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“Big Dipper” Dynamic Contact Angle Curves for Pt-Cured Poly(dimethylsiloxane) on a Thermal Gradient: Inter-relationships of Hydrosilylation, Si–H Autoxidation, and Si–OH Condensation to a Secondary Network
journal contribution
posted on 2019-01-25, 00:00 authored by Kayesh
M. Ashraf, Chenyu Wang, Sithara S. Nair, Kenneth J. WynnePlatinum cure for poly(dimethylsiloxane)
(PDMS) coatings on a thermal
gradient (45–140 °C) was carried out to study the effect
of temperature on surface chemistry and wetting behavior. The motivation
is the 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
(θR) on cure temperature was found for Sylgard 184
(S-PDMS) and a Pt-cured laboratory-prepared analogue (Pt-PDMS) of
known composition. Cure on the thermal gradient gave rise to striking
“Big Dipper” Wilhelmy plate dynamic contact angle 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.
Attenuated total reflectance infrared 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.
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Keywords
hydrosilylationThermal GradientPt-cured siliconessurface chemistrySecondary Network Platinum curePDMSS-PDMSwater roll-off anglescure temperatureθ ΔWilhelmy plate methodgradientPt-cured laboratory-prepared analoguecontact anglesnear-surface chemistryθ RSiCAHcontact angle curvesSylgard 184protein adsorptioncontact angle studies
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