Effect of tetraethoxysilane and tetramethoxysilane (TEOS/TMOS) on melting gel behavior
Creators: name: Al-Marzoki, Kutaiba Hussein
Publisher: name: No Publisher Supplied
Year: 2018

description: Melting gels are silica-based organic-inorganic hybrid gels. A characteristic of so called melting gels is that the gels, which are rigid at room temperature, are able to soften and re-soften at temperatures around 110 C. However, after consolidation at a temperature higher than 150 C, the gels no longer re-soften. In hybrid gels, which show melting gel behavior, the system contains a di-substituted alkoxysilane and a mono-substituted alkoxysilane. Melting gels have many applications such as corrosion protection for metals, alternatives for low-melting sealing glasses, and hermetic packaging for electrochemical devices.   Melting gels with phenyltriethoxysilane (PhTES) and diphenyldiethoxysilane (DPhDES), as well as phenyltrimethoxysilane (PhTMS) and diphenyldimethoxysilane (DPhDMS), have been studied in the past. It is known that melting gels do not form when PhTES or PhTMS is mixed with tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) alone. Nevertheless, it is possible to form melting gels when small amounts of TEOS or TMOS are added to PhTES/DPhDES and PhTMS/DPhDMS gels. This study was carried out to determine the limits of TEOS or TMOS additions, before melting gels lost the ability to soften.    The limit of TEOS or TMOS additions was determined for eight different mixtures. In all cases, the mole ratio of PhTES/DPhDES or PhTMS/DPhDMS was held constant at 1/0.25. The TEOS or TMOS addition was expected to give an increase in the number of bridging oxygens between silicons, -Si-O-Si-, during the hydrolysis and condensation polymerization stage. The miscibility limits were maximum for the gel containing PhTES and DPhDES in ethanol with 50% TEOS in methanol, and minimum for the gel containing PhTMS and DPhDMS in methanol with 5% TEOS in methanol.   An increase in the number of bridging oxygens that result when condensation polymerization is more complete was accompanied by a decrease in the total weight loss measured in thermal gravimetric studies, and to slight decreases in the consolidation temperature. The glass transition temperature recorded using differential scanning calorimetry increased for compositions with increasing TEOS in ethanol or TMOS in methanol. Different solvents, for example, TEOS in methanol, caused transesterification and trends that were not consistent. Helium pycnometer measurements showed that the density of all samples after consolidation increased with increasing the TEOS or TMOS additions. Those with TMOS had higher densities than those with TEOS, due to the higher reactivity and more complete hydrolysis of TMOS. Micro-hardness values for gels increased gradually with increasing TEOS in ethanol or TMOS in methanol. All compositions produced coatings with hydrophobic surfaces, with methoxy substituted compositions being slightly more hydrophobic than ethoxy-substituted compositions.   FTIR spectra revealed the species present in the consolidated gels. The peaks in the spectra were assigned to hydroxyl groups, phenyl and other organic groups, and Si-O-Si bonds in the organically modified glass network. Si-C direct bonds from the substituted siloxanes were retained in the consolidated hybrid glasses. The general trend was that peaks were shifted to higher wavenumbers in systems with TEOS, while peaks were shifted to lower wavenumbers in systems with TMOS, reflecting the faster gelation in TMOS systems.