Molecular insight on gas transport in polymer membranes unveiled by NMR

Significance 

Generally, polymers comprise of a molecular structure built up mainly from a large number of similar units bonded together. This material has found many applications since its discovery. In recent times, development of novel polymer membranes for gas separation applications has motivated intense research. This is due, in part, to the fact that gas separation with polymer membranes is energetically more efficient than conventional separation processes. Nevertheless, despite the intensive effort, the landscape of polymers used in the gas industry has not change significantly over the past two decades. This lack of progress in translating results from lab to plant could be attributed to a variety of reasons, but most significantly to the limited knowledge on gas transport in polymers at the molecular level. Attempts to resolve this issue have led to propose a nuclear magnetic resonance (NMR) method that combines multinuclear spectroscopy and pulsed gradient spin echo (PGSE) in order to get a better understanding of the gas transport in polymer membranes at the molecular scale.

Recently, Drs. Leoncio Garrido and Julio Guzmán from the Institute of Polymer Science and Technology (ICTP) at the Higher Council of Scientific Research (CSIC) in Spain presented a study in which they described the findings on the diffusive behavior of pure and mixed carbon dioxide, ethylene, and methane in membranes of a certain glassy polymer (abbreviated as 6FDA-TMPDA), using 1H and 13C PGSE NMR at several pressures and diffusion times. Also, they compare the diffusivity of those gases in the glassy polymer to that in polydimethylsiloxane (PDMS), a rubbery polymer at the temperature at which the measurements were performed. Their work is currently published in the research journal, Macromolecules 2018.

1H and 13C spin−lattice and spin−spin NMR relaxation times and PGSE diffusion measurements were taken by placing polymer membrane strips inside a specially adapted NMR tube at moderately pressurized gases, in a 9.4 T NMR spectrometer.

The authors reported that the relaxation time measurements showed a mono-exponential behavior for all gases, suggesting that either gas molecules were noting a single environment or they were in fast exchange between possible multiple sites. In addition, PGSE NMR diffusion showed that the diffusion coefficients of 13CO2 in the glassy membrane were independent of the diffusion time in all cases studied, similar to the behavior observed in rubbery membranes. In contrast, the diffusive behavior of methane and ethylene in the glassy membrane was found to be dependent on the diffusion time. Moreover, the presence of carbon dioxide and ethylene altered the behavior of methane, reducing the diffusion time dependence and increasing the magnitude of its diffusion coefficient.

In summary, the study presented an in-depth investigation of the spin−lattice and spin−spin relaxation times and the diffusive behavior of pure and mixed carbon dioxide, methane, and ethylene in glassy membranes of 6FDA-TMPDA using multinuclear spectroscopy and PGSE NMR. A model is proposed to determine the diffusion coefficients of gases exhibiting abnormal (time-dependent) diffusive behavior. Leoncio Garrido and Julio Guzmán finding that the diffusion coefficient of methane increased in the presence of 13CO2 and ethylene, suggests that the sieving properties of the glassy polyimide membranes are compromised even at low concentrations of condensable gases.

Molecular insight on gas transport in polymer membranes unveiled by NMR - Advances in Engineering

About the author

Leoncio Garrido graduated in Chemistry in 1977 and was granted the title Dr. in Chemical Sciences by the University of Valladolid, Spain, in 1982. He did postdoctoral research at the Department of Chemistry, University of Cincinnati, Ohio. After two years in an industrial position in Madrid, Spain, as head of the research and development laboratory, he joined the Department of Radiology at Massachusetts General Hospital, and Harvard Medical School, Boston, Massachusetts, from 1986 to 2000. Then, he moved to the Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas, in Madrid, Spain.

He was the Director of the ICTP from 2001 to 2005, and currently he is a research scientist at the Department of Physical Chemistry, ICTP. He is honorary member of the Institute of Neurosciences, University of La Laguna, Spain, and editorial board member of Scientific Reports.

His research interest includes the development of NMR hardware and methods, with emphasis on diffusion and imaging, to investigate current problems in material science and biomedicine. Also, he is interested in the design of polymer scaffolds for applications in tissue regeneration of the central nervous system.

ORCID ID: 0000-0002-7587-1260

Researcher ID: K-3092-2014

About the author

Julio Guzmán is Ad Honorem Research Professor at the Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas, in Madrid, Spain. After a research stay at CNRS in Lyon, France, he received his PhD from the Universidad Complutense of Madrid, in 1972. Subsequently, he worked three years in an industrial position. Then, he joined the ICTP in 1975 and retired as Research Professor in 2015.

The main interest of his research along these years focused on the study of statistical, mechanical and dielectric properties of polymers, kinetics of polymerization and molecular and ionic transport in polymer membranes.

Dr. Guzmán has published more than 185 scientific papers, holds 2 patents, and has supervised 11 PhD students.

Reference

Leoncio Garrido, Julio Guzmán. Influence of Diffusion Time on the Diffusion Coefficients of Gases in Polymers Determined by Pulsed Gradient Spin Echo NMR. Macromolecules 2018, volume 51, page 8681−8688

Go To Macromolecules 

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