Hari Yadav, An-Tsung Kuo, Shingo Urata, and Wataru Shinoda,*
Langmuir, 35, 14316-14323 (2019).
A
good understanding of the surface hydrophobicity of fluorinated materials is
useful for their application as coating materials. The present study
investigates the surface hydrophobicity of perfluoroalkyl acrylate (PFA) thin
films using molecular dynamics (MD) simulations. Surface hydrophobicity is
characterized by examining the contact angle of a water droplet on PFA surfaces
and the cavity formation free energy in the vicinity of the surface. It is
found that the calculated microscopic contact angles are in good agreement with the
experimental results and partially capture the difference in the hydrophobicity
of the surface arising from the variation of packing density and side chain
length of PFA. The variations of cavity formation free energy in the vicinity
of the surface elucidate that the surface hydrophobicity is mainly governed by
the packing density rather than the chain length of PFA. The hydrophobicity
generally increases with decreasing the packing density to some extent, and
then turns to decrease as further reducing the packing density. At the higher
packing density, the surface hydrophobicity slightly decreases with increasing
the chain length while, at the lower packing density, the surface
hydrophobicity is increased when chain length of PFA is longer than 6 carbons.
Furthermore, we found that the influence of packing density on the surface
hydrophobicity is directly related to the variation of the surface roughness
and chain flexibility. That is, the surface hydrophobicity increases with
increase in the surface roughness, while the chain flexibility plays a
secondary role in the enhancement by affecting the stability of water staying
near the interface. The study provides a significant insight into the local
hydrophobicity and microscopic structure of the PFA surfaces, which would be
useful for the application of surface modification.