UV- and Thermally-Active Bifunctional Gelators Create Surface-Anchored Polymer Networks.

Abstract

A versatile one-step synthesis of surface-attached polymer networks using small bifunctional gelators (SBG), namely 4-azidosulfonylphenethyltrimethoxysilane (4-ASPTMS) and 6-azidosulfonylhexyltriethoxysilane (6-ASHTES) is reported. A thin layer (≈200nm) of a mixture comprising ≈90% precursor polymer and 10% of 4-ASPTMS or 10% 6-ASHTES on a silicon wafer is deposited. Upon UV irradiation (≈l-254nm) or annealing (>100°C) layers, sulfonyl azides (SAz) release nitrogen by forming singlet and triplet nitrenes that concurrently react with any C─H bond in the vicinity resulting in sulfonamide crosslinks. Condensation among tri-alkoxy groups (i.e., methoxy or ethoxy) in bulk connects the SBG units, which completes the crosslinking. Concurrently, when such functionalities react with hydroxyl groups at the surface, which enable the covalent attachment of the crosslinked polymer chains. A systematic investigation on reaction mechanism and gel formation using spectroscopic ellipsometry (SE) and Fourier-transform infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR) is performed. Analogous thermally initiated gelation for both 4-ASPTMS and 6-ASHTES is found. The 6-ASHTES is UV inactive at ≈l-254nm, while the 4-ASPTMS is active and forms gels. The difference is attributed to the aromatic nature of 4-ASPTMS that absorb UV light at ≈l-254nm due to π-π* transition.