Insights into molecular engineering of membranes based on fluorinated polyimide-polyamide miscible blends which do not obey the trade-off rule

This study focuses on the blending technique as a successful approach to improve gas separation performance of a series of polyimide (PI) - polyamide (PA) miscible blends by breaking the trade-off rule. Also, it brings an insight into the physico-chemical characteristics of these blends which were discussed in correlation with the polymer blend composition and structural elements of each component. Thus, the blends were analyzed with respect to their miscibility, morphology, crystallinity, optical transparency, dielectric and mechanical properties. The dielectric spectroscopy and dynamo-mechanical analysis were used to evidence the overall and local chain mobility and their influence on primary and secondary relaxations. The dielectric constant and conductivity of polymer blends were found to be notably higher when compared to the reference PI, increasing with the PA quantity. The gas permeation behavior of the blends was evaluated with respect to the influence of PA content on performances. It was found that gas permeability increases with the rise of PA component, following the simple mixing rule with negative deviation for the highest PA content. The enhanced permeability was obtained with a minimum loss of selectivity. The blend with 50 wt% of PA offered the best option of increased permeability and preserved selectivity.