Creating surface-anchored gradient networks using UV and thermally-active small molecular gelators

We present a versatile one-pot synthesis method for creating surface-anchored gradient networks using 4-(Azidosulfonyl) phenethyl trimethoxysilane (4-ASPTMS). The sulfonyl azide (SAz) group of 4-ASPTMS is UV (≤ 254 nm) and thermally active (≥ 100°C). It thus enables to vary the crosslink density in two opposite directions by activating the SAz groups independently via UV or temperature. We deposit a thin layer (~200 nm) of a mixture comprising ~90% precursor polymer and ~10% of 4-ASPTMS on a silicon wafer. Upon UV irradiation or annealing the layer, SAz releases nitrogen by forming singlet and triplet nitrenes that concurrently reacts with any C-H bond in the vicinity (via C-H insertion crosslinking reaction mechanism), forming the sulfonamide crosslinks. Condensation among trimethoxy groups in bulk connects the 4-ASPTMS units and completes the crosslinking. 4-ASPTMS near the substrate reacts with surface-bound -OH motifs and enables the network's covalent attachment to the substrate. Using such a simple process, we demonstrate the generation of surface-anchored gradient networks exhibiting crosslink density (or stiffness) gradients in orthogonal directions.