Polycarboxylic acids as network modifiers for water durability improvement of inorganic–organic hybrid tin-silico-phosphate low-melting glasses

Abstract

We investigated the water durability of the inorganic–organic hybrid tin-silico-phosphate glasses Me 2SiO–SnO–P 2O 5 (Me designs the organic methyl group) doped with organic acids (salicylic acid (SA), tartaric acid (TA), citric acid (CA) and butane tetracarboxylic acid (BTCA)) containing one or more of carboxylic groups per molecule. The structure, thermal properties and durability of the final glasses obtained via a non-aqueous acid–base reaction were discussed owing to the nature and the concentration of the acid added. 29Si magic angle spinning (MAS) NMR and 31P MAS NMR spectra, respectively, showed clearly a modification of the network in the host glass matrix of the Me 2SiO–SnO–P 2O 5 system. The polycondensation enhancement to form –P–O–Si–O–P– linkages (PSP) and the increase of the Q 2 unit (two bridging oxygens per phosphorus atom) over the Q 3 unit (three bridging oxygens per phosphorus atom) as a function of the acid in the order SA<TA<CA<BTCA, suggest the formation of a chain-like structure which contrasts with the high cross-linkage in the Me 2SiO–SnO–P 2O 5 matrix. In addition, this structural change is accompanied by a decrease of the coefficient of thermal expansion and an increase of the water durability of the glasses with the acids containing a large number of carboxylic groups per molecule. The presence of carboxylic groups of the acid acting as network modifier may retard the movement of water molecules through the glasses due to the steric hindrance strengthening the PSP connections in a chain-like structure.