"Big Dipper" Dynamic Contact Angle Curves for Pt-Cured PDMS on a Thermal Gradient: Interrelationships of Hydrosilylation, Si-H Autoxidation and Si-OH Condensation to a Secondary Network.

Platinum cure for PDMS coatings on a thermal gradient (45 - 140 °C) was carried out to study the effect of temperature on surface chemistry and wetting behavior. Motivation was interest in surfaces with continuous gradients in wettability for applications such as protein adsorption, controlling bacterial adhesion, directional movement of cells, and biosensors. The Wilhelmy plate method and the advancing/receding drop method were employed for determining the positional dependence of θA and θR . A strong dependence of receding contact angles (R) on cure temperature was found for Sylgard 184 (S-PDMS) and a Pt-cured laboratory prepared analog (Pt PDMS) of known composition. Cure on the thermal gradient gave rise to striking "Big Dipper" Wilhelmy plate dynamic contact angle (DCA) curves. High contact angle hysteresis (60-80°) was found for 45 °C cure (CAH = θΔ = θA - θR ) but low CAH for 140 °C cure (10-20°). Drop addition/withdrawal using goniometry identified a similar trend. ATR-IR spectroscopy showed absorptions for Si-OH (3500 cm-1 ) and Si-H (1250 cm-1 ) that were correlated with wetting behavior and near surface chemistry. These studies revealed a complex relationship among hydrosilylation, Si-H autoxidation and condensation of Si-OH. A model for advancing from a single network due to hydrosilylation to a double network for hydrosilylation plus Si-O-Si from condensation of Si-OH best explains evidence from spectroscopic and contact angle studies. These results are relevant to interactions of Pt-cured silicones at bio-interfaces, as receding contact angles determine work of adhesion, as well as applications that benefit from maximum hydrophobicity and minimizing water roll-off angles.