The effect of silica precursor on sol-gel crosslinking: kinetic modeling and experimental analysis

The synthesis of silica materials, including nanofibers, is frequently achieved through the sol-gel process. This method involves the hydrolysis and condensation reactions of precursor molecules, which result in the formation of a cross-linked network. The reaction kinetics and final material structure are significantly affected by factors such as the reagent concentration, temperature and catalyst. Despite the critical need for efficient material tuning, our understanding of how synthesis parameters impact material structure remains largely reliant on trial-and-error experimental approaches. In its most researched form, the sol-gel synthesis starts from tetraethylorthosilicate (TEOS), which has four reactive ethoxy groups. The CTSE research group at Ghent University, where this thesis work was conducted, has made significant advances in the understanding of the kinetics of TEOS-based silica sol-gel synthesis through the integration of advanced kinetic modelling and 29Si nuclear magnetic resonance (NMR) techniques. The first objective of this thesis is to expand the existing knowledge of silica sol-gel systems by investigating the use of a different precursor, methyltriethoxysilane (MTES), which possesses three reactive ethoxy groups. The impact of the number of functional groups on crosslinking reactions remains poorly understood. Gaining insights into this aspect could enhance the use of various precursors in the production of nanofibrous membranes via electrospinning. This thesis work combines kinetic modeling with 29Si NMR analysis to determine the rate coefficients of hydrolysis and condensation reactions during MTES sol-gel synthesis and compare this with TEOS crosslinking. The second objective of this thesis is to link (organo)silica network characteristics to the flow behaviour of the sol. The flow behaviour, such as the viscosity evolution during the reaction, is a key parameter determining the sol electrospinnability but is highly dependent on the sol synthesis conditions. Combining experimental 29Si NMR analysis and kinetic modeling of the network, allows the extraction of intrinsic network characteristics such as conversion and fraction of functional groups. These intrinsic parameters, when linked with the viscosity data, can help compare different precursors and their electrospinnability. This approach aims to improve the understanding of the rheology of silica and organosilica sols and their processability into nanofibers.