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We systematically reduce the crosslink density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride crosslinking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (PA) separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating 15 mol % of the difunctional chain extender, we reduce the crosslink density of the network by a factor of two, which leads to an 80% increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30% increase in the salt passage through the membrane. Surprisingly, this leads to a …