Chapter 1. Multi-scale Molecular Modeling Approaches for Designing/Selecting Polymers used for Developing Novel Membranes

Abstract

During the last decades important progress has been achieved in both materials research and materials processing into devices technologies. Membrane separation technology has profited from the progress in these fields and has emerged and consolidated as an important unit operation in chemical engineering offering advantages in operation simplicity and energetic efficiency over conventional separation units. In particular, the membranes for gas separation that have been developed using mainly polymer materials, compete with other separation processes like cryogenic distillation or adsorption due to their easy operation, small size, low energy consumption and space efficiency. Industrial membranes have been developed using principally a trial-and-error approach based on empirical knowledge. The rapid improvements in numerical simulation and modeling methods and algorithms together with the continuous increase in computer speed and increasingly improvements in supercomputer architectures opens perspectives for accelerating the research and development through contributing to the two inter-related axes of membrane improvements that are materials research and process design and optimisation. Several numerical simulation approaches have been developed and used to improve understanding of different aspects of gas separation by membrane technology. These methods cover a very large range of time and length scales going from atomistic description of polymer membrane to the mesoscale reproduction of membrane morphology and to macroscopic modeling of gas separation by the membrane module. Gas transport phenomena through polymer membranes is modeled on one hand using several numerical approaches to cover the several orders of magnitude of experimentally measured diffusion coefficient, D (D is of the order of 10−6 cm2 s−1 for rubbery polymer membranes and of the order of 10−9 cm2 s−1 for glassy polymer membranes) and on the other describing the gas solubility coefficient, S, in realistic models of the membrane. The selection of the adapted simulation method to a given gas/membrane system depends on the mobility of the polymer chains packed in the amorphous cell model of the membrane as well as the characteristic jumping mechanism of the gas penetrant molecules.The actual status of available commercial software for modeling transport phenomena in polymer membranes, does not allow the development of de novo polymer material design methodologies. 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 membrane manufacturing process. New simulation methods based in coarse-grained techniques as well as mesoscale description of membrane structure and morphology obtained under well defined fabricating constraints are being developed for 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 quantitative structure property relationships (QSPRs) approach has been successfully employed in pharmaceutical, biomedical and fine chemical industries for accelerating the research of compounds exhibiting desirable properties. In order to apply the QSPRs methodology for the discovery of new polymer materials for gas separation by permeation technology, the integration of different aspects of experimental fabrication and computational approaches to simulate these materials is of fundamental importance.