Capillary bonding of membranes by viscous polymers: Infiltration kinetics and mechanical integrity of the bonded polymer/membrane structures

Capillary infiltration of porous medium impacts applications across oil recovery, soil science, and hydrology. The infiltration kinetics is typically captured by a range of models that differ in the approximation of pore structures, fluid properties, and filling ratio. Capillary bonding of a porous membrane by a polymer melt is important for membrane device manufacturing. However, both the capillary infiltration kinetics and the resulting bonding strength or mechanical integrity have not been reported. In this work, we measure the kinetics of capillary infiltration of a viscous polypropylene (PP) in polyethersulfone (PES) membranes with a normal pore size of 200 nm and varying degrees of hydrophilicity. The time-dependent infiltration depth was quantified ex situ by imaging the cross-sections of the bonded PP film/PES membranes. The microscopic details of the bonded interface were characterized by high-resolution nanomechanical imaging, while the contact angles of PP on the PES surfaces were measured by the sessile droplet method. The results show that the infiltration kinetics at 180 °C is better described by the Cai model that incorporates membrane pore structures (porosity, tortuosity, pore size), compared with the basic Lucas Washburn model intended for isolated cylindrical pores. The infiltration kinetics at 200 °C appears significantly slower than the predictions of both models, which is hypothesized to be a result of pore deformation/collapse due to the capillary pressure when the PES approaches the rubbery state. Regardless of bonding temperature, the resulting mechanical integrity of the bonded PP film/PES membrane, as quantified by a modified T-peel test, is dictated by the fracture strength of the membranes and weakly decreases with the increase of infiltration depth.