Chemical evolution of the [formula omitted] gel: A 31P NMR investigation

Abstract

The chemical evolution accompanying the gel heating was studied in the case of the Na 2 O P 2 O 5 = 1.4 system. In order to identify single chemical components of the prepared amorphous materials, the samples obtained by heating the crude product at different temperatures and relevant melted glasses were analyzed by 31P NMR spectroscopy using suitable solvents and in the presence of cation complexing agents, to simplify NMR analysis and to facilitate sample dissolution. Results indicate that the starting gel is composed of monomeric phosphoric units organized in an amorphous structure. These species, on heating, evolve to oligomeric compounds analogous to the products which may be obtained only by keeping the corresponding glasses prepared by melting at high temperature for long times. Gel-derived glass after melting is practically composed of the same species of the glass prepared by the melting method, although the NMR analysis indicates the presence of some residual -OH groups in the gel-derived material.