Multi-scale Molecular Modeling Approaches for Designing/ Selecting Polymers used for Developing Novel Membranes

ELENA TOCCI AND PLUTON PULLUMBI

1.1 Introduction

During the last decade computational chemistry and numerical simulations have had a favorable impact in almost all branches of materials research ranging from phase determination to structural characterization and property prediction. An important effort has been focused on developing simulation tools to describe thermodynamic and transport properties of confined fluids. The present contribution illustrates the benefit of coupling experiment to molecular modeling for selecting novel membrane materials with better separation properties for given gas mixtures as well as the limitations of the existent computational methodologies. New modeling and simulation tools based on multi-scale hierarchical modeling are needed to cope with the complexity of materials and associated phenomena at different length and time scales.

Transport properties of small molecules in amorphous polymer matrices play an important role in many industrial applications such as gas separation of mixtures, packaging applications ranging from food conservation to controlled drug and cosmetics release, to special coatings for protecting specific substrates from gases.

Different aspects of technology and industrial application of polymer membranes from materials research to permeator design to their optimal configuration to enhance processes performance have been discussed in detail in a previous lecture and recently reviewed in the literature. The potential application of a polymer as a separation membrane depends upon the selectivity towards the gas to be separated and the permeate flux. The selectivity determines the product purity and recovery whereas the permeability is related to the productivity of the membranes. This means that both the permeability and the selectivity should be as large as possible. The control of gas permeability and permselectivity of polymer membranes has become a subject of active research with worldwide participation in both industrial and academic laboratories. The design and optimization of polymer membranes used in gas separation applications would be possible if reliable predictions of transport properties could be made rapidly in advance of synthesis and experiment. The actual status of available commercial software for modeling transport phenomena in polymer membranes, does not allow the development of de novo material design approach. This is due not only to formidable time and length scales involved, but also to lack of detailed information on time evolution of the free volume and its distribution as a function of processing history during the manufacturing process. Rapid progress in computational methodology and validation of new simulation tools is improving the understanding of different facets of gas transport in polymer membranes and building the necessary tools for their effective use in materials design. The possibility to predict transport properties of small molecules through polymer matrices permits the rational selection of polymer materials used in these applications and their optimal design. Although there has been reported an increasing number of studies on this subject over the last years, the prediction of transport properties of gas molecules through glassy polymer membranes remains a difficult target. In many of the recent studies reporting molecular simulation predictions of diffusion and solubility of small gas molecules in several membrane models of the same glassy polymer a great scatter of the predicted values is observed. These results clearly indicate that the quality of the packing of the polymer chain into an amorphous cell membrane model strongly impacts the predicted gas transport properties.

The potential application of a polymer as a separation membrane depends upon the possible throughput and the purity of product. This means that both the permeability of the gas that is transported more rapidly and the selectivity should be as large as possible. The permeability coefficient, Pe, of a small molecule through a polymer membrane is defined as:

Pe = D · S (1.1)

the product of the diffusion coefficient, D (kinetic parameter), and of the solubility coefficient, S (thermodynamic parameter). The estimation of these coefficients can be done, either by molecular dynamics (MD) and grand canonical Monte Carlo simulations, or by applying the transition state theory (TST) approach provided that the quality of the membrane amorphous cells used in the calculation represent the real distribution of torsion angles, of the free volume and its distribution, as well as the structural, conformational and volumetric properties of polymer membranes. The selectivity of a polymer membrane for a pair (i, j) of gas molecules is characterized by...