(731f) BIG Dipper Dynamic Contact Angle Curves for Pt-Cure PDMS Gradients

Abstract: Poly(dimethylsiloxane) (PDMS) elastomers have found numerous applications including flexible electronics, cell culture substrates microfluidics and biomedical devices due to ease of processing, fast curing at relatively low temperatures and unique surface and bulk properties. There are no set rules for the cure process. However, in one of our previous works¹, we observed that while cure conditions had little effect on advancing CAs, there were strong effects on receding CAs. Based on these findings, we created a thermal gradient during network formation to observe the positional dependence of receding contact angles. In this method, one end of PDMS coating from Sylgard 184 (S-PDMS) and model Pt-cured silicone elastomer (Pt-PDMS) on coverslips (40´22 mm²) was cured at ³40 °C while the other end was cured at >140 °C for different durations of time (24 h, 48 h and 96 h). A unique “Big Dipper” shaped dynamic water contact angle (DCA) curve was obtained. A high receding CA was obtained on the high temperature (>140 °C) cured end but low receding CA was observed on the low temperature cured end. Constant high advancing CA (110-125 °C) was observed on both ends. DCA probes the surface with the liquid (e.g., water) as the sample slowly immerses in and emerges from the liquid in a container giving rise to a continuous contact angles versus immersion depth or force-distance curves. To our knowledge, this is the first time that the entire surface of a gradient coating was continuously measured by DCA method. The reasons for this varying receding contact angles were examined by goniometry, ATR-IR, water drop mobility and X-ray photoelectron spectroscopy. The results showed that while advancing contact angles were little influenced by curing temperature, there is a pronounced effect on receding contact angles and water drop adhesion. Thus, water drop adhesion can be controlled through control of curing temperature.

Reference:

1. Wang, C.; Nair, S. S.; Veeravalli, S.; Moseh, P.; Wynne, K. J., Sticky or Slippery Wetting: Network Formation Conditions Can Provide a One-Way Street for Water Flow on Platinum-cured Silicone. ACS Appl Mater Interfaces 2016, 8 (22), 14252-62.