Solutions of hybrid silica microgels as precursors of sol–gel coatings

Abstract

A two-step process to synthesize a new type of organic–inorganic hybrid sol, that may be used for a variety of processes related to sol–gel chemistry, is analyzed. The first step consisted of a typical inorganic polycondensation of tetraethoxysilane (TEOS) and 3-methacryloxypropyl-trimethoxysilane (MPMS) leading to a conventional sol. In the second step a convenient amount of a solvent (isopropanol), and a vinyl co-monomer (2-hydroxyethylmethacrylate, HEMA) were added. The free-radical co-polymerization of the CC groups of MPMS and those of the co-monomer was carried out by using a suitable initiator (2,2′-azobis(isobutyronitrile), AIBN). The advance in the organic and inorganic polymerizations was followed employing different experimental techniques: FTIR, ¹³C NMR, ²⁹Si NMR and viscosity measurements. The key to the new process was the amount of solvent added before the second stage such that gelation did not occur after complete consumption of CC groups. This led to a solution of hybrid microgels (a hybrid sol) that was characterized by scanning electron microscopy (SEM) and exhibited the following characteristics: a) it could be stored for prolonged periods at room temperature without gelation, b) it led to extremely fast gelation by solvent evaporation and c) it had a high wettability that allowed the production of coatings over different substrates. Besides, in this new process it is possible to add co- and ter-monomers in the second step to provide a desired functionality to the resulting coating. These concepts may be used to synthesize a variety of sols for the extended field of sol–gel processes.