monomers. Through molecular dynamics compression and relaxation cycles, a virtual amorphous sample was created.  We have evaluated the United Atom and the TraPPE force-fields. Initially, these potentials were judged for quality based on their correspondence to low pressure experimental adsorption, where the effects of swelling are expected to be minimal.

Via grand canonical Monte Carlo (GCMC) simulations, adsorption isotherms and heats of adsorption were calculated.  Despite the complexity of PIM-1, the simulation results demonstrate the effectiveness of our model when compared to experimental data.  The simulations predict CO₂ uptake in qualitative agreement with experimental results.

During the second year of this project we will characterize the influence of adsorbent size, valence asymmetry, and morphology. We shall also study the effects of varying the chain length, and the position of the functional groups. We expect to present the results of this project at national and international conferences, as well as to publish them in recognized journals in the field.