Wettability of smooth and rough surfaces of perfluoroacrylate copolymers
Şimşek, Eren (2006) Wettability of smooth and rough surfaces of perfluoroacrylate copolymers. [Thesis]
In this study, it was primarily shown that random copolymers of perfluoroacrylates can have sufficient hydrophobicity to be effectively employed for the production of ultrahydrophobic surfaces via electrospinning. Presence of small molar amounts of perfluoroacrylate in the copolymer chain resulted in the formation of rather hydrophobic copolymers surfaces that had lower surface free energy than poly(tetrafluoroethylene) due to the orientation of fluorinated groups to the solid-vapor interface on the outermost surface layer. The surface energy measurements of the copolymers indicated that amphiphilic copolymers may have lower surface free energy than the copolymers of perfluoroacrylates with non-polar monomers due to the higher excess of the fluorinated groups on the surface. However, it was also shown for the ultrahydrophobic surfaces of amphiphilic copolymers that reorientation of polar groups to the solid-liquid interface due to water contact on the protrusion tops, where the long interval solid-liquid contact took place, can increase the threshold water sliding angle on the surface remarkably while the high advancing contact angles were maintained for long days. The former was detected to be a direct result of the enhanced adhesive bonds between the three phase contact line and the tops of the solid protrusions, which prevented the receding of the drops; while the latter was attributed to the preserved composite surface structure by the inability of water in penetrating through the hydrophobic walls of the cavities. Consequently, ultrahydrophobicity was lost while superhydrophobic character was preserved. This result one more time showed that high advancing contact angle values do not indicate water repellence all the time. Due to the stability of the styrene-perfluoroacrylate copolymer surface against water exposure, this polymer was chosen to study for further improvements such as enhancing the ultrahydrophobic character. Experiments showed that the effect of applied voltage in electrospinning on the water repellence of surfaces electrospun at low solution concentrations is remarkable. By increasing the applied voltage to high values, a surface on which the microtopology was composed of nanometric beads covering the micron level roughness everywhere was produced. The resultant surface showed no contact angle hysteresis and was perfectly non-wetting, and exhibited no adhesion with water while a pendant drop was made to touch and retreat from the surface.
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