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Effects of post cure treatment in the glass transformation range on the structure and fire behavior of in situ generated silica/epoxy hybrids Abstract A new "in situ" sol-gel synthesis procedure was exploited to produce silica/epoxy nanocomposites with 6 wt.% maximum silica content. 3-Aminopropyltriethoxysilane (APTS) was used as a coupling agent. The experimental results (fouriertransform infrared spectroscopy, FTIR, small-angle X-ray scattering, SAXS, transmission electron microscopy, TEM, nuclear magnetic resonance, NMR, and dynamic mechanical analysis, DMA) support that the structure consists of nanosized silica particles (maximum 1.25 nm in size) embedded in a hybrid co-continuous network. A post cure non-isothermal heating from 15 to 100°C (beyond the Tg of the neat epoxy) caused Tg and storage modulus to increase. The fire behavior, that, owing to severe regulations (i.e., in aerospace engineering), often prevents composites applications, was also studied. The formed silica domains prevented melt dripping phenomena during vertical flame spread tests. Cone calorimetry tests showed a remarkable decrease of the heat release rate (HRR) for all the hybrid systems with respect to the neat cured resin, even at very low silica loadings (i.e., 2 wt.%). This decrease was much more pronounced for the hybrid structures that were not subjected to the post cure thermal treatment. The use of multiple structural investigation techniques allowed to choose among multiple hypothesis and conclude that nanoparticles clustering is the main reason of the effects of the post curing treatments. Graphical Abstract SAXS of the samples before (EPO) and after (EPO_t) post cure thermal treatment in the glass transformation range. Effect of the post cure on the glass transformation temperature (Tg) and heat release rate (HRR) Photos of samples EPO and EPO 2% Si after vertical flame spread tests Highlights• A hybrid co-continuous network embedding nanoparticles (maximum 1.25 nm in size) was obtained;• A post-cure was performed above the glass transformation temperature, T g , of neat epoxy;• The post cure makes T g , storage modulus and Heat Release Rate (HRR) to increase;• No dripping in flame test and up to 40% HRR reduction for only 2 wt.% silica content is observed;• The post cure effects depend on nanoparticles clustering in the glass transformation range.