Surface Segregation of Zinc Oxide Nanoparticles in Polymer Films and Fibers: Implications for Functionalized Fabrics

Fibers and fabrics whose surfaces are decorated with metal oxide particles are finding important applications, including antimicrobial, filtration, and catalytic ones. In the present study, a very simple method has been developed to functionalize polypropylene (PP) surfaces, including those of fibers, with metal oxide nanoparticles (NPs). Extruded fibers and spin-coated films have been prepared from solid mixtures of PP and zinc oxide NPs using either 12 or 250 kDa PP (PP12k and PP250k, respectively). In the case of PP12k films containing 5 wt % ZnO, the segregation of the NPs to the polymer–air interface is detected by the emergence of Zn2p X-ray photoelectron spectroscopy (XPS) signal after either room temperature ozone exposure or annealing at 140 °C in an oxygen-containing atmosphere. For PP250k films, ozone exposure without heating does not induce surface segregation; however, it does occur upon annealing at 160 °C in an oxygen-containing atmosphere. Annealing in air causes the atomic percentages of Zn detected by XPS to increase from 0 to 0.22 and 0.34% for the PP12k and PP250K films, respectively. Cross-sectional scanning electron microscopy of a 3 wt % ZnO in PP12k fiber shows that NP migration to the surface does not deplete the bulk of ZnO, implying that the migration of NPs to the surface occurs from the near-surface region. Adsorption of a phosphonated dye, flavin mononucleotide, on surface-segregated films and fibers is confirmed by fluorescence microscopy, indicating that the ZnO NPs are available for surface reactions. XPS and Fourier transform infrared spectroscopy of PP exposed to ozone or heated in air demonstrate the presence of ketone and carboxylic acid functional groups. Heating of a ZnO surface-segregated PP12k film in ultrahigh vacuum to partially reverse oxidation also reverses surface segregation. It is postulated that surface segregation is enthalpically driven by the attraction of the ZnO NPs to the polar surface functional groups. This work may be useful for the preparation of fibers and fabrics for catalysis/photocatalysis applications.