A relatively new area that involves silicon-containing materials is the synthesis of “ultrastructure” materials, that is materials in which structure can be controlled at the level of around 100 Å. An example of such a synthesis is the “sol-gel” hydrolysis of alkoxysilanes (organosilicates) to give silica, SiO2. The reaction is complicated, involving polymerization and branching, but a typical overall reaction may be written . . . Si(OR)4 + 2H2O → SiO2 + 4ROH (1) . . . where the Si(OR)4 organometallic species is typically tetraethoxysilane (tetraethylorthosilicate) (TEOS, with R being C2H5). In this application, the precursor compound is hydrolyzed and then condensed to polymeric chains, the chains become more and more branched, and finally a continuous highly swollen gel is formed. It is first dried at moderately low temperatures to remove volatile species, and then is fired into a porous ceramic object. It can then be densified, if desired, and machined into a final ceramic part. Not surprisingly, the production of ceramics by this novel route has generated a great deal of interest. Its advantages, over the usual “heat-and-beat” (e.g., sintering) approach to ceramics, is (i) the higher purity of the starting materials, (ii) the relatively low temperatures required, (iii) the possibility of controlling the ultrastructure of the ceramic (to reduce the number of microscopic flaws that lead to brittleness), (iv) the ease with which ceramic coatings can be formed, and (v) the ease with which ceramic alloys can be prepared (for example, by hydrolyzing solutions of both silicates and titanates). This approach has been used to form ceramic-like phases in a wide variety of polymers. The one which has been studied the most in this regard is poly(dimethylsiloxane) (PDMS), the semi-inorganic polymer featured extensively in Chapter 4. This is due to PDMS being in the class of relatively weak elastomers most in need of reinforcement, and being capable of easily absorbing the precursor materials generally used in the sol-gel process. The same hydrolyses can be carried out within a polymeric matrix to generate particles of the ceramic material, typically with an average diameter of a few hundred angstroms. The polymer typically has end groups, such as hydroxyls, that can participate in the hydrolysis-condensation reactions.
Preparation and Electrochemical Characterization of Organic–Inorganic Hybrid Poly(Vinylidene Fluoride)-SiO2 Cation-Exchange Membranes by the Sol-Gel Method Using 3-Mercapto-Propyl-Triethoxyl-Silane
