Sticky or Slippery Wetting: Network Formation Conditions Can Provide a One-Way Street for Water Flow on Platinum-cured Silicone

In the course of studies on Sylgard 184 (SPDMS), we discovered strong effects on receding contact angles (CAs), Or, while cure conditions have little effect on advancing CAs. Network formation at high temperatures resulted in high Bad of 115-120 degrees and high theta(rec) >= 80 degrees. After network formation at low temperatures (<= 25 degrees C), theta(adv) was still high but theta(rec) was 30-50 degrees. Uncertainty about compositional effects on wetting behavior resulted in similar experiments with a model (DDH)-D-V silicone elastomer (Pt-PDMS) composed of a vinyl-terminated poly(dimethylsiloxane) (PDMS) base and a polymeric hydromethylsilane cross-linker. Again, network formation at high temperature (similar to 100 degrees C) resulted in high CAs, while low-temperature curing retained high advancing CAs but gave low receding CAs (theta(rec) 30-50 degrees). These changes in receding CAs translate to strong effects on water adhesion, w(p), which is the actual work required to separate a liquid (water) from a surface: w(p) proportional to (1 + theta(rec)). When the values theta(rec), 84 for high-temperature and theta(rec) 50 for low-temperature network formation are used, w(p) is similar to 1.5 times higher for curing at low temperature. The origin of low receding contact angles was investigated by attenuated total reflectance IR spectroscopy. Absorptions for Si-OH hydrogen bonded to water (3350 cm(-1)) were stronger for low- versus high-temperature curing. This result is attributed to faster hydrosilylation during curing at higher temperatures that consumes Si-H before autoxidation to Si-OH. Sharp bands at 3750 and 3690 cm(-1) due to isolated-Si-OH are more prominent for Pt-PDMS than those for S-PDMS, which may be due to an effect of functionalized nanofiller. To explore the impact of w(p) on water droplet flow, gradient coatings of S-PDMS and Pt-PDMS elastomers were prepared by coating a slide, maintaining opposite ends at high and low temperatures and thus forming a thermal gradient. When the slide was tilted, a droplet moved easily on the high-temperature end (slippery surface) but became pinned at the low-temperature end (sticky surface) and did not move when the slide was rotated 180 degrees. The surface was therefore a one-way street for water droplet flow. Theory provides fundamental understanding for slippery/sticky behavior for gradient S-PDMS and Pt-PDMS coatings. A model for network formation is based on hydrosilylation at high temperature and condensation curing of Si-OH from autoxidation of Si-H at low temperatures. In summary, network formation conditions strongly affect receding contact angles and water adhesion for Sylgard 184 and the filler-free mimic Pt-PDMS. These findings suggest careful control of curing conditions is important to silicones used in microfluidic devices or as biomedical materials. Network-forming conditions also impact bulk mechanical properties for Sylgard 184, but the range that can be obtained has not been critically examined for specific applications.