(459b) Suppressing Substructure Collapse in Carbon Molecular Sieve (CMS) Hollow Fibers

Carbon Molecular Sieve (CMS) membranes exceed the so-called polymer permeability/selectivity trade-off upper bound for most challenging gas-pairs separations. Asymmetric hollow fibers are preferred membrane formats, due to the high surface area to module volume ratios this format enables. During pyrolysis to form CMS, the porous asymmetric polymeric precursor support tends to collapse, thereby reducing productivity. Previous work showed that this support collapse could be overcome for Matrimidâ polyimide precursors by pretreating precursor fibers with vinyltrimethoxysilane (VTMS) (henceforth to be referred as V-treatment). Such V-treated CMS fibers show little support collapse, with much higher permeances as compared to the untreated fibers. CMS fibers pyrolyzed from 6FDA BPDA: DAM (henceforth to be referred as 6F), a fluorinated copolyimide with higher intrinsic permeability than Matrimidâ , shows less sub-structure collapse than Matrimidâ even without V-treatment. The fundamentally different response between these two polyimide precursors reflects a smaller temperatures interval between the glass transition temperature and temperature of the onset of carbonization for the 6F vs. Matrimidâ. Optimization of the V-treatment process under varied pyrolysis conditions will be discussed to illustrate approaches to achieve improved performance relative to the process conditions for Matrimidâ. Fundamental factors controlling performance will be discussed to show how the versatile V-treatment process can be optimized for different precursors, temperatures and advanced pyrolysis options, such as doping with trace O₂ during pyrolysis.