Surface science of elastomeric coatings prepared from alpha,omega-dihydroxypoly(dimethylsiloxane) and the ethoxysiloxane mixture "ES40"

Hybrid poly(dimethylsiloxane) networks have been prepared with the ethoxysi loxane mixture "ES40" that acts as a cross-linker and siliceous domain (SD) precursor through sol-gel chemistry. ES40, with an approximate formula (Si O(OEt)(2))(n),, is a mixture of oligo-ethoxysiloxanes that is much less vol atile than tetraethoxysilane (TEOS) and provides hybrid elastomeric coating s of reproducible composition. With SiOEt from ES40 and SiOH from HO(Me2SiO )(n)H, compositions with SiOEt/SiOH ratios from 5 to 35 (ES40-5x-ES40-35x) were prepared. Solid-state Si-29 NMR spectroscopy was used to determine the relative amounts of PDMS and SD in the hybrid elastomers. The relative amo unts of Q(2), (=SiO)(2)Si(OH)(2); Q(3), (=SiO)(3)Si(OH); and Q(4), (=SiO)(4 )Si, in the SD were estimated by deconvolution of the Si-29 NMR peaks. Both light microscopy and tapping mode atomic force microscopy (TM-AFM) show th at PDMS-SD compositions "5x" and higher slowly develop "island-like" surfac e features while stored at ambient conditions. After I month cure, PDMS-SD surface features are less than or equal to1 mum, while after 6 months featu re size is broadly distributed up to similar to 10 mum. In contrast, surfac e features developed quickly (24 h) with previously reported PDMS-FSD hybri ds, where FSD is fluorinated siliceous domain, from (tridecafluoro-1,1,2,2- tetrahydrooctyl)triethoxysilane. AFM experience demonstrates that PDMS-SD s urface features are often poorly adherent and easily moved compared to the robust phase-separated surface structure of PDMS-FSD materials. Differing d ynamic contact angle (DCA) analysis protocols with water as the interrogati ng 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 c ycle. The stability in water of PDMS-SD materials is strongly compositional ly 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.