Thickness, morphology and structure of sol-gel hybrid films: I - The role of the precursor solution's ageing

The physical properties of inorganic-organic sol-gel hybrid films are determined by a large number of processing parameters, as the initial recipe, the catalysis conditions and the film deposition technique. This work focuses on the influence of the precursor solution's ageing on the quality and structure of hybrid silica-polytetrahydrofuran films, prepared by spin coating. The inorganic precursor, tetraethylorthosilicate, was hydrolysed under acid catalysis and using ethanol as co-solvent. The hydrolysis molar ratio (R) was fixed at 4. Different polymer concentrations of two average molecular weights (M-n = 650 and 2900) were added to the initial colloidal solutions. Ageing took place in open containers. The thickness and surface texture parameters of the films were determined by profilometry, and their structures studied by FTIR. It is shown that the films' thickness increases with the ageing of the precursor solution, more steeply for the hybrid films with higher polymer content and higher M-n. The best films obtained, which present the highest thickness and the lowest relative roughness, are those prepared from the most aged solutions. The FTIR results show that the films' porosity increases with the ageing of the precursor solution, and that the distribution of oligomeric species in solution is maintained upon film deposition. Therefore, the structure of the film is essentially determined by the extension of the condensation reactions until the spinning moment.