Studies of the synergistic effects of crosslink density and crosslink inhomogeneity on crosslinked PEO membranes for CO2-selective separations

Abstract

Crosslinked poly(ethylene oxide) (PEO) membranes are attractive for performing CO2 separations. However, the effects of crosslink density on CO2 separation performance vary widely among reported crosslinked PEO systems, while crosslink inhomogeneity, the uneven distribution of crosslinks in the network, has been largely unexplored. This work reports on a series of PEO-based, model crosslinked membranes with systematically varied crosslink density and crosslink inhomogeneity. Crosslink density was controlled via end-crosslinking commercial Jeffamine® polyetheramines of specific molecular weight of 148, 600, 900, or 2000g/mol with an epoxy terminated PEO; the distribution of crosslinks was controlled by the timed addition of PEO diamine oligomers of differing molecular weight. Three distinct series of networks were produced in increasing crosslink inhomogeneity levels: unimodal, bimodal, and clustered. It was found that while decreasing crosslink density is a prominent factor in increasing CO2 permeability, further improvements in overall CO2 separation performance were realized by adjusting crosslink inhomogeneity. In particular, both the bimodal and clustered films outperformed respective unimodal films in CO2 permeability and CO2/H2 selectivity, suggesting the important role of crosslink inhomogeneity in dictating gas transport properties. These crosslinked PEO films approach or surpass the most recent Robeson upper bounds for pure gas CO2/H2 and CO2/N2 separations, making them attractive membranes for H2 purification and carbon capture.