Thermo-responsive stick-slip behavior of advancing water contact angle on the surfaces of poly( N -isopropylacrylamide)-grafted polypropylene membranes

Wettability of a solid surface is highly important to its practical application, especially for the surface that shows thermo-responsive properties. In this paper, we describe a thermo-responsive stick-slip behavior of water droplets on the surfaces of poly(N-isopropylacrylamide) (PNIPAM)-grafted polypropylene membranes. Field emission scanning electron microscope (FESEM) images elucidate that the morphology of PNIPAM-grafted membrane surface is thermo-responsive, i.e., the surface becomes rougher above the lower critical solution temperature (LCST) of PNIPAM. On the surface of nascent polypropylene membranes, the water droplet shows a smooth motion resulting in advancing and receding water contact angles of 111° and ∼65°, respectively. On the PNIPAM-grafted membrane surfaces, the water droplet shows a stick-slip pattern above the LCST, whereas it advances smoothly below the LCST. This phenomenon is reproducible and can be ascribed to the energy barriers enhanced by the shrink of PNIPAM chains above the LCST. We also find that the slip contact angle decreases from 102° to 92° after several stick-slip cycles. This decrease is attributed to the water adsorption on the grafted PNIPAM layer, which is confirmed by the continuous decrease of the receding water contact angle.