The Wettability of Fluoropolymer Surfaces:  Influence of Surface Dipoles

Abstract

The wettabilities of fluorinated polymers were evaluated using a series of contacting probe liquids ranging in nature from nonpolar aprotic to polar aprotic to polar protic. Fully fluorinated polymers were wet less than partially fluorinated polymers, highlighting the weak dispersive interactions of fluorocarbons. For partially fluorinated polymers, the interactions between the distributed dipoles along the polymer backbone and the dipoles of the contacting liquids were evaluated using both polar and nonpolar probe liquids. The results demonstrate that the surface dipoles of the fluoropolymers generated by substituting fluorine atoms with hydrogen or chlorine atoms can strongly interact with polar contacting liquids. The wettabilities of the partially fluorinated polymers were enhanced by increasing the density of dipoles across the surfaces and by introducing differentially distributed dipoles.

Analysis of the Wettability of Partially Fluorinated Polymers Reveals the Surprisingly Strong Acid-Base Character of Poly(vinylidene Fluoride)

“…26 The alteration of -CH 2 -and -CF 2 (or Cl, H)-, that is, an electronpoor group and an electron-rich group gives rise to a distribution of surface dipoles along the backbones of partially fluorinated polymers. 19 The magnitude of the surface dipoles changes if the distance between the two alternating units is varied. For example, the surface dipoles in ETFE are more widely distributed (i.e., further apart), leading to a relatively low density of dipoles when compared to PVDF.…”

“…The surface dipoles in ECTFE might exhibit multiplicity due to the substitution of CFCl for CF2, leading to a higher density of dipoles than ETFE and thus enhanced wettability. 16,19,27,28 Therefore, the magnitude of the surface dipoles of the following partially fluorinated polymers is expected to increase in this order: ETFE < ECTFE << PVDF.…”

“…16 Lee and coworkers extended Zisman's interpretation and showed that the wettability of a surface can be strongly influenced by the distribution of acidic groups and/or surface dipoles across the interface. [17][18][19] In the thermodynamic polymer blending and sorption process, it is widely accepted that the CF 2 dipoles in PVDF are largely responsible for the intermolecular dipolar interaction. [20][21][22][23][24][25] For the technologically important piezo and pyroelectric properties of PVDF, the polymer must be uniaxially oriented to induce a crystalline β-phase such that C-F dipoles are oriented to form a polar unit cell.…”

“…[20][21][22][23][24][25] With strong surface dipoles caused by alternating CH 2 and CF 2 units, PVDF is expected to possess acidic hydrogens because the fluorine atoms highly polarize the neighboring CH2 groups. 19 In this report, we systematically examine the complex operation of dipole-dipole and acid-base interactions on the surface wettability of partially fluorinated polymers with various contacting liquids. In particular, the non-dispersive interactions arising from surface dipoles and acidity in PVDF will be compared with those in ETFE and ECTFE, which are less polar and possess less acidic hydrogens than PVDF.…”

On-demand droplet loading for automated organic chemistry on digital microfluidics

“…The automated remote operation becomes even more important for radiochemistry, e.g. for positron emission tomography (PET) probe production on digital microfluidics chips [11], in minimizing radiation exposure hazards. Based on the geometry in the top view and cross-sectional views at the electrode edge (e) and the neck (n) regions, the principal radii of curvature at the two sites (r n , r e , R n , R e ) can be determined.…”