Wetting Dynamics of Aqueous Surfactant Solutions on Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 86

Figure 14. Water contact angles for spin-coated Teflon film and industrial PTFE sheets.
Figure 15. Measured and corrected contact angles for a C 12 E 5 drop on Teflon AF (c = cmc).
critical concentration values (cmc) were experimentally determined. Aqueous solu-
tions of these surfactants with desired concentrations in the range 0.0625 to 2 part
of the corresponding cmc were prepared by dilution from the corresponding stock
solutions with reagent-grade water.
In a typical experiment, the drop volume decreases linearly with time begin-
ning at approximately 5 s. Proceeding from the assumption that the drop volume
losses occur due to evaporation caused by relatively low humidity during mea-
surements (40%), the measured contact angles were corrected by assuming volume
constancy [41] shown in Fig. 15.
The amorphous Teflon AF (a copolymer of tetrafluoroethylene and 2,2-
bis(trifluoroethylene)-4,5-difluoro-1,3-dioxole) which is a product of Du Pont, rep-
resents a new class of fluoropolymers. The stability of the surface properties of
Teflon AF films were described in detail [51]. The main conclusion drawn was that
the use of untreated Teflon AF as a model experimental object during long contact
with water is limited by the processes of its swelling and expansion.
Effectiveness of aqueous surfactant solutions at the cmc for model surfactants
used is presented in Fig. 16. Apparently from these experimental data, the following
conclusions arise:
(i) the effectiveness of surfactants strongly depends on the polymer surface used;