A low crosslink density (LCD) network-PIM-1, which offers high compatability with the polymer of intrinsic microporosity PIM-1, is synthesised by a modified PIM-1 polycondensation that combines both a tetrafluoro- and an octafluoro- monomer. To maximise the advantages of utilising such crosslinked PIM-1 fillers in PIM-1-based mixed matrix membranes (MMMs), a grafting route is used to decorate the LCD-network-PIM-1 (dispersed phase) with PIM-1 chains, to further enhance compatability with the PIM-1 matrix. Mixed-gas CO2/CH4 (1:1, v:v) separation results over 160 days of membrane aging confirm the success of a relatively short (24 h) grafting reaction in improving the initial CO2 separation performance, as well as hindering the aging of PIM-1/grafted-LCD-network-PIM-1 MMMs. For MMMs based on a 24 h grafting route, all the gas separation data surpass the 2008 Robeson upper bound by a significant margin, and the 160-day aged membranes show only 29% reduction from the initial CO2 permeability, which is substantially less than the equivalent losses of nearly 70% and 48% for PIM-1 and traditionally-fabricated MMM counterparts, respectively. These results demonstrate the potential of network-PIM components for obtaining much more stable gas separation performance over extended periods of time.