Membranes with excellent stability and stable separation performance under complex or harsh conditions are essential to a successful membrane separation system. Herein, we report a new design of robust crosslinked polymer membranes with unimodal network structures that exhibit superior separation performance surpassing the 2008 upper bound for H₂/CH₄ separation at both ambient (35 °C) and elevated temperatures (up to 180 °C). The crosslinked membranes are based on pentiptycene-containing polybenzoxazole (PPBO) structures prepared via a new controlled crosslinking method, i.e., end-linking of telechelic PPBO oligomers with controlled molecular weight and a thermally curable phenylethynyl group at the chain ends. Using the synthesized oligomer with various molecular weights (4000–12 000 g/mol), the consequent crosslink density of unimodal networks was comprehensively varied. A nonlinear relationship between membrane properties and the crosslink density is observed, which is ascribed to the competing effects between crosslinking-induced densification and the formation of bulky benzoxazole and phenyl groups at the crosslink joints. The concept of unimodal networks exemplifies a novel approach that is able to finely tailor crosslinked microstructures and consequently maximize the membrane separation performance at elevated temperatures.