We tested the use of two hydrophobic methyl-substituted silane precursors, methyltriethoxysilane and dimethyldiethoxysilane, to synthesize methyl-modified silica sols by a two-step method and a cohydrolysis method to produce durable antireflective films with high laser-induced-damage thresholds (LIDTs). Using small-angle x-ray scattering technology, we obtained details of the microstructure of clusters in sol and found various double fractal structural characteristics in the methyl-modified silica clusters; our findings were confirmed by transmission-electron micrographs. Through a magic-angle spin nuclear magnetic resonance study of the corresponding xerogels, we determined the double-fractal microstructure, which we then related to the LIDTs of AR films. The distribution configuration of methyls in clusters determined the double-fractal microstructure of clusters and then the LIDTs of AR films. The LIDTs of films produced by the cohydrolysis method (the highest was 38 J/cm2 for 1-ns, 1064-nm laser action) were much higher than those from the two-step method because of the loose netlike clusters in the former configuration. During the 220-day aging, the transmittance of hydrophobic AR film decreased ∼0.2%. So it is practicable to prepare durable AR films with higher LIDTs than those of normal AR films only by introducing hydrophobic methyls into a Si–O–Si matrix of clusters if an appropriate hydrophobic precursor is chosen.
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