UV-induced switchable wettability between superhydrophobic and superhydrophilic polypropylene surfaces with an improvement of adhesion properties

Polypropylene (PP) surfaces with reversible switching between superhydrophobicity and superhydrophilicity were fabricated by a simple dip-coating method. The reversibility was obtained using a combination of UV and thermal treatment cycles. Superhydrophobic polymeric surfaces were prepared by dipping PP substrates in a hot suspension of xylene containing TiO₂ nanoparticles (NPs) functionalized with trimethoxypropyl silane (TMPSi). This resulted in superhydrophobic PP surfaces with water contact angles (WCA) of 158° that were converted to superhydrophilic (WCA ~0°) by UV irradiation. However, the superhydrophobic state can be easily recovered using a soft thermal treatment (100 °C for 2 h). A continuous switch in the extreme wettability properties of the surfaces was achieved using cycles of UV irradiation and thermal treatments. Additionally, the hydrophilicity obtained by UV illumination improved the weak adhesion that existed between the nanocoating and the PP surface before the UV treatment by about 90 %. Detailed high-resolution X-ray photoelectron spectroscopy data showed that the relative concentration of the hydrophilic component, Ti–OH, in the O 1s signal increased from 32 % to more than 50 % when the surface was irradiated. Simultaneously, the Ti 2p signal showed a reduction of Ti(IV) to Ti(III) after the photochemical treatment leading to a surface hydroxylation and the superhydrophilicity found after UV irradiation. This paper reports the preparation of PP surfaces with excellent controllable wettability with important applications where improvement in the coating adhesion is necessary.