Modified sol-gel hybrid borate structures

A polymeric sol-gel process was used for the synthesis of borate hybrid materials in the system B2O3-PVA-PEG (where PVA=poly(vinyl alcohol) and PEG= poly(ethylene glycol)). The materials subsequently were modified with SiO2 or Al2O3. The synthesis used the following precursors: (CH3O)(3)B, (C2H5O)(4)Si, (C4H9O)(3)Al, PVA, PEG Partial replacement of the polymeric component up to 20 wt % (PVA, PEG) with SiO2 or Al2O3 and with constant B2O3 (5 wt%), results in more homogeneous gel films with better adhesion, transparency and mechanical resistance. DTA, XRD and SEM analysis show that all materials are amorphous. Thermal stability of the modified hybrid structure moves up to 500 degrees C compared with 350 degrees C for the unmodified form. Above these temperatures, hybrid structures decompose, and crystallisation processes start with formation of secondary boron acid (H3BO3), aluminium borate (2Al(2)O(3).3B(2)O(3)) and graphitic carbon. The FTIR method is used to follow structural changes and the mechanism of crosslinking in the gels. The patterns show that the more likely hybrid structure is constructed from BO4, BO3, SiO4, AlO4 sites crosslinked by longer polymeric chains. The presence of mixed bonds results in more homogeneous and stable materials.