Surface Science of Elastomeric Coatings Prepared from α,ω-Dihydroxypoly(dimethylsiloxane) and the Ethoxysiloxane Mixture “ES40”

Abstract

Hybrid poly(dimethylsiloxane) networks have been prepared with the ethoxysiloxane mixture “ES40” that acts as a cross-linker and siliceous domain (SD) precursor through sol−gel chemistry. ES40, with an approximate formula (SiO(OEt)2)n, is a mixture of oligo-ethoxysiloxanes that is much less volatile than tetraethoxysilane (TEOS) and provides hybrid elastomeric coatings of reproducible composition. With SiOEt from ES40 and SiOH from HO(Me2SiO)nH, compositions with SiOEt/SiOH ratios from 5 to 35 (ES40-5x−ES40-35x) were prepared. Solid-state 29Si NMR spectroscopy was used to determine the relative amounts of PDMS and SD in the hybrid elastomers. The relative amounts of Q2, (⋮SiO)2Si(OH)2; Q3, (⋮SiO)3Si(OH); and Q4, (⋮SiO)4Si, in the SD were estimated by deconvolution of the 29Si NMR peaks. Both light microscopy and tapping mode atomic force microscopy (TM-AFM) show that PDMS-SD compositions “5x” and higher slowly develop “island-like” surface features while stored at ambient conditions. After 1 month cure, PDMS-SD surface features are ≤1 μm, while after 6 months feature size is broadly distributed up to ∼10 μm. In contrast, surface features developed quickly (24 h) with previously reported PDMS-FSD hybrids, where FSD is fluorinated siliceous domain, from (tridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane. AFM experience demonstrates that PDMS-SD surface features are often poorly adherent and easily moved compared to the robust phase-separated surface structure of PDMS-FSD materials. Differing dynamic contact angle (DCA) analysis protocols with water as the interrogating fluid show that water contamination affects force vs distance curve (fdc) data for PDMS-SD hybrids. Intrinsic wetting behavior for PDMS-SD hybrids is reproduced only with a protocol where clean water is used for each DCA cycle. The stability in water of PDMS-SD materials is strongly compositionally dependent. Over a period of 70 days, the ES40-14x composition showed the greatest resistance to mass loss with a mass loss rate of 0.08 wt %/month.