P-DNN-C3 and C4 are case studies into this balancing act. They achieve high permeability primarily through methyl groups and large aliphatic rings, which is a relatively underexplored strategy in membrane design. At the same time, these polymers attain unprecedented O2/N2 selectivity via the incorporation of polar groups, such as carbonyls and sulfonyls. By contrast, P-DNN-C1 and C2 each feature an inflexible polycyclic backbone and two trifluoromethyl containing side chains. Amazingly, they demonstrate that our DNN model learns the importance of bulky spherical groups (such as trifluoromethyl groups) for creating steric frustration , which has been recognized in the gas separation community as favoring higher gas permeability .