Creating responsive surfaces with tailored wettability switching kinetics and reconstruction reversibility

We report on the formation of responsive surfaces with tailorable surface reconstruction kinetics and switching hysteresis by thiolene radical addition of mercaptoalkanols with variable lengths to poly(vinylmethylsiloxane) networks. Exposing these elastomer surfaces to water results in rearrangement of the hydrophilic alkanes at the surface. The rearrangement kinetics decreases with increasing number of the methylene spacers (n) in the mercaptoalkanol. The response kinetics is found to be very fast for n = 2 and 6. For instance, upon exposing to water, the water contact angle on 3-mercaptopropanol-based surfaces decreases by approximately 35 degrees at the rate of 2 degrees /s. The high flexibility of the siloxane backbone endows these materials with switching longevity; the materials were able to switch their wettability over 10 cycles with minimum hysteresis. Increasing the number of methylene spacers to n = 11 decreases the surface reorganization dramatically. Formation of semicrystalline regions in such materials (detected via IR) is responsible for initial "sluggish" kinetics and eventual surface "freezing".