DYNAMIC WETTING AND CONTACT-ANGLE HYSTERESIS OF POLYMER SURFACES STUDIES WITH THE MODIFIED WILHELMY BALANCE METHOD

The dynamic wetting behavior of poly(tetrafluoroethylene) (PTFE), polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET), nylon 6, poly(ether urethane) (PU), poly(vinyl alcohol) (PVA), and cellulose was studied by the Wilhelmy balance technique at speeds of immersion from 1 to 50 mm/min. The Wilhelmy method was modified so as to determine contact angles without extrapolation of the loop to the zero immersion depth, employing a rectangular flat sample having a rectangular hole. This modification of the method allowed us to determine the advancing and receding contact angles on the very narrow sample area close to the lower (first) and the upper (second) sample-hole boundaries, theta1 and theta2, respectively. The interaction time of the sample part located at the lower boundary with the wetting liquid (water) was twice as long as that of the upper boundary. No difference was observed between the advancing contact angles measured at the lower and the upper parts of the sample (theta(ADV,1) = theta(ADV,2)) for all the Polymers, displaying that the dried polymer surfaces had no difference in wettability along the sample length. However, the lower part of the sample became more hydrophilic than the upper part during the wetting measurement for PET, PU, nylon 6, PVA, and cellulose, resulting in the difference between the receding contact angles (theta(REC,1) < theta(REC,2)). The effect was attributed to the time-dependent surface reorientation of hydrophilic and hydrophobic groups, occurring upon immersion of the polymer sample in water. A close correlation was observed between the hysteresis of the contact angle and the underwater surface reconstruction of polymers: the strongest hysteresis corresponds to the greatest wettability gradient generated by the time-dependent reorientation process. However, even when the effect of reorientation was zero (PTFE, PE, and PP) or very low (cellulose), the observed hysteresis was still as high as 27-degrees. The contribution of the surface reorientation of polar groups to the observed hysteresis was estimated to amount to 0-25-degrees, depending on the chemical structure of the polymer investigated. The speed of the sample immersion had no detectable effect on the wettability of PTFE, PE, and PP. On the other hand, the advancing contact angle on PET, PU, and nylon 6 increased while the receding contact angle decreased, as the immersion speed became higher. This behavior may be accounted for by referring to a model of macromolecular dynamics at the three-phase boundary.