Curing behavior and microstructure of epoxy-POSS modified novolac phenolic resin with different substitution degree

Easily oxidized group and rigid structure of phenolic resin (PR) network can result in brittleness and low char yield at high temperatures, which limits the widespread applications of PR in harsh environment. Whilst organic-inorganic hybrid material, such as POSS-containing material was introduced to enhance PR, the microstructure and the thermo-mechanical properties of the hybrid material modified PR are not well understood. Therefore, in the work, a novel organic-inorganic hybrid networks (ENH) were prepared with epoxy-polyhedral oligomeric silsesquioxane (EPOSS) modified novolac phenolic resin (EN) and hexamethylenetetramine (HMTA) to further clarify the curing behavior and its relation to microstructure and the resulting thermal-mechanical properties. Here, the curing characteristics of ENH were discussed by usinag advanced isoconversional method and Fourier transform infrared spectroscopy (FTIR). It was revealed that a diversity of crosslinking networks was exhibited after curing ENH, including EPOSS-NR crosslinked, tethered structure, and self-polymerized structure of EPOSS as well as NH networks. When the substitution degree of EPOSS is relatively low, EPOSS serves as a junction point to further improve the crosslinking density of ENH networks, leading to better thermo-mechanical properties, thermal stability and tensile property as well as lower dielectric properties. However, as the self-polymerization of EPOSS increased, the NH network was dramatically disrupted and the thermo-mechanical properties, thermal stability and tensile property of ENH were deteriorated.