Reliable bonding of high-performance membranes onto polymeric supporting structures is critical for capitalizing their potentials within practical filtration applications. The successful bonding typically requires infiltration of the membrane pores by a thermoplastic polymer, driven by capillary pressure and/or external pressure. In this work, we systematically examine the capillary infiltration of a polypropylene (PP) within polyethersulfone (PES) membranes with a highly asymmetric pore structure, a nominal pore size of 20 nm, and varying degrees of hydrophilicity. Most significantly, the infiltration kinetics was strongly influenced by the asymmetric pore structure in two aspects: (1) the time to achieve full infiltration from the large-pore side was approximately 4 times shorter than that from the tight-pore side; (2) When bonding from the tight-pore side, the infiltration depth, L(t) showed L (t) ∼ t1.6, instead of characteristic L (t) ∼ t0.5. The accelerated infiltration rate over time was successfully modelled with the Cai model using depth-dependent pore size that captures the asymmetric pore structure. Furthermore, chemical modification reduced initial infiltration rate only, which is attributed to the reduction in surface porosity. No significant difference in infiltration kinetics at the later stage was observed. Mechanical integrity tests of the bonded samples display complex debonding behaviors including complete peeling, incomplete peeling, and complete membrane failure. The peel force corresponding to membrane failure appeared larger than the other two debonding modes, all of which showed insignificant dependence on the membrane chemistry or infiltration depth. Post-mortem analysis of the completely peeled sample showed PP nanofibers were pulled out of the PES membranes during debonding, emphasizing relatively weak mechanical interlocking due to the low surface porosity. Display omitted •Capillary infiltration kinetics of PP within highly asymmetric PES membranes were determined.•The infiltration kinetics shows strong dependency on the asymmetric pore structure.•The infiltration kinetics is modelled by the Cai model with pore-size gradient.•The bonded PP/PES membrane display complete peeling, incomplete peeling and membrane fracture behaviors.