Sol–Gel Derived Hybrid Thin Films: The Chemistry behind Processing

Abstract

Despite their apparent simple preparation by sol–gel process, silsesquioxanes based materials are quite complex and rich systems. Accordingly, a clear understanding of their elaboration chemistry and its relations to process and properties are important matter. In the particular case of thin film preparation, vitrification must be controlled to ensure high throughput and reliability. Using methylsilsesquioxane based films as a relevant model and a combination of IR spectroscopy, mass spectrometry, and solution 29Si NMR, a meaningful description of the film structure was obtained. Furthermore, vitrification, observed by dynamic mechanical analysis, was shown to depend strongly on the amounts of cages and extended structures present in the films. More interestingly, the proportion of these species can be tuned through a careful control of the elaboration chemistry. On one hand, long aging and high dilution yield a larger amount of cages, which results in a delay of the vitrification. On the other hand, pH appears as the convenient parameter to finely adjust the time span during which high quality films can be prepared from the sols, while preserving the film composition and final properties.

Architecture of Silsesquioxanes

“…8). Accordingly, the progress of condensation from isolated precursors to fully condensed species can be easily followed and the average degree of condensation (DOC) calculated to compare different preparation conditions (Matejka et al 2000;Dong et al 2003;Letailleur et al 2011;Borovin et al 2016a, b). For a given T n environment, the nature of the R group moves the chemical shifts toward lower frequencies with the following order: alkyls > allyl > vinyl~phenyl > H. For instance, the 29 Si chemical shift of [RSiO 1.5 ] 8 POSS has been reported around -65, -71, -80, -80, and -84 ppm for R = CH 2 CH 3 , CH 2 CH=CH 2 , CH=CH 2 , C 6 H 5 , and H, respectively (Cordes et al 2010).…”

“…If a full assignment of these numerous resonances is likely impossible, the 29 Si NMR spectrum can still be used as a fingerprint of the structural features present in the mixture. Letailleur et al followed this approach to conclude that when the pH of the water used to hydrolyzed methyltriethoxysilane was changed from 2 to 3, the same mixture of condensed species were obtained but after a period 20 times longer (Letailleur et al 2011).…”

Ionic liquids as dynamic templating agents for sol–gel silica systems: synergistic anion and cation effect on the silica structured growth

“…This entire process is highly dependent on several parameters like temperature, solvent, pH [11,12], presence of ions [11], air moisture [13], drying process and aging time [14]. The impact caused by changes in these parameters on the competing hydrolysis and condensation reactions, and as a consequence on the final products, is still not fully understood.…”

Indentation and Fracture of Hybrid Sol-Gel Silica Films

“…For a comprehensive understanding of sol-gel technology and hybrid film processing see refs (Brinker & Scherrer, 1990;Haas et al, 1999b;Letailleur et al, 2011).…”