Aluminum polyphosphate gels structural evolution probed by NMR spectroscopy

The aim of this work was to investigate how the structure of aluminum polyphosphate gels change upon aging and drying. This is essential if one is interested in using a gel as a matrix to synthesize organic-inorganic hybrid materials. The liquid and solid samples were characterized by 27Al and 31P NMR spectroscopy. Larger polyphosphate chains make the main contribution to gel formation and the smaller units are expelled into the supernatant solution. Polyphosphate chains undergo hydrolysis and chain scission upon gelation. Samples aged in a moisture-rich environment turn into viscous liquids as a consequence of water uptake, followed by extensive hydrolysis. Samples exposed to low relative humidity environments dry yielding brittle samples in which larger chains are stable. Vacuum-dried samples still contain ca. 30%-weight water.