Epoxy networks reinforced with TiO2 generated by nonhydrolytic sol-gel process: A comparison between in situ and ex situ syntheses to obtain filled polymers

Titania nanoparticles suspended in benzyl alcohol (BzOH) were generated by nonhydrolytic sol-gel process starting from titanium tetrachloride. The obtained suspensions were mixed with 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate epoxy resin, and the formulations were cured in the presence of ytterbium(III) trifluoromethanesulfonate as cationic initiator. The thermally activated cationic ring-opening polymerization produced a three-dimensional network in which the suspending medium BzOH was covalently linked to the epoxy network according to the "activated monomer" mechanism during the propagation step. The presence of titania nanoparticles resulted in a reinforcing and stiffening effect due to both their hydrodynamic effect and, most important, a significantly higher cross-linking density of the composite material with respect to the unfilled epoxy resin. A comparison with similar composites obtained by physical incorporation of preformed titania particles was also reported in order to compare the in situ and ex situ filler generation for preparing nanocomposites