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Glassy 6FDA-based polyimides are attractive membrane materials for many industrially important gas separations; however, plasticization in aggressive natural gas often causes significant loss in CO₂/CH₄ selectivity. In this study, a family of thermally cross-linkable 6FDA-based polyimides containing carboxyl groups were synthesized and cross-linked as dense films via decarboxylation-induced thermal crosslinking. As well as chemical structure, crosslinking temperatures above and below T_g are shown to be synergistic tools. Our results demonstrate that structure evolution during the decarboxylation-induced crosslinking process is affected significantly by the 3,5-diaminobenzoic acid (DABA) content in the polymer backbone. Even for a 50/50 CO₂/CH₄ binary mixture with feed pressure up to 800 psia, decarboxylation-induced crosslinking provides attractive gas separation performance and plasticization …