Collaborative Research: Beyond fluorine: molecular-level understanding of the effect of bromination and chlorination on polymer membrane transport property and long-term stability

Conventional thermal-based chemical separation technologies, such as distillation and absorption, are highly energy inefficient, consuming about 8 GJ per person per year globally. Additionally, these methods produce significant carbon emissions, raising serious environmental and health concerns. Polymer membranes are a more energy-efficient chemical separation technology that can improve energy efficiency by up to 90% when used alone or with traditional methods.

These membranes separate substances based on their ability to move through the membrane material (permeability). Many new membrane materials capable of effectively separating desired gas molecules from mixtures (selectivity) have been developed. However, few of these materials have been adopted in industry due to issues with selectivity, durability, and robustness under real-world conditions.

Challenges include plasticization from contaminants in the feed stream and physical aging, which reduces permeability over time. Improving membrane performance requires a deep understanding of the molecular mechanisms behind selectivity and long-term durability; developing such knowledge is the primary goal of this project. Fluorinated polymer membranes are known for their high permeability and resistance to aging.

Still, they face problems with plasticization and low selectivity and are difficult and expensive to produce. This project will combine experimental and theoretical approaches to explore the potential use of alternative halogenated polymers, including chloro-polymers and bromo-polymers, as robust membrane materials. The study will investigate how chlorination and bromination affect polymer membranes' selectivity and long-term stability.

This research will help scientists and engineers design more effective polymer gas separation membranes using halogenation to optimize selectivity and durability. Additionally, this project will engage students and the public in Oklahoma and Indiana through various STEM activities. These include educational and research opportunities for high school students, seminars for young researchers, the creation of online databases to standardize experimental protocols and improve reproducibility, and training a diverse group of future leaders in chemical engineering.

The investigators hypothesize that carefully controlled degrees of polymer chlorination or bromination are sufficient to simultaneously enhance polymer gas membrane sorption-selectivity, diffusion-selectivity, and long-term stability, in contrast with fluorination, which is often ineffective in enhancing selectivity and plasticization stability. Moreover, there is evidence that the effects of chlorination and bromination on membrane hydrophilicity are profoundly different from those observed upon polymer fluorination.

To test these hypotheses, the team will design, synthesize, and characterize a family of new halogenated polymer materials exhibiting systematically varied degrees of fluorination, chlorination, or bromination. The molecular structure of the newly synthesized halogenated polymers will be examined in detail and correlated with selectivity and long-term stability behaviors using advanced thermodynamic models in conjunction with state-of-the-art experiments, including permeability, plasticization, and physical aging measurements under complex gas mixtures and in the thin film composite membrane configuration.

Successful execution of this research will unlock unique opportunities to design polymer membranes exhibiting previously unattainable selectivity and stability and de-bottleneck molecular separations in areas of strategic importance for the U.S. economy, welfare, and national safety.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.